U.S. patent application number 12/255733 was filed with the patent office on 2009-04-23 for dynamic spacer device and method for spanning a space formed upon removal of an intervertebral disc.
This patent application is currently assigned to SpinalMotion, Inc.. Invention is credited to Yves Arramon, Malan de Villiers, David Hovda, Neville Jansen.
Application Number | 20090105834 12/255733 |
Document ID | / |
Family ID | 40564282 |
Filed Date | 2009-04-23 |
United States Patent
Application |
20090105834 |
Kind Code |
A1 |
Hovda; David ; et
al. |
April 23, 2009 |
Dynamic Spacer Device and Method for Spanning a Space Formed upon
Removal of an Intervertebral Disc
Abstract
A compliant intervertebral spacer according to the present
invention replaces a damage intervertebral disc and functions to
maintain disc height and prevent subsidence with a large surface
area while substantially reducing patient recovery time. The
compliant intervertebral spacer for spanning a space formed by upon
removal of an intervertebral disc includes two end plates sized and
shaped to fit within an intervertebral space and a compliant
connector interconnecting the inner surfaces of the two end plates
in a manner which limits motion between the plates to less than a
total of 5 degrees of motion in any direction. The intervertebral
spacer is configured to permanently maintain the disc space between
the two adjacent discs without the use of bridging bone.
Inventors: |
Hovda; David; (Mountain
View, CA) ; Arramon; Yves; (Sunnyvale, CA) ;
Jansen; Neville; (Waterkloof, ZA) ; de Villiers;
Malan; (Wapadrand, ZA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
SpinalMotion, Inc.
Mountain View
CA
|
Family ID: |
40564282 |
Appl. No.: |
12/255733 |
Filed: |
October 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60981665 |
Oct 22, 2007 |
|
|
|
Current U.S.
Class: |
623/17.16 ;
128/898; 623/17.11 |
Current CPC
Class: |
A61B 17/86 20130101;
A61F 2310/00796 20130101; A61F 2002/30056 20130101; A61F 2310/00431
20130101; A61F 2310/00017 20130101; A61F 2002/30772 20130101; A61F
2250/0032 20130101; A61F 2310/0088 20130101; A61F 2310/00029
20130101; A61F 2310/00071 20130101; A61F 2/30771 20130101; A61F
2310/00604 20130101; A61F 2310/00023 20130101; A61F 2002/30904
20130101; A61F 2002/449 20130101; A61F 2002/30899 20130101; A61F
2310/00407 20130101; A61F 2/4465 20130101; A61F 2310/00131
20130101; A61F 2002/30787 20130101; A61F 2002/30884 20130101; A61F
2310/00976 20130101; A61F 2310/00179 20130101 |
Class at
Publication: |
623/17.16 ;
128/898; 623/17.11 |
International
Class: |
A61F 2/44 20060101
A61F002/44; A61B 19/00 20060101 A61B019/00 |
Claims
1. A method of spanning a space formed by upon removal of an
intervertebral disc, the method comprising: performing a discectomy
to remove disc material between two adjacent vertebral bodies;
placing an intervertebral spacer between the two adjacent vertebral
bodies, the spacer comprising two end plates, each end plate having
a vertebral body contacting surface and an inner surface, and a
connector interconnecting the inner surfaces of the two end plates
in a compliant manner which limits motion between the plates to
less than a total of 5 degrees of motion in any direction; and
maintaining the disc space between the two adjacent vertebral
bodies with the intervertebral spacer without the use of bone graft
or bridging bone.
2. The method of claim 1, wherein the connector is a compliant
connector which allows axial motion between the two end plates.
3. The method of claim 2, wherein the axial motion is limited to
less than 2 mm.
4. The method of claim 1, wherein the connector is a complaint
connector which allows rotation between the two end plates.
5. The method of claim 2, wherein the compliant connector has a
plurality of cuts formed therein to create a unitary flexible
member.
6. The method of claim 5, wherein the two end plates and connector
are formed of a single piece of metal.
7. The method of claim 1, further comprising cutting at least one
slot in each of the adjacent vertebrae and placing a fin into the
slot in each vertebrae.
8. The method of claim 7, wherein the intervertebral spacer is held
in place between the adjacent vertebral bodies by a fixation means,
and wherein the fixation means is at least one of a screw,
serrations, teeth, pyramids or grooves.
9. An intervertebral spacer for spanning a space formed by upon
removal of an intervertebral disc, the spacer comprising: two end
plates sized and shaped to fit within an intervertebral space, each
end plate having a vertebral body contacting surface an inner
surface; a connector interconnecting the inner surfaces of the two
end plates in a compliant manner which limits motion between the
plates to less than a total of 5 degrees of motion in any
direction; and wherein the intervertebral spacer is configured to
permanently maintain the disc space between the two adjacent discs
with the intervertebral spacer without the use of bridging
bone.
10. The spacer of claim 9, wherein the connector is a compliant
connector which allows axial motion between the two end plates.
11. The spacer of claim 10, wherein the axial motion is limited to
less than 2 mm.
12. The spacer of claim 9, wherein the connector is a complaint
connector which allows rotation between the two end plates.
13. The spacer of claim 12, wherein the compliant connector has a
plurality of cuts formed therein to create a unitary compliant
member.
14. The spacer of claim 9, wherein the two end plates and connector
are formed of a single piece of metal.
15. The spacer of claim 9, wherein the connector limits motion
between the plates to less than a total of 3 degrees of motion in
any direction.
16. The spacer of claim 9, further comprising at least one fin
extending from each of the vertebral body contacting surfaces.
17. A method of spanning a space formed by upon removal of an
intervertebral disc, the method comprising: performing a discectomy
to remove disc material between two adjacent vertebral bodies;
placing an intervertebral spacer between the two adjacent vertebral
bodies, the spacer comprising two vertebral body contacting
surfaces and the including means for limiting motion between the
two vertebral body contacting surfaces to less than a total of 5
degrees; and maintaining the disc space between the two adjacent
vertebral bodies with the intervertebral spacer within the
intervertebral space, wherein the vertebral body contacting
surfaces have no holes therein or have holes which cover less than
40 percent of the vertebral body contacting surfaces.
18. The method of claim 17, wherein the intervertebral spacer
allows axial motion between the two end plates.
19. The method of claim 18, wherein the axial motion is limited to
less than 2 mm.
20. The method of claim 17, wherein the intervertebral spacer
includes a compliant portion which allows rotation between the two
end plates.
21. The method of claim 20, wherein the compliant portion has a
plurality of cuts formed therein to create a unitary flexible
member.
22. The method of claim 17, wherein the vertebral body contacting
surfaces have holes which cover less than 25 percent of the
vertebral body contacting surfaces.
23. An intervertebral spacer for spanning a space formed by upon
removal of an intervertebral disc, the spacer comprising: a
compliant spacer body sized and shaped to fit within an
intervertebral space; two opposite vertebral body contacting
surfaces formed on the spacer body; wherein the spacer body limits
motion between the vertebral body contacting surfaces to less than
a total of 5 degrees; and wherein the vertebral body contacting
surfaces have no holes therein or have holes which cover less than
40 percent of the vertebral body contacting surfaces.
24. The spacer of claim 23, wherein the compliant spacer body
allows rotation between the two end plates.
25. The spacer of claim 23, wherein the compliant spacer body
allows axial motion between the vertebral body contacting surfaces
of less than 2 mm.
26. The spacer of claim 23, wherein the compliant spacer body has a
plurality of cuts formed therein to create a unitary flexible
member.
27. The spacer of claim 23, wherein the vertebral body contacting
surfaces and the compliant spacer body are formed as a single
unitary piece.
28. The spacer of claim 24, wherein the spacer body limits
rotational motion between the plates to less than a total of 3
degrees in any direction.
29. The spacer of claim 23, further comprising at least one fin
extending from each of the vertebral body contacting surfaces.
30. The spacer of claim 23, wherein the spacer has holes which
cover less than 25 percent of the vertebral body contacting
surfaces.
31. The spacer of claim 23, wherein the spacer body includes a
plurality of transverse cuts which create a spring element.
32. The spacer of claim 23, wherein the spacer body includes at
least one hole therein and a fixation screw received therein.
33. A method of performing an anterior/posterior fusion, the method
comprising: performing a discectomy to remove disc material between
two adjacent vertebral bodies; anteriorly placing an intervertebral
spacer between the two adjacent discs, the spacer comprising a
compliant spacer body sized and shaped to fit within an
intervertebral space and two opposite vertebral body contacting
surfaces formed on the spacer body wherein the spacer body limits
motion between the vertebral body contacting surfaces to less than
a total of 5 degrees; maintaining the disc space between the two
adjacent discs with the intervertebral spacer; adjusting an angle
between the vertebral bodies posteriorly; and posteriorly placing a
stabilization system to fix the angle between the vertebral
bodies.
34. The method of claim 32, wherein the posteriorly placed
stabilization system includes at least one screw placed into each
of the vertebral bodies and at least one connector
therebetween.
35. The method of claim 32, wherein the method is performed without
the use of bone graft.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application claims priority from U.S.
Provisional Application No. 60/981,665 filed Oct. 22, 2007,
entitled "Method and Spacer Device for Spanning Space Formed Upon
Removal of an Intervertebral Disc," the full disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to medical devices and
methods. More specifically, the invention relates to intervertebral
spacers and methods of spanning a space formed upon removal of an
intervertebral disc.
[0003] Back pain takes an enormous toll on the health and
productivity of people around the world. According to the American
Academy of Orthopedic Surgeons, approximately 80 percent of
Americans will experience back pain at some time in their life. In
the year 2000, approximately 26 million visits were made to
physicians' offices due to back problems in the United States. On
any one day, it is estimated that 5% of the working population in
America is disabled by back pain.
[0004] One common cause of back pain is injury, degeneration and/or
dysfunction of one or more intervertebral discs. Intervertebral
discs are the soft tissue structures located between each of the
thirty-three vertebral bones that make up the vertebral (spinal)
column. Essentially, the discs allow the vertebrae to move relative
to one another. The vertebral column and discs are vital anatomical
structures, in that they form a central axis that supports the head
and torso, allow for movement of the back, and protect the spinal
cord, which passes through the vertebrae in proximity to the
discs.
[0005] Discs often become damaged due to wear and tear or acute
injury. For example, discs may bulge (herniate), tear, rupture,
degenerate or the like. A bulging disc may press against the spinal
cord or a nerve exiting the spinal cord, causing "radicular" pain
(pain in one or more extremities caused by impingement of a nerve
root). Degeneration or other damage to a disc may cause a loss of
"disc height," meaning that the natural space between two vertebrae
decreases. Decreased disc height may cause a disc to bulge, facet
loads to increase, two vertebrae to rub together in an unnatural
way and/or increased pressure on certain parts of the vertebrae
and/or nerve roots, thus causing pain. In general, chronic and
acute damage to intervertebral discs is a common source of back
related pain and loss of mobility.
[0006] When one or more damaged intervertebral discs cause a
patient pain and discomfort, surgery is often required.
Traditionally, surgical procedures for treating intervertebral
discs have involved discectomy (partial or total removal of a
disc), with or without interbody fusion of the two vertebrae
adjacent to the disc. When the disc is partially or completely
removed, it is necessary to replace the excised material to prevent
direct contact between hard bony surfaces of adjacent vertebrae.
Oftentimes, pins, rods, screws, cages and/or the like are inserted
between the vertebrae to act as support structures to hold the
vertebrae and graft material in place while they permanently fuse
together.
[0007] One typical fusion procedure involves inserting a "cage" in
the space usually occupied by the disc to maintain the disc space,
and to prevent the vertebrae from collapsing and impinging the
nerve roots. The cage is used in combination with bone graft
material (either autograft or allograft) such that the two
vertebrae and the graft material will grow together over time
forming bridging bone between the two vertebrae. The fusion process
typically takes 6-12 months after surgery. During in this time
external bracing (orthotics) may be required. External factors such
as smoking, osteoporosis, certain medications, and heavy activity
can prolong or even prevent the fusion process. If fusion does not
occur, patients may require reoperation.
[0008] One known fusion cage is described in U.S. Pat. No.
4,904,261 and includes a horseshoe shaped body. This type cage is
currently available in PEEK (polyetheretherketone). PEEK is used
because it does not distort MRI and CT images of the vertebrae.
However, PEEK is a material that does not allow bone to attach.
Thus, fusion with a PEEK cage requires bridging bone to grow
through the holes in the cage to provide stabilization.
[0009] It would be desirable to achieve immobilization of the
vertebrae and maintain spacing between the adjacent vertebrae
without the associated patient discomfort and long recovery time of
traditional interbody fusion.
[0010] Another problem associated with the typical fusion procedure
is the subsidence of the cage into the vertebral body. The typical
fusion cage is formed with a large percentage of open space to
allow the bone to grow through and form the bridging bone which
immobilizes the discs. However, the large amount of open space
means that the load on each segment of the cage is significantly
higher than if the cage surface area was larger. This results in
the cage subsiding or sinking into the bone over time and allows
the disc space to collapse. In addition, the hard cortical bone on
the outer surface of the vertebral body that transfers load to the
interbody cage or spacer is often scraped, punctured or otherwise
damaged to provide blood to the interbody bone graft to facilitate
bone growth. This damage to the bone is used effectively to promote
bone growth can also lead to subsidence.
[0011] The U.S. Food and Drug Administration approved the use of a
genetically engineered protein, or rhBMP-2, for certain types of
spine fusion surgery. RhBMP-2 is a genetically engineered version
of a naturally occurring protein that helps to stimulate bone
growth, marketed by Medtronic Sofamor Danek, Inc. as InFUSE.TM.
Bone Graft. When InFUSE.TM. is used with the bone graft material it
eliminates the need for painful bone graft harvesting and improves
patients' recovery time. However, InFUSE.TM. adds significantly to
the cost of a typical fusion surgery. Additionally, even with the
bone graft and InFUSE.TM. bone may fail to grow completely between
the two vertebrae or the cage may subside into the vertebrae such
that the fusion fails to achieve its purpose of maintaining disc
height and preventing motion.
[0012] In an attempt to treat disc related pain without fusion
provided by bridging bone, an alternative approach has been
developed, in which a movable, implantable, artificial
intervertebral disc (or "disc prosthesis") is inserted between two
vertebrae. A number of different artificial intervertebral discs
are currently being developed. For example, U.S. Patent Application
Publication Nos. 2005/0021146, 2005/0021145, and 2006/0025862,
which are hereby incorporated by reference in their entirety,
describe artificial intervertebral discs. Other examples of
intervertebral disc prostheses are the LINK SB CHARITE.TM. disc
prosthesis (provided by DePuy Spine, Inc.) the MOBIDISK.TM. disc
prosthesis (provided by LDR Medical), the BRYAN.TM. cervical disc
prosthesis (provided by Medtronic Sofamor Danek, Inc.), the
PRODISC.TM. disc prosthesis or PRODISC-C.TM. disc prosthesis (from
Synthes Stratec, Inc.), the PCM.TM. disc prosthesis (provided by
Cervitech, Inc.), and the MAVERICK.TM. disc prosthesis (provided by
Medtronic Sofomor Danek). Although existing disc prostheses provide
advantages over traditional treatment methods, many patients are
not candidates for an artificial disc due to facet degeneration,
instability, poor bone strength, previous surgery, multi-level
disease, and pain sources that are non-discogenic.
[0013] Therefore, a need exists for improved spacer and method for
spanning a space and maintaining disc spacing between two vertebrae
after removal of an intervertebral disc. Ideally, such improved
method and spacer would avoid the need for growth of bridging bone
across the intervertebral space.
BRIEF SUMMARY OF THE INVENTION
[0014] Embodiments of the present invention provide an
intervertebral spacer with and without shock absorption and methods
of spanning a space formed upon removal of an intervertebral
disc.
[0015] In accordance with one aspect of the present invention, a
method of spanning a space formed by upon removal of an
intervertebral disc includes the steps of: performing a discectomy
to remove disc material between two adjacent vertebral bodies;
placing an intervertebral spacer between the two adjacent vertebral
bodies; and maintaining the disc space between the two adjacent
vertebral bodies with the intervertebral spacer without the use of
bone graft or bridging bone. The spacer includes two end plates,
each end plate having a vertebral body contacting surface and an
inner surface, and a connector interconnecting the inner surfaces
of the two end plates in a compliant manner which limits motion
between the plates to less than a total of 5 degrees of motion in
any direction.
[0016] In accordance with another aspect of the invention, an
intervertebral spacer for spanning a space formed by upon removal
of an intervertebral disc includes two end plates sized and shaped
to fit within an intervertebral space and a connector
interconnecting the inner surfaces of the two end plates in a
compliant manner which limits motion between the plates to less
than a total of 5 degrees of motion in any direction. The
intervertebral spacer is configured to permanently maintain the
disc space between the two adjacent discs without the use of
bridging bone.
[0017] In accordance with a further aspect of the invention, a
method of spanning a space formed by upon removal of an
intervertebral disc includes the steps of: performing a discectomy
to remove disc material between two adjacent vertebral bodies;
placing an intervertebral spacer between the two adjacent vertebral
bodies; and maintaining the disc space between the two adjacent
vertebral bodies with the intervertebral spacer within the
intervertebral space. The spacer includes two vertebral body
contacting surfaces and means for limiting motion between the two
vertebral body contacting surfaces to less than a total of 5
degrees. The vertebral body contacting surfaces have no holes
therein or have holes which cover less than 40 percent of the
vertebral body contacting surfaces.
[0018] In accordance with another aspect of the present invention,
an intervertebral spacer for spanning a space formed by upon
removal of an intervertebral disc includes a compliant spacer body
sized and shaped to fit within an intervertebral space and two
opposite vertebral body contacting surfaces formed on the spacer
body. The spacer body limits motion between the vertebral body
contacting surfaces to less than a total of 5 degrees and the
vertebral body contacting surfaces have no holes therein or have
holes which cover less than 40 percent of the vertebral body
contacting surfaces.
[0019] In accordance with an additional aspect of the present
invention, a method of performing an anterior/posterior fusion,
includes the steps of: performing a discectomy to remove disc
material between two adjacent vertebral bodies; anteriorly placing
an intervertebral spacer between the two adjacent discs;
maintaining the disc space between the two adjacent discs with the
intervertebral spacer; adjusting an angle between the vertebral
bodies posteriorly; and posteriorly placing a stabilization system
to fix the angle between the vertebral bodies. The spacer includes
a compliant spacer body sized and shaped to fit within an
intervertebral space and two opposite vertebral body contacting
surfaces formed on the spacer body wherein the spacer body limits
motion between the vertebral body contacting surfaces to less than
a total of 5 degrees.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of an intervertebral spacer
according to one embodiment of the present invention;
[0021] FIG. 2 is a side cross sectional view of the intervertebral
spacer of FIG. 1;
[0022] FIG. 3 is a bottom view of the intervertebral spacer of FIG.
1;
[0023] FIG. 4 is a top view of the intervertebral spacer of FIG.
1;
[0024] FIG. 5 is a perspective view of an intervertebral spacer
according to another embodiment of the present invention;
[0025] FIG. 6 is a perspective view of an intervertebral spacer
according to an embodiment with added screw fixation;
[0026] FIG. 7 is a perspective view of a further intervertebral
spacer with added screw fixation; and
[0027] FIG. 8 is a perspective view of an alternative embodiment of
another intervertebral spacer.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Various embodiments of the present invention generally
provide for an intervertebral spacer having upper and lower plates
connected by a central connector which provides some limited amount
of axial compliance or rotational motion between the upper and
lower plates. The compliant intervertebral spacer according to the
present invention can maintain disc height and prevent subsidence
with a large surface area while substantially reducing recovery
time by eliminating the need for bridging bone.
[0029] One example of an intervertebral spacer 10 for maintaining
disc height between two adjacent vertebral discs is shown in FIG.
1. The spacer includes two end plates 20, 22, each end plate having
a vertebral body contacting surface 24 and an inner surface 26, and
a compliant connector 30 interconnecting the inner surfaces of the
two end plates. As will be described below some limited rotational
and axial motion may be provided between the two plates to reduce
loading on the vertebral body/spacer interface. The compliance of
the connector 30 as well as some small amount of translation and
rotation is provided by lateral cuts or cuts 70 extending into the
connector 30. The intervertebral spacer 10 when implanted between
two vertebral discs maintains a desirable disc space between the
two adjacent discs similar to that provided by a natural disc and
eliminates the long recovery time required to grow bridging bone
which is required in the traditional fusion surgery.
[0030] Although the connector 30 has been shown as circular in
cross section, other shapes may be used including oval, elliptical,
or rectangular. Although the connector has been shown as a single
member connecting the plates 20, 22 in the center of the plates one
or more compliant connectors may be provided in other
configurations and at other locations. By way of example, a
compliant connector may be the same or substantially the same
diameter and shape as the plates, multiple connectors can be
arranged in a rectangular pattern, or a hollow cylindrical
connector can be used.
[0031] The upper and lower plates 20, 22 and connector 30 may be
constructed from any suitable metal, alloy or combination of metals
or alloys, such as but not limited to cobalt chrome alloys,
titanium (such as grade 5 titanium), titanium based alloys,
tantalum, nickel titanium alloys, stainless steel, and/or the like.
They may also be formed of ceramics, biologically compatible
polymers including PEEK, UHMWPE (ultra high molecular weight
polyethylene) or fiber reinforced polymers. However, when polymer
is used for the spacer 10 the vertebral body contacting surfaces 24
may be coated or otherwise covered with metal for fixation. The
plates 20, 22 and the connector 20 may be formed of a one piece
construction or may be formed of more than one piece, such as
different materials coupled together. When the spacer 10 is formed
of multiple materials, these materials are fixed together to form a
unitary one piece spacer without separately moving parts.
[0032] Different materials may be used for different parts of the
spacer 10 to optimize imaging characteristics. For example, the
plates may be formed of titanium while the connector is formed of
cobalt chromium alloy for improved imaging of the plates. Cobalt
chrome molybdenum alloys when used for the plates 20, 22 may be
treated with aluminum oxide blasting followed by a titanium plasma
spray to improve bone integration. Other materials and coatings can
also be used such as titanium coated with titanium nitride,
aluminum oxide blasting, HA (hydroxylapatite) coating, micro HA
coating, and/or bone integration promoting coatings. Any other
suitable metals or combinations of metals may be used as well as
ceramic or polymer materials, and combinations thereof. Any
suitable technique may be used to couple materials together, such
as snap fitting, slip fitting, lamination, interference fitting,
use of adhesives, welding and/or the like.
[0033] In some embodiments, the outer surface 24 is planar.
Oftentimes, the outer surface 24 will include one or more surface
features and/or materials to enhance attachment of the spacer 10 to
vertebral bone. For example, as shown in FIG. 2, the outer surface
24 may be machined to have serrations 40 or other surface features
for promoting adhesion of the plates 20, 22 to a vertebra. In the
embodiment shown, the serrations 40 are pyramid shaped serrations
extending in mutually orthogonal directions, but other geometries
such as teeth, grooves, ridges, pins, barbs or the like would also
be useful. When the bone integration structures are ridges, teeth,
barbs or similar structures, they may be angled to ease insertion
and prevent migration. These bone integration structures can be
used to precisely cut the bone during implantation to cause
bleeding bone and encourage bone integration. Additionally, the
outer surface 24 may be provided with a rough microfinish formed by
blasting with aluminum oxide microparticles or the like to improve
bone integration. In some embodiments, the outer surface may also
be titanium plasma sprayed or HA coated to further enhance
attachment of the outer surface 24 to vertebral bone.
[0034] The outer surface 24 may also carry one or more upstanding
fins 50, 52 extending in an anterior-posterior direction. The fins
50, 52 are configured to be placed in slots in the vertebral
bodies. Preferably, the fins 50, 52 each have a height greater than
a width and have a length greater than the height. In one
embodiment, the fins 50, 52 are pierced by transverse holes 54 for
bone ingrowth. The transverse holes 54 may be formed in any shape
and may extend partially or all the way through the fins 50, 52. In
alternative embodiments, the fins 50, 52 may be rotated away from
the anterior-posterior axis, such as in a lateral-lateral
orientation, a posterolateral-anterolateral orientation, or the
like.
[0035] The fins 50, 52 provide improved attachment to the bone and
prevent rotation of the plates 20, 22 in the bone. In some
embodiments, the fins 50, 52 may extend from the surface 24 at an
angle other than 90.degree.. For example on one or more of the
plates 20, 22 where multiple fins 52 are attached to the surface 24
the fins may be canted away from one another with the bases
slightly closer together than their edges at an angle such as about
80-88 degrees. The fins 50, 52 may have any other suitable
configuration including various numbers angles and curvatures, in
various embodiments. In some embodiments, the fins 50, 52 may be
omitted altogether. The embodiment of FIG. 1 illustrates a
combination of one plate with a single fin 50 and another plate
with a double fin 52. This arrangement is useful for double level
disc replacements and utilizes offset slots in the vertebral body
to prevent the rare occurrence of vertebral body splitting by
avoiding cuts to the vertebral body in the same plane for
multi-level implants. The combination of the single fin 50 and
double fin 52 can also assist the surgeon in placement of the
spacer in the correct orientation.
[0036] The spacer 10 has been shown with the fins 50, 52 as the
primary fixation feature, however, the fins may also be augmented
or replaced with one or more screws extending through the plates
and into the bone. For example in the spacer 10 of FIG. 1 the upper
fin 50 may be replace with a screw while the two lower fins 52
remain. The plates 20, 22 can be provided with one or a series of
holes to allow screws to be inserted at different locations at the
option of the surgeon. However, the holes should not be of such
size or number that the coverage of the plate 20, 22 is decreased
to such and extent that subsidence occurs. Alternately, the screws
can pass laterally through one or more of the holes in the fins.
When one or more screws are provided, they may incorporate a
locking feature to prevent the screws from backing out. The screws
may also be provided with a bone integration coating.
[0037] Some limited holes 60 may also be provided in the plate as
shown in FIG. 6 to allow bone in growth. However, if the outer
surfaces 24 have holes 60 therein, the holes will cover less than
40 percent of the outer surface 24 which contacts the bone to
prevent subsidence of the plates into the vertebral bodies.
Preferably the holes will cover less than 25 percent, and more
preferably less than 10 percent of the outer bone contacting
surfaces. At the option of the surgeon, when the small holes are
present in the plates 20, 24, bone graft can be placed in the space
between the inner surfaces 26 of the plates to allow bone to grow
through the plates.
[0038] The intervertebral spacer 10 shown herein is configured for
placement in the intervertebral space from an anterior approach. It
should be understood that all approaches can be used including PLIF
(posterior lumbar interbody fusion), TLIF (transverse lumbar
interbody fusion), XLIF (Lateral extracavitary interbody fusion),
ALIF (anterior lumbar interbody fusion), trans-sacral, and other
approaches. The shape of the intervertebral spacer would be
modified depending on the approach. For example, for a posterior
approach, the spacer may include two separate smaller spacers which
are either positioned separately side-by-side in the intervertebral
space or two spacers which are joined together once inside the
intervertebral space. For a lateral approach, the intervertebral
spacer may be formed in a more elongated, kidney bean or banana
shape with a transversely oriented fin.
[0039] As shown in FIG. 1, the intervertebral spacer 10 is provided
with shock absorption or some other limited motion between the two
plates 20, 22 by providing a compliant connector 30. The limited
motion provided by the compliant connector 30 is designed to reduce
to forces on the interface between the outer surfaces 24 and the
bone to improve long term fixation of the spacer. The compliance of
the connector 30 allows motion between the vertebral bodies to be
accommodated by the compliance in the spacer rather than causing
one or both of the vertebral bodies to pull away from the plates
20, 22. The compliant connector 30 provides limited relative motion
between the plates which may include compliance of up to about 2
mm, rotation of less than 5 degrees, and/or translation of up to
about 1 mm.
[0040] In the intervertebral spacer 10 of FIG. 1, the compliance as
well as some small amount of translation and rotation is provided
by thee lateral cuts or slots 70 extending into the connector 30.
The compliant connector 30 is formed as a unitary member with at
least one lateral cut 70 positioned between the inner surfaces 26
to allow the upper and lower plates 20, 22 to move resiliently
toward and away from each other. The unitary or one piece
construction of the spacer 10 provides significant advantages over
multi-part implants both in durability and manufacturability.
However, the spacer 10 can also be formed as multiple parts where
different properties are desired from the different parts, such as
different radiopacities, different strengths, or different
flexibility properties. The lateral cuts 70 in the connector 30
allow the connector to function as a compliant member without
affecting the function of the upper and lower plates of the spacer
10.
[0041] Preferably the connector 30 is made of metal such as
titanium, cobalt chromium alloy, stainless steel, tantalum, nickel
titanium or a combination thereof. These materials also can be
designed to provide a device which is deformable in the elastic
region of the stress/strain curve and will not plastically deform
during compression.
[0042] In the embodiment shown in FIGS. 1 and 2, the lateral cuts
or slits 70 extend into the core in three different directions
which are each 120 degrees from each other. The number of cuts can
be varied to change the amount of compliance of the connector 30.
When a load is applied to the upper and lower plates 20, 22 the
connector 30 will compress with each of the cuts 70 closing and the
total amount of compression possible depending on the number,
arrangement, and height of the cuts. The cuts 70 form cantilevered
portions above and below each of the cuts which function like
cantilevers or leaf springs to allow the connector 30 to be
compressed.
[0043] The connector 30 can be modified with different numbers of
cuts 70 and different cutting directions. There may be one or more
than one cut in each of the cutting directions. The material
remaining after the cuts 70 are made in the connector is called a
column. A shallow cut 70 and a large column provides a stiffer
spacer 10 with more stability in shear (less translation of the
plates), while a deeper cut and smaller column provides a more
compliant spacer and more translation between the plates. The shape
of the columns can be varied to vary the properties of the spacer.
In the embodiment shown the cuts 70 are at least two thirds of the
way through the connector 30 width or diameter, and preferably at
least three quarters of the way through the connector width.
Although the connector 130 has been shown as circular in cross
section, other shapes may be used including oval, elliptical,
rectangular, and others.
[0044] The cuts 70 may be modified to be non uniform to provide
preferential deflection in one or more bending directions.
Preferential deflection is useful to provide increase
anterior--posterior compliance and less lateral compliance or the
other way around.
[0045] FIGS. 1 and 2 illustrate an embodiment of the compliant
spacer 10 with lateral cuts 70 in multiple directions with the
lateral cuts each having a slot width which is substantially
constant along the cuts. This constant width of the cuts 70
provides a device which has a hard stop. However, the lateral cuts
70 can also be designed with varying widths to tailor the
compliance properties of the spacer. A variable stiffness shock
absorbing spacer may be formed with cuts 70 with tapering widths in
which the width of the cuts 70 is smallest where the cut terminates
adjacent the column and is largest at the edge of the connector 30
furthest from the column. In this version, each of the cuts 70 acts
as a non linear spring providing progressively stiffer behavior
upon larger compression. This is due to the fact that progressively
more material on the sides of the cuts 70 is in contact as the
spacer is compressed. The tapered width cuts 70 can provide the
additional benefit of providing a flushing action during operation
that moves any accumulated material out of the cuts.
[0046] As shown in FIGS. 1 and 2, the cuts 70 can include a stress
relief at the ends of the cuts which increases the fatigue life of
the device by reducing the stress concentration at the ends of the
cuts. These stress relief can be provided in any known
configuration.
[0047] An alternative embodiment of a shock absorbing spacer would
include one or more spiral cuts in the connector and a small
central bore through the spacer. A single spiral cut forms a
continuous spring element or multiple spiral cuts provide multiple
spring elements which provides compliance to the core. Two or more
spiral cuts arranged in opposite directions can be formed in the
core. Since it is desirable to limit relative rotation between the
plates 20, 22 multiple spiral cuts in opposite directions can be
used which offset rotation of each other.
[0048] As shown in FIG. 5, some limited holes 60 may also be
provided in the plates 20, 22 to allow bone in growth. Holes
provided in a typical fusion spacer provide a spacer with little
structural support and maximum area for bone growth. Thus, the load
transferred across the disc space per unit area of spacer is quite
high resulting in possible subsidence of the typical spacer. In the
spacer 10 of the present invention, the load transfer is spread
across a larger area. If the outer surfaces 24 have holes 60
therein, the holes will cover less than 40 percent of the outer
surface 24 which contacts the bone to prevent subsidence of the
plates into the vertebral bodies. Preferably the holes will cover
less than 25 percent, and more preferably less than 10 percent of
the outer bone contacting surfaces. At the option of the surgeon,
when the small holes are present in the plates 20, 22, bone graft
can be placed in the space between the inner surfaces 26 of the
plates to encourage bone to grow through the plates. The holes 60,
when present can take on a variety of shapes including circular, as
shown, rectangular, polygonal or other irregular shapes. The holes
60 may extend through the various parts of the spacer including
through the connector or through the fins. The holes 60 may change
shape or size as they pass through portions of the spacer, for
example, holes through the plates and the connector may taper to a
smaller interior diameter. The limited motion provided by the
compliant spacer 10 can stimulate bone growth through the holes 60
in the spacer.
[0049] FIG. 6 shows another embodiment of a spacer 100 having a
single fin 50 on the top and bottom and two screw holes 80 and
corresponding fixation screws 90. The screws 90 extend at an angle
of about 30 to about 60 degrees with respect to the vertebral body
contacting surfaces 24 of the spacer. The spacer 100 also includes
a connector 30 between the vertebral body contacting surfaces 24
which is formed in one piece with the upper and lower plates and
includes cuts 70 providing compliance to the spacer. The fixation
screws 90 can include a locking mechanism, such as a locking thread
or a separate locking member which is inserted into the screw holes
80 after the screws are inserted to prevent backing out of the
screws.
[0050] FIG. 7 illustrates an alternative embodiment of a spacer 110
having a single superior fin 50, two inferior fins 52, and three
alternating holes 80 for receiving bone screws (not shown). The
spacer 110 has multiple fixation structures to provide the patient
near immediate mobility after the fusion procedure. As an
alternative to the alternating angled holes 80, the spacer 110 can
be formed with an anterior flange extending from the top and the
bottom at the anterior side of the plate. This optional flange can
include one or more holes for receiving bone screws placed
laterally. The laterally placed bone screws can prevent
interference in the event of multilevel fusions and are
particularly useful for a cervical fusion where space is more
limited.
[0051] FIG. 8 shows a further alternative embodiment of a spacer
120 having multiple cuts 70a-70d. A first slot 70a on the anterior
edge of the spacer splits into two cuts 70b toward the posterior
edge of the spacer and terminate before the posterior edge of the
spacer (not shown). A pair of third cuts 70c are positioned above
and below the two cuts 70b and extend from the posterior edge of
the spacer, while a larger fourth slot 70d is provided at a center
of the spacer. This arrangement of cuts 70a-70d is designed to
tailor the compliance of the spacer 120 for a particular
application. Other numbers and arrangements of cuts can also be
used.
[0052] According to one embodiment of the invention, the cuts in
the shock absorbing spacer according to any of the embodiments
described herein may be manufactured by wire EDM (electrical
discharge machining), molding, laser cutting, or the like. A number
of lateral cuts 70 can vary from 1 to about 20 for a spacer for
cervical discs and from 1 to about 40 for a spacer for lumbar
discs. A width of the lateral cuts 70 in the direction of the
height of the spacer 10 is about 0.01 mm to about 2 mm, preferably
about 0.05 to about 1 mm. The cuts can be perpendicular to the axis
of the spacer or can be angled. For example a spiral cut can be
angled to provide a conical surface of the cut and provide a limit
on translation of the plates.
[0053] In one embodiment of the present invention, for a cervical
application, the maximum deformation of the shock absorbing spacer
is about 0.1 to about 1 mm, and is preferably about 0.2 to about
0.8 mm. For a lumbar application, the maximum deformation of the
shock absorbing spacer is about 0.1 to about 2 mm, and is
preferably about 0.4 to about 1.5 mm.
[0054] Although motion between the plates 20, 22 of the spacer 10
has been describe herein as provided by lateral cuts, it should be
understood that this motion can be provided in a number of other
known manners, such as use of resilient materials, or movable
joints as long as the motion is limited to the small amount of
motion allowable in a patient requiring a fusion procedure
including compliance or vertical motion between the plates of up to
about 2 mm, rotation between the plates of less than 5 degrees, and
translation between the plates of up to about 1 mm. Preferably, the
motion for a lumbar application is limited to up to about 2 mm of
vertical or axial motion, rotation between the plates of less than
3 degrees, and translation between the plates of up to about 0.5
mm. Preferably, the motion for a cervical application is limited to
up to about 1 mm of vertical or axial motion, rotation between the
plates of less than 3 degrees, and translation between the plates
of up to about 0.5 mm.
[0055] The spacers 10, 100, 110, 120 can be provided in different
sizes, with different plate sizes, angles between plates, lordosis
angles, and heights for different patients or applications. In
addition, the shock absorbing spacer can be provided in different
compliances for different patients. In addition, the compliance
and/or height of the spacer can be adjustable, such as by rotating
an adjustment screw before or after implantation. The spacers
preferably are sized to provide substantial coverage of the
vertebral surfaces. For example in an anterior procedure, the
plates 20, 22 are preferably sized to cover at least 50 percent of
the vertebral surface. In posterior or lateral procedures the
coverage of the vertebral surface may be somewhat smaller due to
the small size of the access area, i.e. the posterior or lateral
spacers may cover about 40 percent or more of the vertebral surface
with a one or two part spacer.
[0056] One common fusion procedure, referred to as an
anterior/posterior fusion, uses of one or more fusion cages to
maintain the disc space while bridging bone grows and also uses a
system of posterior screws and rods for further stabilization.
Fusing both the front and back provides a high degree of stability
for the spine and a large surface area for the bone fusion to
occur. Also, approaching both sides of the spine often allows for a
more aggressive reduction of motion for patients who have deformity
in the lower back (e.g. isthmic spondylolisthesis).
[0057] According to one method of the present invention, the
anterior approach is performed first by removing the disc material
and cutting the anterior longitudinal ligament (which lays on the
front of the disc space). The surgeon then must choose a compliant
spacer based on a size of the vertebrae and an estimated angle
between the vertebral bodies. After the spacer is positioned
anteriorly, the patient is turned over for the implantation of the
posterior stabilization system. In instances where the spacer angle
selected was incorrect or the surgeon would like to alter the angle
of the vertebral bodies to optimize the combined system for the
patient the fusion cage does not allow this modification of the
angle. The compliant fusion spacer of the present invention
provides a particular advantage when used in an anterior/posterior
fusion with a posterior stabilization system that it allows the
angle between the vertebral bodies to be adjusted somewhat, up to 5
degrees, after implantation of the spacer and before the
implantation of the posterior stabilization system to optimize the
system for the patient.
[0058] The intervertebral spacers of the present invention may also
be used with a posterior stabilization system, dynamic rod
stabilization system, or interspinous spacer. In one example, a
posterior intervertebral spacer with two parts can be inserted by a
PLIF or TLIF approach and used with a posterior stabilization
system including screws and rods. This system provides the
advantage of maintenance of disc height and stabilization with an
entirely posterior approach.
[0059] While the exemplary embodiments have been described in some
detail, by way of example and for clarity of understanding, those
of skill in the art will recognize that a variety of modifications,
adaptations, and changes may be employed. Hence, the scope of the
present invention should be limited solely by the appended
claims.
* * * * *