U.S. patent application number 11/189753 was filed with the patent office on 2006-06-08 for supplementation or replacement of a nucleus pulposus of an intervertebral disc.
This patent application is currently assigned to Synthes Inc.. Invention is credited to Michael F. Keane, Thomas P. Schaer, Edward Vresilovic.
Application Number | 20060122704 11/189753 |
Document ID | / |
Family ID | 35241340 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060122704 |
Kind Code |
A1 |
Vresilovic; Edward ; et
al. |
June 8, 2006 |
Supplementation or replacement of a nucleus pulposus of an
intervertebral disc
Abstract
A degenerated nucleus pulposus located in a central core region
of an intervertebral disc within the annulus fibrosus is
supplemented or replaced by a method wherein an amount of a
biocompatible material is introduced into the central core region
by a process including the steps of 1) forming a channel through a
vertebral body adjacent to said intervertebral disc, extending from
an exterior surface of the vertebral body to the central core
region of the annulus fibrosus; 2) introducing an amount of a
biocompatible material through the channel into the central core
region of the annulus fibrosus; 3) pressurizing the biocompatible
material through the channel to a postsurgical pressure sufficient
to alleviate symptoms caused by the degenerated nucleus pulposus;
and 4) sealing the channel while maintaining the sufficient
postsurgical pressure. After sealing the channel, a vertebroplasty
may optionally be performed in the vertebra.
Inventors: |
Vresilovic; Edward;
(Ardmore, PA) ; Keane; Michael F.; (Downingtown,
PA) ; Schaer; Thomas P.; (Landenberg, PA) |
Correspondence
Address: |
MILES & STOCKBRIDGE, P.C.
1751 PINNACLE DRIVE
SUITE 500
MCLEAN
VA
22102
US
|
Assignee: |
Synthes Inc.
|
Family ID: |
35241340 |
Appl. No.: |
11/189753 |
Filed: |
July 27, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60591094 |
Jul 27, 2004 |
|
|
|
Current U.S.
Class: |
623/17.16 |
Current CPC
Class: |
A61B 17/86 20130101;
A61F 2002/444 20130101; A61F 2/4611 20130101; A61F 2002/2839
20130101; A61F 2002/30242 20130101; A61F 2/442 20130101; A61F
2002/4631 20130101; A61F 2230/0069 20130101; A61F 2002/30224
20130101; A61F 2002/4445 20130101; A61F 2/28 20130101; A61F
2002/4627 20130101; A61F 2230/0071 20130101 |
Class at
Publication: |
623/017.16 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Claims
1. A method for replacing or supplementing a nucleus pulposus in an
intervertebral disc, said intervertebral disc having an annulus
fibrosus with a central core containing said nucleus pulposus, said
method comprising: forming a channel through a vertebral body
adjacent to said intervertebral disc, said channel extending from
an exterior surface of said vertebral body to said central core of
said annulus fibrosus; introducing an amount of a biocompatible
material through said channel into said central core of said
annulus fibrosus; exerting pressure on said biocompatible material
through said channel; and sealing said channel while maintaining
said pressure.
2. The method of claim 1, wherein said pressure is sufficient to at
least approximately restore natural function of said intervertebral
disc.
3. The method of claim 1, wherein said pressure is sufficient to at
least approximately restore natural intervertebral spacing.
4. The method of claim 1, wherein said biocompatible material is
pressurized to a pressure approximating natural pressure of the
nucleus pulposus.
5. The method of claim 1, wherein said biocompatible material has a
compressional modulus not greater than about 4 Mpa.
6. The method of claim 1, wherein said biocompatible material is a
hydrogel.
7. The method of claim 6, wherein said hydrogel is comprised of a
material selected from the group consisting of poly(vinyl alcohol)
and an associating hydrogel that is a blend of poly(vinyl alcohol)
and poly(vinylpyrrollidone).
8. The method of claim 1, wherein said biocompatible material is
selected from the group consisting of a polyurethane and a
silicone.
9. The method of claim 1, wherein said channel is sealed with a
plug of a biocompatible sealing material.
10. The method of claim 9, wherein said biocompatible sealing
material is selected from the group consisting of a bone cement, an
autograft, an allograft, and a xenograft.
11. The method of claim 10, wherein said bone cement is selected
from the group consisting of poly(methylmethacrylate), calcium
phosphate, calcium sulfate, calcium carbonate, and
hydroxyapatite
12. The method of claim 1, wherein said channel is sealed with a
plug of biocompatible sealing material and a mechanical plug.
13. The method of claim 12, wherein said mechanical plug is
selected from the group consisting of a bone screw and a barbed
plug.
14. The method of claim 1, wherein said implant is pressurized by
inserting a bone screw or a barbed plug into said channel.
15. The method of claim 1, additionally comprising a step of
performing a vertebroplasty in said vertebral body after sealing
said channel.
16. A restorative implant for an intervertebral disc comprising: a
quantity of a biocompatible material implanted in a central core of
a human intervertebral disc replacing or supplementing a nucleus
pulposus, said quantity of biocompatible material being accessible
through a channel formed in an adjacent vertebra; and a plug
positioned in said channel and exerting pressure on said quantity
of implanted low modulus material.
17. The implant of claim 16, wherein said postsurgical pressure is
sufficient to at least approximately restore natural function of
said intervertebral disc.
18. The implant of claim 16, wherein said postsurgical pressure is
sufficient to at least approximately restore natural intervertebral
spacing.
19. The implant of claim 16, wherein said biocompatible material is
pressurized to a pressure approximating natural pressure of the
nucleus pulposus.
20. The implant of claim 16, wherein said biocompatible material
has a compressional modulus not greater than about 4 Mpa.
21. The implant of claim 16, wherein said biocompatible material
has a Poisson ratio in a range of from about 0.30 to 0.50.
22. The implant of claim 26, wherein said biocompatible material is
a hydrogel.
23. The implant of claim 22, wherein said hydrogel is a comprised
of a material selected from the group consisting of poly(vinyl
alcohol) and an associating hydrogel that is a blend of poly(vinyl
alcohol) and poly(vinylpyrrollidone).
24. The implant of claim 16, wherein said biocompatible material is
selected from the group consisting of a polyurethane and a
silicone.
25. The implant of claim 16 additionally comprising a predetermined
quantity of a generally incompressible material positioned in said
channel between said implanted material and said plug.
26. The implant of claim 25 wherein said generally incompressible
material is selected from the group consisting of bone cement, an
autograft, an allograft, and a xenograft.
27. The implant of claim 16 wherein said plug is selected from the
group consisting of a bone screw and a barbed plug.
28. A restorative implant for an intervertebral disc implanted by
the method of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/591,094, filed Jul. 27, 2004, the entire
disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to methods and apparatus for
replacing or supplementing the natural nucleus pulposus of an
intervertebral disc by using a viscoelastic low-modulus implant
through a transosseus approach, and more particularly to methods
and apparatus for pressurizing a viscoelastic implant within the
intervertebral disc to achieve an appropriate physiologic state of
annulus tension and nucleus pressurization.
[0004] 2. Brief Description of the Prior Art
[0005] Chronic back pain, typically lower back pain, is experienced
by many individuals, and is responsible for much lost time at work
and expense for treatment. Such pain is generally the result of a
pathological condition of an intervertebral disc, caused by injury
or age-related degeneration.
[0006] Current treatment options for lower back pain range from
conservative bed rest to highly invasive surgical procedures
including spinal fusion and discectomy. Spinal fusion, i.e., fusion
or immobilization of the vertebrae on each side of the afflicted
intervertebral disc, is a procedure that offers pain relief and an
increased stability of the fused segment. Discectomy, i.e.,
surgical removal of part of the intervertebral disc is another
surgical option.
[0007] Total disc replacement with a mechanical prosthesis has been
proposed as another option for relief of back pain, and a number of
such mechanical prosthesis have been proposed.
[0008] The human intervertebral disc is comprised of two major
structures, an inner gelatinous nucleus pulposus and an outer
tendinous structure, the annulus fibrosis. Degeneration of the
nucleus leads to degradation and loss of function of the
intervertebral disc, resulting in pain and disability.
Consequently, another surgical option for the relief of lower back
pain is replacement of the nucleus, leaving the annulus intact.
Thus, the aim of nucleus replacement is to relieve pain, to restore
healthy physiologic function to the disc, and to prevent additional
wear on the annulus.
[0009] Normal disc function requires the combined action of the
nucleus pulposus and annulus fibrosus. Consequently, a nucleus
implant should preferably tend to restore the normal mobility of
the disc, restore the disc height and re-create healthy disc
pressure in order to place the annulus fibers back into their
natural state of tension.
[0010] Accordingly, a need exists for a nucleus replacement device
and a method of implantation that substantially reproduces the
synergistic interaction between the nucleus and annulus, thus
restoring the normal mechanical properties and mobility of the
disc.
SUMMARY OF THE INVENTION
[0011] According to the present invention a degenerated nucleus
pulposus contained within a central core of an annulus fibrosus of
an intervertebral disc can be replaced or supplemented by a
procedure that includes: [0012] 1) forming a channel through a
vertebral body adjacent to the intervertebral disc, the channel
extending from an exterior surface of the vertebral body to the
central core of the annulus fibrosus; [0013] 2) introducing an
amount of a biocompatible material through the channel into the
central core of the annulus fibrosus; [0014] 3) pressurizing the
biocompatible material through the channel to a physiologic
pressure sufficient to alleviate symptoms caused by the degenerated
nucleus pulposus; and [0015] 4) sealing the channel while
maintaining the pressure.
[0016] Accordingly, in one aspect of the present invention a method
is provided for replacing or supplementing a degenerated nucleus
pulposus.
[0017] In another aspect, the invention provides a method for
introducing a biocompatible material into the central core of an
annulus fibrosus of an intervertebral disc.
[0018] In another aspect, the invention provides a method for
introducing a hydrogel or other relatively low-modulus material
into the central core of an annulus fibrosus of an intervertebral
disc.
[0019] In another aspect, the invention provides a method for
introducing a pressurized implant into the central core of an
annulus fibrosus of an intervertebral disc.
[0020] In another aspect, the invention provides a surgical
approach to the nucleus for implantation of an implant that will
prevent damage to the annulus.
[0021] In another aspect, the invention provides a transosseus
approach to the nucleus through the superior or inferior vertebral
bodies.
[0022] In another aspect, the invention provides a method of
closing a channel in the vertebral body that will compress a
nucleus replacement into the central core of the annulus fibrosus
to ensure complete filling of the core.
[0023] In another aspect, the invention provides a replacement or
supplement for the nucleus of the natural intervertebral disc that
is designed to work synergistically with the annulus to reproduce
normal disc mechanics.
[0024] In another aspect, the invention provides a postsurgical
configuration of a replaced or supplemented nucleus pulposus that
can reproduce, at least approximately, the mechanical properties of
the normal human intervertebral disc.
[0025] In another aspect, the invention provides an implant that
can improve the function of a degenerated nucleus pulposus of an
intervertebral disc.
[0026] Additional aspects of the invention will be apparent from
the description of the invention that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a lateral elevational cross-section of the
functional spinal motion unit of a human spine, i.e.,
intervertebral disc with superior and inferior vertebrae,
illustrating the first step in the transosseus approach of the
invention, i.e., drilling an access path with a guide wire.
[0028] FIG. 2 is a cross-section of the functional spinal motion
unit illustrating the second step in the transosseus approach,
i.e., over-drilling with a cannulated drill bit.
[0029] FIG. 3 is a cross-section of the functional spinal motion
unit illustrating the third step in the transosseus approach, i.e.,
implantation of the implant through an insertion sheath.
[0030] FIG. 4 is a cross-section of the functional spinal motion
unit illustrating one embodiment of the third and fourth steps in
the transosseus approach, i.e., backfilling and pressurizing the
low-modulus material implant with a bone plug and screw.
[0031] FIG. 5 is a transverse cross section of the functional
spinal motion unit, again illustrating the third and fourth steps
in the transosseus approach of the invention.
[0032] FIG. 6 is a cross section of the functional spinal motion
unit illustrating another embodiment of the third and fourth steps
in the transosseus approach, i.e., backfilling and pressurizing the
low modulus implant with a wedge shaped implant.
[0033] FIG. 7 shows the results of a series of flexion-extension
tests conducted on a spinal motion segment implanted with a
prosthesis by the process of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] According to the invention, an implant that supplements or
replaces the nucleus pulposus of an intervertebral disc is
implanted by a transosseous approach through an adjacent
vertebra.
[0035] In a preferred mode the method of the invention includes the
steps of: [0036] 1) forming a channel through a vertebral body
adjacent to the intervertebral disc, the channel extending from an
exterior surface of the vertebral body to the central core of the
annulus fibrosus; [0037] 2) introducing an amount of a
biocompatible material through the channel into the central core of
the annulus fibrosus; [0038] 3) pressurizing the biocompatible
material through the channel to a physiologic pressure sufficient
to alleviate symptoms caused by the degenerated nucleus pulposus;
and [0039] 4) sealing the channel while maintaining the
pressure.
[0040] In a preferred embodiment of the invention, a relatively
soft, low-modulus implant (i.e., having a low unconstrained
compression modulus) is inserted and pressurized by a transosseous
approach through an adjacent vertebra. The invention will be
described below in terms of such a preferred embodiment; the
skilled practitioner will recognize that the transosseous approach
of the invention may be utilized to insert any appropriate
prosthesis for a nucleus pulposus.
[0041] According to a preferred embodiment of the invention, in a
first step, a channel is formed through a vertebra adjacent, i.e.,
immediately superior or inferior, to the intervertebral disc into
which an implant is to be inserted. The transvertebral channel may
be made by any conventional surgical technique for drilling through
a bony structure such as a vertebra.
[0042] After the channel has been drilled, the surgical site is
prepared, if necessary, by removing a portion or all of the natural
nucleus pulposus. Such removal may be accomplished by conventional
surgical techniques. Thereafter, an amount of a biocompatible
material, preferably a relatively low-modulus material, is inserted
through the transvertebral channel sufficient to fill the central
core of the intervertebral disc and provide the necessary disc
height and intervertebral disc pressure required to at least
approximate the function of the natural intervertebral disc.
[0043] A preferred embodiment of a biocompatible material suitable
for replacement or supplementation of a nucleus pulposus is a
solid, substantially fully hydrated hydrogel as disclosed in
copending U.S. patent application Ser. No. 11/134,309 by Vresilovic
et al., filed May 23, 2005, the entire disclosure of which is
incorporated herein by reference. Another preferred biocompatible
material for use in the method of the invention is a thermogelling
polymer that can be injected in a relatively fluid state to fill
the cavity of a removed or degenerated nucleus pulposus and then
transitions to a solid gel at body temperature, as disclosed in
copending U.S. patent application Ser. No. 10/837,082, by Lowman et
al., filed Apr. 30, 2004, the entire disclosure of which is
incorporated herein by reference. Both of these materials can be
used in the method of the invention to provide a prosthesis that
replaces or supplements a natural nucleus pulposus and establishes
a substantially physiologic pressure state within the
intervertebral disc. Such biocompatible materials can replace or
supplement a nucleus pulposus by substantially filling void volume
in the nucleus region of an intervertebral disc independently of
natural variations in the anatomy of a patient and can be precisely
pressurized by the method of the invention using sealing member as
described herein.
[0044] Further embodiments of biocompatible materials that can be
delivered and pressurized by the method of the invention include
materials that can serve as scaffolds for tissue engineered
constructs, and can incorporate cells, growth factors and other
biologic materials that can promote the regeneration of
intervertebral disc structures such as the nucleus, annulus or
vertebral endplate. Certain conventional materials usable for such
constructs aterials may be sufficiently fluid to be capable of
implantation by the method of the invention, and may therefore be
capable of being implanted while avoiding damage to the annulus
fibrosus and minimizing the possibility of subsequent expulsion.
Furthermore, inasmuch as in vitro studies by Liu et al., Spine
2001, Vol. 26, p. 134; Handa et al., Spine 1997, Vol. 22, p. 1085;
and Grukber et al., Spien 2003, Vol. 28(2), p. 186; the entire
disclosures of which are incorporated herein by reference, have
shown the sensitivity of disc cell function to pressure, the method
of the invention can provide a procedure for the delivery of
biologic functional prostheses while minimizing the danger of
subsequent expulsion and providing for adjusting the
post-implantation pressure to promote initiation and maintenance of
cellular growth and function.
[0045] The inserted material is pressurized, or at least maintained
under pressure, by inserting a sealing member into the
transvertebral channel that seals the channel against leakage of
the material. Accordingly, the sealing member may be inserted at
the end of the implantation to maintain pressure exerted by the
implantation apparatus, or the sealing member may itself exert
pressure on the implanted material as it is advanced through the
transvertebral channel to its final sealing position. In one
preferred embodiment, the sealing member comprises a plug of bone
cement followed by a mechanical plug that is advanced into the
channel to exert an appropriate pressure on the material implanted
in the central core. The mechanical plug may be a conventional
surgical bone screw. Alternatively, the transvertebral channel may
be sealed by a plug of bone cement or other biocompatible material
followed by a barbed plug or wedge forced into the channel to
provide the requisite intradiscal pressure. As an alternative to
the bone cement or other biocompatible material, a bolus of
material can be forced into the channel to pressurize the implant
and the channel sealed, as above, with a bone screw, barbed plug,
or the like. In another embodiment, the sealing element may be
provided by a bone plug and/or screw or wedge made of an
osteoconductive or inductive biomaterial.
[0046] As is well-known in the art, a conventional bone screw can
be moved a precise distance axially dependent on the number of
turns applied to the screw. The advancement per turn can be
determined by the pitch of the screw. Thus, the use of such a screw
makes it possible to adjust the pressure within the cavity
accurately by advancing the distal end of the screw in a precise
manner against the implanted biocompatible material. Various
combinations of screw pitch, length, thread height, and distal end
geometries can be selected to achieve the sensitivity required per
turn and to minimize wear at the implant-screw interface.
[0047] The bone cement used in sealing the transvertebral channel
may be any conventional biocompatible bone cement. Such bone
cements include poly(methyl methacrylate), calcium phosphate,
calcium sulfate, calcium carbonate, hydroxyapatite, and the like.
Alternatively, a natural biomaterial can be used, such as an
autograft, allograft or xenograft.
[0048] An additional embodiment of the method of the invention
incorporates a vertebroplasty procedure performed after the
implantation of the biocompatible material replacing or
supplementing the nucleus. In this alternative embodiment, the
nuclear region of the disc is accessed by the transosseous approach
described above, nucleus material is removed, if desired, to form a
cavity, a biocompatible material is implanted to replace or
supplement the nucleus, and a relatively small sealing element,
such as a small bone screw, is fixed in the cortical bone forming
the shell of vertebral endplate. A substantially conventional
vertebroplasty is then performed by making a cavity in the bone
behind the screw by conventional procesdures used in
vertebroplasty, and the cavity so formed is filled with a
conventional biocompatible bone filler. Such a method of nucleus
replacement or supplementation followed by vertebroplasty may be
desirable to provide the benefits of nucleus replacement to older
patient populations in which the strength of the vertebral body has
been compromised.
[0049] FIGS. 1-6 illustrate the practice of the invention to
provide a pressurized implant within the central core of an
intervertebral disc. FIGS. 1-4 and 6 are side elevational views, in
partial cross-section, showing the implantation of a relatively
low-modulus material into the central core of the intervertebral
disc. FIG. 5 is a plan cross-sectional view, in partial
cross-section, showing the implantation of the implant within the
central core of the intervertebral disc.
[0050] FIGS. 1 and 2 illustrate schematically one procedure for
formation of the transvertebral channel. FIG. 1 shows, in partial
cross-section, a spinal motion segment or unit comprising a
superior vertebra 102, an inferior vertebra 104 with an
intervertebral disc, indicated generally at 106, having an annulus
fibrosus 108 and a central core 110 that contains a nucleus
pulposus. As an indication for the application of the method and
prosthesis of the invention, the natural nucleus pulposus occupying
the central core 110 will have experienced some pathological
condition, such as degeneration, herniation, or the like, or will
have been entirely or at least partially removed because of such a
condition. FIG. 1 shows a first step wherein the path for the
channel is defined by drilling with a guide wire 112 from the
exterior of the vertebra 102 to the central core 110. FIG. 2 shows
a second step wherein the guide wire 112 is overdrilled with a
cannulated drill bit 114 to form a channel 116 extending into the
central core 110. The central core 110 having been thus reached
through the channel 116, the surgical site for receiving an implant
is prepared within the central core 110 by conventional surgical
procedures adapted to be conducted through the narrow channel
116.
[0051] FIG. 3 shows the next step wherein a sheath 118 has been
inserted into the channel 116, and an implant 120 is being passed
through the sheath 118 into the central core 110.
[0052] FIGS. 4-6 illustrate the pressurization of the implant
within the central core 110 of the intervertebral disc.
[0053] FIG. 4 shows a lateral cross-sectional view of an embodiment
of the invention wherein a first quantity 122 of low-modulus
material 120, e.g., a hydrogel, has been inserted into the central
core 110. FIG. 5 shows a superior cross-sectional plan view of the
spinal unit of FIG. 4 taken generally along the line V-V in FIG. 4.
A bone screw 126 has been inserted into the channel to complete the
insertion of the implant 120 into the central core 110 and seal the
low-modulus material 120 therein under pressure. The bone screw 126
may extend entirely through the channel 116, or, as shown, it may
extend into a portion of the channel 116 and exert its pressure
through a second quantity of biocompatible sealing material 124,
which may be, e.g., bone cement or another bolus of low-modulus
material, remaining within the channel 116. The biocompatible
sealing material can include such conventional biomaterials as an
autograft, an allograft, or a xenograft.
[0054] FIG. 6 is a lateral cross-sectional view of another
embodiment of the invention showing an alternative method of
pressurizing and sealing the implant 120 within the central core
110 by driving an appropriate plug, e.g., a barbed plug 128, into
the channel 116. Any other suitable plug, e.g., an expandable plug,
or the like, can be used to seal the channel 116 and pressurize the
implant 120 within the central core 110.
[0055] Any biocompatible material may be implanted according to the
method of the invention. Preferably, the biocompatible material
used to replace or supplement a degenerated nucleus pulposus is a
is a low-modulus material, i.e., a material having a compressional
modulus in the range from about 10 kPa to about 4 MPa, preferably
from about 50 kPa to about 1 Mpa, and more preferably from about
100 kPa to about 200 kPa. The biocompatible low-modulus material
preferably has a relatively high Poisson ratio in order to perform
its function, as discussed below. Such a relatively high Poisson
ratio is that within a range of about 0.30 to about 0.50,
preferably from about 0.40 to about 0.50, and more preferably in
the range from about 0.45 to about 0.50.
[0056] The biocompatible low-modulus material of the preferred
embodiment of the invention may have any non-toxic biocompatible
chemical composition. For example, silicone polymers,
polyurethanes, and hydrogel materials, may be used. Such materials
are known for use in implantable prostheses, and the practitioner
can select a suitable material based on its known properties. A
preferred low-modulus, high Poisson ratio material for implanting
into the central core of the annulus fibrosus to replace or
supplement the nucleus pulposus is a hydrogel such as a poly(vinyl
alcohol)-poly(vinylpyrrolidone) (PVA-PVP) copolymer or an
associating polymer blend such as a mixture of poly(vinyl alcohol)
and poly(vinylpyrrolidone). Such a soft hydrogel, when compressed
in one direction, will expand in directions generally at right
angles to the direction of compression. This phenomenon, known as
the Poisson effect, can effectively approximate the normal
pressurization of the healthy liquid-like nucleus pressing against
the inner wall of the surrounding annulus fibrosus during the
various loading situations the disc encounters. Any biocompatible
hydrogel or other low-modulus material capable of reproducing this
effect is suitable for use in the method of the invention.
Preferably, the biocompatible low-modulus material will have a
compressional modulus not greater than about 4 Mpa, and within the
range defined above. Other suitable hydrogel polymers, for example,
include those disclosed in Ray et al., U.S. Pat. No. 6,132,465; Ray
et al., U.S. Pat. No. 6,602,291; Bao et al, U.S. Pat. No.
5,976,186; Bao et al., U.S. Pat. No. 6,280,475; Marcolongo et al.,
U.S. patent application Ser. No. 10/111,782 (European Patent No.
1229873); Stoy, U.S. Pat. No. 6,264,495; Husson, U.S. Pat. No.
5,919,235; McGuckin, U.S. Published Application No. 2003/0199979;
Trieu, U.S. Pat. No. 6,620,196; Studer, PCT Published Application
No. WO03/084444A1 and U.S. Published Patent Application No.
2005/0119750; and Breslave et al., French Patent 2712486, the
entire disclosure of each of which is incorporated herein by
reference.
[0057] The hydrogel or other low-modulus material may be introduced
into the central core of the annulus fibrosus in any suitable form.
A preferred configuration for the nucleus replacement or
supplementary material is a thin cylindrical shape that can be
inserted through a narrow channel less than 5 mm in diameter, such
as a small hole drilled in an adjacent vertebra. Alternative
preferred shapes for the inserted low-modulus material material
include a generally spherical shape or an ellipsoidal shape that
can be inserted through a narrow channel, preferably less than
about 5 mm in diameter. Curable materials that are introduced into
the region of the nucleus pulposus and cure therein to form a
low-modulus material are also implantable by the process of the
invention, as well as associating polymers of the type disclosed in
Marcolongo et al., U.S. patent application Ser. No. 10/111,782,
referenced above.
[0058] The practice of the invention will be illustrated by the
following example, which is intended to be illustrative and not
limiting.
EXAMPLE
[0059] This example illustrates filling of a nucleus pulposus
cavity in a spinal motion segment by the process of the invention.
Various studies by the inventors have described the Poisson effect
in a hydrogel and its usefulness in approximating the normal
synergistic mechanical properties of the nucleus in interaction
with the annulus. In order to achieve appropriate mechanical
properties, proper filling of the disc core volume is
essential.
[0060] A flexibility experiment was conducted by performing the
process of the invention for replacing the nucleus pulposus. The
flexibility of the spinal motion segment or unit was measured at
various steps in the procedure in order to illustrate the simulated
degeneration and restoration of the nucleus. An appropriate
specimen of an L4/L5 spinal motion segment was selected, including
the L4 and L5 lumbar vertebrae and the intervertebral disc
therebetween with intact annulus fibrosus and nucleus pulposus. The
selected specimen had an essentially normal nucleus pulposus. The
specimen was subjected to measurement of flexibility at four stages
before, during, and after the nucleus replacement procedure by
conducting a simulated flexion-extension series using pure moments.
The torque required for a range of defined angles of flexion and
extension was determined. The results are presented in the chart in
FIG. 7, which shows torque v. degrees of rotation in flexion and
extension.
[0061] The first of the four flexion-extension series was conducted
on the intact healthy disc; the results are shown in Curve 1. The
nucleus was then removed by a generally conventional procedure in
which a trephine of suitable size is used to form an axial aperture
in one of the adjacent vertebrae by removing a plug of bone, the
nucleus is removed through the aperture so created, and the plug of
bone is then reinserted and cemented in position to substantially
restore the vertebral structure. Such a procedure is substantially
similar to methods described by Joshi et al., Journal of Biomedical
Engineering, June 2005, Vol 127; p536-540, the entire disclosure of
which is incorporated herin by reference. The specimen was then
tested through the same applied moments. Accordingly, the second
series simulates a severely degraded nucleus; the results are as
shown in Curve 2. An access channel was then drilled through the
upper vertebra of the specimen spinal motion unit and the specimen
was implanted with a hydrogel implant in two stages. In the first
stage the specimen was implanted with a hydrogel implant prepared
as a thin string (diameter about 3 mm) inserted through a sheath
inserted in the drilled channel in an amount that partially filled
the volume left by the removal of the nucleus pulposus. The
hydrogel was prepared according to the teachings of Marcolongo et
al., U.S. Published Patent Application No. 2004/0220296. The
partially restored spinal motion segment, thereby simulated a
somewhat degenerated nucleus or a nucleus replaced without
pressurization. When the spinal motion segment so restored was
tested, a movement towards normal physiologic values over the range
of motion was found, as shown in Curve 3. Finally, when the
specimen was fully implanted with the hydrogel and the core was
completely filled and pressurized, by inserting a bone screw
according to the invention, close to full restoration of the disc
mechanics was found, as shown in Curve 4.
[0062] The invention having now been described in terms of certain
preferred embodiments it will be understood that modifications and
changes can be made thereto without departing from the spirit and
character thereof.
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