U.S. patent application number 12/240070 was filed with the patent office on 2009-01-22 for spinal facet implants with mating articulating bearing surface and methods of use.
This patent application is currently assigned to FACET SOLUTIONS, INC.. Invention is credited to Alan Chervitz, T. Wade Fallin, Robert W. Hoy, Daniel J. Triplett.
Application Number | 20090024167 12/240070 |
Document ID | / |
Family ID | 37771110 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090024167 |
Kind Code |
A1 |
Chervitz; Alan ; et
al. |
January 22, 2009 |
SPINAL FACET IMPLANTS WITH MATING ARTICULATING BEARING SURFACE AND
METHODS OF USE
Abstract
Superior and/or inferior facets of one or more facet joints may
be replaced by superior and/or inferior facet joint prostheses. In
one embodiment, a kit of superior or inferior prostheses is
provided, in which the prostheses have at least two dimensions that
vary among members of the kit independently of each other. Each
prosthesis may have a bone engaging surface having a surface that
is polyaxially rotatable against a corresponding resection of a
vertebra. Each prosthesis may also have an articulating surface
shaped such that, after attachment to the spine, the replaced or
partially replaced facet joints provide a larger medial-lateral
range of motion when the spine is flexed than when the spine is
extended. Crosslinks may be used to connect left and right
prosthesis together in such a manner that they are stabilized in a
position in which they are seated directly against the
vertebra.
Inventors: |
Chervitz; Alan; (Palm
Harbor, FL) ; Triplett; Daniel J.; (Providence,
UT) ; Fallin; T. Wade; (Hyde Park, UT) ; Hoy;
Robert W.; (Columbus, OH) |
Correspondence
Address: |
MEDICINELODGE INC.
180 SOUTH 600 WEST
LOGAN
UT
84321
US
|
Assignee: |
FACET SOLUTIONS, INC.
Logan
UT
|
Family ID: |
37771110 |
Appl. No.: |
12/240070 |
Filed: |
September 29, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10860543 |
Jun 2, 2004 |
|
|
|
12240070 |
|
|
|
|
60545094 |
Feb 17, 2004 |
|
|
|
60545101 |
Feb 17, 2004 |
|
|
|
Current U.S.
Class: |
606/247 ;
623/17.11 |
Current CPC
Class: |
A61F 2/30771 20130101;
A61F 2002/30663 20130101; A61F 2002/30125 20130101; A61F 2002/30171
20130101; A61B 17/8625 20130101; A61F 2220/0025 20130101; A61F
2230/0023 20130101; A61F 2310/00796 20130101; A61F 2002/30514
20130101; A61F 2230/0021 20130101; A61F 2310/00023 20130101; A61B
17/8695 20130101; A61B 17/7064 20130101; A61F 2220/0041 20130101;
A61F 2310/00239 20130101; A61F 2002/30616 20130101; A61F 2002/30433
20130101; A61F 2230/005 20130101; A61F 2310/00179 20130101; A61F
2002/30448 20130101; A61B 2017/8655 20130101; A61F 2002/30841
20130101; A61F 2002/30934 20130101; A61F 2310/00017 20130101; A61B
17/7067 20130101; A61F 2230/0008 20130101; A61F 2002/30574
20130101; A61F 2/4611 20130101; A61B 17/7049 20130101; A61F
2002/30154 20130101; A61F 2002/30579 20130101; A61F 2002/3085
20130101; A61F 2220/005 20130101; A61F 2310/00131 20130101; A61F
2/4405 20130101; A61F 2310/00203 20130101; A61F 2310/00928
20130101; A61F 2002/3082 20130101; A61F 2002/30156 20130101; A61F
2002/30179 20130101; A61F 2002/30576 20130101; A61F 2002/30578
20130101; A61F 2002/30426 20130101; A61F 2310/00029 20130101 |
Class at
Publication: |
606/247 ;
623/17.11 |
International
Class: |
A61B 17/70 20060101
A61B017/70; A61F 2/44 20060101 A61F002/44 |
Claims
1. A facet prosthesis system for replacing an inferior facet of a
first vertebra of a spine, and a superior facet of a second
vertebra of the spine, the system comprising: an inferior
prosthesis shaped to be coupled to the first vertebra, the inferior
prosthesis having an inferior articulating surface; and a superior
prosthesis shaped to be coupled to the second vertebra, the
superior prosthesis having a superior articulating surface; wherein
the articulating surfaces of the inferior and superior prostheses
are shaped and relatively positioned to articulate against each
other such that a medial-lateral range of relative motion between
the first and second vertebrae decreases significantly with
extension of the spine.
2. The system of claim 1, wherein each of the articulating surfaces
has a curved shape, wherein one of the articulating surfaces has a
cephalad end and a caudal end, and a radius of curvature about an
axis extending generally from the cephalad end to the caudal end,
wherein the radius of curvature changes along the axis to provide
greater clearance between the articulating surfaces when the spine
is flexed.
3. The system of claim 2, wherein the articulating surface of the
superior prosthesis has a concave shape having the cephalad end,
caudal end, and radius of curvature, wherein the radius of
curvature increases toward the cephalad end.
4. The system of claim 1, wherein each of the articulating surfaces
has a curved shape, wherein one of the articulating surfaces has a
cephalad end and a caudal end, and a radius of curvature about an
axis extending generally from the cephalad end to the caudal end,
wherein the prosthesis is shaped such that, when the prosthesis is
coupled to the vertebra, the axis is significantly anteriorly
inclined to provide greater clearance between the articulating
surfaces when the spine is flexed.
5. The system of claim 4, wherein the articulating surface of the
superior prosthesis has a concave shape having the cephalad, caudal
end, and radius of curvature, wherein the axis is anteriorly
inclined toward the cephalad end.
6. The system of claim 5, wherein the axis is inclined at an angle
of at least five degrees with respect to a longitudinal axis of the
spine.
7. The system of claim 1, wherein each of the articulating surfaces
has a curved shape, wherein one of the articulating surfaces has a
cephalad end and a caudal end, and a radius of curvature about an
axis extending generally from the cephalad end to the caudal end,
wherein the radius of curvature varies along a medial-lateral
direction of the articulating surface.
8. The system of claim 7, wherein the articulating surface of the
superior prosthesis has a concave shape having the cephalad end,
caudal end, and radius of curvature, wherein the articulating
surface of the superior prosthesis has a medial end, a lateral end,
and a central portion between the medial and lateral ends, wherein
the radius of curvature is larger toward the medial and lateral
ends than at the central portion.
9. The system of claim 8, wherein the radius of curvature is
substantially infinite toward the medial and lateral ends, such
that the articulating surface of the superior prosthesis has a
curved region proximate the central portion, a first tangent flat
disposed medially of and tangent to the curved region, and a second
tangent flat disposed laterally of and tangent to the curved
region.
10. The system of claim 7, wherein the articulating surface of the
superior prosthesis has a concave shape and the articulating
surface of the inferior prosthesis has a convex shape.
11. The system of claim 10, wherein the articulating surface of the
inferior prosthesis has a three-dimensionally curved elliptical
shape.
12. The system of claim 1, wherein at least one of the articulating
surfaces is constructed of a material selected from a group
consisting of a polymeric material, a polymeric bearing material
attached to a metal substrate, a ceramic bearing material, a metal
bearing material, and combinations thereof.
13. A method for replacing at least one facet of a first vertebra
of a spine, the method comprising: resecting the first vertebra to
provide a resected portion of the first vertebra; and attaching a
coupling portion of a prosthesis to the first resected portion to
position an articulating surface of the prosthesis to articulate
against an articulating surface of a second vertebra; wherein
positioning the articulating surface of the prosthesis comprises
providing a medial-lateral range of relative motion between the
first and second vertebrae that decreases significantly with
extension of the spine.
14. The method of claim 13, wherein the articulating surface of the
prosthesis has a curved shape with a cephalad end and a caudal end,
and a radius of curvature about an axis extending generally from
the cephalad end to the caudal end, wherein the radius of curvature
changes along the axis, wherein providing the medial-lateral range
of relative motion between the first and second vertebrae comprises
providing greater clearance between the articulating surfaces when
the spine is flexed.
15. The method of claim 14, wherein the prosthesis comprises a
superior prosthesis, wherein the articulating surface of the
prosthesis has a concave shape, wherein the radius of curvature
increases toward the cephalad end, wherein resecting the first
vertebra comprises removing at least a portion of a natural
superior facet.
16. The method of claim 13, wherein the articulating surface of the
prosthesis has a curved shape with a cephalad end and a caudal end,
and a radius of curvature about an axis extending generally from
the cephalad end to the caudal end, wherein positioning the
articulating surface of the prosthesis comprises significantly
anteriorly inclining the axis to provide greater clearance between
the articulating surfaces when the spine is flexed.
17. The method of claim 16, wherein the prosthesis comprises a
superior prosthesis, wherein the articulating surface of the
prosthesis has a concave shape, wherein resecting the first
vertebra comprises removing at least a portion of a natural
superior facet, wherein significantly anteriorly inclining the axis
comprises inclining the axis toward the cephalad end.
18. The method of claim 17, wherein significantly anteriorly
inclining the axis comprises inclining the axis at an angle of at
least five degrees with respect to a longitudinal axis of the
spine.
19. The prosthesis of claim 13, wherein the articulating surface of
the prosthesis has a curved shape with a cephalad end and a caudal
end, and a radius of curvature about an axis extending generally
from the cephalad end to the caudal end, wherein the radius of
curvature varies along a medial-lateral direction of the
articulating surface, wherein providing the medial-lateral range of
relative motion between the first and second vertebrae comprises
providing greater clearance between the articulating surfaces when
the spine is flexed.
20. The prosthesis of claim 19, wherein the prosthesis comprises a
superior prosthesis, wherein the articulating surface of the
prosthesis has a concave shape, wherein the articulating surface of
the prosthesis has a medial end, a lateral end, and a central
portion between the medial and lateral ends, wherein the radius of
curvature is larger toward the medial and lateral ends than at the
central portion, wherein resecting the first vertebra comprises
removing at least a portion of a natural superior facet.
21. The prosthesis of claim 20, wherein the radius of curvature is
substantially infinite toward the medial and lateral ends, such
that the articulating surface of the prosthesis has a curved region
proximate the central portion, a first tangent flat disposed
medially of and tangent to the curved region, and a second tangent
flat disposed laterally of and tangent to the curved region.
22. A method for replacing at least one facet of a first vertebra
of a spine, the method comprising: resecting the first vertebra to
provide a resected portion of the first vertebra; and attaching a
coupling portion of a prosthesis to the resected portion to
position an articulating surface of the prosthesis to articulate
against a second articulating surface of a second vertebra; wherein
the articulating surface of the vertebra comprises a cephalad end
and a caudal end and a radius of curvature about an axis extending
generally from the cephalad end to the caudal end, wherein
positioning the articulating surface of the prosthesis comprises
significantly inclining the axis anteriorly toward the cephalad
end.
23. The method of claim 22, wherein the prosthesis comprises a
superior prosthesis, wherein the articulating surface of the
prosthesis has a concave shape, wherein resecting the first
vertebra comprises removing at least a portion of a natural
superior facet.
24. The method of claim 23, wherein significantly anteriorly
inclining the axis comprises inclining the axis at an angle of at
least five degrees with respect to a longitudinal axis of the
spine.
25. The method of claim 22, wherein the articulating surface of the
prosthesis has a medial end, a lateral end, and a central portion
between the medial and lateral ends, wherein the radius of
curvature is larger toward the medial and lateral ends than at the
central portion.
Description
REFERENCE TO PENDING PRIOR APPLICATIONS
[0001] This application is a divisional of the following:
[0002] Pending prior U.S. patent application Ser. No. 10/860,543,
filed Jun. 2, 2004 by Alan Chervitz et al. for SPINAL FACET
IMPLANTS WITH MATING ARTICULATING BEARING SURFACE AND METHODS OF
USE (Attorney's Docket No. FSI-03 NPROV); which claims the benefit
of (i) U.S. Provisional Patent Ser. No. 60/545,094, filed on Feb.
17, 2004 by Alan Chervitz et al. for SPHERICAL ARTICULATING IMPLANT
SURFACE; and (ii) U.S. Provisional Patent Ser. No. 60/545,101,
filed on Feb. 17, 2004 by Alan Chervitz et al. for SPHERICAL
IMPLANT AND BONE BED. The above-identified documents are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to surgical devices and
methods to guide instruments that prepare the surface of bones and
other tissues for implants that replace a damaged, diseased, or
otherwise painful spinal facet joint.
[0005] 2. Description of Related Art
[0006] Traumatic, inflammatory, metabolic, and degenerative
disorders of the spine can produce debilitating pain that can have
severe socioeconomic and psychological effects. One of the most
common surgical interventions today is arthrodesis, or spine
fusion, of one or more motion segments, with approximately 300,000
procedures performed annually in the United States. Clinical
success varies considerably, depending upon technique and
indications, and consideration must be given to the concomitant
risks and complications. For example, Tsantrizos and Nibu have
shown that spine fusion decreases function by limiting the range of
motion for patients in flexion, extension, rotation, and lateral
bending. Furthermore, Khoo and Nagata have shown that spine fusion
creates increased stresses and, therefore, accelerated degeneration
of adjacent non-fused motion segments. Additionally,
pseudoarthrosis, as a result of an incomplete or ineffective
fusion, may reduce or even eliminate the desired pain relief for
the patient. Finally, the fusion device, whether artificial or
biological, may migrate out of the fusion site.
[0007] Recently, several attempts have been made to recreate the
natural biomechanics of the spine by use of an artificial disc.
Artificial discs provide for articulation between vertebral bodies
to recreate the full range of motion allowed by the elastic
properties of the natural intervertebral disc that directly
connects two opposed vertebral bodies.
[0008] However, the artificial discs proposed to date do not fully
address the mechanics of motion of the spinal column. In addition
to the intervertebral disc, posterior elements called the facet
joints help to support axial, torsional and shear loads that act on
the spinal column. Furthermore, the facet joints are diarthroidal
joints that provide both sliding articulation and load transmission
features. The effects of their absence as a result of facetectomy
was observed by Goh to produce significant decreases in the
stiffness of the spinal column in all planes of motion: flexion and
extension, lateral bending, and rotation. Furthermore,
contraindications for artificial discs include arthritic facet
joints, absent facet joints, severe facet joint tropism or
otherwise deformed facet joints, as noted by Lemaire.
[0009] U.S. Pat. No. Re. 36,758 to Fitz discloses an artificial
facet joint where the inferior facet, the mating superior facet, or
both, are resurfaced.
[0010] U.S. Pat. No. 6,132,464 to Martin discloses a spinal facet
joint prosthesis that is supported on the posterior arch of the
vertebra. Extending from this support structure are inferior and/or
superior blades that replace the cartilage at the facet joint. The
Martin prosthesis generally preserves existing bony structures and
therefore does not address pathologies that affect the bone of the
facets in addition to affecting the associated cartilage.
Furthermore, the Martin invention requires a mating condition
between the prosthesis and the posterior arch (also known as the
lamina) that is a thin base of curved bone that carries all four
facets and the spinous process. Since the posterior arch is a very
complex and highly variable anatomic surface, it would be very
difficult to design a prosthesis that provides reproducible
positioning to correctly locate the cartilage-replacing blades for
the facet joints.
[0011] Another approach to surgical intervention for spinal facets
is provided in WO9848717A1 to Villaret. While Villaret teaches the
replacement of spine facets, the replacement is interlocked in a
manner to immobilize the joint.
[0012] It would therefore be an improvement in the art to provide a
vertebral facet replacement device and method that provides a
relatively high degree of mobility in the joint, while effectively
removing the source of arthritic, traumatic, or other disease
mediated pain with a minimum of patient discomfort.
SUMMARY OF THE INVENTION
[0013] In order to overcome the shortcomings of the prior art, the
present invention provides a vertebral facet replacement device and
method that replaces a bony portion of the facets so as to remove
the source of arthritic, traumatic, or other disease mediated pain.
Facet joint replacement in conjunction with artificial disc
replacements represent a holistic solution to recreating a fully
functional motion segment that is compromised due to disease or
trauma. Together, facet joint and disc replacement can eliminate
all sources of pain, return full function and range of motion, and
completely restore the natural biomechanics of the spinal column.
Additionally, degenerative or traumatized facet joints may be
replaced in the absence of disc replacement when the natural
intervertebral disc is unaffected by the disease or trauma.
[0014] Accordingly, in certain embodiments, the present invention
provides an artificial vertebral facet that replaces the cartilage
and a portion of the bone of a facet. Furthermore, the invention
may provide a method for preparing a vertebra for the installation
of an artificial vertebral facet, a method for replacing a spinal
facet, and possibly, a total vertebral facet joint replacement.
[0015] The present invention may provide numerous advantages over
the prior art. One advantage may be that the quality of attachment
of the prosthesis is improved. The present invention may provide a
precise press fit into bones, as opposed to relying on prosthetic
surfaces mating with highly complex and variable external surfaces
of the vertebra, such as the posterior arch or facet. Another
advantage may be that the optional porous coating is placed into
interior bone spaces where porous coatings have proven to achieve
bone ingrowth for excellent long term fixation strength. This
ability to achieve bone ingrowth is uncertain for the prior art
devices that engage the external bone surfaces of the vertebra. Yet
another advantage may lie in the removal of the facet bone
structure; where the facet bone is involved in the disease
pathology or the trauma that compromised the articular or
cartilaginous surface of the facet, resection provides a means for
ensuring that all pain associated with the disease or trauma is
removed.
[0016] The above, and other features and advantages of the present
invention, will become apparent from the following description,
which is to be read in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a perspective view of a portion of the spine;
[0018] FIG. 2 is a lateral view of a facet joint reconstructed in
accordance with the present invention;
[0019] FIG. 3 is a dorsal view of the facet joint shown in FIG.
2;
[0020] FIG. 4 is a perspective view of the implanted left inferior
facet prosthesis shown in FIGS. 2 and 3;
[0021] FIG. 5 is a perspective view of the left inferior facet
prosthesis shown in FIGS. 2 and 3;
[0022] FIG. 6 is a cranial view of the implanted left superior
facet prosthesis shown in FIGS. 2 and 3;
[0023] FIG. 7 is a perspective view of the left superior facet
prosthesis shown in FIGS. 2 and 3;
[0024] FIG. 8 is a perspective view of an alternative implanted
left inferior facet prosthesis;
[0025] FIG. 9 is a perspective view of an alternative left inferior
facet prosthesis;
[0026] FIG. 10 is a lateral view of an alternative reconstructed
facet joint;
[0027] FIG. 11 is a dorsal view of an alternative reconstructed
facet joint;
[0028] FIG. 12 is a perspective view of the implanted left inferior
facet prosthesis shown in FIGS. 10 and 11;
[0029] FIG. 13 is a perspective view of the alternative left
inferior facet prosthesis shown in FIGS. 10 and 11;
[0030] FIG. 14 is a cranial view of the alternative implanted left
superior facet prosthesis shown in FIGS. 10 and 11;
[0031] FIG. 15 is a perspective view of the alternative left
superior facet prosthesis shown in FIGS. 10 and 11;
[0032] FIG. 16 is a perspective view of an alternative bearing
surface for the superior facet prosthesis shown in FIG. 15;
[0033] FIG. 17 is a dorsal view of a single intact vertebra;
[0034] FIG. 18 is a lateral view of the same intact vertebra shown
in FIG. 17;
[0035] FIG. 19 is a dorsal view of the same vertebra of FIG. 17 and
FIG. 18, with a portion of the superior facet resected and a
portion of the inferior facet resected;
[0036] FIG. 20 is a lateral view of the resected vertebra shown in
FIG. 19;
[0037] FIG. 21 is a dorsal view of the same resected vertebra shown
in FIG. 18 and FIG. 19 with a fixation element placed through the
first superior resection surface and into the pedicle bone;
[0038] FIG. 22 is a dorsal view showing the resected vertebra, the
fixation element, and a superior facet prosthesis;
[0039] FIG. 23 is a dorsal view of the vertebra and the implant of
FIG. 23 and also showing the addition of an inferior facet
prosthesis;
[0040] FIG. 24 is a dorsal view of the implant and vertebra of FIG.
23 and also showing the addition of an enlarged head that has the
shape of a locking nut;
[0041] FIG. 25 is a perspective view of a vertebra with an
assembled implant comprising a fixation element, superior facet
prosthesis, and a locking nut;
[0042] FIG. 26 is a perspective, cross-sectioned view of the same
vertebra and implant of FIG. 25 with a cross section aligned with
the axis of the fixation element;
[0043] FIG. 27 is a cranial, cross-sectioned view of the vertebra
and implant of FIG. 25, with the section plane positioned as in
FIG. 26;
[0044] FIG. 28 is a side view of embodiments A, B, C, D, E, and F
of the fixation element, a cross-sectional view of each of
embodiments A, B, C, D, E, and F, and a side view of the enlarged
head in the shape of a locking nut;
[0045] FIG. 28A is a side view of embodiments G, H, I, J, K, and L
of the fixation element with attached enlarged heads, and a
cross-sectional view of each of embodiments G, H, I, J, K, and
L;
[0046] FIG. 29 is a perspective view of a radially expanding
fixation element in its unexpanded state;
[0047] FIG. 30 is a side view and a bottom view of (i) an expanded
radially expanding fixation element and (ii) an unexpanded radially
expanding fixation element;
[0048] FIG. 31 is a perspective cross-sectional view of a vertebra
and a facet implant showing a cross-pin torsionally and axially
securing the fixation element;
[0049] FIG. 32 is a dorsal view of a spinal section showing a top,
middle, and bottom vertebra with unilateral facet replacements on
the right side of the spine section, both between the top and
middle vertebra, and between the middle and bottom vertebra;
[0050] FIG. 33 is a dorsal view of a spine section showing a
superior hemiarthroplasty facet replacement between the top and the
middle vertebra and unilateral replacement between the middle and
the bottom vertebra;
[0051] FIG. 34 is a dorsal view of a spinal section showing an
inferior facet hemiarthroplasty replacement between the top and the
middle vertebra and a unilateral replacement on the right side
between the middle and the bottom vertebra;
[0052] FIG. 35 is a dorsal view of a spinal section showing a
unilateral replacement between the top and middle vertebrae on the
right side, and an inferior facet hemiarthroplasty replacement
between the middle and bottom vertebrae on the same side;
[0053] FIG. 36 is a dorsal view of a spinal section showing a
unilateral replacement between the top and middle vertebrae on the
right side and a superior facet hemiarthroplasty replacement on the
right side between the middle and bottom vertebrae on the same
side;
[0054] FIG. 37 is a spinal section of two vertebrae showing one
inferior facet of the top vertebra and the adjoining superior facet
of the bottom vertebra replaced by an articulating facet
implant;
[0055] FIG. 38 is a perspective view of a curved superior facet
prosthesis;
[0056] FIG. 39 is a perspective view of a superior facet prosthesis
with a bone ingrowth surface;
[0057] FIG. 40 is a perspective view of an inferior facet
prosthesis;
[0058] FIG. 41 is a perspective view of an inferior facet
prosthesis with a bone ingrowth surface;
[0059] FIG. 42 is an exploded, perspective view illustrating the
addition of a locking washer to the construction of the implant
shown in FIG. 25;
[0060] FIG. 43 is a perspective view illustrating the implant of
FIG. 25 with a locking washer fully installed;
[0061] FIG. 44 is a perspective view of the locking washer shown in
FIG. 42;
[0062] FIG. 45 is a perspective view of superior and inferior facet
prostheses held against a vertebra by flexible fixation
elements;
[0063] FIG. 46 is a dorsal view of a bilateral inferior
implant;
[0064] FIG. 47 is perspective view of a vertebra with an
alternative embodiment of a superior facet prosthesis fixed to the
bone by one embodiment of a fixation element;
[0065] FIG. 48 is a perspective, cross-sectional view of the
embodiment of the superior facet prosthesis and fixation element of
FIG. 47 showing the semispherical shape of the resection and the
approximately similarly semispherical shape of the apposition side
of the superior facet prosthesis, as well as an angled resection
and corresponding angled flat on the apposition side of the
superior facet prosthesis in combination with the semispherical
resection;
[0066] FIG. 49 is a perspective view of the resected vertebra
without the superior facet prosthesis attached to the vertebra, in
which the fixation element is installed in the vertebra;
[0067] FIG. 50 is a perspective view of the resected vertebra with
the superior facet prosthesis attached to the vertebra, with the
fixation element installed in the vertebra, but without the locking
fastener shown in FIG. 47;
[0068] FIG. 51 is a top view of the superior facet prosthesis
showing the semispherical shape of the bone apposition side in
combination with the angled flat on the bone apposition side;
[0069] FIG. 52 is a rear view of the superior facet prosthesis
showing the semispherical nut engaging surface on the top of the
area that is design to connect to the fixation element and the
locking nut, or the inferior prosthesis and the fixation
element;
[0070] FIG. 53A is a rear view and a perspective view of a
plurality of superior facet prostheses of a kit;
[0071] FIG. 53B is a top view of an inferior facet prosthesis
according to one embodiment of the invention;
[0072] FIG. 53C is a side view of the inferior facet prosthesis of
FIG. 53B;
[0073] FIG. 53D is a perspective view of a plurality of inferior
facet prostheses of a kit;
[0074] FIG. 53E is a perspective view showing how a superior facet
prosthesis and an inferior facet prosthesis may fit together;
[0075] FIG. 53F is a dorsal view of an L5 superior facet prosthesis
and an L4 inferior facet prosthesis fit on adjacent vertebrae to
articulate against each other;
[0076] FIG. 53G is a posteriolateral view of the implants and
vertebrae shown in FIG. 53F;
[0077] FIG. 53H is a posteriolateral view showing a cross-section
along a first plane cut through the articulation of the implants of
FIG. 53F;
[0078] FIG. 53I is a cephalad view showing a cross-section along a
second plane cut through the articulation of the implants shown in
FIG. 53F;
[0079] FIG. 54 is a dorsal view of a bilateral inferior facet
prosthesis system and a superior facet prosthesis in situ;
[0080] FIG. 55 is a perspective view of the bilateral inferior
facet prosthesis system and the superior facet prosthesis of FIG.
54;
[0081] FIG. 56 is a lateral view of the bilateral inferior facet
prosthesis system and superior facet prosthesis in situ;
[0082] FIG. 57 is a cranial view of the bilateral inferior implant
system in situ;
[0083] FIG. 58 is a bottom view of the bilateral inferior facet
prosthesis system in situ;
[0084] FIG. 59 is rear view of the bilateral inferior facet
prosthesis system in isolation;
[0085] FIG. 60 is a top view of the bilateral inferior facet
prosthesis system in isolation;
[0086] FIG. 61 is a bottom view of the bilateral inferior facet
prosthesis system in isolation;
[0087] FIG. 62 is a perspective view of the right inferior
prosthesis;
[0088] FIG. 63 is a perspective view of various ball-shaped members
of inferior prostheses, the ball-shaped members having differing
surface features, particularly circumferential grooves,
longitudinal grooves, and knurling;
[0089] FIG. 64 is an end view of the ball-shaped members of FIG.
63; and
[0090] FIG. 65 is a dorsal view of the bilateral inferior facet
prosthesis system, in which castle nuts are attached to the left
and right fixation elements.
DETAILED DESCRIPTION OF THE DRAWINGS
[0091] Referring now to FIG. 1, there is shown a perspective view
of a superior vertebra 1 and an inferior vertebra 3, with an
intervertebral disc 2 located in between. The superior vertebra 1
has superior facets 43, inferior facets 6, a posterior arch (or
lamina) 35 and a spinous process 46. The inferior vertebra 3 has
superior facets 7, inferior facets 44, a posterior arch (or lamina)
36 and a spinous process 45. Each of the vertebrae 1, 3 also has a
pair of pedicles 11.
[0092] Referring now to FIG. 2, in a lateral view, the left
inferior facet 6 of the superior vertebra 1 shown in FIG. 1 has
been resected and an inferior facet prosthesis 4 has been attached
to the superior vertebra 1. Similarly, the left superior facet 7 of
the inferior vertebra 3 has been resected and a superior facet
prosthesis 5 has been attached to the inferior vertebra 3.
[0093] FIG. 3 illustrates a dorsal view of the elements shown in
FIG. 2. It can be appreciated that inferior facet prosthesis 4
replicates the natural anatomy when compared to the contralateral
inferior facet 6 of vertebra 1. Similarly, it can be appreciated
that superior facet prosthesis 5 replicates the natural anatomy
when compared to the contralateral superior facet 7 of vertebra 3.
Neither the inferior facet prosthesis 4 nor the superior facet
prosthesis 5 rests on the lamina 35.
[0094] Turning now to FIG. 4, a perspective view of the superior
vertebra 1 with implanted inferior facet prosthesis 4 is provided.
A bone resection on the left side of the superior vertebra 1, shown
as a resection 31, has removed the natural inferior facet 6 at the
bony junction between the inferior facet 6 and the lamina 35. In
this manner, any bone pain associated with a disease, such as
osteoarthritis, or trauma of the left inferior facet 6 will be
eliminated as the involved bony tissue has been osteotomized.
[0095] FIG. 5 illustrates a perspective view of the inferior facet
prosthesis 4. A surface 8 replicates the natural articular surface
of the replaced inferior facet 6. A post 9 provides a mechanism
that can be used to affix the inferior facet prosthesis 4 to the
superior vertebra 1. The post 9 is implanted into the interior bone
space of the left pedicle 11 on the superior vertebra 1 and may or
may not extend into the vertebral body of superior vertebra 1 to
provide additional stability.
[0096] FIG. 6 illustrates a cranial view of the inferior vertebra 3
with the implanted superior facet prosthesis 5. A resection surface
32 represents the bony junction between the natural superior facet
7 and the lamina 36.
[0097] FIG. 7 illustrates a perspective view of the superior facet
prosthesis 5. A surface 38 replicates the natural articular surface
of the replaced superior facet 7. The post 37 provides a mechanism
usable to affix the superior facet prosthesis 5 to the inferior
vertebra 3. The post 37 is implanted into the interior bone space
of the left pedicle 11 (FIG. 6) on the inferior vertebra 3 and may
or may not extend into the vertebral body of the inferior vertebra
3 to provide additional stability.
[0098] When the total facet joint is replaced, as shown in FIGS. 2
and 3, then the surface 8 (FIG. 5) articulates against the surface
38 (FIG. 7) to recreate the natural biomechanics of the spine
motion segment made up of the superior vertebra 1, the inferior
vertebra 3, and the intervertebral disc 2. Neither the inferior
facet prosthesis 4 nor the superior facet prosthesis 5 rests on the
lamina 35 or the lamina 36, respectively.
[0099] FIG. 8 illustrates a perspective view of an alternative
inferior facet prosthesis 10 that may be implanted into the
interior bone space of the lamina 35 of the superior vertebra 1.
The interior bone space is accessed from the resection 31.
[0100] FIG. 9 shows a perspective view of the alternative inferior
facet prosthesis 10, including a fin 13 that extends into the
interior bone space of the 35. A surface 12 replicates the natural
articular surface of the replaced facet.
[0101] The surfaces of the post 9 (FIG. 5), the post 37 (FIG. 7),
and the fin 13 (FIG. 9) may or may not include porous coatings to
facilitate bone ingrowth to enhance the long-term fixation of the
implant. Furthermore, such porous coatings may or may not include
osteoinductive or osteoconductive substances to further enhance
bone remodeling into the porous coating. In this application, the
term "implant" refers to any natural or man-made, fabricated or
unfabricated device or group of devices that may be added to a
human spine. An implant may include one or more prostheses, one or
more fixation devices, and/or other components.
[0102] Referring now to FIG. 10, there is shown a lateral view of a
superior vertebra 14 and an inferior vertebra 16, with an
intervertebral disc 15 located in between. The left inferior facet
of the superior vertebra 14 has been resected and an inferior facet
prosthesis 18 has been attached to superior vertebra 14 via a screw
fastener 17. Similarly, the left superior facet of the inferior
vertebra 16 has been resected and a superior facet prosthesis 19
has been attached to vertebra 16 via a screw fastener 17.
[0103] FIG. 11 illustrates a dorsal view of the elements of FIG.
10. It can be appreciated that inferior facet prosthesis 18
replicates the natural anatomy when compared to the contralateral
inferior facet 22 of the superior vertebra 14. Similarly, it can be
appreciated that superior facet prosthesis 19 replicates the
natural anatomy when compared to the contralateral superior facet
21 of the inferior vertebra 16. Neither the inferior facet
prosthesis 18 nor the superior facet prosthesis 19 rests on the
lamina of the corresponding vertebra 14 or 16.
[0104] Turning now to FIG. 12, there is provided a perspective view
of the superior vertebra 14 with the implanted inferior facet
prosthesis 18. A resection 34 has removed the natural inferior
facet at the bony junction between the inferior facet and the
adjoining lamina. In this manner, any bone pain associated with a
disease, such as osteoarthritis, or trauma of the natural inferior
facet 22 will be eliminated inasmuch as the involved bony tissue
has been osteotomized.
[0105] FIG. 13 illustrates a perspective view of the inferior facet
prosthesis 18. A surface 23 replicates the natural articular
surface of the replaced facet. A flange 25 contacts the pedicle 11
(FIG. 12) and a hole 24 receives the screw fastener 17 to attach
the inferior facet prosthesis 18 to the superior vertebra 14.
[0106] FIG. 14 illustrates a cranial view of the inferior vertebra
16 with the implanted superior facet prosthesis 19. A resection
surface 33 represents the bony junction between the natural
superior facet 21 (FIG. 11) and the corresponding lamina.
[0107] FIG. 15 illustrates a perspective view of the superior facet
prosthesis 19. A surface 27 replicates the natural articular
surface of the replaced facet. A flange 39 contacts the pedicle 11
(FIG. 14) and hole 26 receives a screw fastener 17 to attach the
superior facet prosthesis 19 to the inferior vertebra 16.
[0108] FIG. 16 provides a perspective view of an alternative
superior facet prosthesis 40 with a bearing surface 41 that mounts
to substrate 42. The bearing surface 41 is a biocompatible
polymeric material, such as ultra high molecular weight
polyethylene. Alternatively, the bearing surface can be ceramic,
such as zirconia or alumina. The substrate is a biocompatible metal
alloy, such as an alloy of titanium, cobalt, and/or iron.
[0109] The bearing surface 41 may be formed separately from the
remainder of the superior facet prosthesis 40, so that the bearing
surface 41 and the remainder form components that can be assembled
as needed. A kit of differently-sized prostheses may include
multiple bearing surfaces like the bearing surface 41 that may have
different thicknesses, articulating surface shapes, material
selections, and the like. Such a kit may also include other
differently-sized components designed such that some subset of the
components can be selected and assembled together to provide a
prosthesis having the desired dimensions. Prosthesis kits will be
shown and described in greater detail subsequently.
[0110] Referring to FIG. 17 and FIG. 18, a single intact vertebra
100 is shown. FIG. 17 is a dorsal view of the vertebra 100. FIG. 18
is a lateral view of the same vertebra 100. Similar to the two
vertebrae 1, 3 shown in the portion of the spine illustrated in
FIGS. 1 through 3, the vertebra 100 has posterior anatomy
comprising left and right superior facets 43 on the superior, or
top side in this view of the dorsal vertebra 100, left and right
inferior facets 6 on the inferior or bottom side of the posterior
vertebra 100, left and right transverse processes 105 extending
laterally from the posterior portion of vertebra 100, and left and
right pedicles 11. Each of the superior facets 43 has a superior
articulating surface 145. The posterior portion of vertebra 100
also has a posterior arch (or lamina) 35, and a spinous process 46
that protrudes from the lamina 35 posteriorly, out of the page in
FIG. 17 and to the left in FIG. 18. In FIG. 17, the bony structure
of the superior facets 43 and the inferior facets 6 are intact, as
it would be presented in a vertebra without significant tissue
degeneration or remodeling resulting from facet joint disease.
Although the vertebra 100 is shown in FIG. 17 as a generally
structurally healthy and intact vertebra, if the vertebra 100 were
a diseased vertebra, the vertebra could exhibit signs of facet
joint disease.
[0111] Consequently, structural pathology related to facet joint
disease would likely be visible. For example, the left superior
facet 43 and the right superior facet 43 of the vertebra 100 are
symmetrical in FIG. 17 and FIG. 18. But in the case of a vertebra
100 with only one diseased joint, the facet on the diseased side
would likely be showing pathological signs of disease such as
tissue degeneration or inflammation resulting in an asymmetrical
structural comparison between the two facets.
[0112] Also, in more extreme cases the facet disease could progress
to a state in which the articular process of the facet is eroded or
inflamed resulting in anatomic morphology that is unique to the
pathology of a particular facet joint of an individual patient.
This could present unusual facet morphology that could be different
from what is shown in FIGS. 17 and 18.
[0113] Furthermore, the facet disease could eventually disable the
biomechanics of a patient such that the facet joint is essentially
non-articulating and immobile. In this case, one superior facet of
a first vertebra could essentially be fused to one inferior facet
of a second vertebra. Since the structural pathology of the
diseased facet is variable, a surgeon may determine that the best
bone apposition surface or foundation for securing a facet implant
is a resected bone surface.
[0114] Referring to FIG. 19 and FIG. 20 which are dorsal and
lateral views of the same vertebra shown in FIG. 17 and FIG. 18
after a portion of the right superior facet 43 and a portion of the
right inferior facet 6 have been resected. The removal of a portion
of the superior facet 43 by resection results in a superior facet
resection 111. In the resection shown in FIG. 19 and FIG. 20, the
superior resection 111 has two resulting faces, a first resection
surface 112 and a second resection surface 113. Likewise, the
inferior facet resection results in an inferior facet resection
surface 121.
[0115] Tissue removal tools (not shown) such as a bone burr, rasp,
reamer, mill, saw, rounger, osteotome or similar tools designed to
cut and remove bone tissue can be used to create these resection
surfaces. The surgeon uses anatomic landmarks such as the pedicle
11 or transverse process 105 to align the tissue removal tools in
such a way as to remove the portion of the facet necessary to
provide a superior resection 111 that serves as a bone apposition
surface or foundation to eventually support a superior facet
prosthesis 300, as shown in FIG. 22. The left superior facet 43 is
shown intact in both FIG. 19 and FIG. 20, but a portion of the
right superior facet 43 is resected resulting in the first
resection surface 112 and the adjacent second resection surface 113
(FIG. 19). The shape of the superior resection 111 will vary in
accordance with the structure of the tissue removal tool. In the
embodiment shown in FIG. 19 and FIG. 20, the first resection
surface 112 and the second resection surface 113 are on
approximately perpendicular planes. However, the geometry of the
resection surfaces is a function of the patient anatomy, the
pathology of the diseased tissue, the technique of the surgeon, and
other factors such as the type of tissue removal tools used to
prepare the resection. In general, the first resection surface 112
will be formed in such a way that it will serve as a foundation to
support the superior facet prosthesis 300 (FIG. 22). The second
resection surface 113 or other additional resection surfaces may or
may not be present.
[0116] FIG. 19 and FIG. 20 also show that a portion of the inferior
facet 6 is resected by tissue removal instruments resulting in an
inferior resection surface 121. Such resection is preferably
effected so that resection is confined to the tissue of the
inferior facet 6 and does not extend into the tissue of the
posterior arch (or lamina) 35. In FIGS. 19 and 20, the left
inferior facet 6 is intact, while a portion of the right inferior
facet 6 is resected resulting in the inferior resection surface 121
on the right side. The bone surrounding the inferior resection
surface 121 is contoured by tissue removal tools in a shape
designed to cradle and support an inferior facet prosthesis 400
(FIG. 23) on the medial side such that when the inferior facet
prosthesis 400 is loaded on the lateral side it compresses against
and is supported by the inferior resection surface 121.
[0117] Alternatively, the inferior facet 6 can be resected, and
inferior facet prosthesis 400 sized and shaped, so that inferior
facet prosthesis 400 does not engage the inferior resection surface
121.
[0118] FIG. 21 is a dorsal view of the vertebra 100 with a fixation
element 200 placed through the superior resection 111 and into the
bone of the pedicle 11 to receive the superior facet prosthesis 300
(FIG. 22). The fixation element 200 is aligned and placed into the
pedicle 11, similar to how other pedicle screws for posterior
stabilization involved with vertebrae fusion are placed in the
pedicle 11. In one method, a long guide wire (not shown), with a
diameter sized to fit freely into a cannulation 211 (as also shown
in FIG. 26 and FIG. 27) in the fixation element 200, is placed
through the first resection surface 112 and into the bone of the
pedicle 11. The alignment of the long guide wire can be confirmed
by x-ray. The fixation element 200 is then guided over the guide
wire and driven into the vertebra 100 by a driver (not shown)
engaged with a drive feature 212 (FIG. 21) on a proximal post 230
of the fixation element 200. The fixation element 200 is driven
into the vertebra 100 until a connection feature 213 (e.g., a screw
thread) is just above the first resection surface 112. This
connection feature 213 is eventually used to secure the superior
facet prosthesis 300 to the vertebra 100.
[0119] In a second method for guiding the fixation element 200 into
the pedicle 11, a long guide wire (not shown), with a diameter
sized to fit freely into a cannulation in a bone preparation
instrument (not shown) such as a tap, drill, broach or reamer, is
placed through the first resection surface 112 and into the bone of
the pedicle 11. The alignment of the long guide wire can be
confirmed by x-ray. The bone preparation instrument is then guided
over the guide wire and driven into the bone of the pedicle 11 to
prepare a cavity for the fixation element 200. The guide wire and
bone preparation instrument are then removed and the fixation
element 200 is guided into the prepared cavity in the pedicle 11 by
a driver (not shown) engaged with the drive feature 212 on the
proximal post 230 of the fixation element 200. Like in the first
method, the fixation element 200 is driven into the vertebra until
a connection feature 213 (e.g., a screw thread) is just above the
first resection surface 112. This connection feature 213 is
eventually used to secure the superior facet prosthesis 300 to the
vertebra 100.
[0120] In yet a third method of placing the fixation element 200 in
the pedicle, the surgeon aligns the fixation element 200 with
anatomic landmarks and simply drives the fixation element 200
through the first resected surface 112 and into the pedicle 11. As
with the first and second methods, the fixation element 200 is
driven into the vertebra 100 until a connection feature 213 (e.g.,
a screw thread) is just above the first superior resection surface
112.
[0121] In FIG. 22, a dorsal view illustrates a superior facet
prosthesis 300 placed around the fixation element 200. The superior
facet prosthesis 300 has a facet articulating component 320 that
articulates against the inferior facet articulating surface of the
vertebra above it. The facet articulating component 320 is
preferably formed in the general shape of a blade or wing ear. The
superior facet prosthesis 300 also has a bone apposition surface
322 that has been placed on the first resection surface 112 and an
opening 324 in a flange 323 that surrounds the fixation element
200. The superior facet articulating component 320 has an
articulating surface 321 generally adjacent to the flange 323 that
is oriented in a direction that faces approximately the same
direction that the original anatomic superior articulating surface
145 faced prior to resection.
[0122] This orientation of the articulating surface 321 allows the
superior facet prosthesis 300 to function as either a
hemiarthroplasty implant and articulate against a natural anatomic
inferior facet 6 or act as a portion of a unilateral prosthesis and
articulate against an inferior facet prosthesis 400 on the vertebra
superior (cephalad) to it. No portion of superior facet prosthesis
300 rests on the lamina of the vertebra 100. In this application, a
"unilateral prosthesis" is a prosthesis in which both facets of
only one of the facet joints between adjacent vertebrae are
replaced by prostheses. A "hemiarthroplasty" is a type of
arthroplasty in which one side of an articulating joint surface is
replaced with an artificial implant.
[0123] FIG. 23 is a dorsal view showing the addition of the
inferior facet prosthesis 400 to the construct described in FIG.
22. The inferior facet prosthesis 400 generally has a shape similar
to a longitudinal rod that is curved to match the contour of the
inferior resection 121 (FIGS. 19 and 20). The inferior facet
prosthesis 400 has an opening 410 through its superior end 420 that
is shaped to surround the portion of the fixation element 200 that
protrudes from the first resection surface 112. In FIG. 23, the
inferior facet prosthesis 400 is placed over the superior facet
prosthesis 300. However, the order of the placement of the
prostheses 300, 400 can be reversed such that the inferior
prosthesis 400 is placed on the fixation element 200 first,
followed by the superior prosthesis 300. When only the inferior
facet 6 or the superior facet 43 is being replaced, only the
appropriate (superior or inferior) facet prosthesis 300 or 400 is
placed on the fixation element 200 without the other (inferior or
superior) facet prosthesis 300 or 400.
[0124] Because the various components of the implant are modular,
many combinations of configurations and implant size, structure and
shapes are feasible. For example, in a patient with unusual
anatomy, the inferior facet prosthesis 400 may need to be larger
than expected to conform to a particularly unusual or exceptionally
large morphology of the inferior resection surface 121, and the
superior facet prosthesis 300 may need to have an unusual angle to
its articulating surface 321 to conform to particular anatomic
constraints. If this is the case, the modularity of the system
allows for the surgeon to assemble an implant specifically designed
to match the patient's anatomic structures during the surgery. This
flexibility of a modular implant design allows the implant
manufacturer to accommodate a large variation in anatomic
structures with a limited selection of implant component sizes,
shapes, and material types.
[0125] The modularity of the implant design also allows different
components of the implant to be fabricated from different
materials. Traditionally, bone fixation implants such as the
fixation element 200 are fabricated from biocompatible metals or
alloys that provide sufficient strength and fatigue properties,
such as cobalt chrome alloys, titanium and titanium alloys, and
stainless steels. However, the fixation element 200 may be
fabricated from ceramics, polymers, or biological materials such as
allograft bone, composites, or other biocompatible structural
materials. Likewise the superior facet prosthesis 300 and the
inferior facet prosthesis 400 may be fabricated from metals,
alloys, ceramics, polymers, biological materials, composites, or
other biocompatible structural materials.
[0126] In FIG. 24, a dorsal view illustrates the addition of an
enlarged head 500 to the fixation element 200. The enlarged head
500 is tightened down to force the prostheses 300, 400 against the
bone to stabilize them. The enlarged head 500 shown in FIG. 24 has
a hexagonal geometry on its external surface that is shaped to
accept a driver (not shown) that is used to force an internal
connection feature 520 (e.g., a screw thread) of the enlarged head
500 onto the connection feature 213 of the fixation element 200. In
the case of the threaded embodiment of the connection feature 213,
the enlarged head 500 is provided with a threaded connection
feature 520 and is driven onto the fixation element 200 by turning
the enlarged head 500 and allowing the threads to drive all
components of the implant between the enlarged head 500 and the
first resection surface 112 against the bone at or near the
resection surface 112.
[0127] FIG. 25 is a perspective posterior view of the assembly of
the fixation element 200, the superior facet prosthesis 300, and
the enlarged head 500. The enlarged head 500 has been placed on the
first resection surface 112.
[0128] FIG. 26 is a perspective, cross-sectioned view of the same
construct shown in FIG. 25. The superior facet prosthesis 300, the
enlarged head 500, the fixation element 200, and the vertebra 100
have been cut by a cross-sectioning plane 150 placed along an axis
that passes through the center of the fixation element 200. The
cross-section plane 150 is shown for visualization purposes to
illustrate, using a cross-sectioned view, how the vertebra 100,
fixation element 200, superior facet prosthesis 300 and the
enlarged head 500 engage each other. In actual surgery, it is
highly unlikely that a surgeon would make a cut as illustrated by
the cross-section 150 shown in FIG. 26.
[0129] FIG. 27 is a cranial, section view of the vertebra 100 and
the implant, wherein the cross-section plane 150 is oriented to
face the viewer. In FIG. 27, the fixation element 200 is in the
vertebra 100. The embodiment of the fixation element 200 in FIG. 27
comprises a distal end 220 that is shaped to guide the fixation
element 200 into bone tissue, a bone stabilizing portion 210
adjacent to the distal end, a shaft portion 240 adjacent to the
bone stabilizing portion 210, a connection feature 213 adjacent to
the shaft portion 240, and a drive feature 212.
[0130] The distal end 220 shown in FIG. 27 has a frusto-conical
shape that allows the fixation element 200 to be driven or guided
into the vertebra 100. The distal end 220 could be shaped in the
form of a spade tip, trochar tip, or twist drill tip to assist in
the guidance of the fixation element 200 in the vertebra 100. The
fixation element 200 may also have a cutting flute (not shown)
formed in the distal end 220 to help remove bone tissue and
accommodate the guidance of the fixation element 200 in the
vertebra 100. The bone stabilizing portion 210 helps to secure the
fixation element 200 to the vertebra 100. The bone stabilizing
portion 210 can include various features designed to anchor into
bone such as threads, ribs, grooves, slots, fins, barbs, splines,
bone ingrowth surfaces, roughened surfaces, or any geometric
feature that helps to engage the fixation element 200 with the bone
tissue to help stabilize the fixation element 200. In FIG. 27, the
bone stabilizing portion 210 has a unitary continuous bone thread
231. However, other types of threads such as multiple lead threads,
variable pitched thread, non-uniform pitch thread, buttress thread,
or other thread forms used on bone screws may be used. Because FIG.
27 is a cross-sectional view, the full length of the cannulation
211 is seen passing from the distal end 220 of the fixation element
200 to the proximal post 230 of the fixation element 200.
[0131] The drive feature 212 in the embodiment shown in FIG. 27 is
an internal hex. However, any shape of drive feature 212 that
transmits the loads necessary to drive the fixation element 200
into the vertebra 100 can be formed on the proximal post 230 of the
fixation element 200. The depth of the drive feature 212 formed in
the proximal post 230 of the fixation element 200 is seen in the
cross-sectional view of FIG. 27. The drive feature 212 may be an
internal drive feature such as the hex socket shown in this
embodiment, or an external drive feature with geometry on the
periphery of the proximal post 230 of the fixation element 200 that
engages with a corresponding internal drive feature on a driver
tool (not shown). In this embodiment the depth of the drive feature
212 is slightly longer than its cross-section is wide. This depth
can be adjusted based on the material properties of the fixation
element 200 and the drive tool (not shown).
[0132] The fixation element 200 is fabricated from biocompatible
base materials that provide the necessary structural rigidity and
strength. Examples of base materials that may be used in the
construction of the fixation element 200 include titanium, titanium
alloys, cobalt-chrome alloys, stainless steel alloys, zirconium
alloys, other biocompatible metal materials, biocompatible
ceramics, biocompatible composites, and biocompatible polymers. The
fixation element 200 may also have surface materials formed on the
base material that provide material properties specific to a
particular portion of the fixation element 200. For example, the
bone stabilization portion 210 could be coated with materials that
allow for improved bone ingrowth into the implant surface such as a
hydroxylapatite, bioceramic, Bioglass.RTM., or other calcium
phosphate derived material. The tribological bearing properties of
the material in the areas that the fixation element 200 interfaces
with other artificial elements may be improved by applying surface
hardening techniques to the material of the fixation element 200 in
these areas. Surface hardening techniques known in the materials
science and materials engineering arts such as anodizing, ion
implantation, and other techniques could be applied to these
isolated areas.
[0133] As mentioned previously, the connection feature 213 is
formed on the portion of the fixation element 200 that protrudes
from the first resection surface 112. This connection feature 213
is designed to connect the enlarged head 500 to the fixation
element 200. In the embodiment of the connection feature 213 shown
in FIG. 21, threads 260 are on the external surface of this
proximal section of the fixation element 200. These threads 260
engage with the threads of the internal connection feature 520
(FIG. 27) of the enlarged head 500. Although the connection feature
213 in this embodiment is threaded, other mechanical locking
features (not shown) capable of locking the fixation element 200
and the enlarged head 500 together, such as press fit, taper fit,
bonding fit by cement or glue, interference fit, expansion fit and
mechanical interlocking fit such as a bayonet connection, can be
used as the connection feature 213. A corresponding construction
may then be used as connection feature 520 of the enlarged head
500.
[0134] Also shown in FIG. 27 is a cross-sectional view of the
superior facet prosthesis 300. This embodiment of the superior
facet prosthesis 300 has a flange 323 that has an opening 324 that
receives the fixation element 200. In the assembled and implanted
configuration of this embodiment, the flange 323 is positioned such
that its bone apposition surface 322 makes contact with the first
resection surface 112. Although not shown in this embodiment, other
embodiments of the superior facet prosthesis 300 have structures
(e.g., spikes) that protrude into the first resection surface 112
to help resist torsion and other anatomic loads. Protruding from
the flange 323 at a given angle .alpha., and a given distance X
from the opening 324, is the articulating component 320. The
articulating surface 321 of the facet articulating component 320
replicates the natural articular surface of the replaced facet.
Once the surgeon assesses the anatomy of the superior facet 43 that
is being replaced, a particular superior facet prosthesis 300 is
selected that has the angle .alpha. and the distance X that best
fits the anatomy of the level of vertebra, the left or right side,
and the size of the patient's anatomy being replaced. Thus a kit
containing various sizes and shapes of superior facet prostheses
300 is provided to the surgeon and the surgeon selects the superior
facet prosthesis 300 that best suits the situation.
[0135] After the fixation element 200 and the superior facet
prosthesis 300 are selected and placed, they are locked to the
vertebra 100 by the enlarged head 500. As shown in FIG. 24, the
enlarged head 500 in this embodiment has an internal connection
feature 520 and a hexagonal shaped external drive feature 510 that
is used to drive the enlarged head 500 over the fixation element
200 and against the superior facet prosthesis 300. The specific
shape of the external drive feature 510 is dependent on the mating
shape of the driver (not shown).
[0136] Referring to FIG. 28, side and cross-sectional views
illustrate six different embodiments of fixation elements, which
are labeled A, B, C, D, E, and F. The figure shows a side view of
each fixation element embodiment and a cross-sectional view of each
embodiment to the right of the respective side view. To the left of
the six embodiments is a representative enlarged head 500.
Embodiment A is the threaded fixation element 200 embodiment shown
in FIGS. 26 and 27 and described above. Embodiments B through E are
various designs of fixation elements with non-circular
cross-sections. Embodiment B is a four rib cruciate design with
four longitudinal fins 285 configured to resist torsion when the
fixation element 200 is in the vertebra 100. Embodiment C is an
oval shaped cross-section design that is wider in the first
direction 286 than the second direction 287 to resist torsion. If
the width in the first direction 286 is equal to the width in the
second direction 287, the cross-section shape becomes more of a
circle and bone stabilization portion 210 becomes more of a
press-fit peg. Embodiment D is a square cross-section design with
four approximately perpendicular sides 288. The corners 289 of the
sides 288 help to resist torsion. Embodiment E is a triangular
cross-section design with three sides 291 to resist torsion.
Embodiment F is an anchor-like design that is driven into the
vertebra, with the wire arches or barbs 290 being compressed
against the host bone and applying a radial expansion force so as
to lock the structure to the bone.
[0137] Referring to FIG. 28A, side and cross-sectional views
illustrate six more different embodiments of fixation elements,
which are labeled G, H, I, J, K, and L. FIG. 28A shows a side view
of each fixation element embodiment and a cross-sectional view of
each embodiment to the right of the respective side view. Each
embodiment has an attached or integrally formed enlarged head 500'.
Embodiment G is similar to the threaded fixation element 200
embodiment shown in FIGS. 10, 11, 12 and 24 and described above.
Embodiments H through K are various designs of fixation elements
with non-circular cross-sections. Embodiment H is a four rib
cruciate design with four longitudinal fins 285 configured to
resist torsion when the fixation element is in the vertebra 100.
Embodiment I is an oval shaped cross-section design that is wider
in a first direction 286 than in a second direction 287 to resist
torsion. If the width in the first direction 286 is equal to the
width in the second direction 287, the cross-section shape becomes
more of a circle and the bone stabilization portion 210 becomes
more of a press-fit peg. Embodiment J is a square cross-section
design with four approximately perpendicular sides 288. The corners
289 of the sides 288 help to resist torsion. Embodiment K is a
triangular cross-section design with three sides 291 to resist
torsion.
[0138] Embodiment L is an anchor-like design that is similar to
Embodiment F in FIG. 28, but with an attached or integrally formed
enlarged head 500'. As embodiment L is driven into the vertebra,
wire arches or barbs 290 are compressed and apply radial expansion
force against the wall of the prepared bone and into the pedicle
11, resulting in a locking anchor.
[0139] FIG. 29 is a perspective view of a radially expanding
fixation element 600. The radially expanding fixation element 600
comprises two main elements, an expansion sleeve 620 and a central
element 610 that is inside of the expansion sleeve 620. The
radially expanding fixation element 600 is placed into the vertebra
100 and then the central element 610 is drawn outward relative to
the expansion sleeve 620 resulting in radial expansion of the
fixation element 600. This is shown in FIG. 30.
[0140] Referring to FIG. 30, side and bottom views illustrate the
fixation element 600 of FIG. 29. As a proximal post 630 of the
central element 610 is pulled axially along its longitudinal axis,
and the expansion sleeve is held axially in the bone by compression
fit, talons 621 on the expansion sleeve 620 are radially expanded
outward by a mandrel 660 on the central element 610. The talons or
fingers 621 provide both torsional and axial stability to the
radially expanding fixation element 600. This provides a secure
fixation element for fixation of the remaining implant components.
Furthermore, expansion of the fixation element 600 may cause the
fixation element 600 to center itself within the pedicle 11.
[0141] FIG. 31 is a perspective, cross-sectional view of a
cross-pin element 700 engaged with the fixation element 200 to help
secure the fixation element 200 both torsionally and axially. The
cross-pin element 700 is columnar in shape having a distal end 710,
a midsection 730 (with a length along its longitudinal axis that is
longer than its transverse cross-sectional width), and a proximal
post 720. The distal end 710 is shaped to penetrate through bone
tissue and into a cross hole 280 formed in the fixation element
200. Instrumentation (not shown) is used to align the cross-pin
element 700 with the cross-hole 280 via fixation of the
instrumentation to the drive feature 212 or the cannulation 211 on
the fixation element 200 and alignment of the direction of
insertion of the cross-pin element 700 with the cross-hole 280.
Once the cross-pin element 700 is in place in the bone and through
the fixation element 200, the torsional and axial stability of the
fixation element 200 is improved.
[0142] The various embodiments of the fixation element 200
described above and shown in FIG. 28 through FIG. 31 function in
conjunction with the enlarged head 500 to hold the inferior facet
prosthesis 400 and/or the superior facet prosthesis 300 to their
respective resection surfaces 112, 113, and/or 121. Various
combinations of this modular implant will be described below and
shown in FIGS. 32 through 37. Although these figures illustrate the
use of the fixation element 200 and the enlarged head 500 as the
mechanism for securing the prostheses 300, 400 to the vertebra 100,
other clamping devices such as the screw fastener 17 (FIG. 10) may
be used to mount the prostheses 300, 400 to the bone. For example,
the screw prostheses 17 shown in FIGS. 10 through 12 may pass
through either the opening 324 (FIG. 22) in the superior facet
prosthesis 300 or the opening 410 (FIG. 23) in the inferior facet
prosthesis 400 or through both of these openings 324, 410. The head
of the screw fastener 17 acts as the securing mechanism by pressing
the inferior facet prosthesis 400 and the superior facet prosthesis
300 against their respective resection surfaces 112, 113, and/or
121.
[0143] FIGS. 32 through 37 demonstrate different combinations of
assemblies of facet replacement prostheses. The basic components of
the prosthesis are the fixation element 200, the superior facet
prosthesis 300, the inferior facet prosthesis 400, and the enlarged
head 500. However, as described above, a screw fastener 17 can
replace the fixation element 200 and the enlarged head 500.
[0144] Referring to FIG. 32, a dorsal view illustrates three
sequential layers of vertebrae. A top vertebra 101 is above a
middle vertebra 102, and the middle vertebra 102 is above a bottom
vertebra 103. Portions of some of the facets on the right side of
the vertebrae are replaced by prostheses. With regard to the facet
joint between the top vertebra 101 and the middle vertebra 102, an
inferior facet prosthesis 401 is articulating against a superior
facet prosthesis 302 to form an artificial unilateral joint. The
inferior facet of the middle vertebra 102 is replaced by an
inferior facet prosthesis 402 and the superior facet of the bottom
vertebra 103 is replaced by superior facet prosthesis 303. Thus, a
second unilateral prosthetic joint is formed that is also on the
right side and is located at the level between the middle vertebra
102 and the bottom vertebra 103. FIG. 32 demonstrates the
difference in shape of the inferior facet prosthesis 401 that is
implanted around the fixation element 201 without a superior facet
prosthesis 300 and an inferior facet prosthesis 402 that is
implanted around a fixation element 202 and over a superior facet
prosthesis 302. The opening 410 (not visible) of the inferior facet
prosthesis 401 on the top vertebra 101 in this assembly is offset
more laterally than the opening 410 (not visible) in the inferior
facet prosthesis 402 for the middle vertebra 102. This is because
the fixation element 201 is implanted more laterally on the top
vertebra 101 to preserve more of the superior facet since it is not
replaced by a prosthesis at this level.
[0145] Referring to FIG. 33, a dorsal view illustrates the top
vertebra 101 in intact form, without resection of the facets.
Portions of both the superior and inferior facets on the right side
of the middle vertebra 102 are replaced by a superior facet
prosthesis 302 and an inferior facet prosthesis 402. Only the right
superior facet of the bottom vertebra 103 is replaced (i.e., by a
superior facet prosthesis 303) in FIG. 33. Thus, a hemiarthroplasty
replacement has been performed on the right facet joint between the
top vertebra 101 and the middle vertebra 102 and a unilateral
replacement has been performed between the middle vertebra 102 and
the bottom vertebra 103. The assembly shown in FIG. 33 demonstrates
how the superior facet prosthesis 302 can articulate against the
natural inferior facet 6 and the superior facet prosthesis 303 can
articulate against the inferior facet prosthesis 402.
[0146] FIG. 34 is a dorsal view illustrating how the inferior facet
prosthesis 401 can articulate against the natural superior facet
43, or the inferior facet prosthesis 402 can articulate against the
superior facet prosthesis 303. The right facet joint between the
top vertebra 101 and the middle vertebra 102 is a hemiarthroplasty
replacement with the inferior facet replaced by the inferior facet
prosthesis 401. The right facet joint between the middle vertebra
102 and the bottom vertebra 103 is a unilateral replacement with
the inferior facet replaced by the inferior facet prosthesis 402
and the superior facet of the bottom vertebra 103 replaced by the
superior facet prosthesis 303.
[0147] Referring to FIG. 35, a dorsal view shows another example of
how the superior facet prosthesis 303 can articulate against the
natural inferior facet 6 or the superior facet prosthesis 302 can
articulate against the inferior facet prosthesis 401. In this
assembly of the implant, the right side between the top vertebra
101 and the middle vertebra 102 is a unilateral replacement and the
right side between the middle vertebra 102 and the bottom vertebra
103 is a hemiarthroplasty replacement.
[0148] Referring to FIG. 36, a dorsal view shows another example of
how the inferior facet prosthesis 402 can articulate against the
natural superior facet 43, or the inferior facet prosthesis 401 can
articulate against the superior facet prosthesis 302. The right
facet joint between the top vertebra 101 and the middle vertebra
102 is a unilateral replacement with the inferior facet of the top
vertebra 101 replaced by the inferior facet prosthesis 401 and the
superior facet of the middle vertebra 102 replaced by the superior
facet prosthesis 302. The right facet joint between the middle
vertebra 102 and the bottom vertebra 103 is a hemiarthroplasty
replacement with the inferior facet replaced by the inferior facet
prosthesis 402.
[0149] Referring to FIG. 37, a dorsal view illustrates only one
level, that between the middle vertebra 102 and the bottom vertebra
103, being replaced on the right side. The right facet joint
between the middle vertebra 102 and the bottom vertebra 103 is a
unilateral replacement with the inferior facet of the middle
vertebra 102 replaced by the inferior facet prosthesis 402 and the
superior facet of the bottom vertebra 103 replaced by the superior
facet prosthesis 303.
[0150] FIG. 38 and FIG. 39 show two embodiments of the superior
facet prosthesis. In FIG. 38, a perspective view illustrates an
embodiment in which a curved superior facet prosthesis 305 with a
curved articulating component 330 has a curved articulating surface
331. This curved articulating surface 331 allows for a more
distributed contact load between an inferior facet prosthesis, such
as the inferior facet prosthesis 400 of FIG. 23, and the curved
articulating surface 331. This allows slightly more flexibility in
the position that the surgeon places the curved superior facet
prosthesis 305 than the superior facet prosthesis 300 previously
described. The articulating surface 321 of the superior facet
prosthesis 300 previously described is relatively flat. The
articulating surface 331 of the curved superior facet prosthesis
305 is curved. Since the bearing portion of the inferior facet
prosthesis 400 is columnar, the two prosthesis can be aligned on a
slight mismatch and make more of an anatomic contact if the
articulated surface is curved as in FIG. 38.
[0151] Referring to FIG. 39 a perspective view illustrates a bone
ingrowth feature 390 on a superior facet prosthesis 306. The bone
ingrowth feature 390 can be any surface that allows bone to grow
into the implant between the first resection surface 112 of the
vertebra 100 and the apposition surface 322 of the implant.
Examples of bone ingrowth features 390 include porous coating of
beads or meshes, electrochemically etched shapes and porous pads
pressed onto the implant surface made from tantalum, titanium,
cobalt chrome alloys and/or other biocompatible material such as
hydroxylapatite or calcium phosphate ceramics.
[0152] Referring to FIG. 40, a perspective view shows the inferior
facet prosthesis 400, which is formed in the general shape of a
finger or talon. More particularly, the inferior facet prosthesis
400 is formed with a flange 420 on its superior side shaped to fit
between the enlarged head 500 and either the superior facet
prosthesis 300 or the first resection surface 112. The flange 420
has an opening 410 that is dimensioned to allow the inferior facet
prosthesis 400 to fit over the proximal post 230 of the fixation
element 200 and around the shaft portion 240 of the fixation
element 200. The inferior facet prosthesis 400 also has an inferior
portion 450 on the opposite side of the flange 420 that has a bone
apposition side 440 that is shaped to contact the surface of the
inferior facet resection surface 121 (FIG. 19) and a joint
articulation side 430 that is shaped to articulate against a
natural or prosthetic superior facet.
[0153] Referring to FIG. 41, a perspective view shows an inferior
facet prosthesis 460 also formed in the general shape of a finger
or talon. The inferior facet prosthesis 460 is formed with a
superior end 420 having an opening 410 that is dimensioned and
shaped to accept the fixation element 200. The inferior facet
prosthesis 460 is generally columnar in shape, having a curved
length designed to conform to the prepared anatomy of the vertebra
100. The inferior facet prosthesis 460 of FIG. 41 has an inferior
portion 470, which is shown opposite the superior end 420, and
slightly medially offset from the superior end 420. This medial
offset of the opening 410 relative to the inferior portion 470
allows the inferior facet prosthesis 400 to be anchored to the bone
by the fixation element 200 and secured to the bone by the enlarged
head 500, or the superior facet prosthesis 300 in combination with
the enlarged head 500, at an anatomical position that allows
optimal bone fixation. The inferior facet prosthesis 460 of FIG. 41
has a bone ingrowth surface 441 and a joint articulating side 430
on its inferior end 470. In this embodiment, the bone ingrowth
surface 441 is a textured structure that permits bone cells to grow
into the implant surface. The shape of the bone ingrowth surface
441 can be a uniform textured surface as shown in FIG. 41, or can
be a non-uniform randomized structure such as a open cell foam
structure, a porous beaded structure, a wire mesh structure, an
electrochemical etched structure, or other bone ingrowth structures
known in the design of orthopedic implants. The bone ingrowth
surface 441 is shaped to mate with the inferior resected bone
surface 121 shown in FIG. 19 and FIG. 20.
[0154] FIG. 42 shows an exploded, perspective view of the vertebra
100 with the superior facet prosthesis 300 installed. An additional
locking washer 800 is used to assist in stabilizing the attachment
of the superior facet prosthesis 300 to the first resection surface
112. The construction of the implant assembly shown in FIG. 42 is
similar to that of the assembly shown in FIG. 25 with the addition
of the locking washer 800 that is placed over and around the
proximal post 230 of the fixation element 200.
[0155] Referring to FIG. 43, a perspective view shows the same
implant of FIG. 42 with the enlarged head 500 locked onto the
fixation element 200 and pushing the locking washer 800 against the
superior facet prosthesis 300 and into the bone tissue. This added
bone penetration of the locking washer 800 helps to fix the
superior prosthesis 300 such that the entire assembly is more
mechanically stable with respect to the vertebra 100.
[0156] FIG. 43 shows a further step in the assembly of the implant
construct described in FIG. 42. In FIG. 43, the locking washer 800
is secured over the fixation element 200 and into the bone tissue
by the enlarged head 500. Although this embodiment of the locking
washer 800 is only shown with the superior facet prosthesis 300,
the locking washer 800 can alternatively be used to mechanically
secure the inferior facet prosthesis 400, or the combination of the
inferior facet prosthesis 400 and the superior facet prosthesis
300. In the embodiment of the locking washer 800 shown in FIG. 42
and FIG. 43, the locking washer 800 is placed over the superior
facet prosthesis 300. However, the locking washer 800 may be placed
under the superior facet prosthesis 300, under the inferior facet
prosthesis 400 and the superior facet prosthesis 300, or between
the superior facet prosthesis 300 and the inferior facet prosthesis
400 to stabilize the implant construct.
[0157] FIG. 44 shows a perspective view of the locking washer 800.
The locking washer 800 has a body 805 with an opening 810 that is
dimensioned to fit over the proximal post 230 of the fixation
element 200. The locking washer 800 also has an anti-rotation
feature 820 that mates with either the superior facet prosthesis
300 or the inferior facet prosthesis 400 or a combination of both
the inferior facet prosthesis 400 and the superior facet prosthesis
400. The anti-rotation feature 820 shown in this embodiment is a
flat surface, however, any feature that would rotationally
constrain the locking washer 800 to the other components of the
implant (such as a tab, groove, taper or other geometric shape) can
be formed on the locking washer 800 as an anti-rotation feature.
The locking washer 800 also has prongs 830 that pass into the bone
tissue of the vertebra 100 to help stabilize the implant construct.
The prongs 830 in this embodiment of the locking washer 800 are
elongated protrusions that taper to a tissue penetration tip 840.
The prongs have sidewalls 850 that provide a surface to resist
torsion once the locking washer 800 penetrates the bone tissue. The
prongs 830 may also be simple spikes that are either symmetrical or
asymmetrical in cross-section that protrude from the locking washer
body 805. The shape and length of the locking washer prongs 830 are
dependent on how the locking washer 800 is used. The prongs 830 of
the locking washer 800 that holds only one of the inferior facet
prosthesis 400 or the superior facet prosthesis 300 to the vertebra
100 may be shorter than prongs of a locking washer that holds both
the inferior facet prosthesis 400 and the superior facet prosthesis
300 to the vertebra 100.
[0158] FIG. 45 shows a perspective view of the superior facet
prosthesis 300 and inferior facet prosthesis 400 held to the
vertebra 100 by an adjunctive flexible fixation element 900 and a
secondary flexible fixation element 910. These flexible fixation
elements 900 and/or 910 may be made from such constructs as suture,
braided cable, wire, ribbon, and/or other constructs that have
longer lengths than cross-sections and withstand larger loads in
tension than in compression. The flexible fixation elements 900
and/or 910 may be manufactured from biocompatible metals, alloys
such as cobalt chrome alloys, titanium alloys, stainless steel
alloys, polymers, bioabsorbable materials, composites, or other
materials that are biocompatible and can be formed into a flexible
element structure 900 and/or 910 such as those shown in FIG. 45.
The adjunctive flexible element 900 shown in FIG. 45 is shown
attached to and securing the elongated head 500. A flexible element
attachment portion 580 (e.g., including an opening) mates the
flexible element 900 to the elongated head. However, the adjunctive
flexible fixation element 900 may alternatively or additionally be
attached to the fixation element 200, the superior facet prosthesis
300, the inferior facet prosthesis 400 or any combination of the
above listed elements. A flexible fixation attachment portion 480
(e.g., including an opening) in the inferior facet prosthesis 400
allows the secondary flexible fixation element 910 to secure the
inferior facet prostheses 400 to the vertebra 100. The flexible
fixation elements 900 and/or 910 may be secured to the vertebra 100
by physically wrapping them around anatomic features such as the
posterior arch 35, the spinous process 46, transverse process 105,
or a combination of these anatomic features. The flexible element
900 and the secondary flexible element 910 may also be secured to
the vertebra 100 by bone anchors such as anchors designed to anchor
flexible fixation elements (such as suture, not shown) to bone.
Suture anchors such as threaded suture anchors, barbed suture
anchors, toggle suture anchors or any other means of anchoring a
flexible fixation element to bone may be used to anchor the
flexible fixation element 900 and/or the secondary flexible
fixation element 910 to the vertebra 100.
[0159] FIG. 46 is a dorsal view of a bilateral inferior facet
prosthesis 1000. The bilateral inferior facet prosthesis 1000 is a
one-piece inferior facet prosthesis that has both a right inferior
side 1040 and a left inferior side 1020 connected by a stabilizing
bar 1010. Both the right inferior side 1040 and the left inferior
side 1020 are designed to fix to the top vertebra 101 at the
respective inferior resection surface 121 (FIG. 19) and at the
first resection surface 112. The bilateral inferior facet
prosthesis 1000 allows replacement of both the left and the right
inferior facets. In this embodiment, the bilateral inferior facet
prosthesis 1000 is placed over the left and right fixation elements
200 which extend into the bone of the top vertebra 101. In the
embodiment shown in FIG. 46, the right inferior side 1040 is
articulating against the right superior facet prosthesis 300
attached to the bottom vertebra 102. Also in this embodiment, the
left inferior side 1020 is articulating against the left natural
superior facet 43 of the bottom vertebra 102. The stabilizing bar
1010 of the bilateral inferior prosthesis 1000 is designed to
stabilize the left side 1020 and the right side 1040 so that they
are secure.
[0160] FIG. 47 illustrates a perspective view of a superior facet
prosthesis 1100 coupled to the vertebra 3. The superior facet
prosthesis 1100 has a bone apposition surface (not shown) that has
been placed on a first resection surface 1112 and an opening (not
shown) in a flange 1116 that surrounds a fixation element 1110, and
coupled thereto by a locking fastener such as a castle nut 1114 or
the like. The superior facet prosthesis 1100 has a superior facet
articulating component 1120 with an articulating surface 1122
generally adjacent to the flange 1116. The articulating surface
1122 is oriented in a direction that faces approximately the same
direction that the original anatomic superior articulating surface
faced prior to resection. This orientation of the articulating
surface 1122 allows the superior facet prosthesis 1100 to function
as either a hemiarthroplasty implant by articulating against a
natural anatomic inferior facet 6 or as a unilateral prosthesis by
articulating against an inferior facet prosthesis on the vertebra
superior (cephalad) to it, such as the inferior facet prosthesis 4
shown in FIG. 5, the inferior facet prostheses 10 shown in FIGS. 8
and 9, and the inferior facet prosthesis 400 shown in FIG. 40, as
well as those described below.
[0161] The facet articulating component 1120 is preferably formed
in the general shape of a blade or wing ear, wherein the
articulating surface 1122 has a concave shape. In the embodiment
shown, the articulating surface 1122 curves from an orientation
generally perpendicular to the flange 1116 towards an orientation
generally parallel to the flange 1116 from a distal end 1124
thereof to a proximal end 1126 thereof.
[0162] The concave shape of the articulating surface 1122 provides
more tolerance for a miss-match with the natural anatomic inferior
facet 6 or with the inferior facet prosthesis 4 on the vertebra
superior to it. Functionally, the clearance between the concave
shape of the articulating surface 1122 and the adjacent inferior
facet 6 or inferior facet prosthesis 4 increases as the patient
bends forward (flexion) and decreases as the patient bends backward
(extension). Thus in flexion the patient has more facet movement
allowing for more torsion (twisting) and lateral bending (side to
side movement) than in a neutral stance. As the patient extends,
the articulating members are more constrained in torsion and
lateral bending. This mimics the natural anatomic constraints of
the spinal facets.
[0163] FIG. 48 is a perspective view of the same construct shown in
FIG. 47, but with the implants and the vertebra 3 cut by a
cross-sectioning plane 1130 placed along an axis that passes
through the center of the fixation element 1110. The cross-section
plane 1130 shown cutting through the vertebra 3 and the implant of
FIG. 47 is shown for visualization purposes to illustrate, using a
cross-sectioned view, how the vertebra 3, fixation element 1110,
and superior facet prosthesis 1100 engage each other.
[0164] The fixation element 1110 provides a mechanism that affixes
the superior facet prosthesis 1100 to vertebra 3. Fixation element
1110 is implanted into the interior bone space of the left pedicle
11 (FIG. 6) on the vertebra 3 and may or may not extend into the
vertebral body of vertebra 3 to provide additional stability. The
fixation element 1110 can take the form of a screw (as shown), or
any of the devices shown in FIGS. 28-30. The fixation element 1110
has a drive feature 1140, which is an internal hex in the
embodiment shown in FIG. 48. However, any shape of drive feature
that transmits the loads necessary to drive the fixation element
1110 into the vertebra 3 can be formed on a proximal post 1142 of
the fixation element 1110.
[0165] The depth of the drive feature 1140 formed in the proximal
post 1142 of the fixation element 1110 is seen in the
cross-sectional view of FIG. 48. The drive feature 1140 may be an
internal drive feature such as the hex socket shown in this
embodiment, or an external drive feature with geometry on the
periphery of the proximal post 1142 of the fixation element 1110
that engages with a corresponding internal drive feature on a
driver tool (not shown). The flange 1116 of the superior facet
prosthesis 1100 is secured to the fixation element 1110 by the
castle nut 1114 or the like.
[0166] The flange 1116 of the superior facet prosthesis 1100
includes a coupling portion 1144 having a generally semispherical
bone engaging surface 1150 on the apposition side of the superior
facet prosthesis 1100 that engages a corresponding semispherical
resection 1146 in the bone bed of the pedicle of vertebra 3. The
term "semispherical" relates to a surface that includes some
sectorial portion of a sphere, which may be less than a hemisphere.
A semispherical surface may be concave or convex. A surface that is
semispherical or generally semispherical may have some deviations
from a precise semispherical shape.
[0167] The semispherical resection 1146 may be said to be
"inversely shaped" with respect to the coupling portion because the
semispherical resection 1146 has a generally concave surface that
matches the generally convex surface of the coupling portion 1144.
Although the coupling portion 1144 and the semispherical resection
1146 are semispherical in the embodiment of FIGS. 47 and 48, in
alternative embodiments, they may have a variety of other matched
shapes, including three-dimensional parabolas, ellipsoids, and
other regularly or irregularly curved or flat-sided shapes.
Furthermore, although the coupling portion 1144 is convex and the
semispherical resection 1146 is concave in the embodiment of FIGS.
47 and 48, in alternative embodiments, the shapes may be reversed
so that a coupling portion is concave and a resection is
convex.
[0168] In the embodiment of FIGS. 47 and 48, the coupling portion
1144 is integrally formed with the articulating surface 1122 of the
superior facet articulating component 1120. The coupling portion
1144 may be said to be "attached to" the articulating surface 1122
because in this application, the term "attached" is used broadly to
include parts that are integrally formed with each other as well as
parts that are formed separately and subsequently coupled
together.
[0169] The semispherical resection 1146 in the bone bed allows for
less transverse process to be resected (vs. a flat bone bed
resection). The semispherical resection 1146 in the bone bed also
allows for more stable support of the superior facet prosthesis
1100, than does a flat bone bed resection, as the superior facet
prosthesis 1100 is polyaxially supported in such a way as to resist
any shear forces applied between the semispherical resection 1146
and the coupling portion 1144. In this application, "polyaxial"
refers to a linear or angular force or motion acting with respect
to at least two perpendicular axes. The coupling portion 1144 may
seat directly against the semispherical resection 1146. In this
application, an item that "seats directly against" another is
positioned to abut the other item so that surfaces of the two items
are in contact with each other.
[0170] The coupling portion 1144 has a fixation element receiving
aperture 1148 that can be made slightly larger than a
circumferential diameter of the fixation element 1110 taken in a
direction perpendicular to a longitudinal axis thereof to provide
accurate polyaxial seating of the implant 1100 in relation to the
resected bone bed and fixation element 1110, as well as to provide
increased tolerance for miss-match. An implant engaging end 1154 of
the castle nut 1114 (or other fastener) also has a semispherical
shape for engaging a semispherical nut engaging side of the
coupling portion 1144 of the superior facet prosthesis 1100 at the
final position of the superior facet prosthesis 1100.
[0171] The semispherical shape of the coupling portion 1144 enables
the coupling portion 1144 to move polyaxially against the
semispherical resection 1146. Movement "against" the semispherical
resection refers to movement in which the coupling portion 1144
remains substantially continuously in contact with the
semispherical resection 1146 so as to slide against the
semispherical resection 1146. Accordingly, during installation, a
surgeon can position the coupling portion 1144 against the
semispherical resection 1146 and then pivot the coupling portion
1144 along three perpendicular axes, without removing the coupling
portion 1144 from the semispherical resection 1146. The coupling
portion 1144 simply rotates against the semispherical resection
1146.
[0172] The phrase "polyaxial motion" refers to any combination of
translation and/or rotation along at least two perpendicular axes.
Since the coupling portion 1144 is pivotable with respect to the
semispherical resection 1146 along three perpendicular axes, the
coupling portion 1144 is "tri-axially pivotable" with respect to
the semispherical resection 1146.
[0173] When the superior facet prosthesis 1100 has been rotated to
the proper orientation, the articulating surface 1122 is positioned
for proper articulation against the corresponding inferior facet or
inferior facet prosthesis. The orientation of the coupling portion
1144 may then be fixed with respect to the semispherical resection
1146 by tightening the castle nut 1114 (or another fastener) on the
fixation element 1110, thereby firmly gripping the coupling portion
1144 against the semispherical resection 1146. Accordingly, the
coupling portion 1144 is "selectively polyaxially movable" with
respect to the semispherical resection 1146 because the coupling
portion 1144 is movable with respect to the semispherical resection
1146 along multiple perpendicular axes until the surgeon decides to
fix its disposition.
[0174] In alternative embodiments (not shown) of the invention,
tri-axial pivotal movement need not be provided. Rather, a coupling
portion and a corresponding resection surface may have a
cylindrical, flat-sided, splined, or other shape designed to enable
relative translation in addition to or in place of rotation. In
place of the fixation element receiving aperture 1148, an elongated
fixation element receiving aperture may be used to accommodate
relative translation between the coupling portion and a fixation
element. Alternatively, a coupling portion and a resection surface
may be shaped to provide relative pivotal motion along only one or
two axes.
[0175] In an alternative embodiment the implant engaging end 1154
of the castle nut 1114 (or other fastener) can be deformable such
that the implant engaging end 1154 conforms under pressure to the
adjacent surface of the coupling portion 1144 regardless of the
angle of the surface with respect to the axis of the castle nut
1114. The deformable end can be formed of a plastic such as
polyethylene attached to the metal body of the castle nut 1114, but
is preferably formed of a substance that hardens over time, such as
a fast-curing and biocompatible resin or a material that is heated
prior to insertion into the patient and hardens upon cooling to the
patient's body temperature. The material that hardens over time
provides more stability than the deformable material, though both
provide acceptable results.
[0176] FIG. 48 also shows an angled resection 1112 and
corresponding angled flat 1156 on the apposition side of the
superior facet prosthesis 1100 in combination with the
semispherical resection 1148.
[0177] The surfaces of the apposition side of the coupling portion
1144 and flat 1156, as well as fixation element 1110, may or may
not include porous coatings to facilitate bone ingrowth to enhance
the long-term fixation of the implant. Furthermore, such porous
coatings may or may not include osteoinductive or osteoconductive
substances to further enhance the bone remodeling into the porous
coating.
[0178] FIG. 49 shows a perspective view of the vertebra 3 with a
fixation element 1110 portion implant placed through the
semispherical resection 1146 in the resection surface 1112 and into
the bone of the pedicle 11. The fixation element 1110 is aligned
and placed into the pedicle 11 in a manner similar to that of other
pedicle screws for posterior stabilization vertebrae fusion
procedures.
[0179] In FIG. 50, a perspective view illustrates the superior
facet prosthesis 1100 in place around the fixation element 1110.
The castle nut 1114 has not yet been installed. As shown, the
coupling portion 1144 has a semispherical nut engaging surface
1152.
[0180] FIG. 51 is a top view of the superior facet prosthesis 1100,
particularly showing the curved shape of the articulating surface
1122 and the semispherical bone engaging surface 1150 of the
coupling portion 1144. Additionally, FIG. 51 more clearly
illustrates the angled flat 1156 on the apposition side of the
superior facet prosthesis 1100.
[0181] FIG. 52 is an illustration of a rear view of the superior
facet prosthesis 1100. In this context, "rear" means as viewed from
along the axis of the fixation element receiving aperture 1148.
FIG. 52 particularly shows the curved shape of the articulating
surface 1122 and the semispherical nut engaging surface 1152 of the
coupling portion 1144.
[0182] FIG. 53A shows a kit including a plurality of differently
configured superior facet prostheses 1100, 1162, 1164, 1166, 1168.
View A is a rear view of the superior facet prostheses 1100, 1162,
1164, 1166, 1168, while View B illustrates a perspective view of
the laterally adjacent prosthesis 1100, 1162, 1164, 1166, 1168
rotated 90.degree.. As shown, the superior facet prostheses 1100,
1162, 1164, 1166, 1168 have differing physical dimensions.
[0183] Referring again to FIG. 51, which shows a single superior
facet prosthesis 1100, some of the physical dimensions that change
between the differently sizes superior facet prostheses 1100, 1162,
1164, 1166, 1168 in the kit (FIG. 53A) are a resection angle
(.alpha.), an x offset (X.sub.1), a y offset (Y.sub.1), a facet
angle (.beta.), and a facet articulation radius (R). Exemplary
values for the foregoing will be provided below. Although the
exemplary values relate primarily to L5 superior and L4 inferior,
they may apply to other combinations of vertebrae in the lower back
and/or the sacrum. One or more of these variables can change
between the different superior facet prosthesis sizes.
[0184] P1 is the most medial and anterior point on the articulating
surface 1122. The superior pedicle axis 1170 is the axis that is
colinear with the longitudinal axis of the fixation element 1110
that is positioned through the pedicle 11 nearest to the resected
superior facet (not shown). The superior pedicle axis 1170 extends
through a saddle point S1, which is offset as shown, by an offset
1176, which may be about 2 mm, from the fixation element receiving
aperture 1148. The superior pedicle axis 1170 is parallel with the
direction of the y offset (Y.sub.1). The direction of the x offset
(X.sub.1) is perpendicular to the direction of the y offset
(Y.sub.1). The direction of the x offset (X.sub.1) is generally,
but not precisely, lateral to medial with respect to the central
axis of the patient's spine.
[0185] P4 is the most posterior point on the articulating surface
1122. As shown, P4 is displaced from the saddle point S1 by an x
offset (X.sub.4) and a y offset (Y.sub.4). The direction of the
X.sub.4 offset is parallel to that of the X.sub.1 offset, and the
direction of the Y.sub.4 offset is parallel to that of the Y.sub.1
offset.
[0186] The resection angle (.alpha.) for the superior facet
prostheses 1100 can range from 5.degree. to 85.degree.. However,
the optimal range of the resection angle (.alpha.) for the majority
of patients will range from 30.degree. to 70.degree.. Thus, by way
of example, a family containing nine sets of superior facet
prostheses 1100 can be provided with the resection angles (.alpha.)
varying in increments of 5.degree.. Sets of superior facet
prostheses 1100 would be provided with resection angles (.alpha.)
at 30.degree., 35.degree.. 40.degree., 45.degree., 50.degree.,
55.degree., 60.degree., 65.degree. and 70.degree..
[0187] The x offset (X.sub.1) for the superior facet prosthesis
1100 can range from 5 mm to 30 mm. However, for the majority of
patients, the x offset (X.sub.1) will range from 10 mm to 20 mm.
Therefore a family of superior facet prostheses 1100 can be
provided with the x offset (X.sub.1) varying in increments of 5 mm.
Thus, sets of superior facet prostheses 1100 would be provided with
x offset (X.sub.1) at 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm
to provide superior facet prostheses 1100 that cover the
statistical range for the majority of the population of patients
needing superior facet prostheses 1100.
[0188] The y offset (Y.sub.1) for the superior facet prosthesis
1100 can range from 2 mm to 20 mm. However, for the majority of
patients, the y offset (Y.sub.1) will range from 5 mm to 15 mm.
Therefore a family of superior facet prostheses 1100 can be
provided with the y offset (Y.sub.1) varying in increments of 2 mm.
Thus, sets of superior facet prostheses 1100 would be provided with
y offset (Y.sub.1) at 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, and 15 mm to
provide superior facet prostheses 1100 that cover the statistical
range for the majority of the population of patients needing
superior facet prostheses 1100.
[0189] The x offset (X.sub.4) for the superior facet prosthesis
1100 can range from about 5 mm to about 25 mm. However, for the
majority of patients, X.sub.4 will range from about 8 mm to about
20 mm. A family of superior facet prostheses may be provided with
X.sub.4 values varying in increments of 2 mm. Thus, sets of
superior facet prostheses 1100 would be provided with X.sub.4
values of 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm to
provide superior facet prostheses 1100 that cover the statistical
range for the majority of the population of patients needing
superior facet prostheses 1100.
[0190] The y offset (Y.sub.4) for the superior facet prosthesis
1100 can range from about -5 to about 15 mm. However, for the
majority of patients, Y.sub.4 will range from about -2 mm to about
10 mm. A family of superior facet prostheses may be provided with
Y.sub.4 values varying in increments of 2 mm. Thus, sets of
superior facet prostheses 1100 would be provided with Y.sub.4
values of -2 mm, 0 mm, 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm to provide
superior facet prostheses 1100 that cover the statistical range for
the majority of the population of patients needing superior facet
prostheses 1100.
[0191] The facet angle (.beta.) for the superior facet prosthesis
1100 can range from 50.degree. to 120.degree.. However, for the
majority of patients, the facet angle (.beta.) will range from
60.degree. to 100.degree.. Therefore a family of superior facet
prostheses 1100 can be provided with the facet angle (.beta.)
varying in increments of 5.degree.. Thus, sets of superior facet
prostheses 1100 would be provided with the angle (.beta.) at
60.degree., 65.degree., 70.degree., 75.degree., 80.degree.,
85.degree., 90.degree., 95.degree., and 100.degree. to provide
superior facet prostheses 1100 that cover the statistical range for
the majority of the population of patients needing superior facet
prostheses 1100.
[0192] Once the surgeon assesses the anatomy of the superior facet
that is being replaced, a particular superior facet prosthesis 1100
is selected that has the curvature and overall angle of the
articulating surface 1122, with respect to the flange 1116 that
best fits the anatomy of the level of vertebra, the left or right
side, and the size of the patient's anatomy being replaced. Thus a
kit containing various sizes and shapes of superior facet
prostheses 1100 is provided to the surgeon and the surgeon selects
the superior facet prosthesis 1100 that best suits the
situation.
[0193] According to one example, such a kit may contain nine
prostheses, which may be dimensioned to provide a variety of
combinations of values for .alpha., X.sub.1, Y.sub.1, .beta.,
X.sub.4, Y.sub.4, and R, within the ranges listed above. If
desired, one or more of the above-listed variables may remain
constant over the entire kit. For example, R may have a constant
value, such as 11.5 mm, for all members of the kit.
[0194] The prostheses 1100, 1162, 1164, 1166, 1168 of the kit of
FIG. 53A are not simply scaled up or down, but are varied according
to a number of carefully selected parameters to cover the vast
majority of morphologies occurring in the L5 vertebra. In a similar
manner, a plurality of inferior facet prostheses adapted to replace
inferior facets can be provided either as a separate kit, or in
combination with the kit of FIG. 53A. Such a kit will be shown and
described in connection with FIG. 53D.
[0195] FIGS. 53B and 53C illustrate top and side views,
respectively, of an exemplary inferior facet prosthesis 1172. The
inferior facet prosthesis 1172 has an x offset (X), a y offset (Y),
and a z offset (Z), which are illustrated in FIGS. 53B and 53C. As
shown, the offsets X, Y, and Z run between a saddle point S1 of the
inferior facet prosthesis 1172 and a center point C1 of the
articulation surface 1174. The saddle point S1 of FIGS. 53B and 53C
is defined in a manner similar to that of the superior facet
prosthesis 1100 of FIG. 51.
[0196] As shown in FIGS. 53B and 53C, the inferior facet prosthesis
1172 has a semispherical coupling portion similar to the coupling
portion 1144 of the superior facet prosthesis 1100 introduced in
the description of FIG. 47. Accordingly, the inferior facet
prosthesis 1172 provides the same type of tri-axial pivotal motion
during installation as the coupling portion 1144, as described
previously. The coupling portion of the inferior facet prosthesis
1172 may also be nested in the coupling portion 1144 of the
superior facet prosthesis 1100, or vice versa, to enable
independent polyaxial adjustment of the prostheses 1100, 1170 when
positioned in engagement with a single semispherical resection
1146.
[0197] Referring to FIG. 53D, a perspective view illustrates a kit
of inferior facet prostheses 1180, 1182, 1184, 1186, 1188, 1190.
Again, the physical dimensions can vary between the various
inferior facet prostheses 1180 in the kit of FIG. 53D. These
dimensions may include an inferior resection angle (I.alpha.), an
inferior x offset (X), an inferior y offset (Y), an inferior facet
angle (I.beta.), an inferior facet articulation radius (IR), and an
inferior z offset (Z, from the center of fixation to the center of
the articulation radius).
[0198] The inferior resection angle I.alpha. is the angle of the
flat resection to be made in the vertebra, for example, the
vertebra 101 illustrated in some of the preceding drawings, to
serve as a backing for the articulating surface of the selected
inferior facet prosthesis 1180, 1182, 1184, 1186, 1188, or 1190.
When measured according to the coordinate system established for
the superior facet prosthesis 1100, as illustrated in FIG. 51, the
inferior resection angle I.alpha. may be approximately the same as
the facet angle .beta. for the superior prosthesis 1100 because the
articulation surfaces 1122, 1174 are to be positioned generally
parallel to each other. Due to the clearance between the
articulating surfaces 1122, 1174 and the generally concave and
convex shapes thereof, as long as the selected inferior facet
prosthesis 1180, 1182, 1184, 1186, 1188, or 1190 is placed so that
the articulating surfaces 1122, 1174 are generally parallel to each
other, proper articulation may be expected to occur.
[0199] Thus, the inferior facet prosthesis 1180, 1182, 1184, 1186,
1188, or 1190 may be dimensioned such that la is nearly the same as
.beta., and the orientation of the articulating surface 1174 may be
adjusted as needed to permit the inferior facet prosthesis 1180,
1182, 1184, 1186, 1188, or 1190 to be attached to the corresponding
vertebra 101. Accordingly, I.alpha. need not be determined based on
measurement of the vertebra 101, but may instead be inferred based
on the selection of the superior facet prosthesis 1100, 1162, 1164,
1166, or 1168 and adjusted during installation.
[0200] The inferior facet angle I.beta. may be defined as the angle
of the surface to which the articulating surface 1174 is most
nearly parallel. Due to the shape of the inferior facet prostheses
1180, 1182, 1184, 1186, 1188, or 1190, this angle is the same as
the inferior resection angle I.alpha., when measured according to
the coordinate system of the superior facet prosthesis 1100 of FIG.
51.
[0201] The inferior pedicle axis 1170 is the axis that is collinear
with the longitudinal axis of the fixation element 1110 that is
positioned through the pedicle 11 nearest to the resected inferior
facet (not shown). This axis is parallel with the direction of the
inferior y offset (Y). The direction of the inferior x offset (X)
is perpendicular to the direction of the inferior y offset (Y). The
direction of the inferior x offset (X) is generally lateral to
medial with respect to the central axis of the patient's spine. The
direction of the inferior y offset (Y) is generally anterior to
posterior. The direction of the inferior z offset (Z) is generally
cephalad to caudal.
[0202] The inferior x offset (X) for the inferior facet prosthesis
1180 can range from 0 mm to 20 mm. However, for the majority of
patients, the inferior x offset (X) will range from 2 mm to 16 mm.
Therefore a family of inferior facet prostheses 1180 can be
provided with the inferior x offset (X) varying in increments of 2
mm. Thus, sets of inferior facet prostheses 1180 would be provided
with inferior x offset (X) at 2 mm, 4 mm, 6 mm, 8 mm, 10 mm, 12 mm,
14 mm, and 16 mm to provide inferior facet prostheses 1180 that
cover the statistical range for the majority of the population of
patients needing inferior facet prostheses 1180.
[0203] The inferior y offset (Y) for the inferior facet prosthesis
1180 can range from -15 mm to 5 mm. However, for the majority of
patients, the inferior y offset (Y) will range from -12 mm to 4 mm.
Therefore a family of inferior facet prostheses 1180 can be
provided with the inferior y offset (Y) varying in increments of 2
mm. Thus, sets of inferior facet prostheses 1180 would be provided
with inferior y offset (Y) at -12 mm, -10 mm, -8 mm, -6 mm, -4 mm,
-2 mm, 0 mm, 2 mm, and 4 mm to provide inferior facet prostheses
1180 that cover the statistical range for the majority of the
population of patients needing inferior facet prostheses 1180.
[0204] The inferior facet articulation radius (IR) for the inferior
facet prosthesis 1180 can range from 5 mm to 30 mm. However, for
the majority of patients, the inferior facet articulation radius
(IR) will range from 10 mm to 15 mm. A family of incremented
inferior prostheses may be provided to cover the aforementioned
range. Alternatively, the inferior facet articulation radius (IR)
may be set at a given value, for example, 12 mm, and such a value
may be used in substantially all cases.
[0205] The inferior z offset (Z) for the inferior facet prosthesis
1180 can range from 20 mm to 40 mm. However, for the majority of
patients, the inferior z offset (Z) will range from 25 mm to 31 mm.
Therefore a family of inferior facet prostheses 1180 can be
provided with the inferior z offset (Z) varying in increments of 1
mm. Thus, sets of inferior facet prostheses 1180 would be provided
with inferior z offset (Z) at 25 mm, 26 mm, 27 mm, 28 mm, 29 mm, 30
mm, and 31 mm to provide inferior facet prostheses 1180 that cover
the statistical range for the majority of the population of
patients needing inferior facet prostheses 1180.
[0206] If desired, a kit having ten inferior facet prostheses may
be assembled. Like the prostheses 1100, 1162, 1164, 1166, 1168 of
the kit of FIG. 53A, the prostheses 1180, 1182, 1184, 1186, 1188,
1190 of FIG. 53D are not simply scaled up or down, but are varied
according to a number of carefully selected parameters to cover the
vast majority of morphologies occurring in the L4 vertebra and/or
other vertebrae.
[0207] The parameters of the prostheses 1100, 1162, 1164, 1166,
1168 of FIG. 53A and/or the prostheses 1180, 1182, 1184, 1186,
1188, 1190 of FIG. 53D may include at least two dimensions that
vary among the members of the kit independently of each other.
Dimensions that vary independently of each other need not change
according to any established relationship between the dimensions,
but instead, one may change while the other remains the same
between any two prostheses of the kit.
[0208] FIG. 53E is a perspective view illustrating how a superior
facet prosthesis 1100 and an inferior facet prosthesis 1180 fit
together. The surgeon selects an inferior facet prosthesis that, in
addition to most adequately meeting the anatomy of the patient, has
an articulating surface adapted for articulating with the
articulating surface of the superior facet prosthesis selected.
[0209] FIG. 53F is a dorsal view of a superior facet prosthesis
1100 and an inferior facet prosthesis 1204 attached to the L5 and
L4 lumbar vertebrae 102, 101. In FIG. 53F, the superior facet
prosthesis 1100 is attached to the left side of the L5 vertebra 102
and the inferior facet prosthesis 1204 is attached to the left L4
vertebra 101. The two prostheses 1100, 1204 are positioned on
respective bone resections and oriented such that they articulate
together through the range of motion naturally exhibited between
the L4 and L5 vertebrae 1100, 1204. This range of motion includes
flexion-extension, lateral left and right bending, torsion along a
sagittal axis and combinations and coupling of all these ranges of
motion.
[0210] FIG. 53F shows the prostheses 1100, 1204 and vertebrae 101,
102 in a natural position. The articulation surface 1174 of the
inferior prosthesis 1204 and the articulation surface 1122 of the
superior prosthesis 1100 are in contact in the neutral position.
However, the prostheses 1100, 1204 are shaped to allow anatomic
contact and articulation between the inferior facet articulation
surface 1174 and the superior facet articulation surface 1122
throughout various anatomic ranges of motion.
[0211] Also shown in FIG. 53F are two planes labeled "Plane 1" and
"Plane 2" that that intersect along an axis (not shown) that passes
through the contact areas of the superior facet articulation
surface 1122 and the inferior facet articulation surface 1174.
Plane 1 is parallel to the page of FIG. 53F, and Plane 2 is
perpendicular to the page.
[0212] FIG. 53G is a posteriolaterial view of the same inferior and
superior facet prostheses 1100, 1204 with the planes shown in FIG.
53F. In FIG. 53G, Plane 2 is oriented parallel to the page and
plane 1 is oriented perpendicular to the page. FIG. 53G illustrates
the saddle point (S1) of the vertebra 102 to which the superior
facet prosthesis 1100 is coupled, and the saddle point (S2) of the
vertebra 101 to which the inferior facet prosthesis 1204 is
coupled. The saddle points S1, S2 are displaced from each other
along an x offset (IX) parallel to the axis at which Plane 1 and
Plane 2 intersect, a y offset (IY) extending perpendicular to Plane
2, or out of the page with respect to FIG. 53G, and a z offset (IZ)
extending perpendicular to Plane 1. The offsets IX, IY, and IZ may
be used for implant sizing and/or selection, as will be discussed
subsequently.
[0213] FIG. 53H is a posteriolateral view showing a cross-section
along Plane 2. This cross-section view cuts through the
articulation surfaces 1122, 1174 of the prostheses 1100, 1204,
thereby showing the convex shape of the inferior articulation
surface 1174 against the concave shape of the superior articulation
surface 1122.
[0214] FIG. 53H also illustrates the cephalad and caudal ends 1250,
1252 of the articulation surface 1122 of the superior facet
prosthesis 1100. The articulation surface 1122 has a radius of
curvature 1254 generally about an axis 1256. However, since the
radius of curvature 1254 changes along the articulating surface
1122, the axis 1256 may be the center of curvature for only a
portion of the articulation surface 1122. The radius of curvature
1254 is shown extending from the axis 1256 to the articulation
surface 1122 in FIG. 53H. Furthermore, FIG. 53H illustrates a
longitudinal axis 1258 of the spine in general. The axis 1256 is
angled from the axis 1258 by an offset angle 1259. Since the axis
1256 and the axis 1258 may not both be precisely parallel to Plane
2, the offset angle 1259 may have a component that extends out of
the page with respect to the view of FIG. 53H.
[0215] FIG. 53I is a cephalad view showing a cross-section along
Plane 1. This cross-section cuts through the articulation surfaces
1122, 1174 of the prostheses showing the convex shape of the
inferior articulation surface 1174 against the concave shape of the
superior articulation surface 1122. Each of the articulating
surfaces 1122, 1174 has a curved shape. The articulating surfaces
1122, 1174 of the superior and inferior prostheses 1100, 1204,
respectively, are shaped and relatively positioned to articulate
against each other such that a medial-lateral range of relative
motion between the first and second vertebrae 101, 102 increases
significantly with flexion (i.e., forward bending) of the
spine.
[0216] A "significant" increase in the medial-lateral range of
motion refers to a difference in the range of motion that
approximates the natural motion of the spine to a degree sufficient
to be noticeable by the patient. More precisely, a "significant"
increase may refer to the existence of at least one additional
millimeter of clearance between articulating surfaces of a facet
joint under flexion, as compared to the same facet joint under
extension. Furthermore, a "significant" increase in the
medial-lateral range of motion may refer to the existence of two
additional millimeters of clearance between the articulating
surfaces.
[0217] As shown in FIG. 53H, one of the articulating surfaces 1122,
1174, for example, the articulating surface 1122 of the superior
facet prosthesis 1100, has a cephalad end 1250 and a caudal end
1252. The articulating surface 1122 also has a radius of curvature
1254 about an axis 1256 extending generally from the cephalad end
1250 end to the caudal end 1252. The radius of curvature 1254
changes along the axis 1256 to provide greater clearance between
the articulating surfaces 1122, 1174 when the spine is under
flexion. Similarly, the changing radius of curvature 1254 provides
less clearance between the articulating surfaces 1122, 1174 when
the spine is extended.
[0218] In this embodiment, the articulating surface 1122 is shaped
such that, when the superior facet prosthesis 1100 is coupled to
the vertebra, the axis 1256 is significantly anteriorly inclined at
the cephalad end 1250 to provide greater clearance between the
articulating surfaces 1122, 1174 when the spine is under flexion.
In addition to or in the alternative to variation of the radius of
curvature 1254 from the cephalad end 1250 to the caudal end 1252,
the radius of curvature 1254 could vary along a medial-lateral
direction of the articulating surface.
[0219] More precisely, with brief reference to FIG. 51 again, the
radius of curvature may be larger toward a medial end 1260 and a
lateral end 1262 of the articulating surface 1122 than at a central
portion 1264 thereof. The radius of curvature could also be
substantially infinite toward the medial and lateral ends, such
that the articulating surface of the superior prosthesis has a
curved region 1268 proximate the central portion 1264, a first
tangent flat 1270 disposed medially of and tangent to the curved
region 1268, and a second tangent flat 1272 disposed laterally of
and tangent to the curved region 1268.
[0220] If desired, the inferior facet prosthesis may have an
articulating surface with a three-dimensionally curved, generally
elliptical shape. A three-dimensionally curved, generally
elliptical shape may have the appearance of a stretched spheroid or
the like. Accordingly, a three-dimensionally curved, generally
elliptical shape has a first cross section having a generally
elliptical shape and a second cross section perpendicular to the
first cross section, having a semicircular shape. Alternatively, an
inferior facet prosthesis may have an articulating surface with a
generally cylindrical or semispherical shape, as illustrated in
connection with FIGS. 40, 53B, and 53C, for example.
[0221] According to one alternative embodiment, the articulating
surface of the superior facet prosthesis may have a uniform,
substantially unchanging radius of curvature. The relative
medial-lateral motion between the vertebra and the adjacent
vertebra may still increase significantly with flexion of the spine
due to the curvature of the inferior facet prosthesis. The radius
of curvature of the articulating surface of the inferior facet
prosthesis may change along an axis thereof, either along the
cephalad-caudal direction or along the medial-lateral direction, to
provide greater clearance between the articulating surfaces when
the spine is under flexion. According to yet another alternative,
the variation in motion in the medial-lateral direction may be
obtained, not through a variable radius of curvature, but rather,
through the relative positioning of the superior and inferior facet
prostheses.
[0222] Returning to FIGS. 53F, 53G, 53H, and 53I, the materials
used to construct the articulating surfaces of the prostheses 1100,
1174 may be selected from a group consisting of a polymeric
material, a polymeric bearing material attached to a metal
substrate, a ceramic bearing material, a metal bearing material,
and combinations thereof. A variety of surface coatings,
treatments, and the like may be used to enhance the performance
and/or longevity of the prostheses 1100, 1174.
[0223] The superior facet prosthesis 1100 may be shaped such that,
when the superior facet prosthesis 1100 is coupled to the vertebra
102, the axis 1256 is significantly anteriorly inclined from a
longitudinal axis (not shown) of the spine to provide greater
clearance between the articulating surfaces 1122, 1174 when the
spine is under flexion. In this application, "significantly
anteriorly inclined" refers to the presence of a deliberate offset,
from the longitudinal axis of the spine, that has a meaningful
effect on the facet joint of which the corresponding prosthesis is
a part. The offset angle 1259 between the axis 1256 and the
longitudinal axis 1258 of the spine may range from about
-2.5.degree. to about 14.5.degree.. More precisely, the offset
angle 1259 may range from about 5.degree. to about 10.degree.. Yet
more precisely, the offset angle 1259 may be about
7.25.degree..
[0224] Referring briefly again to FIG. 51 and FIG. 53G, one method
of selecting inferior and superior facet prosthesis will be
described. The appropriate prosthesis of the kit of superior facet
prostheses may be selected by, for example, forming a semicircular
resection centered at a position along the pedicle axis 1170 of the
vertebra 102, at a known displacement from the saddle point S1.
Certain offsets, such as X.sub.1 and X.sub.2, as shown in FIG. 51,
may be measured with between the saddle point S1 and the most
medial and anterior point P1.
[0225] Based on X.sub.1 and X.sub.2, values of the resection angle
.alpha. and the facet angle .beta. may be obtained. The values of
.alpha. and .beta. may be used to select the appropriate superior
facet prosthesis of the kit by, for example, looking up the values
of .alpha. and .beta. on a lookup table or the like. The remaining
dimensions of the selected superior facet prosthesis may thus be
determined based on the combination of .alpha. and .beta..
[0226] The appropriate prosthesis of the kit of inferior facet
prostheses may also be selected by making a limited number of
measurements. More precisely, a semicircular resection may be
formed at a position centered along the pedicle axis of the
vertebra 101, at a known displacement from the saddle point S2. One
or more of the offsets IX, IY, and IZ may be measured between the
resections of the saddle points S1 and S2.
[0227] Based on the values of IX, IY, and/or IZ obtained, the
values of I.alpha. and Z (as illustrated in FIG. 53C) are
determined. The values of I.alpha. and Z may be used to select the
appropriate inferior facet prosthesis of the kit by, for example,
looking up the values of I.alpha. and Z on a lookup table or the
like. The remaining dimensions of the selected inferior facet
prosthesis may thus be determined based on the combination of
I.alpha. and Z.
[0228] The above-described selection method is beneficial because a
relatively small number of linear measurements may be made to
determine which set of prostheses is most appropriate for a given
patient. Ease of measurement is important because the measurements
must generally be performed during surgery. Accordingly, easier,
more rapid measurements enable surgery to be more rapidly and
safely carried out. In alternative embodiments, different
measurement schemes may be carried out, and may include different
linear measurements, angular measurements, and the like. In this
application, measuring the "relative positions" of bony landmarks
may include measurement of linear displacements, angular
displacements, or any combination thereof.
[0229] In alternative embodiments, a kit of superior and/or
inferior prosthesis need not have multiple one-piece prostheses,
but may instead have multiple components designed to be assembled
together to provide a prosthesis having the necessary parameters.
For example, each of a plurality of semispherical bone contacting
portions may be connectable to any of a plurality of articulating
surfaces, via a plurality of connecting members. Selecting a
prosthesis may then entail selecting a bone contacting portion, an
articulating surface, and a connecting member. The bone contacting
portion, articulating surface, and connecting member may then be
coupled together via set screws, adhesives, interference fits, or
the like.
[0230] If desired, the manner in which the various components are
attached together may also be adjustable to enable further
adjustability of the dimensions of a selected prosthesis. Such a
kit of components may also include additional components such as
bearing surfaces, as described in connection with FIG. 16. As yet
another alternative, a single prosthesis may be adjustably
deformed, for example, through the use of a lever-operated manual
press, a hydraulic press, or the like, to provide the desired
dimensions prior to attachment to a patient's vertebra.
[0231] After a semispherical resection 1146 has been formed in a
vertebra and the corresponding prosthesis has been selected, a flat
resection, such as the first resection surface 1112 of FIG. 48, may
be formed. The flat resection may be contiguous with the
semispherical resection 1146, or may be separated from the
semispherical resection 1146 by an expanse of unresected bone. The
determination of which prosthesis to use may also indicate to the
surgeon the proper placement of the flat resection to properly
receive the selected prosthesis. After the flat resection has been
formed, the selected prosthesis may be attached to the vertebra.
The procedure may be the same as or similar to that described above
for installation of the inferior and superior facet prostheses.
[0232] FIG. 54 is a dorsal view of a bilateral inferior facet
prosthesis system 1200 in situ. The bilateral inferior facet
prosthesis system 1200 is a multi-piece inferior and superior facet
prosthesis that has both a right inferior facet prosthesis 1202 and
a left inferior facet prosthesis 1204 connected by a crosslink,
which may take the form of a stabilizing bar 1210. Both the right
inferior facet prosthesis 1202 and the left inferior facet
prosthesis 1204 are designed to be affixed to the top vertebra 101
at the respective inferior facet resection surfaces 121 (FIG.
19).
[0233] The bilateral inferior facet prostheses 1202, 1204 allow
replacement of both the left and the right inferior facets. In this
embodiment, the inferior prostheses are placed over left and right
fixation elements 1232, 1234 that extend into the top vertebra 101.
In the embodiment shown in FIG. 54, the right inferior side is
articulating against a right superior facet prosthesis 1100
attached to the first resection surface 1112 (FIG. 49) of the
bottom vertebra 102. Also in this embodiment, the left inferior
facet prosthesis 1204 is articulating against the left natural
superior facet of the bottom vertebra 102.
[0234] The stabilizing bar 1210 of the bilateral inferior
prosthesis system 1200 is designed to stabilize the left inferior
facet prosthesis 1204 and the right inferior facet prosthesis 1202
so that they are secure. The stabilizing bar 1210 also allows the
left and right inferior facet prostheses 1204, 1202 to support each
other rather than requiring stabilizing members to be coupled to
the spine lamina or the resected inferior facet tissue. Further,
the stabilizing bar 1210 can compress the left and right inferior
facet prostheses 1202, 1204 against the resected bone to improve
bony ingrowth and apposition.
[0235] As also shown in FIG. 54, the stabilizing bar 1210 is
coupled to the left and right inferior prostheses 1202, 1204 by a
gripping mechanism. The gripping mechanism may include any of a
variety of structures, including clips, clamps, adhesive-bonds,
threaded fasteners, and the like. In the embodiment of FIG. 54, the
gripping mechanism includes fore and aft flanges 1212, 1214 that
engage the stabilizing bar 1210 to form a groove-and-rod joint. The
fore and aft flanges 1212, 1214 are compressed together with
threaded turnbuckles 1216, 1218 to pinch the stabilizing bar 1210
there between.
[0236] The pinching action of the flanges 1212, 1214 allows the
distance between the left and right inferior prostheses 1202, 1204
to be adjusted to best suit the anatomy of the patient. During
surgery, the surgeon would use a tool (not shown) to compress the
left and right inferior prostheses 1202, 1204 to the desired
positions and then tighten the turnbuckles 1216, 1218 to secure the
stabilizing bar 1210.
[0237] FIG. 55 is a perspective view of the bilateral inferior
facet prosthesis system 1200. The right inferior facet prosthesis
1202 includes a convex articulating surface 1220 that engages an
articulating surface 1122 of the superior facet prosthesis 1100. In
one embodiment, the articulating surface 1122 of the superior facet
prosthesis 1100 has a concave shape (FIGS. 47, 51).
[0238] In this application, the term "convex" relates to a surface
that bulges outward with a three-dimensional curvature.
Accordingly, a convex surface is not just a sectorial portion of a
cylinder, but rather, has some outward curvature along two
perpendicular directions. A convex surface may be "semispherical,"
or in other words, may include some sectorial portion of a sphere,
which may be less than a hemisphere. However, a convex surface need
not be semispherical, but may instead have contouring that provides
a portion of an oval, elliptical, parabolic, and/or irregular cross
sectional shape. A convex surface also need not be curved in whole
or in part, but may instead have one or more planar portions.
[0239] In this application, "concave" refers to a surface with a
central portion that is recessed with respect to at least two
peripheral portions positioned on either side of the central
portion. A concave surface may be formed by planar regions, curves,
or combinations thereof. The central portion may be recessed along
only one dimension, as with a surface defined by an interior
section of a cylindrical wall. Alternatively, the central portion
may be recessed along two perpendicular dimensions, so that the
central portion is recessed with respect to at least four
peripheral portions arranged around the central portion.
Accordingly, the surface may include a semispherical section, a
three-dimensional parabolic or ellipsoidal section, or any other
three-dimensionally curved shape.
[0240] As another alternative, the central portion of a concave
surface may be recessed along one direction and distended with
respect to a perpendicular direction, so that the concave surface
takes on a shape similar to that of the rounded groove of a pulley
that is designed to receive a rope. Like a convex surface, a
concave surface need not be curved in whole or in part, but may
instead have one or more planar portions.
[0241] FIG. 56 is a lateral view of the bilateral inferior facet
prosthesis system 1200 and superior facet prosthesis 1100. The
right inferior prosthesis 1202 includes a member 1230 upon which
the flanges 1212, 1214 clamp. In the embodiment shown, the member
1230 is a ball-shaped member 1230 upon which the flanges 1212, 1214
clamp to form a ball-and-socket joint. The ball-and-socket joint
and groove-and-rod joint provide multiple degrees of freedom for
variable positioning of the left and right inferior prostheses
1202, 1204. More precisely, the ball-and socket joint enables
tri-axial rotation, i.e., rotation about three perpendicular axes,
until the flanges 1212, 1214 are pressed about the member 1230 to
resist further relative rotation.
[0242] The ball-and-socket joint enables relative motion between
the inferior prostheses 1202, 1204 along the anterior/posterior
directions and along the cephalad/caudal directions. The
groove-and-rod joint enables relative motion between the inferior
prostheses 1202, 1204 along the lateral/medial directions. However,
when the turnbuckles 1216, 1218 are tightened, the displacement
between the ball-shaped members 1230 of the inferior prostheses
1202, 1204 becomes fixed, and the ball-shaped members 1230 are no
longer freely pivotable with respect to the flanges 1212, 1214.
Thus, the relative positions and orientations of the inferior
prostheses 1202, 1204 may be fixed by tightening the turnbuckles
1216, 1218.
[0243] An alternative embodiment replaces the ball shaped member
1230 with a member (not shown) of differing shape and flanges
adapted to engage the alternative member. Other potential shapes
that allow a range of adjustability and movement between the left
and right inferior prostheses 1202, 1204 and the flanges 1212, 1214
prior to clamping include, but are not limited to, columnar and
annular shapes.
[0244] The ball-shaped member 1230 shown in FIG. 56 has several
divots formed thereon. Upon compression of the flanges 1212, 1214,
the flanges deform into the divots to provide enhanced coupling and
resistance to slippage therebetween. An alternative embodiment of
the ball-shaped member 1230 has circumferential or axial splines
(FIG. 63) formed thereon, which "bite" into the flanges 1212, 1214.
Other alternative surface features of the ball-shaped member 1230
include knurling, nubs, grooves, facets, and combinations of any of
the above.
[0245] Similarly, the stabilizing bar 1210 can have surface
features to enhance coupling to the flanges 1212, 1214. Exemplary
surface features include longitudinal splines, knurling, divots,
nubs, and grooves. Splines prevent rotation of the stabilizing bar
1210 with respect to the flanges 1212, 1214. Knurling
advantageously prevents both rotation and translation of the
stabilizing bar 1210 with respect to the flanges 1212, 1214.
[0246] The flanges 1212, 1214 can be formed of a material softer
than that of the stabilizing bar 1210 and the ball-shaped members
1230 to further enhance coupling. Illustrative materials for the
stabilizing bar 1210 and ball-shaped members 1230 are Cobalt-Chrome
(Co--Cr) alloys, Titanium (Ti) and stainless steel alloys. However,
other biocompatible materials such as rigid polymers including PEEK
and PEAK can be formed into the shapes of the stabilization bar
1210, and/or the ball-shaped members 1230. In one alternative
embodiment, the flanges 1212, 1214 are formed integrally with or
rigidly attached to the left and right inferior prostheses 1202,
1204.
[0247] Referring again to FIG. 54, ends of the flanges 1212, 1214
that engage the stabilizing bar 1210 are angled towards each other.
This angling avoids interference with surrounding bone and avoids
interference with the superior facet or the superior facet
prosthesis 1100.
[0248] With continued reference to FIG. 54, it is seen that the
heads of the turnbuckles 1216, 1218 can vary in size. As shown, the
turnbuckle 1216 is larger than the turnbuckle 1218. The larger head
of the turnbuckle 1216 allows the surgeon to exert more torque on
the turnbuckle 1216, thereby allowing a more secure coupling of the
flanges 1212, 1214 to the stabilizing bar 1210. The smaller head of
the turnbuckle 1218 requires less space at the surgical site of the
patient than the larger head of the turnbuckle 1216. Therefore, the
surgeon can select a turnbuckle head having the desired size,
weighing the benefits of more applied torque of the larger head
with the reduced spatial requirements of the smaller head.
[0249] An alternative embodiment replaces the stabilizing bar 1210
with a flexible link, such as a cable of a biocompatible material.
Yet another alternative embodiment includes a stabilizing bar
having threaded ends. Instead of pinching flanges, the threaded
ends of the stabilizing bar extend through flanges of the left and
right inferior prostheses 1202, 1204. Threaded fasteners engage the
threaded ends of the stabilizing bar. The threaded fasteners are
then tightened to provide the desired positioning of the left and
right inferior prostheses 1202, 1204. In another variation, the
stabilizing bar is rotated such that the threads of the stabilizing
bar engage fixed threaded portions of the flanges.
[0250] FIG. 57 is a cranial view of the bilateral inferior facet
prosthesis system 1200.
[0251] FIG. 58 is a bottom in situ view of the bilateral inferior
facet prosthesis system 1200 in situ.
[0252] FIG. 59 is rear view of the bilateral inferior facet
prosthesis system 1200 in isolation.
[0253] FIG. 60 is a top view of the bilateral inferior facet
prosthesis system 1200.
[0254] FIG. 61 is a bottom view of the bilateral inferior facet
prosthesis system 1200.
[0255] FIG. 62 is a perspective view of the right inferior
prosthesis 1204.
[0256] FIGS. 63 and 64 are perspective and end views, respectively,
of various ball-shaped members 1240, 1242, 1244 that may be
included in the inferior prostheses 1202, 1204 in place of the
members 1230, the ball-shaped members 1240, 1242, 1244 having
differing surface features, particularly circumferential grooves
1302, longitudinal grooves 1304, and knurling 1306.
[0257] FIG. 65 is a dorsal view of the bilateral inferior facet
prosthesis system 1200, in which castle nuts 1320 are attached to
the left and right fixation elements 1232, 1234 and to the fixation
member 1110.
[0258] While various embodiments have been described above, it
should be understood that they have been presented by way of
example only, and not by way of limitation. Thus, the breadth and
scope of the invention should not be limited by any of the
above-described exemplary embodiments, but should be defined only
in accordance with the following claims and their equivalents.
* * * * *