U.S. patent application number 12/435087 was filed with the patent office on 2009-11-05 for rail-based modular disc prosthesis.
Invention is credited to Stephen H. Crosbie, Jeffrey C. Felt, Mark A. Rydell.
Application Number | 20090276047 12/435087 |
Document ID | / |
Family ID | 46062833 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090276047 |
Kind Code |
A1 |
Felt; Jeffrey C. ; et
al. |
November 5, 2009 |
RAIL-BASED MODULAR DISC PROSTHESIS
Abstract
A method and apparatus for repairing a damaged intervertebral
disc nucleus in a minimally invasive manner utilizes a modular disc
prosthesis preferably comprised of at least three modular segments
and at least two rails that operably connect adjacent modular
segments. In one embodiment, each modular segment includes a harder
inner portion and a softer outer portion. Preferably, the rails
operate to slidably connect and interlock adjacent modular
segments. A stem portion of the rails that extends outside the
periphery of the body of the prosthesis is removable after
implantation such that the modular segments form an implanted
unitary device that closely mimics the geometry of the disc nucleus
cavity.
Inventors: |
Felt; Jeffrey C.;
(Minnetonka, MN) ; Rydell; Mark A.; (Minnetonka,
MN) ; Crosbie; Stephen H.; (Minnetonka, MN) |
Correspondence
Address: |
PATTERSON, THUENTE, SKAAR & CHRISTENSEN, P.A.
4800 IDS CENTER, 80 SOUTH 8TH STREET
MINNEAPOLIS
MN
55402-2100
US
|
Family ID: |
46062833 |
Appl. No.: |
12/435087 |
Filed: |
May 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11372477 |
Mar 9, 2006 |
7591853 |
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12435087 |
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60685332 |
May 24, 2005 |
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60700459 |
Jul 19, 2005 |
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60660107 |
Mar 9, 2005 |
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Current U.S.
Class: |
623/17.11 |
Current CPC
Class: |
A61F 2002/30016
20130101; A61F 2002/30594 20130101; A61F 2002/30462 20130101; A61F
2002/30235 20130101; A61F 2002/30563 20130101; A61F 2002/30604
20130101; A61F 2250/0018 20130101; A61F 2230/0069 20130101; A61F
2002/30561 20130101; A61F 2210/0061 20130101; A61F 2002/30075
20130101; A61F 2002/30166 20130101; A61F 2230/0028 20130101; A61F
2250/0019 20130101; A61F 2002/444 20130101; A61F 2/4611 20130101;
A61F 2002/4627 20130101; A61F 2002/30677 20130101; A61F 2/4455
20130101; A61F 2002/30971 20130101; A61F 2002/30014 20130101; A61F
2002/30387 20130101; A61F 2220/0075 20130101; A61F 2002/3052
20130101; A61F 2/442 20130101; A61F 2/3094 20130101; A61F
2002/30383 20130101; A61F 2220/0025 20130101 |
Class at
Publication: |
623/17.11 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Claims
1. a modular disc prosthesis that is adapted to be implanted in an
evacuated disc nucleus space, the prosthesis having an expanded
position before implantation and an implanted position after
implantation, the prosthesis comprising: a plurality of modular
segments that, when in the implanted position, are arranged side by
side from a first end of the prosthesis to a second end of the
prosthesis, the plurality of modular segments being configured for
assembly in sequence from the first end to the second end of the
prosthesis and cooperating with each other when in the implanted
position to define a unitary body having a generally continuous
periphery that generally corresponds to the evacuated nucleus disc
space, a first of the plurality of modular segments being integral
with a first rail, a second of the plurality of modular segments
being integral with a second rail, the second modular segment being
slidably connected to the first rail, and a third of the plurality
of modular segments being slidably connected to the second rail,
wherein each of the first and second rails include an extended
portion that extends beyond the generally continuous periphery of
the unitary body of the implanted position, the extended portion of
the first rail being selectively removable from the first of the
plurality of modular segments once the second of the plurality of
modular segments is in the implanted position, and the extended
portion of the second rail being selectively removable from the
second of the plurality of modular segments once the third of the
plurality of modular segments is in the implanted position, wherein
both the second and third modular segments are separated from each
other and slidably connected to the extended portions of the first
and second rails respectively when the prosthesis is in the
expanded position to guide the second and third modular segments
respectively to the implanted position.
2. The modular disc prosthesis of claim 1, wherein a third rail is
integral with the third modular segment, the third rail including
an extended portion that extends beyond the generally continuous
periphery, wherein the modular disc prosthesis further comprises a
fourth modular segment slidably connected to the extended portions
of the third rail when the prosthesis is in the expanded position
to guide the fourth modular segment to the implanted position, the
extended portion of the third rail being selectively removable from
the third of the plurality of modular segments once the fourth of
the plurality of modular segments is in the implanted position.
3. The modular disc prosthesis of claim 1, wherein the extended
portions of the first and second rails each have a rigidity in a
first transverse direction relative to the longitudinal axis of the
respective rail that differs substantially from a rigidity in a
second transverse direction relative to the longitudinal axis of
the respective rail.
4. The modular disc prosthesis of claim 1, wherein each of the
second and third modular segments comprise an inner portion and an
outer portion, each outer portion being operatively coupled with a
portion of the respective inner portion, the inner portions of the
second and third modular segments being slidably engaged with the
first and second rails, respectively.
5. The modular disc prosthesis of claim 4, wherein the inner
portion of the second and third modular segments and the outer
portion of the second and third modular segments are made of
polymers of different durometers.
6. The modular disc prosthesis of claim 1, wherein the first,
second and third modular segments each comprise a moldable polymer
material and the extended portion of each rail comprises an
extrudable polymer material.
7. A modular disc prosthesis that is adapted to be implanted in an
evacuated disc nucleus space, the prosthesis having an expanded
position before implantation and an implanted position after
implantation, the prosthesis comprising: a plurality of modular
segments that, when in the implanted position, are arranged side by
side in generally one of a medial-to-lateral orientation and a
posterior-to-anterior orientation, the plurality of modular
segments being configured for assembly in sequence from a first end
to a second end of the prosthesis along the orientation, a first of
the plurality of modular segments being operatively coupled with
first means for slidably guiding a second modular segment along a
first longitudinal axis when the second modular segment is
manipulated from the expanded position to the implanted position,
the second modular segment being slidably connected to the first
means for slidably guiding the second modular segment along the
first longitudinal axis, the second modular segment being
operatively coupled with second means for slidably guiding a third
modular segment along a second longitudinal axis when the third
modular segment is manipulated from the expanded position to the
implanted position, the third modular segment being slidably
connected to the second means for slidably guiding the third
modular segment along the second longitudinal axis, wherein both
the second and third modular segments are separated from each other
and are slidably connected to the first and second means for
slidably guiding, respectively, when in the expanded position, the
modular segments being in contact with each other when in the
implanted position and defining a unitary body having a generally
continuous periphery that generally corresponds to the evacuated
nucleus disc space, the first means for slidably guiding the second
modular segment extending beyond the generally continuous periphery
and along the first longitudinal axis when the first modular
segment is in the implanted position, the first means for slidably
guiding the second modular segment being selectively detachable
from the first modular segment once the second modular segment is
in the implanted position, the second means for slidably guiding
the third modular segment extending beyond the generally continuous
periphery and along the second longitudinal axis when the second
modular segment is in the implanted position, the second means for
slidably guiding the third modular segment being selectively
detachable from the second modular segment once the third modular
segment is in the implanted position.
8. The modular disc prosthesis of claim 1, wherein the first and
second rails are substantially parallel.
9. The modular disc prosthesis of claim 7 wherein the third modular
segment is operatively coupled with third means for slidably
guiding a fourth modular segment along a third longitudinal axis
when the fourth modular segment is manipulated from the expanded
position to the implanted position, the fourth modular segment
being slidably connected to the third means for slidably guiding
the fourth modular segment along the third longitudinal axis, the
third means for slidably guiding the fourth modular segment
extending beyond the generally continuous periphery and along the
third longitudinal axis when the third modular segment is in the
implanted position, the third means for slidably guiding the fourth
modular segment being selectively removable once the third modular
segment is in the implanted position.
10. A modular disc prosthesis that is adapted to be implanted in an
evacuated disc nucleus space, the prosthesis having an expanded
position before implantation and an implanted position after
implantation, the prosthesis comprising: a plurality of modular
segments that cooperate with each other when in the implanted
position to define a unitary body having a generally continuous
periphery that generally corresponds to the evacuated nucleus disc
space, a first of the plurality of modular segments being integral
with a first rail, the first rail including an extended portion
that extends beyond the generally continuous periphery of the
unitary body of the implanted position, the extended portion of the
first rail having a length, a second of the plurality of modular
segments slidably connected to the first rail and being integral
with a second rail, the second rail including an extended portion
that extends beyond the generally continuous periphery of the
unitary body of the implanted position, and a third of the
plurality of modular segments being slidably connected to the
second rail, wherein both the second and third modular segments are
separated from each other and slidably connected to the extended
portions of the first and second rails respectively when the
prosthesis is in the expanded position to guide the second and
third modular segments respectively to the implanted position, and
wherein the third segment is capable of being separated from the
first segment by a distance that is greater than the length of the
first rail when in the expanded position.
11. The modular disc prosthesis of claim 10, wherein a third rail
is integral with the third modular segment, the third rail
including an extended portion that extends beyond the generally
continuous periphery, wherein the modular disc prosthesis further
comprises a fourth modular segment slidably connected to the
extended portions of the third rail when the prosthesis is in the
expanded position to guide the fourth modular segment to the
implanted position, the extended portion of the third rail being
selectively removable from the third of the plurality of modular
segments once the fourth of the plurality of modular segments is in
the implanted position.
12. The modular disc prosthesis of claim 10, wherein the extended
portions of the first and second rails each have a rigidity in a
first transverse direction relative to the longitudinal axis of the
respective rail that differs substantially from a rigidity in a
second transverse direction relative to the longitudinal axis of
the respective rail.
13. The modular disc prosthesis of claim 10, wherein each of the
second and third modular segments comprise an inner portion and an
outer portion, each outer portion being operatively coupled with a
portion of the respective inner portion, the inner portions of the
second and third modular segments being slidably engaged with the
first and second rails, respectively.
14. The modular disc prosthesis of claim 13, wherein the inner
portion of the second and third modular segments and the outer
portion of the second and third modular segments are made of
polymers of different durometers.
15. The modular disc prosthesis of claim 10, wherein the first,
second and third modular segments each comprise a moldable polymer
material and the extended portion of each rail comprises an
extrudable polymer material.
16. The modular disc prosthesis of claim 10, wherein the first and
second rails are substantially parallel.
17. The modular disc prosthesis of claim 10, wherein the plurality
of modular segments are arranged side by side from a first end of
the prosthesis to a second end of the prosthesis and are configured
for assembly in sequence from the first end to the second end of
the prosthesis.
18. The modular disc prosthesis of claim 10, wherein the extended
portion of the first rail is selectively removable from the first
of the plurality of modular segments once the second of the
plurality of modular segments is in the implanted position.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 11/372,477, filed Mar. 9, 2006, which also claims the benefit
of U.S. Provisional Patent Application No. 60/685,332, filed May
24, 2005, U.S. Provisional Patent Application No. 60/700,459, filed
Jul. 19, 2005, and U.S. Provisional Patent Application No.
60/660,107, filed Mar. 29, 2005, the disclosures of which are
hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to an implantable
prosthesis for repairing damaged intervertebral discs. More
particularly, the present invention relates to a rail-based modular
disc prosthesis of predetermined size and shape.
BACKGROUND OF THE INVENTION
[0003] The spinal motion segment consists of a unit of spinal
anatomy bounded by two vertebral bodies, including the two
vertebral bodies, the interposed intervertebral disc, as well as
the attached ligaments, muscles, and the facet joints. The disc
consists of the end plates at the top and bottom of the vertebral
bones, the soft inner core, called the nucleus and the annulus
fibrosis running circumferentially around the nucleus. In normal
discs, the nucleus cushions applied loads, thus protecting the
other elements of the spinal motion segment. A normal disc responds
to compression forces by bulging outward against the vertebral end
plates and the annulus fibrosis. The annulus consists of collagen
fibers and a smaller amount of elastic fibers, both of which are
effective in resisting tension forces. However, the annulus is not
very effective in withstanding compression and shear forces.
[0004] As people age the intervertebral discs often degenerate.
This degeneration of the intervertebral discs may lead to
degenerative disc disease. Degenerative disc disease of the spine
is one of the most common conditions causing pain and disability in
our population. When a disc degenerates, the nucleus dehydrates.
When a nucleus dehydrates, its ability to act as a cushion is
reduced. Because the dehydrated nucleus is no longer able to bear
loads, the loads are transferred to the annulus and to the facet
joints. The annulus and facet joints are not capable of
withstanding the applied compression and torsional loads, and as
such, they gradually deteriorate. As the annulus and facet joints
deteriorate, many other effects ensue, including the narrowing of
the interspace, bony spur formation, fragmentation of the annulus,
fracture and deterioration of the cartilaginous end plates, and
deterioration of the cartilage of the facet joints. The annulus and
facet joints lose their structural stability and subtle but
pathologic motions occur between the spinal bones.
[0005] As the annulus loses stability it tends to bow out and may
develop a tear allowing nuclear material to extrude. Breakdown
products of the disc and facet joint, including macroscopic chunks,
microscopic particles, and noxious biochemical substances build up.
These breakdown products stimulate sensitive nerve endings in and
around the disc, producing low back pain and sometimes, sciatica.
Affected individuals experience muscle spasms, reduced flexibility
of the low back, and pain when ordinary movements of the trunk are
attempted.
[0006] Degenerative disc disease is irreversible. In some cases,
the body will eventually stiffen the joints of the motion segment,
effectively re-stabilizing the discs. Even in the cases where
re-stabilization occurs, the process can take many years and
patients often continue to experience disabling pain. Extended
painful episodes of longer than three months often leads patients
to seek a surgical solution for their pain.
[0007] Several methods have been devised to attempt to stabilize
the spinal motion segment. Some of these methods include: heating
the annular region to destroy nerve endings and strengthen the
annulus; applying rigid or semi-rigid support members on the sides
of the motion segment or within the disc space; removing and
replacing the entire disc with a non-flexible, articulating
artificial device; removing and replacing the nucleus; and spinal
fusion involving permanently fusing the vertebra adjacent the
affected disc.
[0008] Until recently, spinal fusion has generally been regarded as
the most widely used treatment to alleviate back pain due to
degenerative disc disease. While this treatment is effective at
relieving back pain, all discal motion is lost in the fused spinal
motion segment. The loss of motion in the affected spinal segment
necessarily limits the overall spinal mobility of the patient.
Ultimately, the spinal fusion places greater stress on the discs
adjacent the fused segment as these segments attempt to compensate
for lack of motion in the fused segment, often leading to early
degeneration of these adjacent spinal segments.
[0009] Current developments are focusing on treatments that can
preserve some or all of the motion of the affected spinal segment.
One of these methods to stabilize the spinal motion segment without
the disadvantages of spinal fusion is total disc replacement. Total
disc replacement is a highly invasive and technically demanding
procedure which includes removing the cartilaginous end plates
between the vertebral bone and the disc, large portions of the
outer annulus and the complete inner nucleus. If the entire disc is
removed, typically an artificial prosthesis is placed in the disc
space. Many of the artificial disc prosthesis currently available
consist of a soft polymer to act as the nucleus. The soft polymer
is interposed between two metal plates that are anchored or
attached to the vertebral endplates. A summary of the history of
early development and designs of artificial discs is set forth in
Ray, "The Artificial Disc: Introduction, History and
Socioeconomics," Chpt. 21, Clinical Efficacy and Outcome in the
Diagnosis of Low Back Pain, pgs. 205-225, Raven Press (1992).
Examples of these layered total disc replacement devices are shown,
for example, in U.S. Pat. Nos. 4,911,718, 5,458,643, 5,545,229 and
6,533,818.
[0010] These types of artificial total discs have several
disadvantages. First, because the artificial disc prosthetics are
relatively large, they require relatively large surgical exposures
to accommodate their insertion. The larger the surgical exposure,
the higher the chance of infection, hemorrhage or even morbidity.
Also, in order to implant the prosthetic, a large portion of the
annulus must be removed. Removing a large portion of the annulus
reduces the stability of the motion segment, at least until healing
occurs around the artificial disc. Further, because the devices are
constructed from rigid materials, they can cause serious damage if
they were to displace from the disc space and contact local nervous
or vascular tissues. Another disadvantage is that rigid artificial
disc replacements do not reproduce natural disc mechanics. Finally,
relative movement between the hard surfaces of the metal plates of
many artificial discs and the vertebral bone will tend to cause
erosion of the vertebral endplates. Such endplate erosion can lead
to instability, subsidence, and/or neurological or vascular
damage.
[0011] An alternative to total disc replacement is nuclear
replacement. Like the artificial disc prosthetics, these nuclear
replacements are also inert, somewhat flexible, non-biological
prosthetics. The procedure for implanting a nuclear replacement is
less invasive than the procedure for a total disc replacement and
generally includes the removal of only the nucleus and replacement
of the nucleus with a prosthetic that may be malleable and provide
cushioning that mimics a natural disc nucleus. Examples of the
prosthetics used for nuclear replacement include: the Ray implant
(U.S. Pat. Nos. 4,772,287 and 4,904,260), the Bao implant (U.S.
Pat. No. 5,192,326), the Sulzer spiral implant (U.S. Pat. No.
5,919,235), and the Replication Medical implant (U.S. Pat. No.
6,726,721).
[0012] Nuclear replacements are intended to more closely mimic
natural disc mechanics. To that end, some nuclear replacements
utilize hydrogel because of its water imbibing properties. Hydrogel
is also used because of its ability to expand in situ to permit a
more complete filling of the excavated nuclear cavity. However,
there is a trade-off in that the more expansion the hydrogel
achieves, the less robust the end product will be. As a result,
hydrogel nuclear disc replacements have generally adopted the use
of some formed jacket to contain the hydrogel material. For
example, the Ray implant as described in U.S. Pat. Nos. 4,772,287
and 4,904,260 consists of a block of hydrogel encased in a plastic
fabric casing. The Bao implant as described in U.S. Pat. No.
5,192,326 consists of hydrogel beads enclosed by a fabric shell.
While the use of a jacket can result in better structural integrity
and less potential extravasation of hydrogel material outside of
the nucleus, there is a tendency for jacket encased hydrogel
materials to produce a lateral expansive force against the annulus.
Another problem with this approach is that the hydrogel material
can become too hard for the desired stress response curve of a
replacement disc.
[0013] Another approach to nucleus replacement involves
implantation of a balloon or other container into the nucleus that
is filled filling it with a biocompatible material that hardens in
situ. Examples of this in situ approach to nucleus replacement
include U.S. Pat. Nos. 5,549,679 and 5,571,189. One of the problems
with this approach is that the chemical hardening process is
exothermic and can generate significant amounts of heat that may
cause tissue damage. In addition, there is a possibility that the
balloon may rupture during expansion, causing leakage of material
out of the bone cavity, which may cause undesirable
complications.
[0014] Another technique for nucleus replacement involves
implanting a multiplicity of individual support members one at a
time in the disc space until the cavity is full. Examples of this
approach include U.S. Pat. Nos. 5,702,454 and 5,755,797. Because
each of the individual components is relatively small, there is a
possibility that one or more beads or support members will extrude
out of the cavity.
[0015] From a mechanical perspective, this technique is limited in
the ability to produce consistent and reproducible results because
the location and interaction of the multiplicity of beads or
support members is not controlled and the beads or support members
can shift during and after implantation.
[0016] Accordingly, there is a need for a nuclear prosthesis that
may be inserted using a minimally invasive procedure and that
mimics the characteristics of a natural disc.
SUMMARY OF THE INVENTION
[0017] The present invention provides a method and apparatus for
repairing a damaged intervertebral disc nucleus in a minimally
invasive manner with a modular disc prosthesis. The modular disc
prosthesis preferably comprises at least three modular segments and
at least two rails that operably connect adjacent modular segments.
In one embodiment, each modular segment includes a harder inner
portion and a softer outer portion. Preferably, the rails operate
to slidably connect and interlock adjacent modular segments. A stem
portion of the rails that extends outside the periphery of the body
of the prosthesis is removable after implantation such that the
modular segments form an implanted unitary device that closely
mimics the geometry of the disc nucleus cavity.
[0018] In one embodiment, a modular disc prosthesis that is adapted
to be implanted in an evacuated disc nucleus space includes at
least three modular segments each having a width. The first modular
segment has a first rail extending at least partially along one
side of the width and beyond a periphery of the first modular
segment. The second modular segment is slidably connected to the
first rail on one side of the width and has a second rail extending
at least partially along another side of the width and beyond a
periphery of the second modular segment. The third modular segment
is slidably connected to the second rail on one side of the width.
The prosthesis has an expanded position in which the modular
segments are extended along the first and second rails and
positioned in a generally end to end configuration spaced apart by
the rails prior to implantation. The prosthesis also has an
implanted position in which the modular segments are positioned in
a generally side by side configuration that defines a unitary body
having a generally continuous periphery that generally corresponds
to the evacuated nucleus disc space with at least a portion of the
rails extending beyond the periphery of the body.
[0019] Preferably, each modular segment comprises an inner portion
and an outer portion. The inner portion includes structure that
mates with one of the rails. The outer portion substantially
surrounding the inner portion except for the side having one of the
rails and the side having structure that mates with one of the
rails. In one embodiment, the inner portion of each modular segment
and the outer portion of each modular segment are made of polymers
of different durometers. Preferably, the inner portion of each
modular segment has a compressive modulus from about 70-100 Mpa and
the outer portion of each modular segment has a compressive modulus
from about 6-20 Mpa. The use of a harder inner portion and softer
outer portion as part of an integrated unitary implanted device
permits the modular prosthesis of the present invention to more
closely mimic the stress response of a biological disc nucleus
while simultaneously permitting effective operation of the slidable
relationship between adjacent ones of the modular segments.
[0020] In one embodiment, locking features are provided to ensure
that the modular disc prosthesis is a unitary device both before
and after insertion. To prevent the device from being separated
prior to insertion, locking features may be provided on the rigid
rails to prevent modular segments from being slid back off of the
rails. This ensures that each modular segment is connected in its
proper position and in the proper order. In addition, locking
features may be provided on the modular segments to lock them
together upon insertion. This prevents individual segments from
dislocating from the assembled prosthetic and migrating within the
annulus.
[0021] Another aspect of the present invention comprises a method
for implanting a modular disc prosthesis. Because the modular disc
prosthesis may be implanted one segment at a time, a hole made in
the annulus for implantation of the prosthesis may be a fraction of
the size of the device in its final assembled form. The modular
disc prosthesis is introduced into the patient's intervertebral
space through an access tube and the first modular segment is
inserted into the disc nucleus space through the small hole in the
annulus. The second modular segment is then slid up the first rigid
rail using a pushing tool and into the disc nucleus space until the
second modular segment interlocks with the first modular segment.
The tail stem of the first rigid rail is then severed from the
device. In one embodiment, the tail stem of the rigid rails may be
removed by a cutting mechanism provided preferably as part of the
distal end of the pushing tool. Subsequent modular segments are
slid up the adjoining rigid rail with the pushing tool into the
disc nucleus space and then interlocked with the previously
inserted modular segment in a similar manner. Once all of the
modular segments have been inserted and all of the tail stems
severed, the modular disc prosthesis is fully inserted into the
patient's disc nucleus space and assembled and the access tube and
pushing tool may be withdrawn and the access hole is closed up.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0023] FIG. 1 is a cross-sectional view of a modular disc
prosthesis according to the preferred embodiment of the present
invention in its inserted configuration.
[0024] FIG. 2 is a top view of a modular disc prosthesis according
to the preferred embodiment of the present invention prior to
insertion.
[0025] FIG. 3 is a perspective view of a modular disc prosthesis
according to an alternate embodiment of the present invention prior
to insertion.
[0026] FIG. 4 is a perspective view of a modular disc prosthesis
according to an alternate embodiment of the present invention at a
first stage of insertion.
[0027] FIG. 5 is a perspective a view of a modular disc prosthesis
according to an alternate embodiment of the present invention at a
second stage of insertion.
[0028] FIG. 6 is a perspective view of a modular disc prosthesis
according to an alternate embodiment of the present invention at a
final state of insertion.
[0029] FIG. 7 is a partial perspective view of a portion of a
modular disc prosthesis according to an embodiment of the present
invention.
[0030] [FIG. 8-xx are views of an insertion tube and pushing tool
for use in accordance with one embodiment of the present
invention.]
DETAILED DESCRIPTION OF THE DRAWINGS
[0031] Referring to FIG. 1, there can be seen a cross-sectional
view of a modular disc prosthesis 100 according to the preferred
embodiment of the present invention as configured once inserted
into the body. In this embodiment, modular disc prosthesis 100
comprises first 102, second 104, third 106, fourth 108, and fifth
109 modular segments. Preferably, each modular segment 102, 104,
106, 108, 109 comprises a soft outer portion 102a, 104a, 106a,
108a, 109a and a hard inner portion 102b, 104b, 106b, 108b,
109b.
[0032] In a preferred embodiment, hard inner portions 104b, 106b
and 108b have an I-beam cross-sectional shape that optimizes
flexibility and strength of the hard inner portions. Alternatively,
hard inner portions 104b, 106b, 108b, can have a uniformly shaped
cross-sectional area to reduce any differences in compressibility
of the modular disc prosthesis 100 across the surface area in order
to minimize the potential for stress risers to be created in the
interface between the outer surface of the modular disc prosthesis
100 and the inner surfaces of the disc space cavity. It will be
recognized that various cross-sectional shapes of hard inner
portions 102b, 104b, 106b, 108b and 109b can be utilized in
accordance with the present invention and that the cross-sectional
shapes of the hard inner portions does not need to be
symmetric.
[0033] Hard inner portion 102b of first modular segment 102
includes first segment interlocking portion 116. Hard inner portion
104b of second modular segment 104 includes second segment
interlocking portion 118 and a first slot 128. Hard inner portion
106b of third modular segment 106 includes third segment
interlocking portion 120 and a second slot 130. Hard inner portion
108b of fourth modular segment 108 includes fourth segment
interlocking portion 121 and a third slot 132. Hard inner portion
109b of fifth modular segment 109 includes a fourth slot 133.
[0034] In the preferred embodiment, rails 110, 112, 114, 115 have a
noncircular cross-sectional shape, although it will be understood
that other cross-section shapes could be utilized and that there is
no requirement that all of the rails have similar cross-sectional
shapes. It has been found that the noncircular cross-sectional
shape as shown (corresponding mating C and sideways T
cross-sectional shapes) provides for better alignment of the
modular segments and supports larger insertion forces along the
axis of the rail.
[0035] It will be understood that in a preferred embodiment, the
rails 110, 112, 114, 115 of the present invention have a
non-uniform cross-sectional aspect ratio in terms of the height and
width of the rail. Preferably, the rails have a relative rigidity
along a longitudinal axis of the rail in a dimension of the height
of the rail that is greater than a width of the rail, whereas in a
dimension transverse to the width of the rail the relative rigidity
of the rail permits a greater degree of flexibility such that
succeeding modular segments can be moved laterally with respect to
one another in the expanded position without deforming the rails.
[Insert preferred ranges of dimensions of height and width of the
rails]. This differential rigidity in the two dimensions transverse
to the longitudinal axis of the rail is important in permitting
effective and efficient sliding operation of the adjacent modular
segments.
[0036] Referring to FIG. 2, there can be seen a portion of a
modular disc prosthesis 100 according to the preferred embodiment
of the present invention prior to insertion into the disc nucleus
space. Note that in FIG. 2, modular disc prosthesis 100 is depicted
showing only the hard inner portion 102b, 104b, 106b, 108b, 109b of
each modular segment 102, 104, 106, 108, 109 for convenience of
illustration. However, in the preferred embodiment of the invention
each modular segment would also have soft outer portion as
described above and shown in FIG. 1.
[0037] In alternate embodiments, modular disc prosthesis may
comprise greater or fewer numbers of modular segments and rails, so
long as there are at least three modular segments and two rails.
For example, FIG. 3 depicts a modular disc prosthesis 200 having
four modular segments and three rails.
[0038] Prior to insertion, modular disc prosthesis 100 further
includes first 110, second 112, third 114, and fourth 115 rails.
First modular segment 102 is rigidly attached to first rail 115 at
first segment interlocking portion 116. Second modular segment 104
is slidably attached to first rail 110 at first slot 128 and
rigidly attached to second rail 112 at second segment interlocking
portion 118. Third modular segment 106 is slidably attached to
second rail 112 at second slot 130 and rigidly attached to third
rail 114 at third segment interlocking portion 120. Fourth modular
segment 108 is slidably attached to third rail 114 at third slot
132 and rigidly attached to fourth rail 115 at fourth segment
interlocking portion 121. Fifth modular segment 109 is slidably
attached to fourth rail 115 at fourth slot 133.
[0039] As shown in FIG. 2 and FIG. 3, each rail 110, 112, 114 and
115 or 210, 212 and 214 include a stem portion that extends beyond
a periphery of the body of the prosthesis 100, 200, respectively.
Preferably these stem portions are long enough to permit access
into the evacuated disc nucleus space such that one modular segment
can be positioned inside the evacuated disc nucleus space while the
next modular segment on the rail is still outside of the body. In
an exemplary embodiment, the length of these stem portions ranges
between [insert range of lengths of stem portions].
[0040] As shown in the alternate embodiment of FIG. 3, each rail
210, 212, 214 may further include a retaining portion 222, 224, 226
to keep the device from being separated prior to insertion. The
retaining portions 222, 224 and 226 are configured to prevent the
corresponding rail 210, 212 and 214 from being slid off the rail.
The retaining portions may be molded into the rails or may be
separate pieces or deformations of the rails added during the
manufacture of the device.
[0041] The preferred embodiment is a unitary prosthesis that comes
packaged, sterile, and ready for implantation at the surgical site.
Since the device is fully preformed and delivered as a unitary
implant, the device is under direct operator control until the
prosthetic disc nucleus is completely formed. This unitary design
limits the need for the surgeon to determine how the cavity should
be filled and assures that the components order of insertion and
connection cannot be mixed up. The ability to predetermine the size
of modular disc prosthesis also allows for the nucleus cavity to be
more completely filled and provides a greater degree of control
over the uniformity of the stress response of the implant as
compared to other kinds of minimally invasive implants. In this
regard, it will be understood that the modular disc prosthesis 100
of the present invention may be provided in a variety of different
final assembled volumes and shapes to correspond to different sizes
and shapes of different evacuated disc cavities.
[0042] Modular disc prosthesis is introduced through an access tube
that is inserted partially into the disc nucleus space. As shown in
FIG. 8, access tube is at least 3 inches long and preferably about
6 inches long. Modular segments may be transposed along rails by
means of a pushing tool as shown in FIG. 9. It should be noted that
although the insertion of modular disc prosthesis is described in
relation to a preferred five-segment embodiment or an alternate
four-segment embodiment, embodiments having any other number of
segments would be inserted in a similar fashion.
[0043] Referring again to FIG. 3, there can be seen a modular disc
prosthesis 200 prior to insertion into the body. Upon inserting the
access tube into the disc nucleus space, first modular segment 202
is inserted through the tube and into the disc space. Upon complete
insertion of first modular segment 202, modular disc prosthesis 200
is moved centrally and second modular segment 204 is slid along the
first rail 210 into the disc nucleus space onto first segment
interlocking portion 216 until it is flush with first modular
segment 202. This stage of insertion is depicted in FIG. 4. A stem
portion of first rail 210 is then removed and modular disc
prosthesis 200 is moved centrally again.
[0044] Third modular segment 206 is then slid down the second rail
212 and into the disc nucleus space onto second segment
interlocking portion 218 until it is flush with second modular
segment 204. This configuration is shown in FIG. 5. A stem portion
of second rail 212 is then removed and modular disc prosthesis 200
is moved centrally. Fourth modular segment 208 is slid along third
rail 214 into the disc nucleus space and onto third segment
interlocking portion 220 until it is flush with the other modular
segments 202, 204, 206. Finally, a stem portion of third rail 214
is then removed. This final implanted configuration of modular disc
prosthesis 200 with all modular segments aligned and locked
together is shown in FIG. 6. Modular disc prosthesis 200 is sized
and shaped to conform to the geometry of the disc nucleus
cavity.
[0045] In an alternate embodiment, a keystone approach can be used
to insert modular disc prosthesis such that the last modular
segment inserted into the disc nucleus space is not one of the
outside segments. Instead, the outside segments can be the first
two segments inserted. This creates a bilateral expansion force as
the remaining segments are inserted between the two outside
segments. This helps make a tighter fit within the disc nucleus
space than does the asymmetric lateral force imparted when the
segments are implanted sequentially.
[0046] The stem portions of rails 110, 112, 114, 115 that extend
beyond the periphery of the body of the modular disc prosthesis 100
can be removed by many different techniques. As shown in FIG. 9,
pushing tool may be provided with a cutting mechanism that can
remove the stem portions of the rails. Cutting mechanism may be a
pair of fixed blades located on the distal end of pushing tool. In
this embodiment, the cutting blades would act as a cutting wheel in
which a turning of the handle connected to the blades causes the
blades to circumscribe the rail. Alternatively, the cutting
mechanism can be a clamping means that removes the rails through
twisting or pinching. Stem rails may also be cut off with any other
sharp instrument.
[0047] In another embodiment, the stem portions of the rails may be
provided with a perforation at the junction with each modular
segment such that they can be torn, broken, twisted, or more easily
cut off. Cutting may also be accomplished with a wire loop provided
to the part. Additionally, heat, laser, or any other local energy
source can be used to accomplish the separation. One of skill in
the art will recognize that numerous alternative means exist
whereby stem rails can be severed from modular disc prosthesis.
[0048] Alternatively, modular disc prosthesis may be implanted
using an anterior lateral approach. An anterior lateral approach
allows for a larger insertion opening to be used while still being
minimally invasive. [Insert any further description of this
alternate anterior lateral surgical technique.]
[0049] During insertion, slots 128, 130, 132, 133 slide along the
stem portions of rails 110, 112, 114, 115 and onto segment
interlocking portions 116, 118, 120, 121. Slots 128, 130, 132, 133
and segment interlocking portions 116, 118, 120, 121 may be
provided with locking features to prevent separation of modular
segments 102, 104, 106, 108, 109. Locking features, such as a
series of barbs or studs, may be provided such that once a slot is
slid onto a segment interlocking portion, it cannot be slid back
off of it. A ratchet and pawl may also be used to lock modular
segments together. A ratchet release tool may also be provided in
case separation of modular segments is desired once they are locked
together.
[0050] One example of these locking features is depicted in FIG. 7.
Hard inner portion 304b of each modular segment 304 is provided
with a pair of depressible projections 334 on segment interlocking
portion 318 and a complementary pair of apertures 336 on slot 328.
When slot of a first modular segment is slid onto segment
interlocking portion of a second modular segment, projections are
depressed. When apertures of the first modular segment are
positioned over projections of the second modular segment, the
projections pop through apertures, locking the modular segments
relative to one another. Modular segments may be separated by
depressing the projections and sliding the first modular segment
back off of the second modular segment.
[0051] Alternatively, free movement of modular segments 102, 104,
106, 108, 109 along rails 110, 112, 114, 115 may be allowed until
insertion in the body. It will be understood that, depending upon
the material configuration of the modular prosthetic 100 and the
interface fit, segment interlocking portions 116, 118, 120, 121 may
swell due to hydration to lock in the final configuration. The
feature may be used alone or in combination with a mechanical
locking feature. Alternative methods of locking modular segments
together will be appreciated by those skilled in the art.
[0052] In the preferred embodiment, modular disc prosthesis 100 is
molded from elastomeric biomaterials, preferably polyurethane. Stem
rails 110, 112, 114, 115 and hard inner portions 102b, 104b, 106b,
108b, 109b are made from a hard durometer polyurethane, such as a
polyurethane with shore hardness 55D or above and compressive
modulus of 70 to 100 MPa. Soft outer portions 102a, 104a, 106a,
108a, 109a are made from a soft durometer polyurethane, such as a
polyurethane with a shore hardness ranging from 55D to 18A and a
compression modulus between 6 and 20 MPa.
[0053] In the preferred embodiment, the two different durometer
polyurethanes are co-polymerized to create a chemical bond between
the two portions of each modular segment 102, 104, 106, 108, 109.
In alternate embodiments other polymers such as PEEK, polyethylene,
silicones, acrylates, nylon, polyacetyls, and other similar
engineering polymers may be used for the hard inner portions or the
soft outer portions. For a more detailed description of a preferred
embodiment of the multi-durometer polymer compositions of the
present invention, reference is made to the previously identified
co-pending application entitled, "[fill in final title]".
[0054] In an alternate embodiment, the stem of the tails may be
molded from a harder durometer material than soft outer portion and
hard inner portion of modular segments. Utilizing this approach
allows the rails to be extruded, rather than molded as part of the
modular segments. A bond joint can then be made with the hard inner
portion external to the periphery of the modular segments to form
the unitary design. Extruding the stem portions of the tails makes
modular disc prosthesis easier and less expensive to manufacture
than a completely molded product.
[0055] In the preferred embodiment, the soft outer portion of
modular disc prosthesis is deformable in response to normal
physiological forces of 30 to 300 pounds. Because of this increased
deformability, the prosthesis produces little impingement on the
end plates of the intervertebral disc. As a result, the end plates
do not flatten out over time and conform to the contours of the
implant as is the case with many metal/polymer disc replacement
implants.
[0056] In an alternate embodiment, outer portion of modular disc
prosthesis may be modified to provide for elution of medicants.
Such medicants can include analgesics, antibiotics, or
bioosteologics such as bone growth agents. The solid polymer outer
portion of modular disc prosthesis provides for better and more
controllable elution rates than hydrogel materials can. [Insert
further discussion of elution, including whether there could be
another polymer layer on the very outside that would serve as an
"elution" layer"]
[0057] Various modifications to the disclosed apparatuses and
methods may be apparent to one of skill in the art upon reading
this disclosure. The above is not contemplated to limit the scope
of the present invention, which is limited only by the claims
below.
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