U.S. patent application number 11/968185 was filed with the patent office on 2009-01-15 for prosthetic intervertebral discs.
This patent application is currently assigned to Spinal Kinetics, Inc.. Invention is credited to Thomas A. Afzal, Uriel Hiram Chee, In Haeng Cho, Curtis W. Frank, Sung Kyu Ha, Daniel H. Kim, Kunwoo Lee, Michael L. Reo.
Application Number | 20090018661 11/968185 |
Document ID | / |
Family ID | 34104410 |
Filed Date | 2009-01-15 |
United States Patent
Application |
20090018661 |
Kind Code |
A1 |
Kim; Daniel H. ; et
al. |
January 15, 2009 |
Prosthetic Intervertebral Discs
Abstract
Prosthetic intervertebral discs and methods for using the same
are described. The subject prosthetic discs include upper and lower
endplates separated by a compressible core member. The prosthetic
discs described herein include one-piece, two-piece, three-piece,
and four-piece structures. The subject prosthetic discs exhibit
stiffness in the vertical direction, torsional stiffness, bending
stiffness in the saggital plane, and bending stiffness in the front
plane, where the degree of these features can be controlled
independently by adjusting the components of the discs. The
interface mechanism between the endplates and the core members of
several embodiments of the described prosthetic discs enables a
very easy surgical operation for implantation.
Inventors: |
Kim; Daniel H.; (Mountain
View, CA) ; Afzal; Thomas A.; (Menlo Park, CA)
; Reo; Michael L.; (Redwood City, CA) ; Chee;
Uriel Hiram; (Santa Cruz, CA) ; Cho; In Haeng;
(Seoul, KR) ; Lee; Kunwoo; (Seoul, KR) ;
Frank; Curtis W.; (Cupertino, CA) ; Ha; Sung Kyu;
(Kyunggi-do, KR) |
Correspondence
Address: |
Wheelock Chan LLP
P.O. Box 61168
Palo Alto
CA
94306
US
|
Assignee: |
Spinal Kinetics, Inc.
Sunnyvale
CA
|
Family ID: |
34104410 |
Appl. No.: |
11/968185 |
Filed: |
January 1, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10903276 |
Jul 30, 2004 |
|
|
|
11968185 |
|
|
|
|
10632538 |
Aug 1, 2003 |
7153325 |
|
|
10903276 |
|
|
|
|
Current U.S.
Class: |
623/17.16 ;
623/17.11; 623/17.15 |
Current CPC
Class: |
A61F 2002/30733
20130101; A61F 2/30742 20130101; A61F 2002/30383 20130101; A61F
2002/30841 20130101; A61F 2002/4627 20130101; A61F 2230/0015
20130101; A61F 2310/00023 20130101; A61F 2002/30566 20130101; A61F
2002/30594 20130101; A61F 2310/00017 20130101; A61F 2002/30578
20130101; A61F 2002/30069 20130101; A61F 2002/30133 20130101; A61F
2002/30523 20130101; A61F 2/4611 20130101; A61F 2002/30462
20130101; A61F 2310/00029 20130101; A61F 2/30965 20130101; A61F
2002/4495 20130101; A61F 2/4425 20130101; A61F 2002/3052 20130101;
A61F 2002/30604 20130101; A61F 2002/30884 20130101; A61F 2002/4622
20130101; A61F 2/30767 20130101; A61F 2002/305 20130101; A61F
2002/30904 20130101; A61F 2310/00179 20130101; A61F 2002/448
20130101; A61F 2220/0075 20130101; A61B 17/86 20130101; A61F
2220/0025 20130101; A61F 2002/30563 20130101; A61F 2310/00161
20130101; A61F 2002/4628 20130101; A61F 2/442 20130101; A61L
2430/38 20130101 |
Class at
Publication: |
623/17.16 ;
623/17.11; 623/17.15 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Claims
1. A prosthetic intervertebral disc comprising: a first endplate, a
second endplate having a slot on its internal periphery, a core
member fixedly attached on a first side to said first endplate and
having a flange on a second side opposite said first side, said
flange adapted to engage the slot on said second endplate to
thereby attach said core member to said second endplate.
2. The prosthetic intervertebral disc of claim 1 further comprising
a fixation member for securing said first endplate to a vertebral
body.
3. The prosthetic intervertebral disc of claim 1 further comprising
a fixation member for securing said second endplate to a vertebral
body.
4. The prosthetic intervertebral disc of claim 2 wherein said
fixation member comprises at least one anchoring fin.
5. The prosthetic intervertebral disc of claim 3 wherein said
fixation member comprises at least one anchoring fin.
6. The prosthetic intervertebral disc of claim 2 wherein said
fixation member comprises at least one anchoring spike.
7. The prosthetic intervertebral disc of claim 3 wherein said
fixation member comprises at least one anchoring spike.
8. The prosthetic intervertebral disc of claim 2 wherein said
fixation member comprises a plurality of serrations formed on an
external surface of at least one of said first and said second
endplates.
9. The prosthetic intervertebral disc of claim 3 wherein said
fixation member comprises a plurality of serrations formed on an
external surface of at least one of said first and said second
endplates.
10. The prosthetic intervertebral disc of claim 1 wherein said
flange of said core member and said slot of said second endplate
cooperate to selectively attach said second endplate to said core
member while allowing the flange to slide relatively freely within
the confines of said slot.
11. The prosthetic intervertebral disc of claim 10 wherein said
flange is formed on an inner second endplate attached to said core
member.
12. The prosthetic intervertebral disc of claim 10 further
comprising a retaining lip formed on said second endplate and
adapted to retain said core member on said second endplate in a
manner that allows the flange to slide relatively freely within the
confines of the slot.
13. The prosthetic intervertebral disc of claim 11 further
comprising a plurality of fibers connecting said first endplate to
said inner second endplate.
14. The prosthetic intervertebral disc of claim 1 further
comprising a center core retained within said core member, wherein
said center core comprises one of either polyurethane or silicone.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of application Ser. No.
10/903,276, filed Jul. 30, 2004, which in turn, is a
continuation-in-part of co-pending application Ser. No. 10/632,538,
filed Aug. 1, 2003, which prior applications are incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] The intervertebral disc is an anatomically and functionally
complex joint. The intervertebral disc is composed of three
component structures: (1) the nucleus pulposus; (2) the annulus
fibrosus; and (3) the vertebral endplates. The biomedical
composition and anatomical arrangements within these component
structures are related to the biomechanical function of the
disc.
[0003] The spinal disc may be displaced or damaged due to trauma or
a disease process. If displacement or damage occurs, the nucleus
pulposus may herniate and protrude into the vertebral canal or
intervertebral foramen. Such deformation is known as herniated or
slipped disc. A herniated or slipped disc may press upon the spinal
nerve that exits the vertebral canal through the partially
obstructed foramen, causing pain or paralysis in the area of its
distribution.
[0004] To alleviate this condition, it may be necessary to remove
the involved disc surgically and fuse the two adjacent vertebra. In
this procedure, a spacer is inserted in the place originally
occupied by the disc and it is secured between the neighboring
vertebrae by the screws and plates/rods attached to the vertebra.
Despite the excellent short-term results of such a "spinal fusion"
for traumatic and degenerative spinal disorders, long-term studies
have shown that alteration of the biomechanical environment leads
to degenerative changes at adjacent mobile segments. The adjacent
discs have increased motion and stress due to the increased
stiffness of the fused segment. In the long term, this change in
the mechanics of the motion of the spine causes these adjacent
discs to degenerate.
[0005] To circumvent this problem, an artificial intervertebral
disc replacement has been proposed as an alternative approach to
spinal fusion. Although various types of artificial intervertebral
discs have been developed to restore the normal kinematics and
load-sharing properties of the natural intervertebral disc, they
can be grouped into two categories, i.e., ball and socket joint
type discs and elastic rubber type discs.
[0006] Artificial discs of ball and socket type are usually
composed of metal plates, one to be attached to the upper vertebra
and the other to be attached to the lower vertebra, and a
polyethylene core working as a ball. The metal plates have concave
areas to house the polyethylene core. The ball and socket type
allows free rotation between the vertebrae between which the disc
is installed and thus has no load sharing capability against the
bending. Artificial discs of this type have a very high stiffness
in the vertical direction, they cannot replicate the normal
compressive stiffness of the natural disc. Also, the lack of load
bearing capability in these types of discs causes adjacent discs to
take up the extra loads resulting in the eventual degeneration of
the adjacent discs.
[0007] In elastic rubber type artificial discs, an elastomeric
polymer is embedded between metal plates and these metal plates are
fixed to the upper and the lower vertebrae. The elastomeric polymer
is bonded to the metal plates by having the interface surface of
the metal plates be rough and porous. This type of disc can absorb
a shock in the vertical direction and has a load bearing
capability. However, this structure has a problem in the interface
between the elastomeric polymer and the metal plates. Even though
the interface surfaces of the metal plates are treated for better
bonding, polymeric debris may nonetheless be generated after long
term usage. Furthermore, the elastomer tends to rupture after a
long usage because of its insufficient shear-fatigue strength.
[0008] Because of the above described disadvantages associated with
either the ball/socket or elastic rubber type discs, there is a
continued need for the development of new prosthetic devices.
RELEVANT LITERATURE
[0009] U.S. Pat. Nos. 3,867,728; 4,911,718; 5,039,549; 5,171,281;
5,221,431; 5,221,432; 5,370,697; 5,545,229; 5,674,296; 6,162,252;
6,264,695; 6,533,818; 6,582,466; 6,582,468; 6,626,943; 6,645,248.
Also of interest are published U.S. Patent Application Nos.
2002/0107575, 2003/0040800, 2003/0045939, and 2003/0045940. See
also Masahikio Takahata, Uasuo Shikinami, Akio Minami, "Bone
Ingrowth Fixation of Artificial Intervertebral Disc Consisting of
Bioceramic-Coated Three-dimensional Fabric," SPINE, Vol. 28, No. 7,
pp. 637-44 (2003).
SUMMARY OF THE INVENTION
[0010] Prosthetic intervertebral discs and methods for using such
discs are provided. The subject prosthetic discs include an upper
endplate, a lower endplate, and a compressible core member disposed
between the two endplates.
[0011] In one embodiment, the subject prosthetic discs are
characterized by including top and bottom endplates separated by a
fibrous compressible element that includes an annular region and a
nuclear region. The two plates are held together by at least one
fiber wound around at least one region of the top endplate and at
least one region of the bottom endplate. The subject discs may be
employed with separate vertebral body fixation elements, or they
may include integrated vertebral body fixation elements. Also
provided are kits and systems that include the subject prosthetic
discs.
[0012] In other embodiments, the prosthetic disc comprises an
integrated, single-piece structure. In another embodiment, the
prosthetic disc comprises a two-piece structure including a lower
endplate and a separable upper endplate assembly that incorporates
the core member. The two-piece structure may be a constrained
structure, wherein the upper endplate assembly is attached to the
lower endplate in a manner that prevents relative rotation, or a
partially or semi-constrained structure or an unconstrained
structure, wherein the upper endplate assembly is attached to the
lower endplate in a manner that allows relative rotation. In yet
another, embodiment, the prosthetic disc comprises a three-piece
structure including upper and lower endplates and a separable core
member that is captured between the upper and lower endplates by a
retaining mechanism. Finally, in yet another embodiment, the
prosthetic disc comprises a four-piece structure including upper
and lower endplates and two separable core assemblies which,
together, form a core member.
[0013] Several optional core materials and structures may be
incorporated in each of the prosthetic disc embodiments described
herein. For example, the core member may be formed of a relatively
compliant material, such as polyurethane or silicone, and is
typically fabricated by injection molding. In other examples, the
core member may be formed by layers of fabric woven from fibers. In
still further examples, the core member may comprise a combination
of these materials, such as a fiber-reinforced polyurethane or
silicone. As an additional option, one or more spring members may
be placed between the upper and lower endplates in combination with
the core member, such as in a coaxial relationship in which the
core member has a generally cylindrical or toroidal shape and a
spring is located at its center.
[0014] In the various embodiments, the disc structures are held
together by at least one fiber wound around at least one region of
the upper endplate and at least one region of the lower endplate.
The fibers are generally high tenacity fibers with a high modulus
of elasticity. The elastic properties of the fibers, as well as
factors such as the number of fibers used, the thickness of the
fibers, the number of layers of fiber windings, the tension applied
to each layer, and the crossing pattern of the fiber windings
enable the prosthetic disc structure to mimic the functional
characteristics and biomechanics of a normal-functioning, natural
disc.
[0015] Apparatus and methods for implanting prosthetic
intervertebral discs are also provided. In a first embodiment, the
apparatus includes three implantation tools used to prepare the two
adjacent vertebral bodies for implantation and then to implant the
prosthetic disc. A first tool, a spacer, is adapted to be inserted
between and to separate the two adjacent vertebral bodies to create
sufficient space for implanting the prosthetic disc. A second tool,
a chisel, includes one or more wedge-shaped cutting blades located
on its upper and/or lower surfaces that are adapted to create
grooves in the inward facing surfaces of the two adjacent vertebral
bodies. A third tool, a holder, includes an engagement mechanism
adapted to hold the prosthetic disc in place while it is being
implanted, and to release the disc once it has been implanted.
[0016] In another embodiment, the implantation apparatus includes a
guide member that engages the lower endplate and that remains in
place during a portion of the disc implantation process. A lower
pusher member slidably engages the guide member and is used to
advance the lower endplate into place between two adjacent
vertebrae of a patient's spine. An upper pusher member is
preferably coupled to the lower pusher member and is used to
advance a first chisel into place opposed to the lower endplate
between the two adjacent vertebrae. Once in place, an upward force
is applied to the upper pusher member to cause the first chisel to
engage the upper vertebral body and to create one or more grooves
on its lower surface. A downward force is also applied to the lower
pusher member to cause the lower endplate to engage the lower
vertebral body and to become implanted. The upper pusher member and
first chisel are then removed, as is the lower pusher member.
Preferably, a second chisel is then advanced along the guide member
and is used to provide additional preparation of the upper
vertebral body. After the completion of the preparation by the
first chisel and, preferably, the second chisel, the upper endplate
and core members of the prosthetic disc are implanted using an
upper endplate holder that is advanced along the guide member.
After implantation, the upper endplate holder and guide member are
removed.
[0017] Apparatus and methods for implanting prosthetic
intervertebral discs using minimally invasive surgical procedures
are also provided. In one embodiment, the apparatus includes a pair
of cannulas that are inserted posteriorly, side-by-side, to gain
access to the spinal column at the disc space. A pair of prosthetic
discs are implanted by way of the cannulas to be located between
two vertebral bodies in the spinal column. In another embodiment, a
single, selectively expandable disc is employed. In an unexpanded
state, the disc has a relatively small profile to facilitate
delivery of it to the disc space. Once operatively positioned, it
can then be selectively expanded to an appropriate size to
adequately occupy the disc space. Implantation of the single disc
involves use of a single cannula and an articulating chisel or a
chisel otherwise configured to establish a curved or right angle
disc delivery path so that the disc is substantially centrally
positioned in the disc space. Preferably, the prosthetic discs have
sizes and structures particularly adapted for implantation by the
minimally invasive procedure.
[0018] Other and additional devices, apparatus, structures, and
methods are described by reference to the drawings and detailed
descriptions below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The Figures contained herein are not necessarily drawn to
scale, with some components and features being exaggerated for
clarity.
[0020] FIGS. 1A and 1B provide a three dimensional view of two
different prosthetic discs according to the subject invention.
[0021] FIG. 2 provides a three-dimensional view of a fibrous
compressible element that includes a polymeric nucleus and a
fibrous annulus according to one embodiment of the subject
invention.
[0022] FIGS. 3A to 3C provide different views of a fibrous
component of the fibrous compressible elements according to an
embodiment of the subject invention. FIG. 3C illustrates the manner
in which the 2D fabrics in FIG. 3B are stitched together.
[0023] FIG. 4A provides a three-dimensional top view of a
prosthetic disc according to an embodiment of the present invention
in which the fixation elements are integral to the disc, while FIG.
4B shows the disc of FIG. 4A implanted with the use of bone
screws.
[0024] FIGS. 5A and 5B show the mating interface between disc top
endplate with an upper vertebral body fixation element according to
an embodiment of the subject invention.
[0025] FIGS. 6A and 6B show the mating interface between disc top
endplate with an upper vertebral body fixation element according to
an alternative embodiment of the subject invention. The top
endplate is clamped by a clamping element connected to the upper
vertebral body fixation element through a spring.
[0026] FIG. 7 provides an exploded view of a disc system that
includes both an intervertebral disc and vertebral body fixation
elements, according to an embodiment of the present invention.
[0027] FIGS. 8 and 9 provide views of vertebral body fixation
elements being held in an implantation device according to an
embodiment of the subject invention.
[0028] FIG. 10 provides a view of disc implantation device and disc
according to an embodiment of the subject invention.
[0029] FIG. 11 provides sequential views of a disc being replaced
with a prosthetic disc according to a method of the subject
invention.
[0030] FIG. 12 provides a cross-sectional view of a prosthetic disc
having a one-piece structure.
[0031] FIG. 13A provides a three-dimensional view of a prosthetic
disc having a one-piece structure including a single anchoring fin
on each of the upper and lower endplates.
[0032] FIG. 13B provides a three-dimensional view of a prosthetic
disc having a one-piece structure including three anchoring fins on
each of the upper and lower endplates.
[0033] FIG. 13C provides a three-dimensional view of a prosthetic
disc having a one-piece structure including a serrated surface on
each of the upper and lower endplates.
[0034] FIG. 13D provides a three-dimensional view of a prosthetic
disc having a one-piece structure including a superior dome.
[0035] FIG. 13E provides a three-dimensional view of the prosthetic
disc having a one-piece structure of FIG. 13D, having no superior
dome.
[0036] FIG. 13F provides a three-dimensional cross-sectional view
of the prosthetic disc having a one-piece structure shown in FIG.
13D.
[0037] FIG. 13G provides a three-dimensional view of a prosthetic
disc having a one-piece structure design without a gasket retaining
ring.
[0038] FIG. 13H provides a three-dimensional cross-sectional view
of the prosthetic disc having a one-piece structure shown in FIG.
2G.
[0039] FIG. 13I provides a cross-sectional view of an upper
endplate of a prosthetic disc having a one-piece structure design
without a gasket retaining ring.
[0040] FIG. 13J provides an inset view of a portion of the upper
endplate shown in FIG. 2I.
[0041] FIG. 13K provides a cross-sectional illustration of a
prosthetic disc having a one-piece structure design with a center
spring.
[0042] FIG. 13L provides a three-dimensional cross-sectional
illustration of the prosthetic disc having a one-piece structure
shown in FIG. 13K.
[0043] FIGS. 14A and B provide illustrations of uni-directional and
bi-directional fiber winding patterns.
[0044] FIGS. 15A-C provide illustrations of an annular capsule.
[0045] FIG. 16 provides a three-dimensional view of a prosthetic
disc having a two-piece structure.
[0046] FIG. 17 provides a three-dimensional view of an outer lower
endplate of the prosthetic disc shown in FIG. 16.
[0047] FIG. 18 provides a cross-sectional view of a prosthetic disc
having a two-piece constrained structure.
[0048] FIG. 19 provides a three-dimensional view of a prosthetic
disc having a two-piece unconstrained structure.
[0049] FIG. 20 provides a cross-sectional view of a prosthetic disc
having a two-piece unconstrained structure.
[0050] FIG. 21 provides a three-dimensional view of a prosthetic
disc having a three-piece structure.
[0051] FIG. 22 provides a three-dimensional view of a lower
endplate of the prosthetic disc shown in FIG. 21.
[0052] FIG. 23 provides a cross-sectional view of a prosthetic disc
having a three-piece structure.
[0053] FIG. 24A provides a three-dimensional view of a core
assembly for a prosthetic disc having a three-piece structure.
[0054] FIG. 24B provides a three-dimensional view of another core
assembly for a prosthetic disc having a three-piece structure.
[0055] FIG. 24C provides a three-dimensional view of another core
assembly for a prosthetic disc having a three-piece structure.
[0056] FIG. 25A provides a cross-section view of a fiber reinforced
core assembly.
[0057] FIG. 25B provides a cross-section view of another fiber
reinforced core assembly.
[0058] FIG. 25C provides a cross-section view of another fiber
reinforced core assembly.
[0059] FIG. 26 provides a three-dimensional view of a stacked
fabric core assembly.
[0060] FIG. 27 provides a cross-sectional view of a stacked fabric
core assembly.
[0061] FIG. 28A provides a three-dimensional view of a stacked
fabric core assembly.
[0062] FIG. 28B provides a three-dimensional view of another
stacked fabric core assembly.
[0063] FIG. 28C provides a three-dimensional view of another
stacked fabric core assembly.
[0064] FIG. 29 provides a three-dimensional view of a prosthetic
disc having a four-piece structure.
[0065] FIG. 30 provides a cross-sectional view of a prosthetic disc
having a four-piece structure.
[0066] FIG. 31 provides an expanded view of a core assembly for a
prosthetic disc having a four-piece structure.
[0067] FIG. 32 provides a three-dimensional view of a prosthetic
disc having a four-piece structure.
[0068] FIG. 33 provides a cross-sectional view of a prosthetic disc
having a four-piece structure.
[0069] FIG. 34 provides a three-dimensional view of a prosthetic
disc having a four-piece structure.
[0070] FIG. 35 provides an expanded view of a core assembly for a
prosthetic disc having a four-piece structure.
[0071] FIG. 36A provides a perspective view of a spacer.
[0072] FIG. 36B provides a perspective view of the head portion of
the spacer shown in FIG. 36A.
[0073] FIG. 37A provides a perspective view of a double-sided
chisel.
[0074] FIG. 37B provides a top view of the head portion of the
double-sided chisel shown in FIG. 37A.
[0075] FIG. 38A provides a perspective view of a holder.
[0076] FIG. 38B provides a perspective view of the head portion of
the holder shown in FIG. 38A.
[0077] FIG. 39 provides a perspective view of a guide member.
[0078] FIG. 40 provides a perspective view of a first chisel and
lower endplate insert apparatus.
[0079] FIG. 41 provides a perspective view of an upper endplate
holder.
[0080] FIG. 42 provides a perspective view of a second chisel.
[0081] FIG. 43A provides an illustration of a method step of
advancing a first chisel and outer lower endplate.
[0082] FIG. 43B provides an illustration showing a pair of adjacent
vertebrae during an implantation procedure.
[0083] FIG. 44A provides an illustration of a method step of
providing a force separating a first chisel and an outer lower
endplate.
[0084] FIG. 44B provides an illustration of a pair of adjacent
vertebrae during the method step shown in FIG. 44A.
[0085] FIG. 45A provides an illustration of a guide member and
outer lower endplate.
[0086] FIG. 45B provides an illustration of a pair of vertebrae
with an outer lower endplate implanted onto the lower vertebra.
[0087] FIG. 46A provides an illustration of a method step of
advancing a second chisel.
[0088] FIG. 46B provides an illustration of a pair of adjacent
vertebrae during the method step shown in FIG. 46A.
[0089] FIG. 47A provides an illustration of a guide member and
outer lower endplate.
[0090] FIG. 47B provides an illustration of a pair of vertebrae
with an outer lower endplate implanted onto the lower vertebra.
[0091] FIG. 48A provides an illustration of a method step of
advancing a prosthetic disc upper subassembly.
[0092] FIG. 48B provides an illustration of a pair of adjacent
vertebrae during the method step shown in FIG. 48A.
[0093] FIG. 49A provides an illustration of a method step of
withdrawing an upper endplate holder and guide member.
[0094] FIG. 49B provides an illustration of a pair of vertebrae
with a prosthetic disc having been implanted therebetween.
[0095] FIG. 50A provides a three-dimensional view of a preferred
prosthetic disc for use with a minimally invasive surgical
procedure.
[0096] FIG. 50B provides a three-dimensional view of another
preferred prosthetic disc for use with a minimally invasive
surgical procedure.
[0097] FIG. 51 provides an illustration of a minimally invasive
surgical procedure for implanting a pair of prosthetic discs.
[0098] FIG. 52A provides an illustration of an alternative
minimally invasive surgical procedure for implanting a prosthetic
disc.
[0099] FIG. 52B provides a schematic illustration of a dual
prosthetic disc having a mechanism for separating the discs after
implantation.
[0100] FIG. 53 provides a cross-sectional schematic illustration of
an anti-creep compression member.
[0101] FIG. 54 provides a cross-sectional illustration of a
mechanism for deploying and retracting fins and/or spikes located
on prosthetic disc endplate.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
[0102] Before the present invention is described, it is to be
understood that this invention is not limited to particular
embodiments described, as such may, of course, vary. It is also to
be understood that the terminology used herein is for the purpose
of describing particular embodiments only, and is not intended to
be limiting, since the scope of the present invention will be
limited only by the appended claims.
[0103] Where a range of values is provided, it is understood that
each intervening value, to the tenth of the unit of the lower limit
unless the context clearly dictates otherwise, between the upper
and lower limit of that range and any other stated or intervening
value in that stated range is encompassed within the invention. The
upper and lower limits of these smaller ranges may independently be
included in the smaller ranges is also encompassed within the
invention, subject to any specifically excluded limit in the stated
range. Where the stated range includes one or both of the limits,
ranges excluding either or both of those included limits are also
included in the invention.
[0104] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
any methods and materials similar or equivalent to those described
herein can also be used in the practice or testing of the present
invention, the preferred methods and materials are now described.
All publications mentioned herein are incorporated herein by
reference to disclose and describe the methods and/or materials in
connection with which the publications are cited.
[0105] It must be noted that as used herein and in the appended
claims, the singular forms "a", "an", and "the" include plural
referents unless the context clearly dictates otherwise.
[0106] The publications discussed herein are provided solely for
their disclosure prior to the filing date of the present
application. Nothing herein is to be construed as an admission that
the present invention is not entitled to antedate such publication
by virtue of prior invention. Further, the dates of publication
provided may be different from the actual publication dates which
may need to be independently confirmed.
[0107] As will be apparent to those of skill in the art upon
reading this disclosure, each of the individual embodiments
described and illustrated herein has discrete components and
features which may be readily separated from or combined with the
features of any of the other several embodiments without departing
from the scope or spirit of the present inventions.
[0108] Prosthetic intervertebral discs, methods of using such
discs, apparatus for implanting such discs, and methods for
implanting such discs are described herein. It is to be understood
that the prosthetic intervertebral discs, implantation apparatus,
and methods are not limited to the particular embodiments
described, as these may, of course, vary. It is also to be
understood that the terminology used herein is for the purpose of
describing particular embodiments only, and is not intended to be
limiting, since the scope of the present inventions will be limited
only by the appended claims.
[0109] The following description includes three Parts. Part A
contains a description of a first set of embodiments of the subject
prosthetic intervertebral discs, a review of representative methods
for using the prosthetic discs, and a review of systems and kits
that include the subject prosthetic discs. The embodiments
described in Part A are those illustrated in FIGS. 1-11. Part B
contains a description of a second set of embodiments of the
subject prosthetic intervertebral discs, methods for using the
discs, and apparatus and methods for implanting the discs. The
embodiments described in Part B are those illustrated in FIGS.
12-54. Each of the descriptions contained in Parts A and B will be
understood to be complete and comprehensive in its own right, as
well as describing structures, features, and methods that are
suitable for use with those described in the other Part. Part C
includes additional information about the descriptions contained
herein.
Part A
I. Prosthetic Intervertebral Disc
[0110] As summarized above, the subject invention is directed to a
prosthetic intervertebral disc. By prosthetic intervertebral disc
is meant an artificial or manmade device that is configured or
shaped so that it can be employed as a replacement for an
intervertebral disc in the spine of a vertebrate organism, e.g., a
mammal, such as a human. The subject prosthetic intervertebral disc
has dimensions that permit it to substantially occupy the space
between two adjacent vertebral bodies that is present when the
naturally occurring disc between the two adjacent bodies is
removed, i.e., a void disc space. By substantially occupy is meant
that it occupies at least about 75% by volume, such as at least
about 80% by volume or more. The subject discs may have a roughly
bean shaped structure analogous to naturally occurring
intervertebral body discs which they are designed to replace. In
many embodiments the length of the disc ranges from about 15 mm to
about 50 mm, such as from about 18 mm to about 46 mm, the width of
the disc ranges from about 12 mm to about 30 mm, such as from about
14 mm to about 25 mm and the height of the disc ranges from about 3
mm to about 13 mm, such as from about 5 mm to about 12 mm.
[0111] The subject discs are characterized in that they include
both an upper (or top) and lower (or bottom) endplate, where the
upper and lower endplates are separated from each other by a
fibrous compressible element, where the combination structure of
the endplates and fibrous compressible element provides a
prosthetic disc that functionally closely mimics real disc. A
feature of the subject prosthetic discs is that the top and bottom
endplates are held together by at least one fiber, e.g., of the
fibrous compressible element, wound around at least one portion of
each of the top and bottom endplates. As such, the two endplates
(or planar substrates) are held to each other by one or more fibers
that are wrapped around at least one domain/portion/area of the
upper endplate and lower endplate such that the plates are joined
to each other.
[0112] Two different representative intervertebral discs are shown
in FIGS. 1A and 1B. As can be seen in FIGS. 1A and 1B, prosthetic
discs 10 each include a top endplate 11 and a lower endplate 12.
Top and bottom endplates 11 and 12 are planar substrates, where
these plates typically have a length from about 12 mm to about 45
mm, such as from about 13 mm to about 44 mm, a width of from about
11 mm to about 28 mm, such as from about 12 mm to about 25 mm and a
thickness of from about 0.5 mm to about 4 mm, such as from about 1
mm to about 3 mm. The top and bottom endplates are fabricated from
a physiologically acceptable material that provides for the
requisite mechanical properties, where representative materials
from which the endplates may be fabricated are known to those of
skill in the art and include, but are not limited to: titanium,
titanium alloys, stainless steel, cobalt/chromium, etc.; plastics
such as polyethylene with ultra high molar mass (molecular weight)
(UHMW-PE), polyether ether ketone (PEEK), etc.; ceramics; graphite;
etc. As shown in FIGS. 1A and 1B, separating the top and bottom
endplates is a fibrous compressible element 17. The thickness of
the fibrous compressible element may vary, but ranges in many
embodiments from about 2 mm to about 10 mm, including from about 3
mm to about 8 mm.
[0113] The disc is further characterized in that it includes an
annular region 13 (i.e., annulus), which is the region, domain or
area that extends around the periphery of the disc, and a nuclear
region (i.e., nucleus) 14, which is the region, domain or area in
the center of the disc and surrounded by the annulus.
[0114] While in the broadest sense the plates may include a single
region around which a fiber is wound in order to hold the plates
together, in many embodiments the plates have a plurality of such
regions. As shown in FIGS. 1A and 1B, endplates 11 and 12 include a
plurality of slots 15 through which fibers, e.g., of the fibrous
compressible element, may be passed through or wound, as shown. In
many embodiments, the number of different slots present in the
periphery of the device ranges from about 4 to about 36, such as
from about 5 to about 25. As shown in FIGS. 1A and 1B, at least one
fiber 16 of the fibrous compressible element is wrapped around a
region of the top and bottom plates, e.g., by being passed through
slots in the top and bottom plates, in order to hold the plates
together.
[0115] The fibrous compressible elements, 17, are typically made up
of one or more fibers, where the fibers are generally high tenacity
fibers with a high modulus of elasticity. By high tenacity fibers
is meant fibers that can withstand a longitudinal stress without
tearing asunder of at least about 50 MPa, such as at least about
250 MPa. As the fibers have a high modulus of elasticity, their
modulus of elasticity is typically at least about 100 MPa, usually
at least about 500 MPa. The fibers are generally elongate fibers
having a diameter that ranges from about 3 mm to about 8 mm, such
as about 4 mm to about 7 mm, where the length of each individual
fiber making up the fibrous component may range from about 1 m to
about 20 m, such as from about 2 m to about 15 m.
[0116] The fibers making up the fibrous compressible elements may
be fabricated from any suitable material, where representative
materials of interest include, but are not limited to: polyester
(e.g., Dacron), polyethylene, polyaramid, carbon or glass fibers,
polyethylene terephthalate, arcrylic polymers, methacrylic
polymers, polyurethane, polyurea, polyolefin, halogenated
polyolefin, polysaccharide, vinylic polymer, polyphosphazene,
polysiloxane, and the like.
[0117] The fibrous compressible elements made up of one or more
fibers wound around one or more regions of the top or bottom plates
may make up a variety of different configurations. For example, the
fibers may be wound in a pattern that has an oblique orientation to
simulate the annulus of intact disc, where a representative oblique
fiber configuration or orientation is shown in FIG. 1A. The number
of layers of fiber winding may be varied to achieve similar
mechanical properties to an intact disk. Where desired, compliancy
of the structure may be reduced by including a horizontal winding
configuration, as shown in FIG. 1B.
[0118] In certain embodiments, the fibrous compressible element 20
has a fibrous component 21 limited to the annular region of the
disc 22, e.g., to the region along the periphery of the disc. FIG.
2 provides a representation of this embodiment, where the fibrous
component is limited solely to the annular region of the disc and
includes both oblique and horizontal windings. Also shown is a
separate polymeric component 23 present in the nucleus. The fiber
windings of the various layers of fiber may be at varying angles
from each other where the particular angle for each layer may be
selected to provide a configuration that best mimics the natural
disc. Additionally, the tension placed on the fibers of each layer
may be the same or varied.
[0119] In yet other embodiments the fibrous component of the
fibrous compressible element may extend beyond the annular region
of the disc into at least about a portion, if not all, of the
nucleus. FIG. 3A provides a view of a fibrous component 30 that
occupies both the annular and nuclear regions of the disc, where
the annular region of the disc is made up of fiber windings that
are both oblique and horizontal, as described above, while the
nucleus of the disc is occupied by fibers woven into a
three-dimensional network that occupies the nuclear space. Instead
of a three-dimensional network structure, one may have multiple two
dimensional layers' of interwoven fibers stacked on top of each
other, as shown in FIG. 3B, where the multiple stacked layers may
be stitched to each other, as shown in FIG. 3C. By adjusting one or
more parameters of the fibrous component, such as the density of
the fibers, number of layers, frequency of stitching, the wrapping
angle of each fiber layer, and the like, the mechanical properties
of the fibrous component can be tailored as desired, e.g., to mimic
the mechanical properties of a natural intervertebral disc. Also
shown in FIGS. 3B and 3C is the outline of a polymeric component 32
in which the fibrous component 30 is embedded.
[0120] In certain embodiments, the fibrous compressible element
further includes one or more polymeric components. The polymeric
component(s), when present, may be fabricated from a variety of
different physiologically acceptable materials. Representative
materials of interest include, but are not limited to: elastomeric
materials, such as polysiloxane, polyurethane, poly(ethylene
propylene) copolymer, polyvinylchloride, poly(tetrafluoro ethylene)
and copolymers, hydrogels, and the like.
[0121] The polymeric component may be limited to particular
domains, e.g., the annular and/or nucleus domains, or extend
throughout the entire region of the fibrous compressible elements
positioned between the two endplates. As such, in certain
embodiments the polymeric component is one that is limited to the
nuclear region of the disc, as shown in FIG. 2. In FIG. 2, fibrous
compressible element 20 includes a distinct fibrous component 21
that is located in the annular region of the disc 22, while
polymeric component 23 is located in the nuclear region of the
disc. In other embodiments, the polymeric component is located in
both the annular and nuclear regions. In yet other embodiments, the
polymeric component may be located solely in the annular
region.
[0122] Depending on the desired configuration and mechanical
properties, the polymeric component may be integrated with the
fibrous component, such that at least a portion of the fibers of
the fibrous component is embedded in, e.g., complexed with, at
least a portion of the polymeric component. In other words, at
least a portion of the fibrous component is impregnated with at
least a portion of the polymeric component. For example, as shown
in FIG. 3B, stacked two-dimensional layers of the fibrous component
30 are present inside the polymeric component 32, such that the
fibrous component is impregnated with the polymeric component.
[0123] In those configurations where the fibrous and polymeric
components are present in a combined format, e.g., as shown in FIG.
3B, the fibers of the fibrous component may be treated to provide
for improved bonding with the polymeric component. Representative
fiber treatments of interest include, but are not limited to:
corona discharge, O.sub.2 plasma treatment, oxidation by strong
acid (HNO.sub.3, H.sub.2SO.sub.4). In addition, surface coupling
agents may be employed, and/or a monomer mixture of the polymer may
be polymerized in presence of the surface-modified fiber to produce
the composite fiber/polymeric structure.
[0124] As indicated above, the devices may include one or more
different polymeric components. In those embodiments where two or
more different polymeric components are present, any two given
polymeric components are considered different if they differ from
each other in terms of at least one aspect, e.g., composition,
cross-linking density, and the like. As such, the two or more
different polymeric components may be fabricated from the same
polymeric molecules, but differ from each other in terms of one or
more of: cross-linking density; fillers; etc. For example, the same
polymeric material may be present in both the annulus and nucleus
of the disc, but the crosslink density of the annulus polymeric
component may be higher than that of the nuclear region. In yet
other embodiments, polymeric materials that differ from each other
with respect to the polymeric molecules from which they are made
may be employed.
[0125] By selecting particular fibrous component and polymeric
component materials and configurations, e.g., from the different
representative formats described above, a disc with desired
functional characteristics, e.g., that mimics the functional
characteristics of the naturally occurring disc, may be
produced.
[0126] Representative particular combinations of interest include,
but are not limited to, the following: [0127] 1. Biocompatible
polyurethane, such as Ethicon Biomer, reinforced with Dacron
poly(ethylene terephthalate) fiber, or Spectra polyethylene fiber,
or Kevlar polyaramide fiber, or carbon fiber. [0128] 2.
Biocompatible polysiloxane modified styrene-ethylene butylene block
copolymer sold under C-Flex tradename reinforced with Dacron
poly(ethylene terephthalate) fiber, or Spectra polyethylene fiber,
or Kevlar polyaramide fiber, or carbon fiber. [0129] 3.
Biocompatible Silastic silicone rubber, reinforced with Dacron
poly(ethylene terephthalate) fiber, or Spectra polyethylene fiber,
or Kevlar polyaramide fiber, or carbon fiber.
[0130] In using the subject discs, the prosthetic disc is fixed to
the vertebral bodies between which it is placed. More specifically,
the upper and lower plates of the subject discs are fixed to the
vertebral body to which they are adjacent. As such, the subject
discs are employed with vertebral body fixation elements during
use. In certain embodiments, the vertebral body fixation elements
are integral to the disc structure, while in other embodiments the
vertebral body fixation elements are separate from the disc
structure.
[0131] A representative embodiment of those devices where the
vertebral body fixation elements are integral with the disc
structure is depicted in FIGS. 4A and 4B. FIG. 4A shows device 40
made up of top and bottom endplates 41 and 42. Integrated with top
and bottom endplates 41 and 42 are vertebral body fixation elements
43 and 44. The vertebral body fixation elements include holes
through which bone screws may be passed for fixation of the disc to
upper and lower vetrebral bodies 47 and 48 upon implantation, as
represented in FIG. 4B.
[0132] In an alternative embodiment, the disc does not include
integrated vertebral body fixation elements, but is designed to
mate with separate vertebral body fixation elements, e.g., as
depicted in FIG. 7. In other words, the disc is structured to
interface with separate vertebral body fixation elements during
use. Any convenient separate vertebral body fixation element may be
employed in such embodiments, so long as it stably positions the
prosthetic disc between two adjacent vertebral bodies.
[0133] One representative non-integrated vertebral body fixation
element according to this embodiment is shown in FIGS. 5A and 5B.
FIG. 5A provides a representation of the upper plate 50 of a
prosthetic disc mated with a vertebral body fixation element 51, as
the structures would appear upon implantation. Vertebral body
fixation element 51 is a horseshoe shaped structure having spikes
55 at locations corresponding to the cortical bone of vertebrae and
porous coating to enhance bone fixation. The fixation element 51
also has gear teeth 52 such that corresponding gear teeth 53 of the
disc upperplate 50 can slide through the gear contact resulting in
the right location of prosthetic disc with respect to the fixation
element. The gear teeth have a shape such that only inward movement
of the upper plate upon implantation is possible. Also present are
slots 56 in the spiked fixation elements next to the gear teeth
that provide for the elastic deformation of the whole teeth area
upon implantation and desirable clearance between mating gear teeth
of the disc and fixation element so that incoming gear teeth of the
disc can easily slide into the fixation element.
[0134] In the embodiment shown in FIG. 5A, as the disc is pushed
into the fixation element, the protruded rail 57 on the disc slides
along the corresponding concave rail-way 58 on the fixation element
until the protruded rail on the most front side is pushed into the
corresponding concave rail-way on the fixation element, as shown in
FIG. 5B. This rail interface is devised to prevent the
upward/downward movement of the top disc endplate and the bottom
disc endplate with respect to the corresponding fixation element.
This interface between the fixation elements and the top and bottom
endplates of the disc enables an easy surgical operation.
Specifically, the fixation elements are transferred together to the
disc replacement area (disc void space) with an instrument and
pushed in the opposite directions toward the vertebrae until they
are fixed to the vertebrae, and then the prosthetic disc is
transferred by the instrument between the fixation elements and
simply pushed inward until the stoppers mate the corresponding
stoppers. The prosthetic disc can also be easily removed after
long-term use. For its removal, the gear teeth on the fixation
element are pushed to reduce the gap of the slot so that the gear
engagement between the disc endplate and the fixation element is
released.
[0135] An alternative embodiment is depicted in FIGS. 6A and 6B. In
the embodiment shown in FIGS. 6A and 6B, the fixation element 61
and the endplate 62 have a different mating interface from that
depicted in FIGS. 5A and 5B. As shown in FIGS. 6A and 6B, the gear
teeth in the endplate are brought in contact with the corresponding
gear teeth of the clamping element 63 that is attached to the
fixation element 61 through a spring 64. In this mechanism, the
slots next to the gear teeth shown in the embodiment depicted in
FIGS. 5A and 5B are replaced by a spring attached to the fixation
element and this spring deformation provides the necessary recess
of the clamping element as the disc endplate is pushed in upon
implantation. The gear teeth contact between the endplate and the
clamping element allows one way sliding. The disc endplates and the
fixation elements have the rail interface as in FIGS. 5A and 5B to
prevent the vertical movement.
II. Systems
[0136] Also provided are systems that include at least one
component of the subject prosthetic discs, as described above. The
systems of the subject invention typically include all of the
elements that may be necessary and/or desired in order to replace
an intervertebral disc with a prosthetic disc as described above.
As such, at a minimum the subject systems include a prosthetic disc
according to the present invention, as described above. In
addition, the systems in certain embodiments include a vertebral
body fixation element, or components thereof, e.g., the fixation
elements shown in FIGS. 5A to 6B, bone screws for securing
integrated fixation elements as shown in FIGS. 4A and 4B, and the
like. The subject systems may also include special delivery
devices, e.g., as described in greater detail below.
[0137] One specific representative system of particular interest is
depicted in FIG. 7. The system 70 of FIG. 7 is depicted as an
exploded view, and includes upper and lower fixation elements 71A
and 71B, and disc 74 made up of top and bottom endplates 72A and
72B, as well as the fibrous compressible element 75, made up of
both a fibrous component 73 and polymeric component 76 of the
prosthetic disc.
III. Methods of Use
[0138] Also provided are methods of using the subject prosthetic
intervertebral discs and systems thereof. The subject prosthetic
intervertebral discs and systems thereof find use in the
replacement of damaged or dysfunctional interverterbral discs in
vertebrate organisms. Generally the vertebrate organisms are
"mammals" or "mammalian," where these terms are used broadly to
describe organisms which are within the class mammalia, including
the orders carnivore (e.g., dogs and cats), rodentia (e.g., mice,
guinea pigs, and rats), lagomorpha (e.g. rabbits) and primates
(e.g., humans, chimpanzees, and monkeys). In many embodiments, the
subjects will be humans.
[0139] In general, the devices are employed by first removing the
to be replaced disc from the subject or patient according to
standard protocols to produce a disc void space. Next, the subject
prosthetic disc is implanted or positioned in the disc void space,
resulting in replacement of the removed disc with the prosthetic
disc. This implantation step may include a vertebral body fixation
element implantation substep, a post implantation vertebral body
securing step, or other variations, depending on the particular
configuration of the prosthetic device being employed. In addition,
the implantation step described above may include use of one or
more implantation devices (or disc delivery devices) for implanting
the system components to the site of implantation.
[0140] A representative implantation protocol for implanting the
device depicted in FIG. 7 is now provided. First, the spine of a
subject is exposed via a retroperitoneal approach after sterile
preparation. The intervertebral disc in trauma condition is
removed, and the cartilage endplates above and below the disc are
also removed to the bony end plates to obtain the bleeding surface
for the bone growth into porous cavities in the spiked fixation
elements 71A and 72A. The gap resulting from these removals is
measured and the proper artificial disk assembly is chosen
according to the measurement.
[0141] The spiked fixation element plates are loaded onto a
delivery instrument 80 as shown in FIGS. 8 and 9 such that relative
location and orientation between the upper spiked fixation element
plate and the lower spiked fixation element plate are kept at a
desired configuration. This configuration can be realized by
providing appropriate mating features on the instrument and the
corresponding mating features on the spiked plates. One of the
possible mating features would be the pocket of the instrument and
the corresponding external faces of the spiked plates as shown in
FIG. 8. The pocket has the same internal face as the external face
of spiked plates but with a slightly smaller size such that the
spiked plate fits tightly into the pocket of the instrument. The
instrument together with the spiked fixation plates is delivered to
the area where the disc was removed and the spiked plates are
pushed against the vertebra using the distracting motion of the
instrument as shown in FIG. 9.
[0142] Once the spiked fixation plates are firmly fixed to the
vertebra, the prosthetic disc 75 is held by a different tool and
inserted into the implanted spiked fixation plates such that its
gear teeth go through the matching gear teeth on the spiked
fixation plates. FIG. 10 shows the tool holding the disc. The
grippers in FIG. 10 hold the fiber area of the disc when it is in
grasp position. The disc accommodating the grippers has the
circular concave area in contact with the disc and is pushed into
the spiked fixation plates through this contact. When the disc is
inserted all the way into the spiked plates, the protruded rails on
the disc at its most front side are in contact with the female
railway of the spiked fixation plates and the disc is secured
between the spiked fixation plates and therefore the vertebra.
[0143] The above-described protocol is depicted in FIG. 11.
[0144] The above specifically reviewed protocol is merely
representative of the protocols that may be employed for implanting
devices according to the subject invention.
IV. Kits
[0145] Also provided are kits for use in practicing the subject
methods, where the kits typically include one or more of the above
prosthetic intervertebral disc devices (e.g., a plurality of such
devices in different sizes), and/or components of the subject
systems, e.g., fixation elements or components thereof, delivery
devices, etc. as described above. The kit may further include other
components, e.g., site preparation components, etc., which may find
use in practicing the subject methods.
[0146] In addition to above-mentioned components, the subject kits
typically further include instructions for using the components of
the kit to practice the subject methods. The instructions for
practicing the subject methods are generally recorded on a suitable
recording medium. For example, the instructions may be printed on a
substrate, such as paper or plastic, etc. As such, the instructions
may be present in the kits as a package insert, in the labeling of
the container of the kit or components thereof (i.e., associated
with the packaging or sub-packaging) etc. In other embodiments, the
instructions are present as an electronic storage data file present
on a suitable computer readable storage medium, e.g. CD-ROM,
diskette, etc. In yet other embodiments, the actual instructions
are not present in the kit, but means for obtaining the
instructions from a remote source, e.g. via the internet, are
provided. An example of this embodiment is a kit that includes a
web address where the instructions can be viewed and/or from which
the instructions can be downloaded. As with the instructions, this
means for obtaining the instructions is recorded on a suitable
substrate.
[0147] It is evident from the above discussion and results that the
subject invention provides a significantly improved prosthetic
intervertebral disc. Significantly, the subject discs closely
imitate the mechanical properties of the fully functional natural
discs that they are designed to replace. The subject discs exhibit
stiffness in the vertical direction, torsional stiffness, bending
stiffness in saggital plane, and bending stiffness in front plane,
where the degree of these features can be controlled independently
by adjusting the components of the discs, e.g., number of layers of
fiber winding, pattern of fiber winding, distribution of
impregnated polymer, and the types of impregnated polymers, etc.
The fiber reinforced structure of the subject discs prevents the
fatigue failure on the inside polymer and the surface treatment on
the fiber of certain embodiments eliminates the debris problem,
both of which are major disadvantages experienced with certain
"rubber-type" artificial disks. The interface mechanism between the
fixation plates and the disc plates of certain embodiments of the
subject invention, e.g., as shown in FIG. 7, enables a very easy
surgical operation. The surgeon simply needs to push the disc
inward after fixing the spiked fixation plates onto the vertebrae.
Such embodiments also enable easy removal of the disc in case the
surgery brings about an ill effect. The gear teeth on the fixation
elements are easily pushed from outside such that the gear
engagement between the disc endplates and the fixation elements is
released and the disc endplates are pulled out from the spiked
plates. In view of the above and other benefits and features
provided by the subject invention, it is clear that the subject
invention represents a significant contribution to the art.
Part B
[0148] With reference to the embodiments illustrated in FIGS.
12-54, the subject prosthetic discs include upper and lower
endplates separated by a core member. In one embodiment, the
prosthetic disc comprises an integrated, single-piece structure. In
another embodiment, the prosthetic disc comprises a two-piece
structure including a lower endplate, and an upper endplate and the
core member. The core may be assembled or integrated with either or
the two endplates. The two-piece structure may be a constrained
structure, wherein the upper endplate assembly is attached to the
lower endplate in a manner that prevents relative rotation.
Alternatively, the structure may be a semi-constrained or an
unconstrained structure, wherein the upper endplate assembly is
attached to the lower endplate in a manner that allows relative
rotation. In yet another embodiment, the prosthetic disc comprises
a three-piece structure including upper and lower endplates and a
separable core member that is captured between the upper and lower
endplates by a retaining mechanism. Finally, in yet another
embodiment, the prosthetic disc comprises a four-piece structure
including upper and lower endplates and two separable core
assemblies which, together, form a core member. Those of ordinary
skill in the art will recognize that five-piece, six-piece, or
other multi-piece structures may be constructed by further division
of the core member and/or the upper and lower endplates, or by the
provision of additional components to the structure.
[0149] The implantation apparatus and methods are adapted to
implant the prosthetic discs between two adjacent vertebral bodies
of a patient. In a first embodiment, the apparatus includes three
implantation tools used to prepare the two adjacent vertebral
bodies for implantation and then to implant the prosthetic disc. A
first tool, a spacer, is adapted to be inserted between and to
separate the two adjacent vertebral bodies to create sufficient
space for implanting the prosthetic disc. A second tool, a chisel,
includes one or more wedge-shaped cutting blades located on its
upper and/or lower surfaces that are adapted to create grooves in
the inward facing surfaces of the two adjacent vertebral bodies. A
third tool, a holder, includes an engagement mechanism adapted to
hold the prosthetic disc in place while it is being implanted, and
to release the disc once it has been implanted.
[0150] In another embodiment, the implantation apparatus includes a
guide member that engages the lower endplate and that remains in
place during a portion of the disc implantation process. A lower
pusher member slidably engages the guide member and is used to
advance the lower endplate into place between two adjacent
vertebral bodies of a patient's spine. An upper pusher member is
preferably coupled to the lower pusher member and is used to
advance a first chisel into place opposed to the lower endplate
between the two adjacent vertebral bodies. Once in place, an upward
force is applied to the upper pusher member to cause the first
chisel to engage the upper vertebral body and to chisel one or more
grooves into its lower surface. A downward force is also applied to
the lower pusher member to cause the lower endplate to engage the
lower vertebral body and to become implanted. The upper pusher
member and first chisel are then removed, as is the lower pusher
member. Preferably, a second chisel is then advanced along the
guide member and is used to provide additional preparation of the
upper vertebral body. After the completion of the preparation by
the first chisel and, preferably, the second chisel, the upper
endplate and core members of the prosthetic disc are implanted
using an upper endplate holder that is advanced along the guide
member. After implantation, the upper endplate holder and guide
member are removed.
[0151] As will be apparent to those of skill in the art upon
reading this disclosure, each of the individual embodiments
described and illustrated herein has discrete components and
features which may be readily separated from or combined with the
features of any of the other several embodiments without departing
from the scope or spirit of the present inventions.
I. Prosthetic Intervertebral Discs
[0152] The prosthetic intervertebral discs are preferably
artificial or manmade devices that are configured or shaped so that
they can be employed as replacements for an intervertebral disc in
the spine of a vertebrate organism, e.g., a mammal, such as a
human. The subject prosthetic intervertebral discs have dimensions
that permit them to substantially occupy the space between two
adjacent vertebral bodies that is present when the naturally
occurring disc between the two adjacent bodies is removed, i.e., a
disc void space. By substantially occupy is meant that the
prosthetic disc occupies a sufficient volume in the space between
two adjacent vertebral bodies that the disc is able to perform some
or all of the functions performed by the natural disc for which it
serves as a replacement. In certain embodiments, subject prosthetic
discs may have a roughly bean shaped structure analogous to
naturally occurring intervertebral body discs. In many embodiments,
the length of the prosthetic discs range from about 15 mm to about
50 mm, preferably from about 18 mm to about 46 mm, the width of the
prosthetic discs range from about 12 mm to about 30 mm, preferably
from about 14 mm to about 25 mm, and the height of the prosthetic
discs range from about 3 mm to about 15 mm, preferably from about 5
mm to about 14 mm.
[0153] The prosthetic discs include upper and lower endplates
separated by a core member. The resulting structure provides a
prosthetic disc that functionally closely mimics a natural
disc.
A. One-Piece Structure
[0154] Representative prosthetic intervertebral discs 100 having
one-piece structures are shown in FIGS. 12 through 15. The
prosthetic disc includes an upper endplate 110, a lower endplate
120, and a core member 130 retained between the upper endplate 110
and the lower endplate 120. One or more fibers 140 are wound around
the upper and lower endplates to attach the endplates to one
another. (For clarity, the fibers 140 are not shown in all of the
Figures. Nevertheless, fibers 140, as shown, for example, in FIG.
12, are present in and perform similar functions in each of the
embodiments described herein.) The fibers 140 preferably are not
tightly wound, thereby allowing a degree of axial rotation,
bending, flexion, and extension by and between the endplates. The
core member 130 may be provided in an uncompressed or a
pre-compressed state. An annular capsule 150 is optionally provided
in the space between the upper and lower endplates, surrounding the
core member 130 and the fibers 140. The upper endplate 110 and
lower endplate 120 are generally flat, planar members, and are
fabricated from a physiologically acceptable material that provides
substantial rigidity. Examples of materials suitable for use in
fabricating the upper endplate 110 and lower endplate 120 include
titanium, titanium alloys, stainless steel, cobalt/chromium, etc.,
which are manufactured by machining or metal injection molding;
plastics such as polyethylene with ultra high molar mass (molecular
weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are
manufactured by injection molding or compression molding; ceramics;
graphite; and others. Optionally, the endplates may be coated with
hydroxyapatite, titanium plasma spray, or other coatings to enhance
bony ingrowth.
[0155] As noted above, the upper and lower endplates typically have
a length of from about 12 mm to about 45 mm, preferably from about
13 mm to about 44 mm, a width of from about 11 mm to about 28 mm,
preferably from about 12 mm to about 25 mm, and a thickness of from
about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3
mm. The sizes of the upper and lower endplates are selected
primarily based upon the size of the void between adjacent
vertebral bodies to be occupied by the prosthetic disc.
Accordingly, while endplate lengths and widths outside of the
ranges listed above are possible, they are not typical.
[0156] The upper surface of the upper endplate 110 and the lower
surface of the lower endplate 120 are preferably each provided with
a mechanism for securing the endplate to the respective opposed
surfaces of the upper and lower vertebral bodies between which the
prosthetic disc is to be installed. For example, in FIG. 12, the
upper endplate 110 includes a plurality of anchoring fins 111a-b.
The anchoring fins 111a-b are intended to engage mating grooves
that are formed on the surfaces of the upper and lower vertebral
bodies to thereby secure the endplate to its respective vertebral
body. The anchoring fins 111a-b extend generally perpendicularly
from the generally planar external surface of the upper endplate
110, i.e., upward from the upper side of the endplate as shown in
FIG. 12. In the FIG. 12 embodiment, the upper endplate 110 includes
three anchoring fins 111a-c, although only two are shown in the
cross-sectional view. A first of the anchoring fins, 111a, is
disposed near an external edge of the external surface of the upper
endplate and has a length that approximates the width of the upper
endplate 110. A second of the anchoring fins, 111b, is disposed at
the center of external surface of the upper endplate and has a
relatively shorter length, substantially less than the width of the
upper endplate 110. Each of the anchoring fins 111a-b has a
plurality of serrations 112 located on the top edge of the
anchoring fin. The serrations 112 are intended to enhance the
ability of the anchoring fin to engage the vertebral body and to
thereby secure the upper endplate 110 to the spine.
[0157] Similarly, the lower surface of the lower endplate 120
includes a plurality of anchoring fins 121a-b. The anchoring fins
121a-b on the lower surface of the lower endplate 120 are identical
in structure and function to the anchoring fins 111a-b on the upper
surface of the upper endplate 110, with the exception of their
location on the prosthetic disc. The anchoring fins 121a-b on the
lower endplate 120 are intended to engage mating grooves formed on
the lower vertebral body, whereas the anchoring fins 111a-b on the
upper endplate 110 are intended to engage mating grooves on the
upper vertebral body. Thus, the prosthetic disc 100 is held in
place between the adjacent vertebral bodies.
[0158] The anchoring fins 111, 121 may optionally be provided with
one or more holes or slots 115, 125. The holes or slots help to
promote bony ingrowths that bond the prosthetic disc 100 to the
vertebral bodies.
[0159] Turning to FIGS. 13A-C, there are shown several alternative
mechanisms for securing the endplates to the respective opposed
surfaces of the upper and lower vertebral bodies between which the
prosthetic disc is to be installed. In FIG. 13A, each of the upper
endplate 110 and lower endplate 120 is provided with a single
anchoring fin 111, 121. The anchoring fins 111, 121 are located
along a center line of the respective endplates, and each is
provided with a plurality of serrations 112, 122 on its upper edge.
The single anchoring fins 111, 121 are intended to engage grooves
formed on the opposed surface of the upper and lower vertebral
bodies, as described above. In FIG. 13B, each of the upper endplate
110 and lower endplate 120 is provided with three anchoring fins
111a-c, 121a-c. The FIG. 13B prosthetic disc is the same as the
prosthetic disc shown in FIG. 1, but it is shown in perspective
rather than cross-section. Thus, the structure and function of the
anchoring fins 111a-c and 121a-c are as described above in relation
to FIG. 12. Finally, in FIG. 13C, each of the upper endplate 110
and lower endplate 120 is provided with a plurality of serrations
113, 123 over a portion of the exposed external surface of the
respective endplate. The serrations 113, 123 are intended to engage
the opposed surfaces of the adjacent vertebral bodies to thereby
secure the endplates in place between the vertebral bodies. The
serrations 113, 123 may be provided over the entire external
surface of each of the upper and lower endplates, or they may be
provided over only a portion of those surfaces. For example, in
FIG. 13C, the serrations 113 on the upper surface of the upper
endplate 110 are provided over three major areas, a first area 113a
near a first edge of the upper endplate 110, a second area 113b
near the center of the upper endplate 110, and a third area near a
second edge of the endplate 113c.
[0160] Turning to FIG. 54, in an optional embodiment, the anchoring
fins 111 are selectively retractable and extendable by providing a
deployment mechanism 160 that is associated with the upper endplate
110. A similar mechanism may be used on the lower endplate 120. The
deployment mechanism includes a slider 161 that slides within a
channel 162 formed in the upper endplate 110. The channel 162
includes a threaded region 163, and the slider 161 includes
matching threads 164, thereby providing a mechanism for advancing
the slider 161 within the channel 162. As the slider 161 is
advanced within the channel 162, a tapered region 165 engages the
bottom surface of a deployable fin 166. Further advancement of the
slider 161 causes the deployable fin 166 to be raised upward within
a slot 167 on the upper surface of the upper endplate 110.
Reversing the deployment mechanism 160 causes the fin 166 to
retract. The deployment mechanism 160 may also be used in
conjunction with spikes, serrations, or other anchoring devices. In
an alternative embodiment, the threaded slider 161 of the
deployment mechanism may be replaced with a dowel pin that is
advanced to deploy the fin 166. Other advancement mechanisms are
also possible.
[0161] Returning to FIG. 12, the upper endplate 110 contains a
plurality of slots 114 through which the fibers 140 may be passed
through or wound, as shown. The actual number of slots 114
contained on the endplate is variable. Increasing the number of
slots will result in an increase in the circumferential density of
the fibers holding the endplates together. In addition, the shape
of the slots may be selected so as to provide a variable width
along the length of the slot. For example, the width of the slots
may taper from a wider inner end to a narrow outer end, or visa
versa. Additionally, the fibers may be wound multiple times within
the same slot, thereby increasing the radial density of the fibers.
In each case, this improves the wear resistance and increases the
torsional and flexural stiffness of the prosthetic disc, thereby
further approximating natural disc stiffness. In addition, the
fibers 140 may be passed through or wound on each slot, or only on
selected slots, as needed. Two exemplary winding patterns are shown
in FIGS. 14A and 14B. In FIG. 14A, the fibers 140 are wound in a
uni-directional manner, which closely mimics natural annular fibers
found in a natural disc. In FIG. 14B, the fibers 140 are wound
bi-directionally. Other winding patterns, either single or
multi-directional, are also possible.
[0162] As described above, the purpose of the fibers 140 is to hold
the upper endplate 110 and lower endplate 120 together and to limit
the range-of-motion to mimic the range-of-motion of a natural disc.
Accordingly, the fibers preferably comprise high tenacity fibers
with a high modulus of elasticity, for example, at least about 100
MPa, and preferably at least about 500 MPa. By high tenacity fibers
is meant fibers that can withstand a longitudinal stress of at
least 50 MPa, and preferably at least 250 MPa, without tearing. The
fibers 140 are generally elongate fibers having a diameter that
ranges from about 100 .mu.m to about 500 .mu.m, and preferably
about 200 .mu.m to about 400 .mu.m. Optionally, the fibers may be
injection molded with an elastomer to encapsulate the fibers,
thereby providing protection from tissue ingrowth and improving
torsional and flexural stiffness, or the fibers may be coated with
one or more other materials to improve fiber stiffness and wear.
Additionally, the core may be injected with a wetting agent such as
saline to wet the fibers and facilitate the mimicking of the
viscoelastic properties of a natural disc.
[0163] The fibers 140 may be fabricated from any suitable material.
Examples of suitable materials include polyester (e.g.,
Dacron.RTM.), polyethylene, polyaramid, poly-paraphenylene
terephthalamide (e.g., Kevlar.RTM.), carbon or glass fibers,
polyethylene terephthalate, acrylic polymers, methacrylic polymers,
polyurethane, polyurea, polyolefin, halogenated polyolefin,
polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and
the like.
[0164] The fibers 140 may be terminated on an endplate by tying a
knot in the fiber on the superior surface of an endplate.
Alternatively, the fibers 140 may be terminated on an endplate by
slipping the terminal end of the fiber into a slot on an edge of an
endplate, similar to the manner in which thread is retained on a
thread spool. The slot may hold the fiber with a crimp of the slot
structure itself, or by an additional retainer such as a ferrule
crimp. As a further alternative, tab-like crimps may be machined
into or welded onto the endplate structure to secure the terminal
end of the fiber. The fiber may then be closed within the crimp to
secure it. As a still further alternative, a polymer may be used to
secure the fiber to the endplate by welding. The polymer would
preferably be of the same material as the fiber (e.g., PE, PET, or
the other materials listed above). Still further, the fiber may be
retained on the endplates by crimping a cross-member to the fiber
creating a T-joint, or by crimping a ball to the fiber to create a
ball joint.
[0165] The core member 130 is intended to provide support to and to
maintain the relative spacing between the upper endplate 110 and
lower endplate 120. The core member 130 is made of a relatively
compliant material, for example, polyurethane or silicone, and is
typically fabricated by injection molding. A preferred construction
for the core member includes a nucleus formed of a hydrogel and an
elastomer reinforced fiber annulus. For example, the nucleus, the
central portion of the core member 130, may comprise a hydrogel
material such as a water absorbing polyurethane, polyvinyl alcohol
(PVA), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP),
polyacrylamide, silicone, or PEO based polyurethane. The annulus
may comprise an elastomer, such as silicone, polyurethane or
polyester (e.g., Hytrel.RTM.), reinforced with a fiber, such as
polyethylene (e.g., ultra high molecular weight polyethylene,
UHMWPE), polyethylene terephthalate, or poly-paraphenylene
terephthalamide (e.g., Kevlar.RTM.).
[0166] The shape of the core member 130 is typically generally
cylindrical or bean-shaped, although the shape (as well as the
materials making up the core member and the core member size) may
be varied to obtain desired physical or performance properties. For
example, the core member 130 shape, size, and materials will
directly affect the degree of flexion, extension, lateral bending,
and axial rotation of the prosthetic disc.
[0167] The annular capsule 150 is preferably made of polyurethane
or silicone and may be fabricated by injection molding, two-part
component mixing, or dipping the endplate-core-fiber assembly into
a polymer solution. A preferred annular capsule 150 is shown in
FIGS. 15A-C. As shown, the annular capsule is generally
cylindrical, having an upper circular edge 153, a lower circular
edge 154, and a generally cylindrical body 155. In the embodiment
shown in the Figures, the body 155 has two bellows 156a-b formed
therein. Alternative embodiments have no bellows, one bellow, or
three or more bellows. A function of the annular capsule is to act
as a barrier that keeps the disc materials (e.g., fiber strands)
within the body of the disc, and that keeps natural in-growth
outside the disc.
[0168] Additional examples of the one-piece structure embodiment of
the prosthetic disc are illustrated in FIGS. 13D-F. Each of these
embodiments includes an upper endplate 110, lower endplate 120, and
a core member 130, as described above. The upper endplate 110
includes an outer portion 110a and an inner portion 110b, and the
lower endplate also includes an outer portion 120a and an inner
portion 120b. The inner and outer portions of each of the endplates
are bonded to each other by methods known to those of skill in the
art. Each of the endplates 110, 120 also includes anchoring fins
111a-c, 121a-c on the upper surface of the upper endplate 110 and
the lower surface of the lower endplate 120, as also described
above. Additionally, with reference to FIG. 13D, a superior dome
116 is provided on the upper surface of the upper endplate 110. The
superior dome 116 is a generally convex portion that extends upward
from the upper surface of the upper endplate 110. The superior dome
116 is optional, and functions by filling space between the upper
endplate 110 and the vertebral body upon implantation to help
approximate the upper endplate 110 to the natural anatomy. The size
and shape of the superior dome 116 may be varied according to need.
As shown in FIG. 13D, the superior dome 116 is generally convex and
has a maximum height (distance above the generally flat upper
surface portion of the upper endplate) of approximately one-half
the height of the anchoring fin 111b. The superior dome 116 may be
centered in the middle of the upper endplate 110, as shown in FIG.
2D, or it may be shifted to one side or another, depending on
need.
[0169] With particular reference to FIG. 13F, a polymer film 170 is
sandwiched between the outer portion 10a and inner portion 10b of
the upper endplate 110, and another polymer film 170 is sandwiched
between the outer portion 120a and inner portion 120b of the lower
endplate 120. The polymer films 170 are adapted to tightly adhere,
either mechanically or chemically, to the fibers 140 wound through
the slots 114, 124 formed in the upper endplate 110 and lower
endplate 120.
[0170] FIGS. 13D-F provide additional detail concerning the annular
capsule 150. As shown there, the annular capsule 150 seals the
interior space between the upper and lower endplates. The annular
capsule 150 is retained on the disc by a pair of retaining rings
151 that engage a mating pair of external facing grooves 152 on the
upper and lower endplates. (See FIG. 13F). Although the retaining
rings may be of any suitable cross-section (e.g., round,
triangular, square, etc.), the examples shown in FIG. 13F have a
rectangular cross-section. The rectangular shape is believed to
provide relatively better gasket retention and is more easily
manufactured.
[0171] FIGS. 13G and 13H illustrate still further examples of the
one-piece structure embodiment of the prosthetic disc. In the
examples shown there, the upper endplate 110 includes an outer
portion 110a and an inner portion 110b. Similarly, the lower
endplate 120 includes an outer portion 120a and an inner portion
120b. The two portions 110a-b, 120a-b of each of the upper and
lower endplates mate together to form the integrated upper endplate
110 and lower endplate 120. Preferably, the two portions 110a-b,
120a-b of the upper and lower endplates 110, 120 are joined
together by welding, e.g., laser welding or some similar process.
An advantage that may be obtained with this structure is the
ability to retain the annular capsule 150 (not shown in FIGS.
13G-H) without the need for a separate retaining ring. For example,
the upper edge of the annular capsule may be captured and retained
between the outer portion 110a and inner portion 110b of the upper
endplate 110 when they are attached to one another. Similarly, the
lower edge of the annular capsule may be captured and retained
between the outer portion 120a and inner portion 120b of the lower
endplate 120 when those components are attached to one another. In
this manner, the annular capsule is held in place between the upper
and lower endplates by the compression forces retaining the upper
and lower edges of the annular capsule.
[0172] An optional structure for retaining the annular capsule 150
is illustrated in FIGS. 13I-J. There, an upper endplate 110 is
shown including an outer portion 110a and an inner portion 110b.
The upper surface of the inner portion 110b of the upper endplate
110 is provided with an annular groove 117 that extends about the
periphery of the inner portion 110b. The annular groove 117
cooperates with the bottom surface of the outer portion 110a of the
upper endplate 110 to create an annular space 118. A similar
structure, not shown in the drawings, may be provided on the lower
endplate 120. The annular capsule 150 (not shown in FIGS. 13I-J)
may advantageously be formed having a bead, i.e., a ball-like
termination about its upper and lower edge, (also not shown in the
drawings), that occupies the annular space 118 formed on the upper
and lower endplates 110, 120. The cooperation of the annular space
118 with the bead formed on the annular capsule 150 creates a
stronger and more secure retaining force for retaining the upper
and lower edge of the annular capsule 150 by the upper and lower
endplates 110, 120. Alternatively, the annular capsule may be
retained by adhesives with or without the endplate compression
already described.
[0173] Another optional feature of the present invention is the
placement of the fibers in a state of tensile fatigue upon
fabrication so as to minimize long-term wear. For example, in the
embodiment of FIGS. 13I-J, a material 131 such as a metal plate or
a polymer film may be positioned within space 119 of upper portion
110a of the endplate and between the fibers 127 and the surface of
the endplate. The material may initially be in a form, e.g., gel or
emulsion, so as to coat and impregnate the fibers. With such
material, the fibers are caused to impinge upon the endplate
thereby reducing their susceptibility to movement during use of the
disc. As an additional optional feature, each of the endplates may
be made up of two plates that are selectively rotationally
displaceable relative to each other. In this structure, a slight
rotation of one of the plates relative to the other has the effect
of changing the size and/or shape of the slots formed on the
combined endplate. Thus, the user is able to select a desired set
of dimensions of the slots.
[0174] FIGS. 13K-L illustrate another optional feature that may be
incorporated in the one-piece structure embodiment of the
prosthetic disc. In the examples shown there, a spring 180 is
located coaxially with the core member 130 between the upper
endplate 110 and lower endplate 120. In this example, the core
member 130 is in the form of a toroid, thus having a space at its
center. The spring 180 is placed in the space at the center of the
core member 130, with each being retained between the upper
endplate 110 and lower endplate 120. The spring 180 provides a
force biasing the two endplates apart, and having performance
characteristics and properties that are different from those
provided by the core member 130. Those characteristics may be
varied by, for example, selecting a spring 180 having different
dimensions, materials, or a different spring constant. In this way,
the spring 180 provides an additional mechanism by which the
performance of the prosthetic disc may be varied in order to
approximate that of a natural disc.
[0175] Turning to FIGS. 50A-B, additional examples of the one-piece
structure embodiment of the prosthetic discs are shown. The discs
illustrated in FIGS. 50A-B are particularly adapted in size and
shape for implantation by minimally invasive surgical procedures,
as described below. Aside from their size and shape, the structures
of the examples shown in FIGS. 50A-B are similar to those described
above, including an upper endplate 110, lower endplate 120, a core
member 130, and an annular capsule 150. Each of the upper and lower
endplates 110, 120 is provided with an anchoring fin 111, 121
extending from its surface over most of the length of the endplate.
Although not shown in the drawings, these examples also preferably
include fibers 140 wound between and connecting the upper endplate
110 to the lower endplate 120.
[0176] In the example shown in FIG. 50A, a single elongated core
member is provided, whereas the example structure shown in FIG. 50B
has a dual core including two generally cylindrical core members
130a, 130b. It is believed that the dual core structure (FIG. 50B)
better simulates the performance characteristics of a natural disc.
In addition, the dual core structure is believed to provide less
stress on the fibers 140 relative to the single core structure
(FIG. 50A). Each of the exemplary prosthetic discs shown in FIGS.
50A-B has a greater length than width. Exemplary shapes to provide
these relative dimensions include rectangular, oval, bullet-shaped,
or others. This shape facilitates implantation of the discs by the
minimally invasive procedures described below.
[0177] The one-piece structure embodiment of the prosthetic disc is
implanted by a surgical procedure. After removing the natural disc,
grooves are formed in the superior and inferior vertebrae between
which the prosthetic disc is to be implanted. The prosthetic disc
is then inserted into the void, while aligning the anchoring fins
111, 121 with the grooves formed on the vertebral bodies. The
anchoring fins cause the prosthetic disc to be secured in place
between the adjacent vertebral bodies. The prosthetic disc has
several advantages over prior art artificial discs, as well as over
alternative treatment procedures such as spinal fusion. For
example, the prosthetic discs described herein provide compressive
compliance similar to that of a natural spinal disc. In addition,
the motions in flexion, extension, lateral bending, and axial
rotation are also restricted in a manner near or identical to those
associated with a natural disc.
B. Two-Piece Structure
[0178] Representative prosthetic intervertebral discs 200 having
two-piece structures are shown in FIGS. 16 through 20. The
components and features included in the two-piece prosthetic discs
are very similar to those of the one-piece disc described above. A
primary difference between the devices is that the two-piece
prosthetic disc contains two separable components, whereas the
one-piece prosthetic disc contains a single, integrated structure.
In particular, and as described more fully below, the lower
endplate of the two-piece prosthetic disc is separated into an
inner lower endplate 220a, and an outer lower endplate 220b (see
FIGS. 16-20), whereas there is only a single lower endplate 120 in
the one-piece disc (see FIGS. 12 and 13A-C).
[0179] Turning to FIGS. 16-20, the two-piece prosthetic disc
includes two primary, separable components: the outer lower
endplate 220b, and an upper subassembly 205. In a first embodiment
of the two-piece prosthetic disc, shown in FIGS. 16-18, the upper
subassembly 205 is constrained, i.e., it cannot freely rotate in
relation to the outer lower endplate 220b. In a second embodiment
of the two-piece prosthetic disc, shown in FIGS. 19-20, the upper
subassembly 205 is unconstrained, i.e., it can substantially freely
rotate in relation to the outer lower endplate 220b.
[0180] The upper subassembly includes the inner lower endplate
220a, an upper endplate 210, and a core member 230 retained between
the upper endplate 210 and the inner lower endplate 220a. One or
more fibers 240 are wound around the upper and inner lower
endplates to attach the endplates to one another. The fibers 240
preferably are not tightly wound, thereby allowing a degree of
axial rotation, bending, flexion, and extension by and between the
endplates. The core member 230 is preferably pre-compressed. An
annular capsule 250 is optionally provided in the space between the
upper and inner lower endplates, surrounding the core member 230
and the fibers 240. Alternatively, an outer ring or gasket (not
shown in the drawings) may optionally be provided in place of the
annular capsule 250.
[0181] The upper endplate 210 and outer lower endplate 220b are
generally flat, planar members, and are fabricated from a
physiologically acceptable material that provides substantial
rigidity. Examples of materials suitable for use in fabricating the
upper endplate 210 and outer lower endplate 220b include titanium,
titanium alloys, stainless steel, cobalt/chromium, etc., which are
manufactured by machining or metal injection molding; plastics such
as polyethylene with ultra high molar mass (molecular weight)
(UHMWPE), polyether ether ketone (PEEK), etc., which are
manufactured by injection molding or compression molding; ceramics;
graphite; and others. Optionally, the endplates may be coated with
hydroxyapatite, titanium plasma spray, or other coatings to enhance
bony ingrowth.
[0182] As noted above, the upper and outer lower endplates
typically have a length of from about 12 mm to about 45 mm,
preferably from about 13 mm to about 44 mm, a width of from about
11 mm to about 28 mm, preferably from about 12 mm to about 25 mm,
and a thickness of from about 0.5 mm to about 4 mm, preferably from
about 1 mm to about 3 mm. The sizes of the upper and outer lower
endplates are selected primarily based upon the size of the void
between adjacent vertebral bodies to be occupied by the prosthetic
disc. Accordingly, while endplate lengths and widths outside of the
ranges listed above are possible, they are not typical.
[0183] The upper surface of the upper endplate 210 and the lower
surface of the outer lower endplate 220b are preferably each
provided with a mechanism for securing the endplate to the
respective opposed surfaces of the upper and lower vertebral bodies
between which the prosthetic disc is to be implanted. For example,
as shown in FIGS. 16 and 18-20, the upper endplate 210 includes a
plurality of anchoring fins 211a-c. The anchoring fins 211a-c are
intended to engage mating grooves that are formed on the surfaces
of the upper and lower vertebral bodies to thereby secure the
endplate to its respective vertebral body. The anchoring fins
211a-c extend generally perpendicular from the generally planar
external surface of the upper endplate 210, i.e., upward from the
upper side of the endplate as shown in FIG. 16. In the FIG. 16
embodiment, the upper endplate 210 includes three anchoring fins
211a-c. The first and third of the anchoring fins, 211a and 211c,
are disposed near the external edges of the external surface of the
upper endplate 210 and have lengths that approximate the width of
the upper endplate 210. The second of the anchoring fins, 211b, is
disposed at the center of external surface of the upper endplate
and has a relatively shorter length, substantially less than the
width of the upper endplate 210. Each of the anchoring fins 211a-c
has a plurality of serrations 212 located on the top edge of the
anchoring fin. The serrations 212 are intended to enhance the
ability of the anchoring fin to engage the vertebral body and to
thereby secure the upper endplate 210 to the vertebral body.
[0184] The lower surface of the outer lower endplate 220b includes
a plurality of anchoring spikes 221. The anchoring spikes 221 on
the lower surface of the outer lower endplate 220b are intended to
engage the surface of the lower vertebral body, while the anchoring
fins 211a-c on the upper endplate 210 are intended to engage mating
grooves on the upper vertebral body. Thus, the prosthetic disc 200
is held in place between the adjacent vertebral bodies.
[0185] Alternatively, the upper endplate 210 and outer lower
endplate 220b of the two-piece prosthetic disc may employ one of
the alternative securing mechanisms shown in FIGS. 13A-C. As
described above, in FIG. 13A, each of the upper endplate 110 and
lower endplate 120 is provided with a single anchoring fin 111,
121. The anchoring fins 111, 121 are located along a center line of
the respective endplates, and each is provided with a plurality of
serrations 112, 122 on its upper edge. The single anchoring fins
111, 121 are intended to engage grooves formed on the opposed
surface of the upper and lower vertebral bodies, as described
above. In FIG. 13B, each of the upper endplate 110 and lower
endplate 120 is provided with three anchoring fins 111a-c, 121a-c.
The FIG. 13B prosthetic disc is the same as the prosthetic disc
shown in FIG. 12, but it is shown in perspective rather than
cross-section. Thus, the structure and function of the anchoring
fins 111a-c and 121a-c are as described above in relation to FIG.
12. Finally, in FIG. 13C, each of the upper endplate 110 and lower
endplate 120 is provided with a plurality of serrations 113, 123
over a portion of the exposed external surface of the respective
endplate. The serrations 113, 123 are intended to engage the
opposed surfaces of the adjacent vertebral bodies to thereby secure
the endplates in place between the vertebral bodies. The serrations
113, 123 may be provided over the entire external surface of each
of the upper and lower endplates, or they may be provided over only
a portion of those surfaces. For example, in FIG. 13C, the
serrations 113 on the upper surface of the upper endplate 110 are
provided over three major areas, a first area 113a near a first
edge of the upper endplate 110, a second area 113b near the center
of the upper endplate 110, and a third area near a second edge of
the endplate 113c.
[0186] Turning to FIG. 54, in an optional embodiment, the anchoring
fins 111 are selectively retractable and extendable by providing a
deployment mechanism 160 that is associated with the upper endplate
110. A similar mechanism may be used on the lower endplate 120. The
deployment mechanism includes a slider 161 that slides within a
channel 162 formed in the upper endplate 110. The channel 162
includes a threaded region 163, and the slider 161 includes
matching threads 164, thereby providing a mechanism for advancing
the slider 161 within the channel 162. As the slider 161 is
advanced within the channel 162, a tapered region 165 engages the
bottom surface of a deployable fin 166. Further advancement of the
slider 161 causes the deployable fin 166 to be raised upward within
a slot 167 on the upper surface of the upper endplate 110.
Reversing the deployment mechanism 160 causes the fin 166 to
retract. The deployment mechanism 160 may also be used in
conjunction with spikes, serrations, or other anchoring devices. In
an alternative embodiment, the threaded slider 161 of the
deployment mechanism may be replaced with a dowel pin that is
advanced to deploy the fin 166. Other advancement mechanisms are
also possible.
[0187] Returning to FIG. 18, the upper endplate 210 contains a
plurality of slots 214 through which the fibers 240 may be passed
through or wound, as shown. The actual number of slots 214
contained on the endplate is variable. Increasing the number of
slots will result in an increase in the circumferential density of
the fibers holding the endplates together. Additionally, the fibers
may be wound multiple times within the same slot, thereby
increasing the radial density of the fibers. In each case, this
improves the wear resistance and increases the torsional and
flexural stiffness of the prosthetic disc, thereby further
approximating natural disc stiffness. In addition, the fibers 240
may be passed through or wound on each slot, or only on selected
slots, as needed.
[0188] As described above, the purpose of the fibers 240 is to hold
the upper endplate 210 and lower endplate 220 together and to limit
the range-of-motion to mimic the range-of-motion of a natural disc.
Accordingly, the fibers preferably comprise high tenacity fibers
with a high modulus of elasticity, for example, at least about 100
MPa, and preferably at least about 500 MPa. By high tenacity fibers
is meant fibers that can withstand a longitudinal stress of at
least 50 MPa, and preferably at least 250 MPa, without tearing. The
fibers 240 are generally elongate fibers having a diameter that
ranges from about 100 .mu.m to about 500 .mu.m, and preferably
about 200 .mu.m to about 400 .mu.m. Optionally, the fibers may be
injection molded with an elastomer to encapsulate the fibers,
thereby providing protection from tissue ingrowth and improving
torsional and flexural stiffness.
[0189] The fibers 240 may be fabricated from any suitable material.
Examples of suitable materials include polyester (e.g.,
Dacron.RTM.), polyethylene, polyaramid, poly-paraphenylene
terephthalamide (e.g., Kevlar.RTM.), carbon or glass fibers,
polyethylene terephthalate, acrylic polymers, methacrylic polymers,
polyurethane, polyurea, polyolefin, halogenated polyolefin,
polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and
the like.
[0190] The fibers 240 may be terminated on an endplate by tying a
knot in the fiber on the superior surface of an endplate.
Alternatively, the fibers 240 may be terminated on an endplate by
slipping the terminal end of the fiber into a slot on an edge of an
endplate, similar to the manner in which thread is retained on a
thread spool. The slot may hold the fiber with a crimp of the slot
structure itself, or by an additional retainer such as a ferrule
crimp. As a further alternative, tab-like crimps may be machined
into or welded onto the endplate structure to secure the terminal
end of the fiber. The fiber may then be closed within the crimp to
secure it. As a still further alternative, a polymer may be used to
secure the fiber to the endplate by welding. The polymer would
preferably be of the same material as the fiber (e.g., PE, PET, or
the other materials listed above). Still further, the fiber may be
retained on the endplates by crimping a cross-member to the fiber
creating a T-joint, or by crimping a ball to the fiber to create a
ball joint.
[0191] The core member 230 is intended to provide support to and to
maintain the relative spacing between the upper endplate 210 and
inner lower endplate 220a. The core member 230 is made of a
relatively compliant material, for example, polyurethane or
silicone, and is typically fabricated by injection molding. A
preferred construction for the core member 230 includes a nucleus
formed of a hydrogel and an elastomer reinforced fiber annulus. For
example, the nucleus, the central portion of the core member 230,
may comprise a hydrogel material such as tecophilic water absorbing
polyurethane, polyvinyl alcohol (PVA), polyethylene oxide (PEO),
polyvinylpyrrolidone (PVP), polyacrylamide, silicone, or PEO based
polyurethane. The annulus may comprise an elastomer, such as
silicone, polyurethane or polyester (e.g., Hytrel.RTM.), reinforced
with a fiber, such as polyethylene, polyethylene terephthalate, or
poly-paraphenylene terephthalamide (e.g., Kevlar.RTM.).
[0192] The shape of the core member 230 is typically generally
cylindrical or bean-shaped, although the shape (as well as the
materials making up the core member and the core member size) may
be varied to obtain desired physical or performance properties. For
example, the core member 230 shape, size, and materials will
directly affect the degree of flexion, extension, lateral bending,
and axial rotation of the prosthetic disc.
[0193] The annular capsule 250 is preferably made of polyurethane
or silicone and may be fabricated by injection molding, two-part
component mixing, or dipping the endplate-core-fiber assembly into
a polymer solution. Alternatively, an outer ring or gasket (not
shown in the drawings) may optionally be provided in place of the
annular capsule 250.
[0194] The upper subassembly 205 is configured to be selectively
attached to the outer lower endplate 220b. As shown, for example,
in FIGS. 3 and 6, the edges 225 of the inner lower endplate 220a
have a size and shape adapted to engage slots 226 formed on the
upper surface of the outer lower endplate 220b. Accordingly, the
upper subassembly 205 will slide onto the outer lower endplate
220b, with the inner lower endplate edges 225 engaging the outer
lower endplate slots 226.
[0195] At this point, the differences between the constrained,
semi-constrained and unconstrained embodiments of the two-piece
prosthetic disc will be described. Turning first to the constrained
embodiment shown in FIGS. 16-18, once the upper subassembly 205 is
fully advanced onto the outer lower endplate 220b--i.e., once the
leading edge 225 of the inner lower endplate 220a engages the back
portion of the slot 226 of the outer lower endplate 220b--a tab 261
on the bottom surface of the inner lower endplate 220a engages a
notch 262 on the top surface of the outer lower endplate 220b (see
FIG. 18), thereby locking the upper subassembly 205 to the outer
lower endplate 220b. The tab 261 and notch 262 are squared
surfaces, thereby preventing relative rotation between the inner
lower endplate 220a and outer lower endplate 220b. Additionally,
the edges 225 of the inner lower endplate 220a and the mating slots
226 of the outer lower endplate 220b include mating straight
portions 227 and 228, respectively, which also tend to inhibit
rotation of the inner lower endplate 220a relative to the outer
lower endplate 220b.
[0196] Turning next to the unconstrained embodiment shown in FIGS.
19-20, the outer lower endplate 220b is provided with a raised lip
271. The raised lip 271 is slightly downwardly displaceable, i.e.,
the raised lip 271 will deflect downwardly when force is applied to
it. Accordingly, when the upper subassembly is being attached to
the outer lower endplate 220b, the raised lip will displace
downwardly to allow the edges 225 of the inner lower endplate 220a
to engage the slots 226 of the outer lower endplate 220b. Once the
upper subassembly 205 is fully advanced onto the outer lower
endplate 220b--i.e., once the leading edge 225 of the inner lower
endplate 220a engages the back portion of the slot 226 of the outer
lower endplate 220b--the raised lip 271 snaps back into place, as
shown in FIG. 20, thereby locking the upper subassembly 205 to the
outer lower endplate 220b. Notably, the raised lip 271 and inner
lower endplate 220a include rounded surfaces, thereby allowing
relative rotation between the inner lower endplate 220a and outer
lower endplate 220b. Additionally, the edges 225 of the inner lower
endplate 220a and the mating slots 226 of the outer lower endplate
220b do not include the mating straight portions 227, 228 of the
constrained embodiment. Thus, in the unconstrained embodiment of
the two-piece prosthetic disc, as shown in FIGS. 19 and 20, the
upper subassembly 205 is capable of substantially free rotation
relative to the outer lower endplate 220b.
[0197] The two-piece structure embodiment of the prosthetic disc is
implanted by a surgical procedure. After removing the natural disc,
the outer lower endplate 220b is placed onto and anchored into the
inferior vertebral body within the void between the two adjacent
vertebral bodies previously occupied by the natural disc. Next,
grooves are formed in the superior vertebral body. The upper
subassembly 205 of the prosthetic disc is then inserted into the
void, while aligning the anchoring fins 211 with the grooves formed
on the superior vertebral body, and while sliding the inner lower
endplate 220a into the outer lower endplate 220b in a manner that
the edges 225 of the inner endplate 220a engage the slots 226 of
the outer endplate 220b. The anchoring fins cause the prosthetic
disc to be secured in place between the adjacent vertebral
bodies.
[0198] The two-piece prosthetic disc has several advantages over
prior art artificial discs, as well as over alternative treatment
procedures such as spinal fusion. For example, the two-piece
prosthetic discs described herein provide compressive compliance
similar to that of a natural spinal disc. In addition, the motions
in flexion, extension, lateral bending, and axial rotation are also
restricted in a manner near or identical to those associated with a
natural disc.
C. Three-Piece Structure
[0199] A representative prosthetic intervertebral disc 300 having a
three-piece structure is shown in FIGS. 21 through 23. The
prosthetic disc includes an upper endplate 310, a lower endplate
320, and a core assembly 330 retained between the upper endplate
310 and the lower endplate 320.
[0200] The upper endplate 310 and lower endplate 320 are generally
flat, planar members, and are fabricated from a physiologically
acceptable material that provides substantial rigidity. Examples of
materials suitable for use in fabricating the upper endplate 310
and lower endplate 320 include titanium, titanium alloys, stainless
steel, cobalt/chromium, etc., which are manufactured by machining
or metal injection molding; plastics such as polyethylene with
ultra high molar mass (molecular weight) (UHMWPE), polyether ether
ketone (PEEK), etc., which are manufactured by injection molding or
compression molding; ceramics; graphite; and others. Optionally,
the endplates may be coated with hydroxyapatite, titanium plasma
spray, or other coatings to enhance bony ingrowth.
[0201] As noted above, the upper and lower endplates typically have
a length of from about 12 mm to about 45 mm, preferably from about
13 mm to about 44 mm, a width of from about 11 mm to about 28 mm,
preferably from about 12 mm to about 25 mm, and a thickness of from
about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3
mm. The sizes of the upper and lower endplates are selected
primarily based upon the size of the void between adjacent
vertebral bodies to be occupied by the prosthetic disc.
Accordingly, while endplate lengths and widths outside of the
ranges listed above are possible, they are not typical. The upper
surface of the upper endplate 310 and the lower surface of the
lower endplate 320 are preferably each provided with a mechanism
for securing the endplate to the respective opposed surfaces of the
upper and lower vertebral bodies between which the prosthetic disc
is to be implanted. For example, in FIGS. 21 and 23, the upper
endplate 310 includes an anchoring fin 311. The anchoring fin 311
is intended to engage a mating groove that is formed on the surface
of the upper vertebral body to thereby secure the endplate to the
vertebral body. The anchoring fin 311 extends generally
perpendicularly from the generally planar external surface of the
upper endplate 310, i.e., upward from the upper side of the
endplate as shown in FIGS. 21 and 23. As shown in the Figures, the
anchoring fin 311 is disposed at the center of external surface of
the upper endplate 310 and has a length that is slightly shorter
than the width of the upper endplate 310. Although not shown in the
Figures, the anchoring fin 311 may be provided with a plurality of
serrations located on the top edge of the anchoring fin. The
serrations are intended to enhance the ability of the anchoring fin
to engage the vertebral body and to thereby secure the upper
endplate 310 to the spine.
[0202] Similarly, the lower surface of the lower endplate 320
includes an anchoring fin 321. The anchoring fin 321 on the lower
surface of the lower endplate 320 is identical in structure and
function to the anchoring fin 311 on the upper surface of the upper
endplate 310, with the exception of its location on the prosthetic
disc. The anchoring fin 321 on the lower endplate 320 is intended
to engage a mating groove formed on the lower vertebral body,
whereas the anchoring fin 311 on the upper endplate 310 is intended
to engage a mating groove on the upper vertebral body. Thus, the
prosthetic disc 300 is held in place between the adjacent vertebral
bodies.
[0203] Alternatively, the upper endplate 310 and lower endplate 320
of the three-piece prosthetic disc may employ one of the
alternative securing mechanisms shown in FIGS. 13A-C. As described
above in relation to the one-piece prosthetic device shown in FIG.
13A, each of the upper endplate 110 and lower endplate 120 is
provided with a single anchoring fin 111, 121. The anchoring fins
111, 121 are located along a center line of the respective
endplates, and each is provided with a plurality of serrations 112,
122 on its upper edge. The single anchoring fins 111, 121 are
intended to engage grooves formed on the opposed surface of the
upper and lower vertebral bodies, as described above. In FIG. 13B,
each of the upper endplate 110 and lower endplate 120 is provided
with three anchoring fins 111a-c, 121a-c. The FIG. 13B prosthetic
disc is the same as the prosthetic disc shown in FIG. 12, but it is
shown in perspective rather than cross-section. Thus, the structure
and function of the anchoring fins 111a-c and 121a-c are as
described above in relation to FIG. 12. Finally, in FIG. 13C, each
of the upper endplate 110 and lower endplate 120 is provided with a
plurality of serrations 113, 123 over a portion of the exposed
external surface of the respective endplate. The serrations 113,
123 are intended to engage the opposed surfaces of the adjacent
vertebral bodies to thereby secure the endplates in place between
the vertebral bodies. The serrations 113, 123 may be provided over
the entire external surface of each of the upper and lower
endplates, or they may be provided over only a portion of those
surfaces. For example, in FIG. 13C, the serrations 113 on the upper
surface of the upper endplate 110 are provided over three major
areas, a first area 113a near a first edge of the upper endplate
110, a second area 113b near the center of the upper endplate 110,
and a third area near a second edge of the endplate 113c.
[0204] Turning to FIG. 54, in an optional embodiment, the anchoring
fins 111 are selectively retractable and extendable by providing a
deployment mechanism 160 that is associated with the upper endplate
110. A similar mechanism may be used on the lower endplate 120. The
deployment mechanism includes a slider 161 that slides within a
channel 162 formed in the upper endplate 110. The channel 162
includes a threaded region 163, and the slider 161 includes
matching threads 164, thereby providing a mechanism for advancing
the slider 161 within the channel 162. As the slider 161 is
advanced within the channel 162, a tapered region 165 engages the
bottom surface of a deployable fin 166. Further advancement of the
slider 161 causes the deployable fin 166 to be raised upward within
a slot 167 on the upper surface of the upper endplate 110.
Reversing the deployment mechanism 160 causes the fin 166 to
retract. The deployment mechanism 160 may also be used in
conjunction with spikes, serrations, or other anchoring devices. In
an alternative embodiment, the threaded slider 161 of the
deployment mechanism may be replaced with a dowel pin that is
advanced to deploy the fin 166. Other advancement mechanisms are
also possible.
[0205] The core assembly 330 is intended to provide support to and
to maintain the relative spacing between the upper endplate 310 and
lower endplate 320. The core assembly 330 provides compressive
compliance to the three-piece prosthetic disc, as well as providing
limited translation, flexion, extension, and lateral bending by and
between the upper endplate 310 and lower endplate 320. The core
assembly 330 further provides substantially unlimited rotation by
and between the upper endplate 310 and the lower endplate 320.
[0206] The core assembly 330 includes a top cap 331, a bottom cap
332, a sidewall 333, and a core center 334 held by and retained
between the top cap 331, bottom cap 332, and sidewall 333. The top
cap 331 and bottom cap 332 are generally planar, and are fabricated
from a physiologically acceptable material that provides
substantial rigidity. Examples of materials suitable for use in
fabricating the top cap 331 and bottom cap 332 include titanium,
titanium alloys, stainless steel, cobalt/chromium, etc., which are
manufactured by machining or metal injection molding; plastics such
as polyethylene with ultra high molar mass (molecular weight)
(UHMWPE), polyether ether ketone (PEEK), etc., which are
manufactured by injection molding or compression molding; ceramics;
graphite; and others. The core center 334 is made of a relatively
compliant material, for example, polyurethane or silicone, and is
typically fabricated by injection molding. The shape of the core
center 334 is typically generally cylindrical or bean-shaped,
although the shape (as well as the materials making up the core
center and the core member size) may be varied to obtain desired
physical or performance properties. For example, the core member
334 shape, size, and materials will directly affect the degree of
flexion, extension, lateral bending, and axial rotation of the
prosthetic disc.
[0207] The top cap 331 and bottom cap 332 each preferably includes
a generally concave indentation 336 formed at a center point of the
cap. The indentations 336 are intended to cooperate with a pair of
retainers formed on the internal surfaces of the endplates to
retain the core assembly 330 in place between the retainers, as
described more fully below.
[0208] The top cap 331 and bottom cap 332 preferably contain a
plurality of slots 335 spaced radially about the surface of each of
the caps. One or more fibers 340 are wound around the top cap 331
and bottom cap 332 through the slots 335 to attach the endplates to
one another. The fibers 340 preferably are not tightly wound,
thereby allowing a degree of axial rotation, bending, flexion, and
extension by and between the top cap 331 and bottom cap 332. The
core center 334 is preferably pre-compressed. The actual number of
slots 335 contained on each of the top cap 331 and bottom cap 332
is variable. Increasing the number of slots will result in an
increase in the circumferential density of the fibers holding the
endplates together. Additionally, the fibers may be wound multiple
times within the same slot, thereby increasing the radial density
of the fibers. In each case, this improves the wear resistance and
increases the torsional and flexural stiffness of the prosthetic
disc, thereby further approximating natural disc stiffness. In
addition, the fibers 340 may be passed through or wound on each
slot, or only on selected slots, as needed.
[0209] The purpose of the fibers 340 is to hold the top cap 331 and
bottom cap 332 together and to limit the range-of-motion to mimic
the range-of-motion of a natural disc. Accordingly, the fibers
preferably comprise high tenacity fibers with a high modulus of
elasticity, for example, at least about 100 MPa, and preferably at
least about 500 MPa. By high tenacity fibers is meant fibers that
can withstand a longitudinal stress of at least 50 MPa, and
preferably at least 250 MPa, without tearing. The fibers 140 are
generally elongate fibers having a diameter that ranges from about
100 .mu.m to about 500 .mu.m, and preferably about 200 .mu.m to
about 400 .mu.m. Optionally, the fibers may be injection molded
with an elastomer to encapsulate the fibers, thereby providing
protection from tissue ingrowth and improving torsional and
flexural stiffness.
[0210] The fibers 340 may be fabricated from any suitable material.
Examples of suitable materials include polyester (e.g.,
Dacron.RTM.), polyethylene, polyaramid, poly-paraphenylene
terephthalamide (e.g., Kevlar.RTM.), carbon or glass fibers,
polyethylene terephthalate, acrylic polymers, methacrylic polymers,
polyurethane, polyurea, polyolefin, halogenated polyolefin,
polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and
the like.
[0211] The fibers 340 may be terminated on an endplate by tying a
knot in the fiber on the superior surface of an endplate.
Alternatively, the fibers 340 may be terminated on an endplate by
slipping the terminal end of the fiber into a slot on an edge of an
endplate, similar to the manner in which thread is retained on a
thread spool. The slot may hold the fiber with a crimp of the slot
structure itself, or by an additional retainer such as a ferrule
crimp. As a further alternative, tab-like crimps may be machined
into or welded onto the endplate structure to secure the terminal
end of the fiber. The fiber may then be closed within the crimp to
secure it. As a still further alternative, a polymer may be used to
secure the fiber to the endplate by welding. The polymer would
preferably be of the same material as the fiber (e.g., PE, PET, or
the other materials listed above). Still further, the fiber may be
retained on the endplates by crimping a cross-member to the fiber
creating a T-joint, or by crimping a ball to the fiber to create a
ball joint.
[0212] The sidewall 333 is preferably made of polyurethane or
silicone and may be fabricated by injection molding, two-part
component mixing, or dipping the core assembly into a polymer
solution. Alternatively, an outer ring or gasket (not shown in the
drawings) may optionally be provided in place of the sidewall
333.
[0213] As noted above, the core assembly 330 is selectively
retained between the upper endplate 310 and the lower endplate 320.
A preferred mechanism for retaining the core assembly 330 between
the two endplates is illustrated in FIGS. 21 through 23. For
example, the upper endplate 310 is provided with a retainer 313
formed on the interior surface of the upper endplate 310. The
retainer 313 is a convex body formed at the center of the internal
surface of the upper endplate 310 that extends into the space
between the upper endplate 310 and lower endplate 320 when the
endplates are implanted into the patient. A similar retainer 323 is
formed on the opposed internal surface of the lower endplate 320.
Each of the retainers 313, 323 is preferably of generally
semi-spherical shape, and each is preferably formed from the same
material used to fabricate the upper and lower endplates 310,
320.
[0214] As shown, for example, in FIG. 23, the retainers 313, 323
formed on the internal surfaces of the endplates cooperate with the
indentations 336 formed on the external surfaces of the top cap 331
and bottom 332 of the core assembly 330 to hold the core assembly
in place between the endplates. The amount of retaining force
holding the core assembly 330 in place will depend on several
factors, including the materials used to form the endplates and the
core assembly, the size and shape of the core assembly, the
distance separating the two endplates, the size and shape of each
of the retainers and indentations, and other factors. Any one or
all of these factors may be varied to obtain desired results.
Typically, the retaining force will be sufficient to hold the core
assembly in place, while still allowing each of the endplates to
rotate substantially freely relative to the core assembly.
[0215] Turning to FIGS. 24A-C, three embodiments of the core
assembly 330 are illustrated. In a first embodiment, shown in FIG.
24A, the core assembly 330 is provided with a through hole 337,
i.e., the central portion of the core assembly 330 is hollow. In
this embodiment, although there are no indentations 336, the
through hole 337 creates a shoulder 338 on each of the top cap 331
and bottom cap 332. The shoulders 338 have a size selected to
suitably engage the retainers 313, 323 formed on the endplates. In
a second embodiment, the core assembly 330 is provided with
indentations 336 and the core center 334 extends throughout the
internal volume of the core assembly. Finally, in a third
embodiment, the core assembly 330 is provided with indentations
336, but the core center 334 occupies only a central portion of the
internal volume of the core assembly 330.
[0216] Turning to FIGS. 25A-C, the core assembly may optionally
include a plurality of reinforcing fibers 360 distributed
throughout the body of the core assembly. The fibers 360 may be
fabricated from any suitable material. Examples of suitable
materials include polyester (e.g., Dacron), polyethylene,
polyaramid, carbon or glass fibers, polyethylene terephthalate,
acrylic polymers, methacrylic polymers, polyurethane, polyurea,
polyolefin, halogenated polyolefin, polysaccharide, vinylic
polymer, polyphosphazene, polysiloxane, and the like. The
reinforcing fibers 360 provide additional strength to the core
assembly. The fiber reinforcement is made by injecting core center
material around the fibers formed in the shape of the core center.
Exemplary core shapes are shown in FIGS. 25A-C, and include a core
assembly 330 having a through hole 337 (FIG. 25A), a core assembly
330 having indentations 336 on each of the top and bottom surfaces
(FIG. 25B), and a core assembly 330 having a toroidal shape (FIG.
25C).
[0217] The fibers 360 may be terminated on an endplate by tying a
knot in the fiber on the superior surface of an endplate.
Alternatively, the fibers 360 may be terminated on an endplate by
slipping the terminal end of the fiber into a slot on an edge of an
endplate, similar to the manner in which thread is retained on a
thread spool. The slot may hold the fiber with a crimp of the slot
structure itself, or by an additional retainer such as a ferrule
crimp. As a further alternative, tab-like crimps may be machined
into or welded onto the endplate structure to secure the terminal
end of the fiber. The fiber may then be closed within the crimp to
secure it. As a still further alternative, a polymer may be used to
secure the fiber to the endplate by welding. The polymer would
preferably be of the same material as the fiber (e.g., PE, PET, or
the other materials listed above). Still further, the fiber may be
retained on the endplates by crimping a cross-member to the fiber
creating a T-joint, or by crimping a ball to the fiber to create a
ball joint.
[0218] Turning next to FIGS. 26, 27, and 28A-C, the core assembly
may optionally be formed of stacks of reinforcing fabric having no
silicone, polyurethane, or other polymeric component. As shown in
FIG. 26, woven fibers 370 are formed into sheets of fabric that are
compressed into a stack to form a core body. The woven fibers 370
may be formed of materials such as polyester (e.g., Dacron),
polyethylene, polyaramid, carbon or glass fibers, polyethylene
terephthalate, acrylic polymers, methacrylic polymers,
polyurethane, polyurea, polyolefin, halogenated polyolefin,
polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and
the like. FIG. 27 is a cross-sectional view of a woven fiber core
body. FIG. 28A illustrates a woven fiber core body 330 having a
through hole 337 similar to the structure described previously.
Similarly, FIG. 28B illustrates a woven fiber core body 330 having
indentations 336 on its upper and lower surfaces. Finally, FIG. 28C
illustrates a woven fiber core body 330 having a toroidal
shape.
[0219] The three-piece structure embodiment of the prosthetic disc
is implanted by a surgical procedure. After removing the natural
disc, grooves are formed in the superior and inferior vertebrae
between which the prosthetic disc is to be implanted. The upper
endplate 310 and lower endplate 320 are then each implanted into
the void, while aligning the anchoring fins 311 321 with the
grooves formed on the vertebral bodies. The anchoring fins cause
the prosthetic disc to be secured in place between the adjacent
vertebral bodies. After the upper endplate 310 and lower endplate
320 are implanted, the core assembly 330 is engaged between the
endplates to complete the implantation.
[0220] The three-piece prosthetic disc has several advantages over
prior art artificial discs, as well as over alternative treatment
procedures such as spinal fusion. For example, the prosthetic discs
described herein provide compressive compliance similar to that of
a natural spinal disc. In addition, the motions in flexion,
extension, lateral bending, and axial rotation are also restricted
in a manner near or identical to those associated with a natural
disc.
D. Four-Piece Structure
[0221] Representative prosthetic intervertebral discs 400 having
four-piece structures are shown in FIGS. 29 through 35. The
prosthetic discs include an upper endplate 410, a lower endplate
420, and a two-piece core assembly 430 retained between the upper
endplate 410 and the lower endplate 420.
[0222] The upper endplate 410 and lower endplate 420 are generally
flat, planar members, and are fabricated from a physiologically
acceptable material that provides substantial rigidity. Examples of
materials suitable for use in fabricating the upper endplate 410
and lower endplate 420 include titanium, titanium alloys, stainless
steel, cobalt/chromium, etc., which are manufactured by machining
or metal injection molding; plastics such as polyethylene with
ultra high molar mass (molecular weight) (UHMWPE), polyether ether
ketone (PEEK), etc., which are manufactured by injection molding or
compression molding; ceramics; graphite; and others. Optionally,
the endplates may be coated with hydroxyapatite, titanium plasma
spray, or other coatings to enhance bony ingrowth.
[0223] As noted above, the upper and lower endplates typically have
a length of from about 12 mm to about 45 mm, preferably from about
13 mm to about 44 mm, a width of from about 11 mm to about 28 mm,
preferably from about 12 mm to about 25 mm, and a thickness of from
about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3
mm. The sizes of the upper and lower endplates are selected
primarily based upon the size of the void between adjacent
vertebral bodies to be occupied by the prosthetic disc.
Accordingly, while endplate lengths and widths outside of the
ranges listed above are possible, they are not typical
[0224] The upper surface of the upper endplate 410 and the lower
surface of the lower endplate 420 are preferably each provided with
a mechanism for securing the endplate to the respective opposed
surfaces of the upper and lower vertebral bodies between which the
prosthetic disc is to be implanted. For example, as shown in FIGS.
30 and 32, the upper endplate 410 includes an anchoring fin 411.
The anchoring fin 411 is intended to engage a mating groove that is
formed on the surface of the upper vertebral body to thereby secure
the endplate to the vertebral body. The anchoring fin 411 extends
generally perpendicularly from the generally planar external
surface of the upper endplate 410, i.e., upward from the upper side
of the endplate as shown in FIGS. 30 and 32. As shown in the
Figures, the anchoring fin 411 is disposed at the center of
external surface of the upper endplate 410 and has a length that is
slightly less than the width of the upper endplate 410. Although
not shown in the Figures, the anchoring fin 411 may be provided
with a plurality of serrations located on its top edge. The
serrations are intended to enhance the ability of the anchoring fin
to engage the vertebral body and to thereby secure the upper
endplate 410 to the spine.
[0225] Similarly, the lower surface of the lower endplate 420
includes an anchoring fin 421. The anchoring fin 421 on the lower
surface of the lower endplate 420 is identical in structure and
function to the anchoring fin 411 on the upper surface of the upper
endplate 410, with the exception of its location on the prosthetic
disc. The anchoring fin 421 on the lower endplate 420 is intended
to engage a mating groove formed on the lower vertebral body,
whereas the anchoring fin 411 on the upper endplate 410 is intended
to engage a mating groove on the upper vertebral body. Thus, the
prosthetic disc 400 is held in place between the adjacent vertebral
bodies.
[0226] Alternatively, the upper endplate 410 and lower endplate 420
of the three-piece prosthetic disc may employ one of the
alternative securing mechanisms shown in FIGS. 13A-C. As described
above in relation to the one-piece prosthetic device shown in FIG.
13A, each of the upper endplate 110 and lower endplate 120 is
provided with a single anchoring fin 111, 121. The anchoring fins
111, 121 are located along a centerline of the respective
endplates, and each is provided with a plurality of serrations 112,
122 on its upper edge. The single anchoring fins 111, 121 are
intended to engage grooves formed on the opposed surface of the
upper and lower vertebral bodies, as described above. In FIG. 13B,
each of the upper endplate 110 and lower endplate 120 is provided
with three anchoring fins 111a-c, 121a-c. The FIG. 13B prosthetic
disc is the same as the prosthetic disc shown in FIG. 12, but it is
shown in perspective rather than cross-section. Thus, the structure
and function of the anchoring fins 111a-c and 121a-c are as
described above in relation to FIG. 12. Finally, in FIG. 13C, each
of the upper endplate 110 and lower endplate 120 is provided with a
plurality of serrations 113, 123 over a portion of the exposed
external surface of the respective endplate. The serrations 113,
123 are intended to engage the opposed surfaces of the adjacent
vertebral bodies to thereby secure the endplates in place between
the vertebral bodies. The serrations 113, 123 may be provided over
the entire external surface of each of the upper and lower
endplates, or they may be provided over only a portion of those
surfaces. For example, in FIG. 13C, the serrations 113 on the upper
surface of the upper endplate 110 are provided over three major
areas, a first area 113a near a first edge of the upper endplate
110, a second area 113b near the center of the upper endplate 110,
and a third area near a second edge of the endplate 113c.
[0227] Turning to FIG. 54, in an optional embodiment, the anchoring
fins 111 are selectively retractable and extendable by providing a
deployment mechanism 160 that is associated with the upper endplate
110. A similar mechanism may be used on the lower endplate 120. The
deployment mechanism includes a slider 161 that slides within a
channel 162 formed in the upper endplate 110. The channel 162
includes a threaded region 163, and the slider 161 includes
matching threads 164, thereby providing a mechanism for advancing
the slider 161 within the channel 162. As the slider 161 is
advanced within the channel 162, a tapered region 165 engages the
bottom surface of a deployable fin 166. Further advancement of the
slider 161 causes the deployable fin 166 to be raised upward within
a slot 167 on the upper surface of the upper endplate 110.
Reversing the deployment mechanism 160 causes the fin 166 to
retract. The deployment mechanism 160 may also be used in
conjunction with spikes, serrations, or other anchoring devices. In
an alternative embodiment, the threaded slider 161 of the
deployment mechanism may be replaced with a dowel pin that is
advanced to deploy the fin 166. Other advancement mechanisms are
also possible.
[0228] FIG. 29 illustrates yet another alternative mechanism for
securing the upper and lower endplates to the vertebral bodies. As
shown in the Figure, the upper endplate 410 may be provided with a
plurality of anchoring spikes 419 spaced over the external surface
of the endplate. The anchoring spikes 419 are adapted to engage the
internal surface of the vertebral body. Although not shown in FIG.
29, the external surface of the lower endplate 420 may optionally
be provided with similar anchoring spikes to secure the lower
endplate to the internal surface of the inferior vertebral
body.
[0229] The core assembly 430 is intended to provide support to and
to maintain the relative spacing between the upper endplate 410 and
lower endplate 420. The core assembly 430 provides compressive
compliance to the four-piece prosthetic disc, as well as providing
limited translation, flexion, extension, and lateral bending by and
between the upper endplate 410 and lower endplate 420. The core
assembly 430 further provides substantially unlimited rotation by
and between the upper endplate 410 and the lower endplate 420.
[0230] The core assembly 430 includes an upper core member 430a and
a lower core member 430b, 430c. Two embodiments of the core
assembly 430 of the four-piece prosthetic disc are shown in FIGS.
29 through 35. In the first embodiment, shown in FIGS. 29 through
32, both the upper core member 430a and the lower core member 430b
include a core structure having top and bottom caps, slots, fibers,
a core center, and an annular capsule. In the second embodiment,
shown in FIGS. 33 through 35, the upper core member 430a is
identical to that of the first embodiment, but the lower core
member 430c is, instead, a solid structure. These structures are
described more fully below.
[0231] The upper core member 430a includes a top cap 431a, a bottom
cap 432a, a sidewall 433a, and a core center 434a held by and
retained between the top cap 431a, bottom cap 432a, and sidewall
433a. The top cap 431a and bottom cap 432a are generally planar,
and are fabricated from a physiologically acceptable material that
provides substantial rigidity. Examples of materials suitable for
use in fabricating the top cap 431a and bottom cap 432a include
titanium, titanium alloys, stainless steel, cobalt/chromium, etc.,
which are manufactured by machining or metal injection molding;
plastics such as polyethylene with ultra high molar mass (molecular
weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are
manufactured by injection molding or compression molding; ceramics;
graphite; and others. The core center 434a is made of a relatively
compliant material, for example, polyurethane or silicone, and is
typically fabricated by injection molding. The shape of the core
center 434a is typically generally cylindrical or bean-shaped,
although the shape (as well as the materials making up the core
center and the core member size) may be varied to obtain desired
physical or performance properties. For example, the core member
434a shape, size, and materials will directly affect the degree of
flexion, extension, lateral bending, and axial rotation of the
prosthetic disc.
[0232] The bottom cap 432a preferably includes a generally convex
retainer 437a formed at a center point of the bottom cap 432a. The
retainer 437a is intended to cooperate with an indentation 436b,
436c formed on the upper surface of the lower core member 430b,
430c to create an engagement between the upper core member 430a and
the lower core member 430b, 430c, as described more fully
below.
[0233] The top cap 431a and bottom cap 432a preferably contain a
plurality of slots 435a spaced radially about the surface of each
of the caps. One or more fibers 440 are wound around the top cap
431a and bottom cap 432a through the slots 435a to attach the top
and bottom caps to one another. The fibers 440 preferably are not
tightly wound, thereby allowing a degree of axial rotation,
bending, flexion, and extension by and between the top cap 431a and
bottom cap 432a. The core center 434a is preferably pre-compressed.
The actual number of slots 435a contained on each of the top cap
431a and bottom cap 432a is variable. Increasing the number of
slots will result in an increase in the circumferential density of
the fibers holding the endplates together. Additionally, the fibers
may be wound multiple times within the same slot, thereby
increasing the radial density of the fibers. In each case, this
improves the wear resistance and increases the torsional and
flexural stiffness of the prosthetic disc, thereby further
approximating natural disc stiffness. In addition, the fibers 440
may be passed through or wound on each slot, or only on selected
slots, as needed.
[0234] The purpose of the fibers 440 is to hold the top cap 431a
and bottom cap 432a together and to limit the range-of-motion to
mimic the range-of-motion of a natural disc. Accordingly, the
fibers preferably comprise high tenacity fibers with a high modulus
of elasticity, for example, at least about 100 MPa, and preferably
at least about 500 MPa. By high tenacity fibers is meant fibers
that can withstand a longitudinal stress of at least 50 MPa, and
preferably at least 250 MPa, without tearing. The fibers 440 are
generally elongate fibers having a diameter that ranges from about
100 .mu.m to about 500 .mu.m, and preferably about 200 .mu.m to
about 400 .mu.m. Optionally, the fibers may be injection molded
with an elastomer to encapsulate the fibers, thereby providing
protection from tissue ingrowth and improving torsional and
flexural stiffness.
[0235] The fibers 440 may be fabricated from any suitable material.
Examples of suitable materials include polyester (e.g.,
Dacron.RTM.), polyethylene, polyaramid, poly-paraphenylene
terephthalamide (e.g., Kevlar.RTM.), carbon or glass fibers,
polyethylene terephthalate, acrylic polymers, methacrylic polymers,
polyurethane, polyurea, polyolefin, halogenated polyolefin,
polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and
the like.
[0236] The fibers 440 may be terminated on an endplate by tying a
knot in the fiber on the superior surface of an endplate.
Alternatively, the fibers 440 may be terminated on an endplate by
slipping the terminal end of the fiber into a slot on an edge of an
endplate, similar to the manner in which thread is retained on a
thread spool. The slot may hold the fiber with a crimp of the slot
structure itself, or by an additional retainer such as a ferrule
crimp. As a further alternative, tab-like crimps may be machined
into or welded onto the endplate structure to secure the terminal
end of the fiber. The fiber may then be closed within the crimp to
secure it. As a still further alternative, a polymer may be used to
secure the fiber to the endplate by welding. The polymer would
preferably be of the same material as the fiber (e.g., PE, PET, or
the other materials listed above). Still further, the fiber may be
retained on the endplates by crimping a cross-member to the fiber
creating a T-joint, or by crimping a ball to the fiber to create a
ball joint.
[0237] The sidewall 433a is preferably made of polyurethane or
silicone and may be fabricated by injection molding, two-part
component mixing, or dipping the core assembly into a polymer
solution. Alternatively, an outer ring or gasket (not shown in the
drawings) may optionally be provided in place of the sidewall
433a.
[0238] As shown, for example, in FIGS. 29 through 35, the top cap
431a of the upper core member 430a includes an edge member 438a
that is adapted to engage a groove 416 formed on the perimeter of
the internal surface of the upper endplate 410 to provide an
engagement mechanism for attaching the upper core member 430a to
the upper endplate 410. For example, the upper core member 430a is
slid into the upper endplate 410, as shown by the arrows in FIG.
29. After the upper endplate 430a is fully advanced, i.e., once the
leading edge of the edge member 438a contacts the interior of the
groove 416 on the upper endplate 410, a tab 461a on the upper
surface of the top cap 431a engages a slot 462a on the lower
surface of the upper endplate 410 (see FIGS. 30 and 33), thereby
locking the upper core member 430a in place within the upper
endplate 410.
[0239] Turning to the first embodiment of the lower core member
430b, shown in FIGS. 29 through 32, the lower core member 430b
includes a top cap 431b, a bottom cap 432b, a sidewall 433b, and a
core center 434b held by and retained between the top cap 431b,
bottom cap 432b, and sidewall 433b. The top cap 431b and bottom cap
432b are generally planar, and are fabricated from a
physiologically acceptable material that provides substantial
rigidity. Examples of materials suitable for use in fabricating the
top cap 431b and bottom cap 432b include titanium, titanium alloys,
stainless steel, cobalt/chromium, etc., which are manufactured by
machining or metal injection molding; plastics such as polyethylene
with ultra high molar mass (molecular weight) (UHMWPE), polyether
ether ketone (PEEK), etc., which are manufactured by injection
molding or compression molding; ceramics; graphite; and others. The
core center 434b is made of a relatively compliant material, for
example, polyurethane or silicone, and is typically fabricated by
injection molding. The shape of the core center 434b is typically
generally cylindrical or bean-shaped, although the shape (as well
as the materials making up the core center and the core member
size) may be varied to obtain desired physical or performance
properties. For example, the core member 434b shape, size, and
materials will directly affect the degree of flexion, extension,
lateral bending, and axial rotation of the prosthetic disc.
[0240] The top cap 431b preferably includes a generally concave
indentation 436b formed at a center-point of the top cap 431b. The
indentation 436b is intended to cooperate with the retainer 437a
formed on the lower surface of the upper core member 430a to create
an engagement between the upper core member 430a and the lower core
member 430b, as described more fully below.
[0241] The top cap 431b and bottom cap 432b preferably contain a
plurality of slots 435b spaced radially about the surface of each
of the caps. One or more fibers 440 are wound around the top cap
431b and bottom cap 432b through the slots 435b to attach the top
and bottom caps to one another. The fibers 440 preferably are not
tightly wound, thereby allowing a degree of axial rotation,
bending, flexion, and extension by and between the top cap 431b and
bottom cap 432b. The core center 434b is preferably pre-compressed.
The actual number of slots 435b contained on each of the top cap
431b and bottom cap 432b is variable. In addition, the fibers 440
may be passed through or wound on each slot, or only on selected
slots, as needed.
[0242] The purpose of the fibers 440 is to hold the top cap 431b
and bottom cap 432b together. Accordingly, the fibers preferably
comprise high tenacity fibers with a high modulus of elasticity,
for example, at least about 100 MPa, and preferably at least about
500 MPa. By high tenacity fibers is meant fibers that can withstand
a longitudinal stress of at least 50 MPa, and preferably at least
250 MPa, without tearing. The fibers 440 are generally elongate
fibers having a diameter that ranges from about 100 .mu.m to about
500 .mu.m, and preferably about 200 .mu.m to about 400 .mu.m.
[0243] The fibers 440 may be fabricated from any suitable material.
Examples of suitable materials include polyester (e.g., Dacron),
polyethylene, polyaramid, carbon or glass fibers, polyethylene
terephthalate, acrylic polymers, methacrylic polymers,
polyurethane, polyurea, polyolefin, halogenated polyolefin,
polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and
the like.
[0244] The sidewall 433b is preferably made of polyurethane or
silicone and may be fabricated by injection molding, two-part
component mixing, or dipping the core assembly into a polymer
solution. Alternatively, an outer ring or gasket (not shown in the
drawings) may optionally be provided in place of the sidewall
433b.
[0245] As shown, for example, in FIGS. 29 through 32, the bottom
cap 432b of the lower core member 430b includes an edge member 438b
that is adapted to engage a groove 426 formed on the perimeter of
the internal surface of the lower endplate 420 to provide an
engagement mechanism for attaching the lower core member 430b to
the lower endplate 420. For example, the lower core member 430b is
slid into the lower endplate 420, as shown by the arrow in FIG. 16.
After the lower endplate 430b is fully advanced, i.e., once the
leading edge of the edge member 438b contacts the interior of the
groove 426 on the lower endplate 420, a tab 461b on the lower
surface of the bottom cap 432b engages a slot 462b on the upper
surface of the lower endplate 420 (see FIG. 30), thereby locking
the lower core member 430b in place within the lower endplate
420.
[0246] Turning to the second embodiment of the lower core member
430c, shown in FIGS. 33 through 35, the lower core member 430c is
formed of a solid structure having none of the top and bottom caps,
sidewall, core center, or fibers that are included in the first
embodiment of the lower core member 430b. The second embodiment of
the lower core member 430c has an identical external shape and size
to that of the first embodiment of the lower core member 430b,
including having an edge member 438c that engages the groove 426 on
the internal surface of the lower endplate 420. A tab 461c is
configured to selectively engage the notch 462c formed on the upper
internal surface of the lower endplate 420. An indentation 436c is
formed on the central upper surface of the lower core member 430c,
and is adapted to engage the retainer 437a formed on the upper core
member 430a.
[0247] Examples of materials suitable for use in fabricating the
second embodiment of the lower core member 430c include titanium,
titanium alloys, stainless steel, cobalt/chromium, etc., which are
manufactured by machining or metal injection molding; plastics such
as polyethylene with ultra high molar mass (molecular weight)
(UHMWPE), polyether ether ketone (PEEK), etc., which are
manufactured by injection molding or compression molding; ceramics;
graphite; and others.
[0248] The four-piece structure embodiment of the prosthetic disc
is implanted by a surgical procedure. After removing the natural
disc, grooves are formed in the superior and inferior vertebrae
between which the prosthetic disc is to be implanted (only in the
situation where the endplates are provided with anchoring fins).
The upper endplate 410 and lower endplate 420 are then each
implanted into the void, while aligning the anchoring fins 411, 421
with the grooves formed on the vertebral bodies. The anchoring fins
cause the prosthetic disc to be secured in place between the
adjacent vertebral bodies. After the upper endplate 410 and lower
endplate 420 are put in position, the core assembly 430 is engaged
between the endplates to complete the implantation.
[0249] The four-piece prosthetic disc has several advantages over
prior art artificial discs, as well as over alternative treatment
procedures such as spinal fusion. For example, the prosthetic discs
described herein provide compressive compliance similar to that of
a natural spinal disc. In addition, the motions in flexion,
extension, lateral bending, and axial rotation are also restricted
in a manner near or identical to those associated with a natural
disc.
E. Fabric Tubes
[0250] The one-piece, two-piece, three-piece, and four-piece
structures of the prosthetic discs described above include upper
and lower endplates that are attached to each other by fibers wound
around the endplates. In an alternative embodiment, the fiber
component is provided in a fabric cylinder or tubing of woven or
knitted form, rather than as individual fibers. The fabric tubing
extends between and structurally connects the upper and lower
endplates.
[0251] In a first example, a single fabric tube may be provided in
place of the wound fibers. The fabric tube may be attached at its
upper edge to the upper endplate, and at its lower edge to the
lower endplate. For example, through-holes may be provided in each
of the endplates to allow the fabric tubing (or individually woven
fibers) to pass through and to be secured by knots or crimping on
the external surfaces of the endplates. Alternatively, the fabric
tube may be attached to each endplate by a peripheral metal or
plastic ring that is fixed to the interior surfaces of the
endplates.
[0252] In another example, two or more tubes of fabric may be
provided between and interconnecting the upper and lower endplates.
The two or more fabric tubes may be attached to the endplates by
through-holes, as described above, or by press-fit, adhesion, weld,
or injection molding integration to a progressively smaller metal
or plastic ring with tubing circumferentially affixed to it. This
structure creates an assembly of two or more concentric layers of
fabric tubing. Alternatively, the concentric tubes may be
terminated by collecting each tubing end together and crimping or
sewing them together, then fixing the collected ends to the upper
and lower endplates. As a still further alternative, an injection
molded lid may be fabricated in a manner in which the lid captures
the terminal ends of each of the fabric tubes during the injection
molding process.
[0253] In a particularly preferred embodiment, multiple concentric
fabric tubes are provided. Each of the fabric tubes may be formed
from a fabric of material different from the other tubes (e.g, PET,
PE, PTFE, Polyamide, etc.), or from a fabric having different
material properties. This provides the ability to construct
prosthetic discs having a range of performance characteristics.
[0254] As an alternative, the tubing may be comprised of a fiber
reinforced elastomeric material rather than a fabric alone. For
example, a polyurethane, PDMS, polyester, or other elastomer may be
integrated with a fabric or with individual fibers to create a
tubing that attaches and interconnects the upper and lower
endplates.
F. Anti-Creep Compression Member
[0255] Turning to FIG. 53, an optional anti-creep compression
member 135 is shown. The anti-creep member 135 is intended to
prevent "creeping" of the core due to vertical compression and
lateral expansion of the core, which occurs due to extended from
wear. The anti-creep member 135 is preferably used in connection
with a toroidal shaped core member 130, 230, 330, 430 of any of the
one-, two-, three-, or four-piece structures of the prosthetic
disc. The anti-creep element 135 includes a post 136 extending
downward from the upper endplate 110, 210, 310, 410 and a mating
receptacle or cup 137 extending upward from the lower endplate 120,
220, 320, 420. Alternatively, the post 136 may extend upward from
the lower endplate and the receptacle 137 may extend downward from
the upper endplate. A spring 138 is located within the receptacle
138. The post 136 is slightly conical in shape, and the receptacle
137 has a slightly larger diameter than the post 136 in order to
receive the post 136 within the receptacle 137. The slightly
conical shapes of the post 136 and cup 137 are preferred in order
to accommodate lateral bending (side-to-side), flexion (forward),
and extension (backward) of the upper and lower endplates relative
to one another. The spring is pre-loaded to provide a force biasing
the two endplates apart.
G. Advantages of the Present Prosthetic Intervertebral Discs
[0256] It is evident from the above discussion and results that the
present invention provides significantly improved prosthetic
intervertebral discs. Significantly, the subject discs closely
imitate the mechanical properties of the fully functional natural
discs that they are intended to replace.
[0257] More specifically, the modes of spinal motion may be
characterized as compression, shock absorption (i.e., very
rapid-compressive loading and unloading), flexion (forward) and
extension (backward), lateral bending (side-to-side), torsion
(twisting), and translation and sublaxation (motion of axis). The
prosthetic discs described herein semi-constrain each mode of
motion, rather than completely constrain or allow a mode to be
unconstrained. In this manner, the present prosthetic discs closely
mimic the performance of natural discs. The tables below provide
data that illustrates this performance.
TABLE-US-00001 TABLE 1 One-Piece Structure Lumbar Prosthetic Disc
Compared to Natural Human Disc and Ball & Socket Design
Prosthetic Disc Compressive Mode of Motion Ball & Socket
Prosthetic Disc Prosthetic Disc Properties Human Spine Design Core
Only Core & Fiber Stiffness (N/mm) .apprxeq.1288 Very large
800-1600 850-1650 ROM (mm) 0.50 .apprxeq.0 0.61 0.50 Ult. Load (N)
3952 >5900 >5900 >5900
TABLE-US-00002 TABLE 2 One-Piece Structure Cervical Prosthetic Disc
Compared to Natural Human Disc and Ball & Socket Design
Prosthetic Disc Compressive Mode of Motion Ball & Socket
Prosthetic Disc Prosthetic Disc Properties Human Spine Design Core
Only Core & Fiber Stiffness (N/mm) .apprxeq.737 Very large
100-950 150-1000 ROM (mm) 0.70 +/- 0.03 .apprxeq.0 .apprxeq.0.87
.apprxeq.0.60 Ult. Load (N) .apprxeq.1600 >5900 >9000
>9000
[0258] The subject discs exhibit stiffness in the axial direction,
torsional stiffness, bending stiffness in the saggital plane, and
bending stiffness in the front plane, where the degree of these
features can be controlled independently by adjusting the
components of the discs. The interface mechanism between the
endplates and the core members of several embodiments of the
described prosthetic discs enables a very easy surgical operation.
In view of the above and other benefits and features provided by
the subject inventions, it is clear that the subject inventions
represent a significant contribution to the art.
II. Implantation Apparatus and Methods
A. Conventional (Non-Minimally Invasive Method)
[0259] The prosthetic intervertebral discs may be implanted into a
patient's spine using the apparatus and methods described herein.
This description will focus on use of apparatus to implant one- and
two-piece prosthetic discs, although the apparatus may also be used
to implant other embodiments of the prosthetic disc with little or
no modification, as will be appreciated by a person of skill in the
art. In addition, and as described below, the method may
incorporate less than all of the apparatus components described
below.
[0260] The prosthetic discs are implanted surgically between two
adjacent vertebrae, an upper vertebra and a lower vertebra, in a
patient's spinal column. The vertebrae to be treated are exposed
using conventional surgical procedures. After exposure, the natural
vertebral disc is removed, leaving a void space between the two
adjacent vertebrae. The prosthetic intervertebral disc is then
implanted using the apparatus and methods described below.
1. Implantation Tools
[0261] In a first embodiment, and in reference to FIGS. 36-38, the
implantation tools include a spacer 810, a two-sided chisel 830,
and a holder 850.
[0262] Turning first to FIGS. 36A-B, the spacer 810 includes a
proximal handle 812, a shaft 814, and a head portion 816. The
handle 812 is adapted to be easily grasped by the user during the
implantation procedure. The shaft 814 is preferably cylindrical and
smaller in cross-section than the handle. The head portion 816 has
a size and shape adapted to perform its function of being inserted
between and separating two adjacent vertebral bodies. In the
embodiment shown in the Figures, the head portion 816 has a
generally trapezoidal shape when viewed from above or below, with a
leading edge 817 being generally parallel to, but having a shorter
length than the trailing edge 818. Other shapes may be used. The
head portion has a thickness "h". The thickness "h" may be varied
according to need, i.e., the thickness "h" will impact the ability
of the user to insert the head portion 816 between the two
vertebral bodies, and also the amount by which the head portion 816
will be able to separate the two bodies. Thus, a spacer 810 with a
head portion 816 of relatively large or small thickness "h" may be
used depending on the need. The edges 819 of the head portion 816
are generally rounded to allow the head portion 816 to be more
easily inserted between the two vertebral bodies.
[0263] Turning next to FIGS. 37A-B, the two-sided chisel 830
includes a handle 832, a shaft 834, and a head portion 836. The
handle 832 is adapted to be easily grasped by the user during the
implantation procedure. The shaft 834 is preferably cylindrical and
smaller in cross-section than the handle. The head portion 836 has
a size and shape adapted to perform its function of being inserted
between and creating grooves on the two adjacent vertebral bodies.
In the embodiment shown in the Figures, the head portion 836 has a
generally trapezoidal shape when viewed from above or below, with a
leading edge 837 being generally parallel to, but having a shorter
length than the trailing edge 838. Other shapes may be used. The
head portion has a thickness "h". The thickness "h" may be varied
according to need, i.e., the thickness "h" will impact the ability
of the user to be able to insert the head portion 836 between the
two vertebral bodies and to cut grooves on the two bodies. Thus, a
chisel 830 with a head portion 836 of relatively large or small
thickness "h" may be used depending on the need.
[0264] The chisel 830 includes a plurality of wedge-shaped blades
839 formed on the upper and lower surfaces of the head portion 836.
The blades 839 of the chisel 830 are adapted to create grooves in
the lower surface of the upper vertebra and on the upper surface of
the lower vertebra being treated. In the embodiment shown in the
Figures, the chisel 830 includes three blades 839 on each of the
upper and lower surfaces. More or fewer blades may be provided.
Optimally, the number, shape, and orientation of the blades 839 on
the surfaces of the chisel 830 are selected to match those of the
anchoring fins provided on the surfaces of the prosthetic disc to
be implanted.
[0265] Turning next to FIGS. 38A-B, the holder 850 includes a
handle 852, a shaft 854, and a head portion 856. The handle 852 is
adapted to be easily grasped by the user during the implantation
procedure. The shaft 854 is preferably cylindrical and smaller in
cross-section than the handle. The head portion 856 has a size and
shape adapted to perform its function of retaining the prosthetic
disc on an end thereof in order to implant the disc between the two
adjacent vertebral bodies.
[0266] The head portion 856 of the holder 850 includes a proximal
body portion 857 and two arms 858a-b extending distally from the
body portion 857. The body portion 857 has a generally square
shape, and its distal end includes a slightly concave section 859
at its center that provides a space for receiving a portion of the
prosthetic disc. Each of the arms 858a-b also includes a slightly
recessed portion 860a-b that is adapted to engage the side surfaces
of the prosthetic disc in order to facilitate holding the disc in
place during the implantation procedure. The body portion also
includes engagement pins 861 on its distal surface, which
engagement pins 861 are adapted to engage mating holes provided on
the prosthetic disc.
[0267] In an alternative embodiment, and in reference to FIGS.
39-42, the implantation tools include a guide 500, a lower pusher
520 connected to a first chisel 540, an upper endplate holder 560,
and a second chisel 580.
[0268] Turning first to FIG. 39, the guide 500 serves the purposes
of, first, positioning and retaining the lower endplate 220b in
place on the lower of the two adjacent vertebrae being treated,
and, second, guiding one or more of the other implantation tools to
their proper locations for performing their functions. In the
preferred embodiment, the guide 500 comprises a generally flat,
elongated member 501 having a first end 502, a second end 503, and
a pair of raised sides 504, 505. Each of the raised sides 504, 505
includes an inwardly facing portion 504a, 505a that extends back
over the elongated member 501 on a plane slightly above that of the
elongated member. Each of the inwardly facing portions 504a, 505a
of the pair of raised sides 504, 505 thereby forms a groove 506,
507 that extends along the length of the guide 500. As described
below, the grooves 506, 507 may be used to guide one or more of the
other implantation tools in cooperation with a flange provided on
those other tools.
[0269] Extending from the first end 502 of the guide 500 are a pair
of lower endplate rods 508, 509. Each of the lower endplate rods
508, 509 is a generally cylindrical rod that extends outward from
the first end 502 of the guide 500 in the plane of the elongate
member 501 or parallel to that plane. The sizes of the lower
endplate rods 508, 509--e.g., lengths, cylindrical diameters--are
not critical, provided that the rods are of sufficient size to be
capable of performing the function of engaging and retaining the
lower endplate 220b, as described more fully below.
[0270] Turning to FIG. 40, in the preferred embodiment, a spacer
tool 570 includes a combination of a lower pusher 520 and a first
chisel 540 attached to a base member 530. The lower pusher 520
includes a pair of lower pusher rods 521a, 521b. Each of the lower
pusher rods 521a, 521b is connected at a first end to the base 530.
At a second end, a cross-member 522 extends between and connects to
each of the pair of lower pusher rods 521a, 521b. The two lower
pusher rods 521a, 521b are thus held in a generally parallel
relation to one another and extend outward from the base 530. At
the end opposite the base 530, each of the lower pusher rods 521a,
521b is attached to a lower endplate insert 523. The lower endplate
insert 523 includes a flange 524 along its edge that is adapted to
engage the matching slot 226 found on an outer lower endplate 220b
of a two-piece prosthetic disc, such as those described herein.
[0271] In the preferred embodiment, each of the lower pusher rods
521a, 521b and the cross-member 522 are generally cylindrical rods.
The cross-sectional shape and size of the rods are not critical,
such that the lower pusher rods 521a, 521b are capable of advancing
the lower endplate during the implantation procedure, as described
more fully below.
[0272] In the preferred embodiment illustrated in FIG. 40, the base
530 includes a block-shaped bottom portion 531. The bottom portion
531 of the base 530 is the portion of the base to which the pusher
rods 521a, 521b of the lower pusher 520 are attached. The bottom
portion 531 shown in FIG. 40 has a generally block-shaped body,
although the size and shape of the bottom portion 531 are not
critical.
[0273] Extending upward from the top surface of the bottom portion
are two flanges, a tall flange 532 and a short flange 533. A pivot
pin 534 is located at the upper end of the tall flange 532. The
pivot pin 534 extends through a hole in the upper end of the tall
flange 532, and is able to rotate around its pivot axis. A pair of
upper pusher rods 541a, 541b are attached to the pivot pin 534,
with one of the two upper pusher rods 541a attached to a first end
of the pivot pin 534, and the other upper pusher rod 541b attached
to the opposite end of the pivot pin 534. At the end of the upper
pusher rods 541a, 541b opposite the pivot pin 534, the upper pusher
rods 541a, 541b are attached to a first chisel 540. In addition, a
cross-member 542 attaches to and interconnects the pair of upper
pusher rods 541a, 541b near the end to which the first chisel 540
is attached.
[0274] A ratchet key 535 is extends through a hole in the short
flange 533. The ratchet key 535 is able to rotate around its
longitudinal axis within the hole in the short flange. The ratchet
key 535 includes a grasping portion 536 extending from one side of
the short flange 533, and a gear portion (not shown in the Figures)
extending from the opposite side of the short flange 533. An
elongated guide rail 537 extends beneath the gear portion of the
ratchet key 535 and generally between the pair of upper pusher rods
541a, 541b and the pair of lower pusher rods 521a, 521b. The guide
rail 537 includes a plurality of teeth 538 formed on its upper
side, which teeth are adapted to engage the gear portion of the
ratchet key 535. Thus, by rotating the ratchet key 535, a user is
able to advance or withdraw the guide rail 537.
[0275] A separator 515 is attached to an end of the guide rail 537.
The separator 515 is a generally flat member that is disposed
generally transversely to the guide rail 537. A pair of upper
grooves 516a, 516b are formed on the top edge of the separator 515.
The upper grooves 516a, 516b have a size and are located so as to
slidably engage the upper pusher rods 541a, 541b. Similarly, a pair
of lower grooves 517a, 517b are formed on the bottom edge of the
separator 515. The lower grooves 517a, 517b have a size and are
located so as to slidably engage the lower pusher rods 521a, 521b.
Thus, as shown in FIG. 40, the separator is able to be advanced or
withdrawn along the lengths of the upper pusher rods 541a, 541b and
lower pusher rods 521a, 521b by turning the ratchet key 535.
Turning the ratchet key 535 causes the gear portion of the ratchet
key 535 to engage the teeth 538 on the guide rail 537. With
reference to the perspective illustrated in FIG. 40, rotating the
ratchet key clockwise will cause the guide rail 537 and the
separator 515 to withdraw, i.e., to draw nearer to the base 530.
Alternatively, rotating the ratchet key 535 counter-clockwise will
cause the guide rail 537 and the separator to advance, i.e., to
move away from the base 530.
[0276] As best seen in the illustration in FIG. 40, the separator
515 has a partial height, h, that is defined as the distance
between the bottom edge of the upper grooves 516a, 516b and the top
edge of the lower grooves 517a, 517b. The partial height h of the
separator 515 is less than the distance separating the upper pusher
rods 541a, 541b and lower pusher rods 521a, 521b at the point that
they attach to the base 530. The partial height h of the separator
is greater than the height of the prosthetic disc or, stated
otherwise, the partial height h of the spacer is greater than the
post-operative distance separating the two adjacent vertebrae being
treated. Thus, as explained more fully below, the separator 515 has
a partial height h that is suitable for expanding the distance
separating the first chisel 540 and the lower endplate insert 523
as the separator 515 is advanced during the implantation
procedure.
[0277] The first chisel 540 is attached to the ends of each of the
upper pusher rods 541a, 541b opposite the tall flange 532. The
first chisel 540 includes a generally flat plate portion 543 and
one or more wedge-shaped blades 544 extending upward from the flat
plate portion 543. The blades 544 of the first chisel are adapted
to create grooves in the lower surface of the upper vertebra being
treated. The flat plate portion 543 of the first chisel is
preferably relatively thin in relation to the height of the
prosthetic disc, thereby allowing the first chisel to be inserted
between the two adjacent vertebrae after the natural disc has been
removed.
[0278] Turning to FIG. 41, the upper endplate holder 560 includes a
pusher block 561 attached to the end of a push rod 563. The pusher
block 561 has a generally flat front surface 562 that is adapted to
engage the trailing surface of the upper endplate of the prosthetic
disc, as described more fully below. In addition, the upper
endplate holder 560 includes a pair of outer engagement pins 564
extending outward from the front surface 562, and a center
engagement pin 565 also extending outward from the front surface.
The outer engagement pins 564 and center engagement pin 565 are
each generally cylindrical in shape, and relatively short in length
relative to the size of the push rod 563. The engagement pins 564,
565 are intended to engage and retain the upper endplate of the
prosthetic disc during the implantation procedure, as explained
more fully below.
[0279] Turning to FIG. 42, the second chisel 580 includes a
generally flat plate portion 583 attached to the end of a push rod
582. One or more wedge-shaped blades 584 attach to and extend
upward from the top surface of the flat plate portion 583. Similar
to the blade 544 of the first chisel 540, the blades 584 of the
second chisel are adapted to create grooves in the lower surface of
the upper vertebra being treated. The flat plate portion 583 of the
second chisel is preferably thicker than the flat plate portion 543
of the first chisel 540, and is generally about the same thickness
as the height of the prosthetic disc. A flange 585 extends outward
from the bottom of the second chisel 580. The flange 585 has a size
and is oriented such that it will engage the grooves 506, 507 on
the guide member 500 during the implantation procedure.
2. Implantation Procedures
a. First Embodiment
[0280] A preferred implantation procedure utilizes the spacer 810,
chisel 830, and holder 850 shown in FIGS. 36-38. As discussed
above, the procedure described herein is in relation to
implantation of a one-piece prosthetic disc. This description is
intended to illustrate the apparatus and methods described herein,
however, and is not intended to be limiting.
[0281] A first step of the procedure is to expose the two adjacent
vertebrae to be treated by conventional surgical procedures and to
remove the natural disc. Once the natural disc has been removed,
the spacer 810 is advanced and its head portion 816 is placed
between the two adjacent vertebrae in order to separate them. After
the vertebrae are adequately separated, the spacer 810 is
withdrawn.
[0282] The two-sided chisel 830 is then advanced and its head
portion 836 is placed between the vertebral bodies. Because of the
size of the head portion 836 relative to the axial space between
the vertebrae, the wedge-shaped blades 839 engage the inward-facing
surfaces of the vertebrae, creating grooves on those surfaces
simultaneously. After the grooves are formed as needed, the
two-sided chisel is withdrawn.
[0283] A prosthetic disc is then installed on the distal end of the
holder 850. Optimally, the arms 858a-b of the holder 850 engage the
side surfaces of the prosthetic disc, and the proximal side of the
disc butts up against the distal face of the body portion 857 of
the holder 850. In this position, the holder is able to retain the
prosthetic disc and hold it in place. The prosthetic disc is then
advanced by the holder into the disc space between the two
vertebrae. Optimally, the anchoring fins on the external surfaces
of the prosthetic disc are aligned with the grooves formed in the
upper and lower vertebrae as the disc is implanted. Once the disc
has been satisfactorily located, the holder 850 is withdrawn,
leaving the disc in place.
b. Second Embodiment
[0284] An alternative implantation procedure is illustrated in
FIGS. 43 through 49. The preferred procedure utilizes the
implantation tools described above in relation to FIGS. 39-42. As
discussed above, the procedure described herein is in relation to
implantation of a two-piece prosthetic disc. This description is
intended to illustrate the apparatus and methods described herein,
however, and is not intended to be limiting.
[0285] Turning first to FIGS. 43A-B, after the two adjacent
vertebrae to be treated are exposed by conventional surgical
procedures and the natural disc is removed, the guide member 500,
lower pusher 520, and upper pusher rods 541a, 541b are advanced in
the direction of arrow "A" toward the void space between the two
adjacent vertebrae 601, 602 until the outer lower endplate 220b and
first chisel 540 are located between the two adjacent vertebrae
601, 602 (see FIG. 43B).
[0286] At this point in the procedure, the distance "d" between the
vertebrae 601, 602 is insufficient to accommodate the prosthetic
disc. Accordingly, as shown in FIGS. 44A-B, a force is applied to
separate the first chisel 540 and the outer lower endplate 220b,
e.g., as represented by arrows "B" in FIGS. 44A-B. The separating
force is applied by advancing the separator 515 away from the base
member in the apparatus shown in FIG. 40 by the method described
above. The upward force by the first chisel 540 causes the
wedge-shaped blades 544 of the first chisel to embed in the lower
surface of the upper vertebra 601, creating grooves in that
surface. Similarly, the downward force by the lower endplate insert
523 and outer lower endplate 220b cause the anchor fins 221 on the
lower surface of the outer lower endplate 220b to embed in the
upper surface of the lower vertebra 602. Thus, by advancing the
separator 515 between the upper pusher rods 541a, 541b and lower
pusher rods 521a, 521b, the user is able to implant the outer lower
endplate 220b onto the lower vertebra 602 and to create a set of
grooves in the upper vertebra 601 that will accommodate the
anchoring fins 211 on the upper endplate of the prosthetic
disc.
[0287] After the separating forces are applied as described above,
the first chisel apparatus is withdrawn, as shown in FIGS. 45A-B.
More particularly, the lower pusher 520 is withdrawn, thereby
withdrawing the lower endplate insert 523 from the outer lower
endplate 220b, leaving the outer lower endplate 220b implanted onto
the lower vertebra 602. Also, the upper pusher rods 541a, 541b are
withdrawn, thereby withdrawing the first chisel 540, leaving one or
more grooves formed on the upper vertebra 601 (see FIG. 45B). The
guide member 500 remains in place to facilitate additional
procedures described below.
[0288] After the first chisel apparatus is withdrawn, the second
chisel 580 is advanced into the space between the two vertebrae
601, 602, as shown in FIGS. 46A-B. Preferably, the second chisel is
advanced (see arrows "A") into the void space by advancing the push
rod 583. Upon entry into the void space, the wedge-shaped blades
584 on the top surface of the second chisel engage the grooves
formed in the lower surface of the upper vertebra 601 by the first
chisel 540. Advantageously, the flange 585 on the bottom surface of
the second chisel 580 engages and rides in the grooves 506, 507 on
the guide member 500 as the second chisel is being advanced,
thereby guiding the second chisel 580 into place.
[0289] As noted above, the second chisel 580 preferably has a
thickness that is similar to the height of the upper endplate
assembly of the two-piece prosthetic disc. Thus, advancing the
second chisel 580 into the void space between the two adjacent
vertebrae 601, 602 ensures that the void space is adequately
prepared for implanting the remaining portion of the prosthetic
disc. In addition, if the second chisel 580 has a snug fit within
the void space, this will further confirm that a prosthetic disc of
the appropriate size and shape is being used.
[0290] After the second chisel 580 has been advanced and engages
the lower surface of the upper vertebra 601, it is withdrawn, once
again leaving behind the outer lower endplate 220b implanted onto
the lower vertebra 601 and the guide member 500 engaged with the
outer lower endplate 220b. (See FIGS. 47A-B).
[0291] Once the pair of vertebrae 601, 602 have been adequately
prepared for implantation of the remaining portions of the
prosthetic disc, the upper subassembly 205 of the prosthetic disc
is implanted using the upper endplate holder 560. (See FIGS.
48A-B). The upper endplate holder 560 is advanced in the direction
"A" by the push rod 563 until the upper subassembly 205 engages the
outer lower endplate 220b and is locked in place by the tab 261 and
notch (not shown). At this point, the anchoring fins 211 on the
upper subassembly engage the grooves formed on the lower surface of
the upper vertebra 601, thereby helping to retain the prosthetic
disc in place between the two adjacent vertebrae 601, 602. Turning
to FIGS. 49A-B, the upper endplate holder 560 and the guide member
500 are then withdrawn, leaving the prosthetic disc in place. FIG.
49A provides additional detail showing the manner by which the
engagement pins 564, 565 of the upper endplate holder engages a set
of mating holes 206 formed in the trailing edge of the upper
subassembly 205. Similarly, FIG. 49A shows the manner by which the
lower endplate rods 508, 509 engage the mating holes 215 formed on
the trailing edge of the outer lower endplate 220b.
[0292] In an alternative method particularly adapted for implanting
the one-piece structure prosthetic discs 100 described herein,
implantation of the prosthetic disc is achieved without using the
guide member 500, through use of only the second chisel 580, the
spacer tool 570, and a modified upper endplate holder 560'. The
spacer tool 570 is used, as described above, to separate the
adjacent vertebral bodies to provide space for the prosthetic disc.
The second chisel 580 is also used in the manner described above to
provide grooves on the internal surface of the vertebral bodies to
accommodate the fins on the prosthetic disc. The modified upper
endplate holder 560' has a similar structure to the endplate holder
560 shown in FIG. 41, but is provided with an additional set of
engagement pins 564', 565' for engaging mating holes provided on
the lower endplate 120 of the one-piece prosthetic disc 100. The
modified upper endplate holder 560' is used to advance the
prosthetic disc 100 into place between the adjacent vertebrae, then
is withdrawn.
B. Minimally Invasive Implantation
[0293] A minimally invasive surgical implantation method is
illustrated in FIG. 51. The minimally invasive surgical
implantation method may be performed using a posterior approach,
rather than the anterior approach used for conventional lumbar disc
replacement surgery.
[0294] Turning to FIG. 51, the a pair of cannulas 700 are inserted
posteriorly to provide access to the spinal column. More
particularly, an small incision is made and a pair of access
windows are created through the lamina 610 of one of the vertebrae
on each side of the vertebral canal to access the natural vertebral
disc to be replaced. The spinal cord 605 and nerve roots 606 are
avoided or mobilized to provide access. Once access is obtained,
each of the cannulas 700 is inserted. The cannulas 700 may be used
to remove the natural disc by conventional means. Alternatively,
the natural disc may have already been removed by other means prior
to insertion of the cannulas.
[0295] Once the natural disc has been removed and the cannulas 700
located in place, a pair of prosthetic discs are implanted between
adjacent vertebral bodies. In the preferred embodiment, the
prosthetic discs have a shape and size adapted for the minimally
invasive procedure, such as the elongated one-piece prosthetic
discs 100 described above in relation to FIGS. 50A-B. A prosthetic
disc 100 is guided through each of the two cannulas 700 (see arrows
"C" in FIG. 51) such that each of the prosthetic discs is implanted
between the two adjacent vertebral bodies. In the preferred method,
the two prosthetic discs 100 are located side by side and spaced
slightly apart between the two vertebrae. Optionally, prior to
implantation, grooves are created on the internal surfaces of one
or both of the vertebral bodies in order to engage anchoring fins
located on the prosthetic discs 100. The grooves may be created
using a chisel tool adapted for use with the minimally invasive
procedure.
[0296] Optionally, a third prosthetic disc may be implanted using
the methods described above. The third prosthetic disc is
preferably implanted at a center point, between the two prosthetic
discs 100 shown in FIG. 51. The third disc would be implanted prior
to the two discs shown in the Figure. Preferably, the disc would be
implanted by way of either one of the cannulas, then rotated by
90.degree. to its final load bearing position between the other two
prosthetic discs. The first two prosthetic discs 100 would then be
implanted using the method described above.
[0297] An alternative minimally invasive implantation method and
apparatus is illustrated schematically in FIGS. 52A-B. In this
alternative implantation method, a single cannula 700 is used. The
cannula is inserted on one side of the vertebral canal in the
manner described above. Once the cannula is inserted, a chisel is
used to create a groove 701 having a 90.degree. bend on the
interior surfaces of the two adjacent vertebral bodies. Thus, the
terminal portion of the groove 702 is perpendicular to the axis
defined by the insertion cannula 700.
[0298] Turning to FIG. 52B, a dual prosthetic disc 710 structure is
shown. The dual disc 710 includes a pair of one-piece structure
prosthetic discs 100a-b identical in structure to those described
above in relation to FIGS. 50A-B. The two prosthetic discs 100a-b
of the dual disc 710 are joined by a separating mechanism 711. The
separating mechanism 711 is accessed remotely by the surgeon after
the dual disc 710 has been implanted into a patient's spinal
column, and is adapted to drive the two prosthetic discs 100a-b
apart once they are implanted. The separating mechanism 711 may be
a screw device such as a worm screw, a ratcheting mechanism, a
spring, or any other mechanism suitable for providing the
capability of applying a separating force between the two
prosthetic discs 100a-b of the dual disc 710. Preferably, an
anchoring fin 111 is provided on only one of the prosthetic discs
100a. Thus, when the dual disc 710 is implanted, the anchoring fin
111 of the first prosthetic disc 100a will retain the first disc
100a in place while the ratcheting mechanism 711 causes the second
disc 100b to be separated spatially from the first disc 100a, as
shown by the arrow "D".
[0299] The subject devices and systems may be provided in the form
of a kit for performing the methods of the present invention. The
kits may include instructions for using the various devices and
systems.
Part C
I. Information Concerning the Descriptions Contained Herein
[0300] It is to be understood that the inventions that are the
subject of this patent application are not limited to the
particular embodiments described, as such may, of course, vary. It
is also to be understood that the terminology used herein is for
the purpose of describing particular embodiments only, and is not
intended to be limiting, since the scope of the present invention
will be limited only by the appended claims.
[0301] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which these inventions belong.
Although any methods and materials similar or equivalent to those
described herein can also be used in the practice or testing of the
present inventions, the preferred methods and materials are herein
described.
[0302] All patents, patent applications, and other publications
mentioned herein are hereby incorporated herein by reference in
their entireties. The patents, applications, and publications
discussed herein are provided solely for their disclosure prior to
the filing date of the present application. Nothing herein is to be
construed as an admission that the present invention is not
entitled to antedate such publication by virtue of prior invention.
Further, the dates of publication provided may be different from
the actual publication dates which may need to be independently
confirmed.
[0303] As will be apparent to those of skill in the art upon
reading this disclosure, each of the individual embodiments
described and illustrated herein has discrete components and
features which may be readily separated from or combined with the
features of any of the other several embodiments without departing
from the scope or spirit of the present inventions.
[0304] The preceding merely illustrates the principles of the
invention. It will be appreciated that those skilled in the art
will be able to devise various arrangements which, although not
explicitly described or shown herein, embody the principles of the
invention and are included within its spirit and scope.
Furthermore, all examples and conditional language recited herein
are principally intended to aid the reader in understanding the
principles of the invention and the concepts contributed by the
inventors to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions. Moreover, all statements herein reciting principles,
aspects, and embodiments of the invention as well as specific
examples thereof, are intended to encompass both structural and
functional equivalents thereof. Additionally, it is intended that
such equivalents include both currently known equivalents and
equivalents developed in the future, i.e., any elements developed
that perform the same function, regardless of structure. The scope
of the present invention, therefore, is not intended to be limited
to the exemplary embodiments shown and described herein. Rather,
the scope and spirit of present invention is embodied by the
appended claims.
* * * * *