U.S. patent application number 10/201838 was filed with the patent office on 2002-12-05 for prosthetic spinal disc nucleus having a shape change characteristic.
This patent application is currently assigned to Raymedica, Inc.. Invention is credited to Assell, Robert L., Ray, Charles D..
Application Number | 20020183848 10/201838 |
Document ID | / |
Family ID | 23096832 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020183848 |
Kind Code |
A1 |
Ray, Charles D. ; et
al. |
December 5, 2002 |
Prosthetic spinal disc nucleus having a shape change
characteristic
Abstract
A prosthetic spinal disc nucleus comprising a hydrogel core
surrounded by a constraining jacket. The hydrogel core is
configured to expand from a dehydrated state to a hydrated state.
In the dehydrated state, the hydrogel core has a shape selected to
facilitate implantation through an anulus opening. Further, in the
hydrated state, the hydrogel core has a shape corresponding
generally to a portion of a nucleus cavity, the hydrated shape
being different from the dehydrated shape. Upon hydration, the
hydrogel core transitions from the dehydrated shape to the hydrated
shape.
Inventors: |
Ray, Charles D.;
(Williamsburg, VA) ; Assell, Robert L.; (Mendota
Heights, MN) |
Correspondence
Address: |
DICKE, BILLIG & CZAJA
701 Building, Suite 1250
701 Fourth Avenue South
Minneapolis
MN
55415
US
|
Assignee: |
Raymedica, Inc.
|
Family ID: |
23096832 |
Appl. No.: |
10/201838 |
Filed: |
July 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10201838 |
Jul 24, 2002 |
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09286047 |
Apr 5, 1999 |
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Current U.S.
Class: |
623/17.12 |
Current CPC
Class: |
A61F 2/441 20130101;
A61F 2002/30075 20130101; A61F 2002/449 20130101; A61F 2/442
20130101; A61F 2002/30092 20130101; A61F 2002/448 20130101; A61F
2002/30461 20130101; A61F 2220/0075 20130101; A61F 2002/3008
20130101; A61F 2210/0061 20130101; A61F 2/3094 20130101; A61F
2250/0098 20130101; A61F 2210/0014 20130101 |
Class at
Publication: |
623/17.12 |
International
Class: |
A61F 002/44 |
Claims
What is claimed is:
1. A spinal nucleus implant for replacement of at least a portion
of nucleus pulposus tissue removed from a spinal disc of a living
vertebrate to restore function of said spinal disc and related
vertebral joint, and implantable into the cavity created by said
removal of nucleus pulposus tissue, which comprises: a swellable,
biomimetic plastic, having a hydrophobic phase having high
crystallinity and low water content and with hydrophilic phase
having low crystallinity and high water content, said biomrimetic
plastic having an inherent shape in which it has a relaxed polymer
network in a state of full hydration, having an insertion shape in
which it is at least partially dehydrated to a xerogel state and
formable into a compacted mode for maximum efficiency of surgical
insertion, and capable of anisotropic expansion due to partial
rehydration in situ into an indwelling shape that substantially
conforms to the size and shape of said cavity and is capable of
osmotic movement of liquid therethrough in response to external
pressure change to thereby increase and decrease liquid content in
its hydrated state, said anisotropically swellable biomimetic
plastic having preferred swelling in a vertical plane and
suppressed swelling or minimal swelling in horizontal planes
2. The spinal nucleus implant of claim 1, wherein said implant is
anisotropically deformable in its said indwelling shape having
preferred deformability in a vertical plane and suppressed
deformability in horizontal planes under compression in the
vertical plane.
3. The spinal nucleus implant of claim 1, wherein said swellable,
biomimetic plastic is at least partially hydrated in its insertion
xerogel state.
4. The spinal nucleus implant of claim 1, wherein said swellable,
biomimetic plastic has been formed in a physiologically safe form
by being plasticized with a non-toxic liquid in its insertion
xerogel state.
5. The spinal nucleus implant of claim 4, wherein said non-toxic
liquid is present at a concentration less than 50% by weight of the
plasicized anisotropicafly swellable, biomimnetic plastic.
6. The spinal nucleus implant according to claim 3, wherein said
non-toxic liquid is water.
7. The spinal nucleus implant according to claim 1, wherein said
swellable, biomimetic plastic is a dehydrated anisotropically
swellable plastic wherein both said hydrophobic phase and said
hydrophilic phase each have hydrophobic and hydrophilic aspects and
said hydrophobic phase is a less hydrophilic phase having higher
content of hydrophobic groups and said hydrophilic phase is a less
hydrophobic phase having higher content of hydrophilic groups,
relative to one another.
8. The spinal nucleus implant according to claim 7, wherein said
anisotropically swellable, biomimetic plastic comprises
non-degradable polymer with a carbon-carbon backbone.
9. The spinal nucleus implant according to claim 7, wherein said
less hydrophilic phase is a crystalline phase containing nitrile
groups.
10. The spinal nucleus implant according to claim 7, wherein said
hydrophilic phase has hydrophilic groups which are selected from a
group consisting of hydroxyl, carboxyl, carboxylate, amide,
N-substituted amide, amidine and N-substituted amidine.
11. The spinal nucleus implant according to claim 1, wherein said
swellable, biomimetic plastic has water content more than 70% by
weight in said state of fully hydration by deionized water.
12. The spinal nucleus implant according to claim 1, wherein said
more hydrophilic phase is substantially discrete hydrophilic
domains dispersed in a substantially continuous less hydrophilic
domain.
13. The spinal nucleus implant according to claim 1, wherein both
the hydrophilic phase and the hydrophobic phase are substantially
continuous hydrophilic domains and hydrophobic domains forming an
interpenetrating network.
14. The spinal nucleus implant according to claim 1, wherein said
hydrophobic phase contains crystalline polymer phase detectable by
x-ray diffraction.
15. The spinal nucleus implant according to claim 7, wherein said
more hydrophobic phase is substantially discrete crystalline
domains dispersed in a substantially continuous more hydrophilic
domain.
16. The spinal nucleus implant according to claim 1 wherein said
swellable, biomimetic plastic has hydrophilic lubricious
surface.
17. The spinal nucleus implant according to claim 16, wherein said
surface is formed in a gradiented manner with increasing carboxylic
groups from the center of said implant towards its outer
surface.
18. The spinal nucleus implant according to claim 1 wherein said
implantable device has at least the two following structural
components: (a) an inner core from said swellable plastic; and (b)
an outer jacket that is surrounding said core and made from said
swellable plastic which is, in its fully hydrated state, less
swellable than said inner core.
19. The spinal nucleus implant according to claim 1, including at
least one reinforcing element from a substantially non-swellable
material embedded in said swellable, biomimetic plastic.
20. The spinal nucleus implant according to claim 18, and further
including at least one reinforcing element from a substantially
non-swellable material embedded in said swellable, biomimnetic
plastic wherein said at least one reinforcing element is located
between said jacket and said core.
21. The spinal nucleus implant according to claim 19, wherein said
at least one reinforcing element is made from an implantable
material selected from the group consisting of metal, metal alloys,
carbon, ceramics, polymer and combinations thereof.
22. The spinal nucleus implant according to claim 18, wherein said
inner core is adherent to and connected to said outer jacket.
23. The spinal nucleus implant according to claim 19, wherein said
reinforcing element has a general shape of a cylinder.
24. A surgical implant procedure for replacing at least a portion
of nucleus pulposus tissue removed from a spinal disc of a living
vertebrae to restore function of said spinal disc and related
vertebral joint, which comprises: (a) creating a spinal nucleus
implant in the form of an anisotropically swellable, biomimetic
xerogel plastic, having a two phase structure with a hydrophobic
phase having high crystallinity and low water content and with
hydrophilic phase having low crystallinity and high water content,
said xerogel plastic being capable of anisotropic expansion by
rehydration into an inherent shape in which it has a relaxed
polymer network in a state of full hydration, and being capable of
osmotic movement of liquid therethrough in response to external
pressure change to thereby increase and decrease liquid content in
its hydrated state said anisotropically swellable biomimetic
plastic having preferred swelling in a vertical plane and minimal
swelling or suppressed swelling in horizontal planes; (b)
surgically removing at least a portion of nucleus pulposus tissue
from a spinal disc of a living vertebrae to create a cavity; and
(c) implanting said spinal nucleus implant into said nucleus
pulposus cavity in an at least partially hydrated state.
25. The surgical implant procedure according to claim 24, wherein
said spinal nucleus implant, in said fully hydrated state, has
volume substantially larger than volume of said cavity vacated by
the removal of nucleus pulposus tissue.
26. The surgical implant procedure according to claim 24, wherein
said spinal nucleus implant, in said fully hydrated state, has a
cross-section area substantially equivalent to the cross-section
area of said cavity vacated by the removal of nucleus pulposus
tissue, and height substantially larger than the height of said
cavity, the "height" being the dimension substantial parallel with
the spinal axis and "cross-section area" being the area lateral to
the spinal axis.
27. The surgical implant procedure according to claim 24, wherein
said xerogel plastic swells in situ substantially more in the
direction of the spinal axis than in lateral direction.
28. The surgical implant procedure according to claim 24, wherein
said xerogel plastic is implanted in an anisotropically dehydrated
state in which its volume is less than 50% of the volume of said
cavity vacated by the removal of nucleus pulposus tissue.
29. The surgical implant procedure according to claim 28, wherein
said xerogel plastic in its anisotropically dehydrated state has
the shape optimized for insertion into the cavity through a small
incision in the annulus fibrosus, said shape being an approximate
shape of a cylindrical body.
30. The surgical implant procedure according to claim 28, wherein
said anisotropically dehydrated state is achieved by anisotropical
deformation of said xerogel.
31. The surgical implant procedure according to claim 30, wherein
said anisotropical deformation is achieved by heating the xerogel
above its glass transition temperature, exposing it to deforming
stress in a selected direction, and cooling it down under its glass
transition temperature while still exposed to said deforming
stress.
32. The surgical implant procedure according to claim 30, wherein
said anisotropical deformation is achieved by forming said xerogel
by drying the hydrated swellable plastic under a restraining
stress, preventing shrinking of xerogel in one or more selected
directions.
33. The surgical implant procedure according to claim 32, wherein
said restraining stress is an external stress caused by applying
pressure in axial direction during the dehydration process.
34. The surgical implant procedure according to claim 24, wherein
said hydrated implant is under axial stress substantially more
deformable in axial direction than in lateral direction.
35. A spinal nucleus implant for replacement of at least a portion
of nucleus pulposus tissue removed from a spinal disc of a living
vertebrate to restore function of said spinal disc and related
vertebral joint, and implantable into the cavity created by said
removal of nucleus pulposus tissue, which comprises: a swellable,
biomimetic plastic, having a hydrophobic phase having high
crystallinity and low water content and with hydrophilic phase
having low crystallinity and high water content, said biomrimetic
plastic having an inherent shape in which it has a relaxed polymer
network in a state of full hydration, having an insertion shape in
which it is at least partially dehydrated to a xerogel state and
formable into a compacted mode for maximum efficiency of surgical
insertion, and capable of anisotropic expansion due to partial
rehydration in situ into an indwelling shape that substantially
supports the size and shape of said cavity and is capable of
osmotic movement of liquid therethrough in response to external
pressure change to thereby increase and decrease liquid content in
its hydrated state, said anisotropically swellable biomimetic
plastic having preferred swelling in a vertical plane and
suppressed minimal swelling or swelling in horizontal planes.
36. A surgical implant procedure for replacing at least a portion
of nucleus pulposus tissue removed from a spinal disc of a living
vertebrae to restore function of said spinal disc and related
vertebral joint, which comprises: (a) creating a spinal nucleus
implant in the form of an anisotropically swellable, biomimetic
xerogel plastic, having a two phase structure with a hydrophobic
phase having high crystallinity and low water content and with
hydrophilic phase having low crystallinity and high water content,
said xerogel plastic being capable of anisotropic expansion by
rehydration into an inherent shape in which it has a relaxed
polymer network in a state of full hydration, and being capable of
osmotic movement of liquid therethrough in response to external
pressure change to thereby increase and decrease liquid content in
its hydrated state said anisotropically swellable biomimetic
plastic having preferred swelling in a vertical plane and minimal
swelling or suppressed swelling in horizontal planes; (b)
surgically removing at least a portion of nucleus pulposus tissue
from a spinal disc of a living vertebrae to create a cavity; and
(c) implanting said spinal nucleus implant into said nucleus
pulposus cavity in a less than hydrated state.
37. A spinal nucleus implant for replacement of at least a portion
of nucleus pulposus tissue removed from a spinal disc of a living
vertebrate to restore function of said spinal disc and related
vertebral joint, and implantable into the cavity created by said
removal of nucleus pulposus tissue, which comprises: a hydrogel
having an insertion shape formable into a compacted mode for
maximum efficiency of surgical insertion, and capable of
anisotropic expansion due to partial rehydration in situ into an
indwelling shape that supports the size and shape of said cavity
and is capable of osmotic movement of liquid therethrough in
response to external pressure change to thereby increase and
decrease liquid content in its hydrated state.
38. The spinal nucleus implant of claim 37, wherein the hydrogel
has an indwelling shape that substantially conforms to the size and
shape of said cavity.
39. The spinal nucleus implant of claim 37, wherein the hydrogel
has preferred swelling in a vertical plane and minimal swelling or
suppressed swelling in horizontal planes.
40. The spinal nucleus implant of claim 37, wherein said implant is
anisotropically deformable in its said indwelling shape having
preferred deformability in a vertical plane and suppressed
deformability in horizontal planes under compression in the
vertical plane.
41. The spinal nucleus implant of claim 37, wherein said hydrogel
is substantially dehydrated in its insertion state.
42. The spinal nucleus implant according to claim 37, wherein said
implantable device has at least the two following structural
components: (a) an inner core from said hydrogel; and (b) an outer
jacket that is surrounding said core and made from said hydrogel
which is, in its fully hydrated state, less swellable than said
inner core.
43. A surgical implant procedure for replacing at least a portion
of nucleus pulposus tissue removed from a spinal disc of a living
vertebrae to restore function of said spinal disc and related
vertebral joint, which comprises: (a) creating a spinal nucleus
implant in the form of an anisotropically swellable hydrogel
capable of osmotic movement of liquid therethrough in response to
external pressure change to thereby increase and decrease liquid
content in a hydrated state; (b) surgically removing at least a
portion of nucleus pulposus tissue from a spinal disc of a living
vertebrae to create a cavity; and (c) implanting said spinal
nucleus implant into said nucleus pulposus cavity in a less than
hydrated state.
44. The surgical implant procedure of claim 43, wherein the
hydrogel has preferred swelling in a vertical plane and minimal
swelling or suppressed swelling in horizontal planes.
45. The surgical implant procedure of claim 43, wherein the step of
implanting includes implanting the spinal nucleus implant in a
substantially dehydrated state.
46. The surgical implant procedure of claim 43, wherein the step of
implanting includes implanting the spinal nucleus implant in a
dehydrated state.
47. The surgical implant procedure according to claim 43, wherein
said hydrogel swells in situ substantially more in the direction of
the spinal axis than in lateral direction.
48. The surgical implant procedure according to claim 43, wherein
said hydrogel is implanted in an anisotropically dehydrated state
in which its volume is less than 50% of the volume of said cavity
vacated by the removal of nucleus pulposus tissue.
49. The surgical implant procedure according to claim 48, wherein
said hydrogel in its anisotropically dehydrated state has the shape
optimized for insertion into the cavity through a small incision in
the annulus fibrosus, said shape being an approximate shape of a
cylindrical body.
50. The surgical implant procedure according to claim 48, wherein
said anisotropically dehydrated state is achieved by anisotropical
deformation of said hydrogel.
51. The surgical implant procedure according to claim 50, wherein
said anisotropical deformation is achieved by forming said hydrogel
by drying the hydrated hydrogel under a restraining stress,
preventing shrinking of the hydrogel in one or more selected
directions.
52. The surgical implant procedure according to claim 51, wherein
said restraining stress is an external stress caused by applying
pressure in axial direction during the dehydration process.
53. The surgical implant procedure according to claim 43, wherein
said hydrated implant is under axial stress substantially more
deformable in axial direction than in lateral direction.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a prosthetic spinal disc
nucleus. More particularly, it relates to a prosthetic spinal disc
nucleus having a pre-implant shape for facilitating implantation
and a different, post-implant shape for restoring proper spacing
anatomical configuration of an intradiscal space.
[0002] The vertebral spine is the axis of the skeleton upon which
all of the body parts "hang". In humans, the normal spine has seven
cervical, twelve thoracic and five lumbar segments. The lumbar
segments sit upon a sacrum, which then attaches to a pelvis, in
turn supported by hip and leg bones. The bony vertebral bodies of
the spine are separated by intervertebral discs, which act as
joints, but allow known degrees of flexion, extension, lateral
bending and axial rotation.
[0003] The typical vertebra has a thick interior bone mass called
the vertebral body, with a neural (vertebral) arch that arises from
a posterior surface of the vertebral body. Each narrow arch
combines with the posterior surface of the vertebral body and
encloses a vertebral foramen. The vertebral foramina of adjacent
vertebrae are aligned to form a vertebral canal, through which the
spinal sac, cord and nerve rootlets pass. The portion of the neural
arch that extends posteriorly and acts to protect a posterior side
of the spinal cord is known as the lamina. Projecting from the
posterior region of the neural arch is a spinous process. The
central portions of adjacent vertebrae are each supported by an
intervertebral disc.
[0004] The intervertebral disc primarily serves as a mechanical
cushion between the vertebral bones, permitting controlled motions
within vertebral segments of the axial skeleton. The normal disc is
a unique, mixed structure, comprised of three component tissues:
The nucleus pulposus ("nucleus"), the anulus fibrosus ("anulus"),
and two opposing vertebral end plates. The two vertebral end plates
are each composed of thin cartilage overlying a thin layer of hard,
cortical bone which attaches to the spongy, richly vascular,
cancellous bone of the vertebral body. The end plates thus serve to
attach adjacent vertebrae to the disc. In other words, a
transitional zone is created by the end plates between the
malleable disc and the bony vertebrae.
[0005] The anulus of the disc is a tough, outer fibrous ring that
binds together adjacent vertebrae. This fibrous portion, which is
much like a laminated automobile tire, is generally about 10 to 15
millimeters in height and about 15 to 20 millimeters in thickness.
The fibers of the anulus consist of 15 to 20 overlapping multiple
plies, and are inserted into the superior and inferior vertebral
bodies at roughly a 30 degree angle in both directions. This
configuration particularly resists torsion, as about half of the
angulated fibers will tighten when the vertebrae rotate in either
direction, relative to each other. The laminated plies are less
firmly attached to each other.
[0006] Immersed within the anulus, positioned much like the liquid
core of a golf ball, is the nucleus. The anulus and opposing end
plates maintain a relative position of the nucleus in what can be
defined as a nucleus cavity. The healthy nucleus is largely a
gel-like substance having a high water content, and similar to air
in a tire, serves to keep the anulus tight yet flexible. The
nucleus-gel moves slightly within the anulus when force is exerted
on the adjacent vertebrae with bending, lifting, etc.
[0007] The nucleus and the inner portion of the anulus have no
direct blood supply. In fact, the principal nutritional source for
the central disc arises from circulation within the opposing
vertebral bodies. Microscopic, villous-like fingerlings of the
nuclear and anular tissue penetrate the vertebral end plates and
allow fluids to pass from the blood across the cell membrane of the
fingerlings and then inward to the nuclear tissue. These fluids are
primarily body water and the smallest molecular weight nutrients
and electrolytes.
[0008] The natural physiology of the nucleus promotes these fluids
being brought into, and released from, the nucleus by cyclic
loading. When fluid is forced out of the nucleus, it passes again
through the end plates and then back into the richly vascular
vertebral bodies. The cyclic loading amounts to daily variations in
applied pressure on the vertebral column (e.g., body weight and
muscle pull) causing the nucleus to expel fluids, followed by
periods of relaxation and rest, resulting in fluid absorption or
swelling by the nucleus. Thus, the nucleus changes volume under
loaded and non-loaded conditions. Further, the resulting tightening
and loosening effect on the anulus stimulates the normal anulus
collagen fibers to remain healthy or to regenerate when torn, a
process found in all normal ligaments related to body joints.
Notably, the ability of the nucleus to release and imbibe fluids
allows the spine to alter its height and flexibility through
periods of loading or relaxation. Normal loading cycling is thus an
effective nucleus and inner anulus tissue fluid pump, not only
bringing in fresh nutrients, but perhaps more importantly, removing
the accumulated, potentially autotoxic by-products of
metabolism.
[0009] The spinal disc may be displaced or damaged due to trauma or
a disease process. A disc herniation occurs when the anulus fibers
are weakened or torn and the inner tissue of the nucleus becomes
permanently bulged, distended, or extruded out of its normal,
internal anular confines. The mass of a herniated or "slipped"
nucleus can compress a spinal nerve, resulting in leg pain, loss of
muscle control, or even paralysis. Alternatively, with discal
degeneration, the nucleus loses its water binding ability and
deflates, as though the air had been let out of a tire.
Subsequently, the height of the nucleus decreases, causing the
anulus to buckle in areas where the laminated plies are loosely
bonded. As these overlapping laminated plies of the anulus begin to
buckle and separate, either circumferential or radial anular tears
may occur, which may contribute to persistent and disabling back
pain. Adjacent, ancillary spinal facet joints will also be forced
into an overriding position, which may create additional back
pain.
[0010] Whenever the nucleus tissue is herniated or removed by
surgery, the disc space will narrow and may lose much of its normal
stability. In many cases, to alleviate pain from degenerated or
herniated discs, the nucleus is removed and the two adjacent
vertebrae surgically fused together. While this treatment
alleviates the pain, all discal motion is lost in the fused
segment. Ultimately, this procedure places greater stress on the
discs adjacent the fused segment as they compensate for the lack of
motion, perhaps leading to premature degeneration of those adjacent
discs. A more desirable solution entails replacing in part or as a
whole the damaged nucleus with a suitable prosthesis having the
ability to complement the normal height and motion of the disc
while stimulating the natural disc physiology.
[0011] The first prostheses embodied a wide variety of ideas, such
as ball bearings, springs, metal spikes and other perceived aids.
These prosthetic discs were designed to replace the entire
intervertebral disc space and were large and rigid. Beyond the
questionable efficacy of those devices was the inherent
difficulties encountered during implantation. Due to their size and
inflexibility, these first generation devices required an anterior
implantation approach as the barriers presented by the lamina and,
more importantly, the spinal cord and nerve rootlets during
posterior implantation, could not be avoided. Recently, smaller and
more flexible prosthetic nucleus devices have been developed. With
the reduction in prosthesis size, the ability to work around the
spinal cord and nerve rootlets during posterior implantation has
become possible.
[0012] Generally speaking, these reduced size prostheses are
intended to serve as a replacement for the natural nucleus. In
other words, the anulus and end plates remain intact, and the
prosthesis implanted within the nucleus cavity. It is generally
believed that this approach facilitates healing of the anulus.
Unfortunately, however, inherent design characteristics of these
prostheses may in fact damage the anulus. For example, Bao et al.,
U.S. Pat. No. 5,047,055, discloses a prosthetic nucleus made of a
hydrogel material that is implanted into the intradiscal space in a
dehydrated state. Following implant, the hydrogel material hydrates
and expands without constraint to, at least in theory, a shape
conforming to the natural nucleus. Similarly, Bao et al., U.S. Pat.
No. 5,192,326, describes a prosthetic nucleus comprised of a solid
hydrogel core or of a multiplicity of hydrogel beads surrounded by
a membrane. Once again, this prosthesis is implanted into the disc
space in a dehydrated state, subsequently hydrating, at least in
theory, to a shape conforming to the natural nucleus. The
prostheses of Bao, as well as other similar products, rely solely
upon the natural anulus to constrain expansion of the hydrogel
core. Obviously, this essentially uncontrolled expansion imparts a
lateral force directly upon the anulus. In most situations, the
anulus is already damaged, and any additional forces placed on the
anulus by the prosthesis may impede healing and even cause further
deterioration. Further, it is virtually impossible to accurately
orientate the dehydrated prostheses of Bao within the nucleus
cavity due to the confined environment.
[0013] As previously described, an important feature of a
prosthetic nucleus is that the anulus is not entirely removed upon
implantation. Normally, however, an opening of some type must be
created through the anulus. The prosthetic nucleus is then passed
through this opening for implantation into the nucleus cavity.
Because creation of this opening traumatizes the anulus, it is
highly desirable to minimize its size. Unfortunately, however, most
prosthetic nucleus devices currently available do not account for
this generally accepted implantation technique. For example, a
relatively rigid prosthesis configured to approximate a shape of
the natural nucleus requires an extremely large opening in the
anulus in order for the prosthetic device to "pass" into the
nucleus cavity. Further, a hydrogel-based prosthesis, such as that
described in Bao, minimizes, at least in part, the size of the
anulus opening in that the hydrogel prosthesis is implanted in a
dehydrated state, thereby having a reduced overall size. However,
even in the dehydrated state, the Bao prosthesis still has a shape
generally conforming to that of a natural nucleus. As a result,
regardless of orientation, a relatively blunt surface is presented
to the anulus when attempting to implant. This blunt surface is not
conducive to insertion through the anulus opening. In fact, the
blunt surface may impede implantation, thereby requiring an
enlarged opening in the anulus.
[0014] In addition to the above-described concern for minimizing
stress on the anulus, anatomical variations of the nucleus cavity
should also be considered. Generally speaking, each intradiscal
space has a greater transverse diameter (as defined by the anulus)
at a posterior side than at an anterior side. Additionally, the
intradiscal space varies in height (as defined by the opposing end
plates) from posterior side to anterior side. In this regard, each
intradiscal space has a relatively unique height configuration. For
example, the L3-L4 intradiscal space has a slightly greater height
at a central area in comparison to the posterior and anterior
sides. The L4-L5 intradiscal space displays a more dramatic
increase in central height. Finally, the L5-S1 intradiscal space
increases in height from the posterior side to the anterior side.
Effectively, each intradiscal space can be generally referred to as
having an anterior area. With these dimensional variations in mind,
a "standard" or single-sized prosthesis likely will not meet the
anatomical needs of each and every intradiscal space. This is
especially true for a single, rigid prosthesis design sized to
encompass the entire intradiscal space that therefore does not
recognize the general distinction between the anterior area and the
posterior area. A prosthetic nucleus that fails to account for the
anatomical variation in height of the nucleus cavity may also
result in an uneven load distribution across the prosthesis and
therefore poor spacing performance.
[0015] Finally, restoring the nutrition-flushing cycle of a natural
disc is important for a prosthetic spinal disc nucleus to be
successful. As previously described, most of the nutrition for the
inner anulus and nucleus is provided by diffusion through the end
plates of the vertebral bodies and by the important pumping action
between the partially loaded and fully loaded conditions of the
disc. If the nutritional cycle is impeded, a variety of
degenerative changes may occur. Nutrition to the inner disc slowly
ceases, resulting in intradiscal build-up of acids and autotoxins,
and other changes. This is followed by anular fiber and nucleus
degeneration, shrinkage of the nucleus, segmental laxity, spur
formation, disc space collapse and perhaps spontaneous fusion.
Significantly disabling back pain may also develop. Thus, a
prosthetic nucleus sized to encompass the entire nucleus cavity
prevents the fluid pumping action of the disc space from occurring,
and will not result in complete healing.
[0016] Degenerated, painfully disabling intraspinal discs are a
major economic and social problem for patients, their families,
employers and the public at large. Any significant means to correct
these conditions without further destruction or fusion of the disc
may therefore serve an important role. Other means to replace the
function of a degenerated disc have major problems such as complex
surgical procedures, unproven efficacy, placing unnecessary and
possibly destructive forces on an already damaged anulus, etc.
Therefore, a substantial need exists for a prosthetic spinal disc
nucleus formed to facilitate implantation through an anulus opening
while providing necessary intradiscal support following
implant.
SUMMARY OF THE INVENTION
[0017] The present invention provides an elongated prosthetic
spinal disc nucleus for implantation within a nucleus cavity
defined by opposing end plates and an anulus, and a method of
manufacturing such a prosthesis. In one preferred embodiment, the
prosthesis is comprised of a formed hydrogel core surrounded by a
constraining jacket.
[0018] The hydrogel core is configured to expand from a dehydrated
state to a hydrated state. In this regard, the hydrogel core has a
dehydrated shape in the dehydrated state and a hydrated shape in
the hydrated state. The dehydrated shape is configured to
facilitate insertion of the prosthetic spinal disc nucleus through
an opening in the anulus. Further, the dehydrated shape is
generally different from the hydrated shape, which in one preferred
embodiment relates to size characteristics of the nucleus
cavity.
[0019] The constraining jacket surrounds the hydrogel core and
constrains expansion upon hydration. The constraining jacket is
preferably flexible but substantially inelastic. Further, in one
preferred embodiment, the constraining jacket has a generally fixed
maximum volume that is less than the volume of the nucleus
cavity.
[0020] The method of manufacturing a prosthetic spinal disc nucleus
in accordance with the present invention includes providing a
hydrogel material that expands from a dehydrated state to a
hydrated state. The hydrogel material is then formed into a
hydrogel core having a first shape in the hydrated state. The
hydrogel core is inserted into a constraining jacket and reshaped
to have a second shape in the dehydrated state, the second shape
being different from the first shape. In this regard, the hydrogel
core is configured to transition from the second shape to the first
shape upon hydration. In one preferred embodiment, reshaping the
hydrogel core to have a second shape in the dehydrated state
includes forcing the hydrogel core to an elongated shape defined by
a leading end, a trailing end and a central portion, the hydrogel
core tapering from the central portion to the leading end. This
taper facilitates insertion of the leading end of the hydrogel
core, otherwise encompassed by the constraining jacket, through an
opening in the anulus.
[0021] The prosthetic spinal disc nucleus is implanted into the
nucleus cavity with the hydrogel core in a dehydrated state. In one
preferred embodiment, in the dehydrated state, the hydrogel core
has a tapered leading end to facilitate insertion through an
opening in the anulus. Once inserted, the prosthetic spinal disc
nucleus is preferably transversely orientated within the nucleus
cavity, and the hydrogel core is allowed to hydrate. During
hydration, the hydrogel core transitions from the dehydrated shape
to a predetermined hydrated shape. The hydrated shape preferably
conforms with a general anatomical spacing of the particular disc
space. For example, in one preferred embodiment, the hydrogel core
is wedge shaped in the hydrated state, having a variable height
corresponding generally to a shape of the nucleus cavity.
[0022] Another aspect of the present invention relates to a
prosthetic spinal disc nucleus for implantation into a nucleus
cavity of a spinal disc. The nucleus cavity has a height defined by
an opposing pair of end plates and an outer periphery defined by an
anulus. The prosthetic spinal disc nucleus comprises a formed
hydrogel core surrounded by a constraining jacket. The formed
hydrogel core is configured to expand from a dehydrated state to a
hydrated state. The hydrogel core has a streamlined shape in the
dehydrated state and a generally wedge shape in the hydrated state.
Further, the hydrogel core is configured to transition from the
streamlined shape to the wedge shape upon hydration. The
constraining jacket is flexible but substantially inelastic, having
a generally fixed maximum volume that is less than a volume of the
nucleus cavity. With this configuration, the constraining jacket
allows the hydrogel core to transition from the streamlined shape
to the wedge shape upon hydration. However, the constraining jacket
limits expansion of the hydrogel core in the hydrated state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1A is a perspective view of a prosthetic spinal disc
nucleus in a dehydrated state, including a cutaway view showing a
portion of a hydrogel core, in accordance with the present
invention;
[0024] FIG. 1B is a side, sectional view of the prosthetic spinal
disc nucleus of FIG. 1A along the line 1B-1B;
[0025] FIG. 1C is a top, sectional view of the prosthetic spinal
disc nucleus of FIG. 1A along the line 1C-1C;
[0026] FIG. 1D is a perspective view of the prosthetic spinal disc
nucleus of FIG. 1A in a hydrated state;
[0027] FIGS. 2A and 2B are perspective views of an alternative
prosthetic spinal disc nucleus, including a cutaway view showing a
portion of a hydrogel core, in accordance with the present
invention;
[0028] FIGS. 3A and 3B are perspective views of an alternative
prosthetic spinal disc nucleus, including a cutaway view showing a
portion of a hydrogel core, in accordance with the present
invention;
[0029] FIGS. 4A and 4B are perspective views of an alternative
prosthetic spinal disc nucleus, including a cutaway view showing a
portion of a hydrogel core, in accordance with the present
invention;
[0030] FIG. 5 is an elevated view of a spinal segment including a
degenerated discal area;
[0031] FIG. 6 is a posterior view of a portion of a human spine,
showing an opening through an anulus;
[0032] FIGS. 7A and 7B illustrate implantation of a prosthetic
spinal disc nucleus into a discal segment through an opening in the
anulus;
[0033] FIG. 8 is a top, sectional view of a disc space having a
prosthetic spinal disc nucleus implanted in a dehydrated state;
[0034] FIG. 9 is a lateral, sectional view of a disc space having
one implanted prosthetic spinal disc nucleus, and a second,
partially implanted prosthetic spinal disc nucleus;
[0035] FIG. 10 is a top, sectional view of a disc space having two
prosthetic spinal disc nuclei implanted and in a hydrated state;
and
[0036] FIG. 11 is a lateral, sectional view of a human spine having
several prosthetic spinal disc nuclei implanted and in a hydrated
state.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] One preferred embodiment of a prosthetic spinal disc nucleus
20 is shown in FIG. 1A. The prosthetic spinal disc nucleus 20 is
comprised of a hydrogel core 22 and a constraining jacket 24. The
constraining jacket 24 is secured about the hydrogel core 22 by
closures 26 located at opposite ends of the constraining jacket
24.
[0038] As will be made more clear below, the prosthetic spinal disc
nucleus 20 of the present invention is described with reference to
a first, pre-implant shape and a second, post-implant shape. To
this end, because the hydrogel core 22 is dehydrated prior to
implant and hydrated following implant, the pre-implant shape can
also be referred to as a dehydrated shape; whereas the post-implant
shape is referred to as a hydrated shape. As a point of reference,
FIGS. 1A-1C depict the dehydrated shape; whereas FIG. 1D depicts
the hydrated shape.
[0039] In a preferred embodiment, the hydrogel core 22 is
configured to imbibe fluids, expanding from a dehydrated state
(shown in FIG. 1A) to a hydrated state (FIG. 1D). In this regard,
the hydrogel core 22 is preferably formulated as a mixture of
hydrogel polyacrylonitrile. In particular, acrylamide and
acrylonitrile (block co-polymer) are used. Alternatively, the
hydrogel core 22 can be any hydrophilic acrylate derivative with a
unique multi-block co-polymer structure or any other hydrogel
material having the ability to deform and reform in a desired
fashion in response to placement and removal of loads. Even
further, a biologically safe polymer that can imbibe fluids while
maintaining its structure under various stresses is acceptable. For
example, the hydrogel core 22 can be formulated as a mixture of
polyvinyl alcohol and water. Much like a normal nucleus, the
hydrogel core 22 will initially swell from a dehydrated state as it
absorbs fluid. When hydrated, the hydrogel core 22 will have a
water content of 25-90 percent. The hydrogel material used for the
hydrogel core 22 in the preferred embodiment is manufactured under
the trade name HYPAN.RTM. by Hymedix International, Inc. of Dayton,
New Jersey.
[0040] As shown in FIG. 1A, the hydrogel core 22 defines a leading
end 28, a central portion 30 and a trailing end 32. As described in
greater detail below, the leading end 28 and the trailing end 32
are in reference to a preferred orientation of the prosthetic
spinal disc nucleus 20 during an implantation procedure. For the
purposes of this disclosure, directional terminology such as
"leading" and "trailing" are with reference to one possible
orientation of the prosthetic spinal disc nucleus 20 during
implantation. It should be understood, however, that due to its
unique sizing, the prosthetic spinal disc nucleus 20 can be
orientated in any direction relative to a nucleus cavity (not
shown) or the world in general. As such, the directional terms are
provided for purposes of illustration only, and should not be
interpreted as limitations.
[0041] As a point of reference, the prosthetic spinal disc nucleus
20 is defined by a width (x-axis in FIGS. 1A and 1C), a length
(y-axis in FIGS. 1A-1C) and a height (z-axis in FIGS. 1A and 1B).
With this in mind, the hydrogel core 22, and thus the prosthetic
spinal disc nucleus 20, is fabricated to assume a streamlined shape
in the dehydrated state. The term "streamlined" is with reference
to the hydrogel core 22 being configured, in the dehydrated state,
to taper or decrease in height (z-axis) from the central portion 30
to the leading end 28, as shown most clearly in FIG. 1B (side,
cross-sectional view). In one preferred embodiment, in the
dehydrated state, the hydrogel core 22 is further configured to
taper or decrease in height (z-axis) from the central portion 30 to
the trailing end 32. With this preferred embodiment, then, opposing
sides of the hydrogel core 22 are generally convex, resulting in
the generally convexo-convex shape of FIG. 1B. While the taper or
decrease in height (z-axis) is preferably uniform, other designs
are acceptable. In general terms, a side sectional view of the
hydrogel core 22 defines a leading profile 34 terminating at the
leading end 28 and a trailing profile 36 terminating at the
trailing end 32. The "streamlined" shape in the dehydrated state
relates to the leading profile 34 being conical, tapering in height
to the leading end 28. Further, in a preferred embodiment, the
trailing profile 36 is also conical.
[0042] In addition to the above-described streamlined shape, in one
preferred embodiment, a top, cross-sectional view (FIG. 1C) shows
the central portion 30 of the hydrogel core 22 as being curved.
More particularly, and with reference to FIG. 1C, opposing sides of
the hydrogel core 22 curve in a generally symmetrical fashion from
the leading end 28 to the trailing end 32. Alternatively, the
opposing side may be linear, non-symmetrical etc.
[0043] Completely surrounding the hydrogel core 22 is the
constraining jacket 24. The constraining jacket 24 is preferably a
flexible tube made of tightly woven high molecular weight, high
tenacity polymeric fabric. In a preferred embodiment, high
molecular weight polyethylene is used as the weave material for the
constraining jacket 24. However, polyester or any high tenacity
polymeric material can be employed, and carbon fiber yarns, ceramic
fibers, metallic fibers, etc., also are acceptable.
[0044] The constraining jacket 24 is preferably made of fibers that
have been highly orientated along their length. As a result, the
constraining jacket 24 material, while flexible, has little
elasticity or stretch. The constraining jacket 24 defines a
generally fixed maximum volume, including a generally fixed length
(y-axis of FIGS. 1A-1C). In one preferred embodiment, the generally
fixed maximum volume of the constraining jacket 24 is less than a
theoretical volume of the hydrogel core 22 if allowed to completely
hydrate without constraint. Thus, because the hydrogel core 22 has
a natural, fully hydrated volume greater than the constraining
jacket 24, the constraining jacket 24 will be tight about the
hydrogel core 22 when hydrated, as described in greater detail
below. Additionally, the volume differential between the
constraining jacket 24 and the hydrated hydrogel core 22 serves to
extend the useful life of the prosthetic spinal disc nucleus 20. In
particular, the constraining jacket 24 effectively prevents the
hydrogel core 22 from reaching its natural hydration level.
Consequently, the hydrogel core 22 will have a constant affinity
for imbibing additional fluid. Finally, as shown in FIGS. 1B and
1C, the hydrogel core 22 is preferably configured such that in the
dehydrated state, the hydrogel core 22 has a length approximating
the generally fixed maximum length of the constraining jacket 24.
Thus, the hydrogel core 22 causes the constraining jacket 24 to be
relatively taut along its length (y-axis). Notably, the hydrogel
core 22 in the dehydrated state does not encompass the entire
available volume of the constraining jacket 24.
[0045] The preferred woven construction of the constraining jacket
24 creates a plurality of small openings 38. Each of the plurality
of small openings 38 is large enough to allow bodily fluids to
interact with the hydrogel core 22 otherwise maintained within the
constraining jacket 24. However, each of the plurality of small
openings 38 is small enough to prevent the hydrogel core 22 from
escaping. Each of the plurality of small openings 38 preferably has
an average diameter of about 10 micrometers, although other
dimensions are acceptable. In this regard, although the
constraining jacket 24 has been described as having a woven
configuration, any other configuration having a semi-permeable or
porous attribute can be used. Finally, the constraining jacket 24
material preferably allows for tissue in-growth and is textured to
provide a grip or purchase within a disc space (not shown).
[0046] As indicated above, the hydrogel core 22 is configured to
expand from the dehydrated shape, shown in FIGS. 1A-1C, to a
hydrated shape, shown in FIG. 1D, following implant. Manufacture of
the hydrogel core 22 is described in greater detail below.
Generally speaking, however, the hydrogel core 22 is constructed
such that the hydrated shape is different from the dehydrated
shape. In other words, the hydrogel core 22 has a streamlined shape
in the dehydrated state to facilitate implant, and preferably has a
shape generally corresponding to the shape of a portion of a
nucleus cavity (not shown) in the hydrated state. One example of
the hydrated prosthetic spinal disc nucleus 20 is shown in FIG. 1D.
In the hydrated state, the hydrogel core 22, and thus the
prosthetic spinal disc nucleus 20, defines an anterior face 50
(partially hidden in FIG. 1D), a posterior face 52, and opposing
end plate faces 54, 56 (partially hidden in FIG. 1D). The opposing
end plate faces 54, 56 may also be referred to as a superior face
and an inferior face, respectively. For the purposes of this
disclosure, directional terminology such as "anterior,"
"posterior," "superior," and "inferior" are with reference with one
possible orientation of the prosthetic spinal disc nucleus 20
within a nucleus cavity (not shown). It should be understood,
however, that due to its unique sizing, the prosthetic spinal disc
nucleus 20 can be orientated in any direction relative to a nucleus
cavity or the world in general. As such, the directional terms are
provided for purposes of illustration only, and should not be
interpreted as limitations. As a point of reference, FIG. 1D again
identifies the leading end 28 and the trailing end 32.
[0047] A comparison of the prosthetic spinal disc nucleus 20 in the
dehydrated state (FIG. 1A) with that of the hydrated state (FIG.
1D) graphically illustrates the preferred transition in shape of
the hydrogel core 22. The hydrogel core 22 has transitioned, upon
hydration, from the streamlined configuration of FIG. 1A to a
rectangular configuration of FIG. 1D. In particular, the hydrogel
core 22 in the hydrated state does not taper from the central
portion 30 to the leading end 28 or the trailing end 32. Instead,
the hydrogel core 22 has a relatively uniform height (z-axis in
FIG. 1D). In other words, with hydration, the hydrogel core 22
transitions from the substantially convexo-convex cross-sectional
shape of FIG. 1B to the rectangular (or plano-plano) shape of FIG.
1D. Further, in the hydrated state, the central portion 30 of the
hydrogel core 22 is no longer curved along its length, as
previously described with reference to the preferred embodiment of
FIG. 1C. As described in greater detail below, the prosthetic
spinal disc nucleus 20 in the hydrated state is uniquely designed
to generally adhere to the spacing requirements of a particular
disc space (not shown).
[0048] The desired dehydrated and hydrated shapes of the prosthetic
spinal disc nucleus 20, and in particular the hydrogel core 22, are
generated during manufacture. First, the hydrogel core 22 is
formulated. In the preferred embodiment, the selected hydrogel
material has an inherent shape memory attribute. An appropriate
volume of hydrogel material, dissolved or suspended in a solvent,
is poured into a mold having a shape corresponding to the desired
hydrated shape. For example, to achieve the rectangular
configuration of the prosthetic spinal disc nucleus 20 of FIG. 1D,
the hydrogel material is poured into a mold having a rectangular
shape. Once cast, a solvent exchange process is performed,
replacing the solvent with water such that the hydrogel material
hydrates to a maximum hydration level, thereby creating the
hydrogel core 22. As a result of this solvent exchange process, a
rectangular, hydrated shape is imparted into the shape memory of
the hydrogel core 22.
[0049] In the hydrated state, the hydrogel core 22 is relatively
soft. To aid in ensuring proper placement of the prosthetic spinal
disc nucleus 20 within an intervertebral disc space and to review
the stability of the prosthetic spinal disc nucleus 20 during
follow-ups, a radiopaque wire (not shown) may be forced into the
hydrogel core. The radiopaque wire is preferably made of a
platinum-iridium material, but can be any other material having
radiopaque and biologically inert characteristics. Notably, the
preferred platinum-iridium material is visible by normal,
inexpensive x-ray procedures, as well as by computer-generated
imaging.
[0050] The hydrogel core 22 is then preferably placed in an oven
and dehydrated, resulting in an under-sized, rectangular-shaped
body. The hydrogel core 22, in a dehydrated state, is then inserted
into the constraining jacket 24.
[0051] Prior to insertion of the hydrogel core 22, the constraining
jacket 24 is an elongated, open-ended tube, and does not include
the closures 26. The dehydrated hydrogel core 22 is inserted
axially into the constraining jacket 24 through one of the open
ends and centrally positioned. The open ends of the constraining
jacket 24 are then secured by forming the closures 26. For example,
the material at the open ends may be folded and then closed by
sewing a dense, bar-tack stitch at a position near the hydrogel
core 22. The bar-tack stitch material is preferably the same high
tenacity, high polymeric material, such as a high molecular weight
polyethylene, as is used for the constraining jacket 24. By
employing the same material for both the constraining jacket 24 and
the bar-tack stitch, the biocompatibility of the entire prosthetic
spinal disc nucleus 20 is ensured. Any excess material is removed
from the constraining jacket 24 by a thermal cut. This thermal cut
fuses the potentially fraying ends of the constraining jacket 24
distal the stitching.
[0052] Following closure of the constraining jacket 24 about the
hydrogel core 22, the prosthetic spinal disc 20, and in particular
the hydrogel core 22, is rehydrated. In this regard, the hydrogel
core 22 is allowed to hydrate and expand to a volumetric limit of
the constraining jacket 24.
[0053] Assuming the constraining jacket 24 and the closures 26 do
not fail, the hydrogel core 22 is then "conditioned". This
conditioning amounts to at least three compressive loads being
applied across the length of the prosthetic spinal disc nucleus 20.
The selected magnitude of the compressive loads relates to an in
vivo compressive load normally encountered by a patient. In this
regard, the magnitude of in vivo compressive loads varies from
patient to patient and is a function of a patient's size and spinal
level. For example, published literature has stated that the normal
standing or sitting compressive load on the discal area is 1.8
multiplied by the patient's body weight. Further, the maximum
compressive load placed on the lumbar discal area during normal,
daily activities is 3.6 multiplied by the patient's body weight.
The conditioning, therefore, will consist of a series of
compressive loads being placed on the prosthetic spinal disc
nucleus 20 equivalent to a maximum of 1.8 multiplied by a typical
body weight; up to a maximum of 3.6 multiplied by a typical body
weight.
[0054] With reference to FIG. 1D, the compressive loads are applied
along a plane substantially normal to the opposing end plate faces
54, 56. To accomplish this effect, the hydrogel core 22 is
preferably maintained within a clamp configured to maintain the
rectangular shape of the hydrogel core 22.
[0055] As a result of the above-described conditioning, in
combination with other elements such as size, shape, etc., the
hydrogel core 22, and thus the prosthetic spinal disc nucleus 20,
will have a known load bearing ability. The resulting hydrogel core
22 is viscoelastic, having a defined a cross-sectional area and
thickness, as well as a defined compression modules of elasticity.
Due to conditioning, the hydrogel core 22, and thus the prosthetic
spinal disc nucleus 20, will consistently adhere to a known change
in height in response to various loads. The conditioning ensures
that the hydrogel core 22 is deformable, but essentially is not
compressible.
[0056] Following conditioning, the hydrogel core 22 is reshaped and
dehydrated. More particularly, the prosthetic spinal disc nucleus
20 is placed into a mold having a streamlined shape corresponding
to the shape of the hydrogel core 22 shown in FIGS. 1A-1C. The
streamlined-shaped mold is secured about the prosthetic spinal disc
nucleus 20 and exerts a pressure onto the hydrogel core 22. The
mold containing the prosthetic spinal disc nucleus 20 is preferably
placed in an oven to expedite dehydration of the hydrogel core 22.
Following this processing, the dehydrated hydrogel core 22 assumes
the streamlined shape previously described. Once again, following
reshaping and in the dehydrated state, the hydrogel core 22 has a
length (y-axis in FIGS. 1B and 1C) approximating the generally
fixed maximum length of the constraining jacket 24. Thus, the
constraining jacket 24 is relatively taut along its length (y-axis
in FIGS. 1A-1C). Upon hydration, the hydrogel core 22 will expand
to the shape shown in FIG. 1A due to the shape memory attribute of
the hydrogel material.
[0057] Prior to implant, the prosthetic spinal disc nucleus 20 is
preferably, but not necessarily, maintained, in a dehydrated state,
within a retaining tube (not shown) sized to maintain the generally
streamlined shape of the hydrogel core 22. The retaining tube is
preferably made of implantable grade stainless steel, but can be
any other surgically safe material such as polyethylene. The
prosthetic spinal disc nucleus 20 and its retaining tube may be
packaged in a dry foam. The entire surgical package is sterilized
in a tray, via gas, steam or other form of sterilization. Once
conditioned, reshaped and sterilized, the dehydrated prosthetic
spinal disc nucleus 20 is ready for implantation into a human disc
space (not shown).
[0058] Importantly, the above-described manufacturing process
allows for the production of the prosthetic spinal disc nucleus
having a number of different hydrated shapes. For example, as
described in greater detail below, one advantage of a prosthesis of
the present invention is the general conformance, upon hydration,
to the anatomical shape of a general area or a compartment of a
disc space. For example, the prosthetic spinal disc nucleus 20 has
been shown as, in the hydrated state, generally assuming a
rectangular shape. It should be understood, however, that an
individual disc space or intradiscal area/compartment may present
additional anatomical variations. In recognition of these
anatomical variances, the prosthetic spinal disc nucleus 20 in
accordance with the present invention may be manufactured to assume
other shapes in the hydrated state. For example, one alternative
embodiment of a prosthetic spinal disc nucleus 70 is shown in FIGS.
2A and 2B.
[0059] The prosthetic spinal disc nucleus 70 is shown in a hydrated
state in FIG. 2A, and a dehydrated state in FIG. 2B. The prosthetic
spinal disc nucleus 70 is highly similar to the prosthetic spinal
disc nucleus 20 (FIG. 1A) previously described and is comprised of
a hydrogel core 72 surrounded by a constraining jacket 74. The
constraining jacket 74 is secured about the hydrogel core 72 by
closures 76. The hydrogel core 72 has a leading end 78, trailing
end 80 and central portion 82, defined most clearly in the
dehydrated state (FIG. 2B). In the hydrated state (FIG. 2A), the
central portion 82, and thus the prosthetic spinal disc nucleus 70,
more accurately defines an anterior face 84 (shown partially in
FIG. 2A), a posterior face 86, and opposing end plate faces 88, 90
(shown partially in FIG. 2A).
[0060] The prosthetic spinal disc nucleus 70 is fabricated to
assume an elongated wedge shape in the hydrated state. In other
words, in the hydrated state, the anterior face 84, the posterior
face 86 and the opposing end plate faces 88, 90 are substantially
rectangular, whereas the leading end 78 and the trailing end 80 are
tapered or wedge shaped. Thus, in the hydrated state, the
prosthetic spinal disc nucleus 70 has a height (z-axis in FIG. 2B)
increasing from the posterior face 86 to the anterior face 84. For
this reason, it should be understood that the alternative
prosthetic spinal disc nucleus 70 can be referenced as a "tapered
prosthetic spinal disc nucleus," whereas the prosthetic spinal disc
nucleus 20 (FIGS. 1A-1D) can be referred to as a "rectangular
prosthetic spinal disc nucleus."
[0061] Other than being configured to have a different shape in the
hydrated state, the prosthetic spinal disc nucleus 70 is identical
to the prosthetic spinal disc nucleus 20 (FIGS. 1A-1D). In a
dehydrated state (FIG. 2B), the prosthetic spinal disc nucleus 70
has the same streamlined shape as the prosthetic spinal disc
nucleus 20 shown in FIG. 1D. Thus, the prosthetic spinal disc
nucleus 70 is manufactured in a highly similar fashion, except that
a different mold is used during initial formation of the hydrogel
core 72. Subsequent reshaping of the hydrogel core 72 results in
the streamlined shape of FIG. 2B. Due to a shape memory attribute
of the hydrogel core 72, upon hydration, the hydrogel core 72 will
transition from the dehydrated, streamlined shape of FIG. 2B to the
hydrated, tapered shape of FIG. 2A.
[0062] Yet another alternative embodiment of a prosthetic spinal
disc nucleus 100 is shown in FIGS. 3A-3B. As a point of reference,
FIG. 3A depicts the prosthetic spinal disc nucleus 100 in a
hydrated state; whereas FIG. 3B is a dehydrated configuration. The
prosthetic spinal disc nucleus 100 is highly similar to previous
embodiments and includes a hydrogel core 102 and a constraining
jacket 104. The constraining jacket 104 is secured about the
hydrogel core 102 by closures 106. As seen most distinctly in the
dehydrated state (FIG. 3B), the hydrogel core 102 is defined by a
leading end 108, a trailing end 110 and a central portion 112. In
the hydrated state (FIG. 3A), the central portion 112, and thus the
prosthetic spinal disc nucleus 100, defines an anterior face 114
(partially hidden in FIG. 3A), a posterior face 116 and opposing
end plate faces 118, 120 (partially hidden in FIG. 3A).
[0063] The composition and fabrication of the hydrogel core 102 and
the constraining jacket 104 is virtually identical to that
previously described. The actual shape of these components differs
somewhat. In particular, with reference to FIG. 3A, in the hydrated
state the prosthetic spinal disc nucleus 100 is configured to
assume an angled, wedge shape. For this reason, the alternative
prosthetic spinal disc nucleus 100 can be referred to as an "angled
prosthetic spinal disc nucleus." In particular, the anterior face
114 and the posterior face 116 are substantially rectangular, the
posterior face 116 being larger than the anterior face 114.
Further, the leading end 108 and the trailing end 110 are wedge
shaped. Finally, the opposing end plate faces 118, 120 are
approximately trapezoidal or wedge-shaped. With this configuration,
in the hydrated state, the angled prosthetic spinal disc nucleus
100 tapers in height (z-axis) from the posterior face 116 to the
anterior face 114. The rate of change in height is preferably
relatively uniform. Additionally, the angled prosthetic spinal disc
nucleus 100 tapers in length (y-axis) from the posterior face 116
to the anterior face 114. In the hydrated state, then, the angled
prosthetic spinal disc nucleus 100 is highly similar to the
previously described tapered prosthetic spinal disc nucleus 70
(FIG. 2B), except for the generally trapezoidal shape of the
opposing end plate faces 118, 120.
[0064] The preferred hydrated shape of the angled prosthetic spinal
disc nucleus 100 is accomplished by, for example, use of a
correspondingly shaped mold as part of the above-described
manufacturing process. Similarly, the preferred dehydrated shape
(FIG. 3B) of the angled prosthetic spinal disc nucleus 100 is
generated by reshaping the hydrogel core 102. For example, the
hydrogel core 102 may be placed in a streamlined-shaped mold and
compressed while dehydrating. Regardless of the exact manufacturing
technique, the resulting dehydrated angled prosthetic spinal disc
nucleus 100 is preferably substantially convexo-convex, tapering in
height (z-axis) from the central portion 112 to the leading end 108
and the trailing end 110. Notably, to achieve the desired hydrated
shape of FIG. 3A, the hydrogel core 102 may taper in length
(y-axis) in the dehydrated state such that the hydrogel core 102 of
FIG. 3B differs slightly from the hydrogel core 72 of FIG. 2B,
although the dehydrated hydrogel core 102 preferably renders the
constraining jacket 104 relatively taut along its length. Due to a
shape memory characteristic of the hydrogel core 102, upon
hydration, the hydrogel core 102 will transition from the
dehydrated, streamlined shape of FIG. 3B to the hydrated, angled
shape of FIG. 3A.
[0065] Yet another alternative embodiment of a prosthetic spinal
disc nucleus 130 is shown in FIGS. 4A and 4B. As a point of
reference, FIG. 4A depicts the prosthetic spinal disc nucleus 130
in a hydrated state; whereas FIG. 4B is a dehydrated configuration.
The alternative prosthetic spinal disc nucleus 130 is highly
similar to previous embodiments and includes a hydrogel core 132
and a constraining jacket 134 secured about the hydrogel core 132
by closures 136. As depicted most distinctly in the dehydrated
state (FIG. 4B), the hydrogel core 132 is defined by a leading end
138, a trailing end 140 and a central portion 142. In the hydrated
state (FIG. 4A), the central portion 142, and thus the prosthetic
spinal disc nucleus 130 defines an anterior face 144, a posterior
face 146 (partially hidden in FIG. 4A) and opposing end plate faces
148, 150 (partially hidden in FIG. 4A).
[0066] The composition and fabrication of the hydrogel core 132 and
the constraining jacket 134 is virtually identical to that
previously described. The actual shape of these components upon
hydration differs somewhat. In particular, with reference to FIG.
4A, in the hydrated state, the prosthetic spinal disc nucleus 130
is configured to assume an angled, wedge-shape. This shape has a
reverse angular configuration when compared to the angled
prosthetic spinal disc nucleus 100 (FIG. 3A). For this reason, the
prosthetic spinal disc nucleus 130 can be referred to as a "reverse
angle prosthetic spinal disc nucleus." The reverse angle prosthetic
spinal disc nucleus 130, in the hydrated state, tapers in length
(y-axis) from the posterior face 146 to the anterior face 144,
preferably with a relatively uniform rate of change in length, such
that the opposing end plate faces 148, 150 are approximately
trapezoidal. Additionally, the reverse angle prosthetic spinal disc
nucleus 130 tapers in height (z-axis) from the anterior face 144 to
the posterior face 146, preferably with a relatively uniform rate
of change in height, such that the leading end 138 and the trailing
end 140 are approximately trapezoidal.
[0067] As with previous embodiments, the unique shape of the
reverse angle prosthetic spinal disc nucleus 130 shown in FIG. 4A
is achieved only upon hydration. In accordance with the above
described manufacturing technique, however, in a dehydrated state,
the reverse angle prosthetic spinal disc nucleus 130 assumes the
streamlined shape shown in FIG. 4B. The preferred dehydrated shape
of the reverse angle prosthetic spinal disc nucleus 130 is created
during the above-described reshaping procedure. The resulting
hydrogel core 132, in the dehydrated state, is preferably
substantially convexo-convex, tapering in height (z-axis) from the
central portion 142 to the leading end 138 and the trailing end
140. Similar to the angled prosthetic spinal disc nucleus 100 (FIG.
3B), the hydrogel core 132 of FIG. 4B has a slight taper in length
(y-axis) in the dehydrated state, although is preferably configured
to maintain the constraining jacket 134 in a taut position along
its length (y-axis). Due to a shape memory characteristic of the
hydrogel core 132, upon hydration, the hydrogel core 132 will
transition from the dehydrated, streamlined shape of FIG. 4B to the
hydrated, reverse angle shape of FIG. 4A.
[0068] As should be apparent from the above discussion, a
prosthetic spinal disc nucleus in accordance with the present
invention can be configured to assume a number of different shapes
in a hydrated state. In the dehydrated state, however, a prosthetic
spinal disc nucleus in accordance with the present invention will
have the streamlined shape shown best in FIG. 1. To this end, the
hydrated shape will generally correspond with the anatomical
variations presented by a portion of a particular disc space. U.S.
patent application Ser. No. 09/090,820, the teachings of which are
incorporated herein by reference, describes the dimensional
characteristics of several different prosthetic spinal disc nucleus
devices in a hydrated state in greater detail. It should be
understood, however, that a prosthetic spinal disc nucleus in
accordance with the present invention may assume any other shape in
the hydrated state, so long as a streamlined, dehydrated shape is
provided.
[0069] Regardless of which embodiment of the above-described
prosthetic spinal disc nucleus 20, 70, 100 or 130 is employed, the
preferred method of implantation is identical. For example, FIGS.
5-9 depict implantation of a pair of prosthetic nuclei, including
the tapered prosthetic spinal disc nucleus 70 (FIGS. 2A and 2B) and
the angled prosthetic spinal disc nucleus 100 (FIGS. 3A and 3B)
into a damaged disc space 160, for example at disc level L4/L5. The
disc space 160 separates two adjacent vertebrae 162 and includes an
anulus 164 and a nucleus region or cavity 166 (shown best in FIGS.
7A and 7B). Proper positioning is achieved by first performing a
laminectomy in a targeted lamina area 168. A passage 170 is created
through a posterior side of the anulus 164, such as by a simple
incision or removal of a radial plug. If necessary, excess material
is removed from the nucleus cavity 166 to create room for the
prosthetic spinal disc nuclei 70, 100. Although in this example a
single passage 170 is illustrated and discussed, a pair of passages
may alternatively be used. Further, while a generally posterior
technique has been identified, insertion through any portion of the
anulus 164 is acceptable.
[0070] The tapered prosthetic spinal disc nucleus 70 (FIGS. 2A and
2B) and the angled prosthetic spinal disc nucleus 100 (FIGS. 3A and
3B) are then implanted into the nucleus cavity 166 via the passage
170. In this particular example, for reasons made clear below, the
angled prosthetic spinal disc nucleus 100 will be implanted within
an anterior area 172 of the disc space 160; whereas the tapered
prosthetic spinal disc nucleus 70 will be implanted within a
posterior area 174. With the preferred posterior implantation
technique, then, the angled prosthetic spinal disc nucleus 100 is
implanted first.
[0071] Insertion of the angled prosthetic spinal disc nucleus 100
is shown in greater detail in FIGS. 7A and 7B. During implantation,
the angled prosthetic spinal disc nucleus 100 is in a dehydrated
state, thereby assuming a streamlined shape (FIG. 3B). As shown in
FIG. 7A, the angled prosthetic spinal disc nucleus 100 is directed
toward the anulus 164 such that the leading end 108 extends through
the passage 170. As previously described, in the dehydrated state,
the leading end 108 tapers in height (relative to a "height" of the
nucleus cavity 166 defined by the adjacent vertebrae 162). With
this tapered profile, the leading end 108 easily passes through the
passage 170 of the anulus 164, thereby facilitating implantation of
the angled prosthetic spinal disc nucleus 100. Because the
constraining jacket 104 is relatively taut along its length (via
the unique shape of the dehydrated hydrogel core 102), the
constraining jacket 104 will not fold back on to itself or
otherwise impede insertion through the passage 170.
[0072] Following insertion, the angled prosthetic spinal disc
nucleus 100 is preferably rotated to extend transversely within the
nucleus cavity 166. In this regard, as shown in FIG. 7B, where the
hydrogel core 102 (in the dehydrated state) is formed to have a
slight curve along its length, this transverse orientation will
occur more naturally. Regardless, following rotation, the angled
prosthetic spinal disc nucleus 100 is positioned within the
anterior area 172 of the nucleus cavity 166. If necessary, a rod
and mallet (not shown) may be used to force the angled prosthetic
spinal disc nucleus 100 into the position shown in FIG. 8.
[0073] The tapered prosthetic spinal disc nucleus 70 is then
similarly implanted through the passage 170 in the anulus 164. As
shown in FIG. 9, in a dehydrated state, the leading end 78 of the
tapered prosthetic spinal disc nucleus 70 presents a tapered
profile so as to facilitate insertion through the passage 170. Once
inserted, the tapered prosthetic spinal disc nucleus 70 is rotated
to extend transversely within the nucleus cavity 166, positioned
within the posterior area 174 as shown in FIG. 10, which, for ease
of illustration, depicts the nuclei 70, 100 in a hydrated
state.
[0074] Notably, in certain situations, it may be desirable to
slightly separate the adjacent vertebrae 162 to facilitate
insertion of the prosthetic spinal disc nuclei 70, 100. With this
approach, a pair of passages 170 through the anulus 164 is
required. An inflatable jack, lamina spreader or similar tool (not
shown) is inserted through one of the passages 170 and inflated to
jack apart the adjacent vertebrae 162. Once separation sufficient
to insert the angled prosthetic spinal disc nucleus 100 is
achieved, the angled prosthetic spinal disc nucleus 100 is inserted
through the passage 170 otherwise not occupied by the tool. The
tool is then removed, and the tapered prosthetic spinal disc
nucleus 70 is placed through one of the passages 170.
[0075] The angled prosthetic spinal disc nucleus 100 is positioned
such that the anterior face 114 is adjacent an anterior side of the
anulus 164. The posterior face 116, conversely, is centrally
located within the nucleus cavity 166. Thus, the angled prosthetic
spinal disc nucleus 100 is generally positioned within the anterior
area 172 of the nucleus cavity 166. The tapered prosthetic spinal
disc nucleus 70 is positioned such that the posterior face 86 is
adjacent a posterior side of the anulus 164, whereas the anterior
face 84 is centrally located within the nucleus cavity 166. Thus,
the tapered prosthetic spinal disc nucleus 70 is positioned within
the posterior area 174 of the nucleus cavity 166.
[0076] As shown in FIGS. 10 and 11, upon hydration, the tapered
prosthetic spinal disc nucleus 70 and the angled prosthetic spinal
disc nucleus 100 are sized and orientated to generally conform to
the transverse geometry of the respective areas of the nucleus
cavity 166. It should be recognized, however, that orientation and
selection of the prosthetic spinal disc nuclei can and will vary
depending upon an individual disc space. For example, the
rectangular prosthetic spinal disc nucleus 20 (FIGS. 1A-1D) and/or
the reverse angle prosthetic spinal disc nucleus 130 (FIGS. 4A and
4B) may be used instead of the tapered prosthetic spinal disc
nucleus 70 or the angled prosthetic spinal disc nucleus 100.
Further, the particular prosthetic spinal disc nucleus 20, 70, 100,
130 employed may be rotated 180 degrees. Thus, for example, the
angled prosthetic spinal disc nucleus 100 may be positioned in the
posterior area 174 such that the anterior face 114 is adjacent the
posterior side of the anulus 164, whereas the posterior face 116 is
centrally located within the nucleus cavity 166. Simply stated, any
combination, location or orientation of the prosthetic spinal disc
nuclei 20, 70, 100, 130 disclosed can be used. In this regard, FIG.
11 shows the prosthetic spinal disc nuclei 20, 70, 100 and 130 in
different locations and between different vertebrae, including an
L-3 vertebrae 175, an L-4 vertebrae 176, an L-5 vertebrae 178 and
an S-1 vertebrae 180. As should be evident from these examples, the
particular prosthetic spinal disc nuclei will be selected such that
in a hydrated state, the prosthesis corresponds generally to an
anatomical shape of a particular side or portion of the disc space
in question.
[0077] Following implantation, each of the prosthetic spinal disc
nuclei 20, 70, 100 or 130 functions as an intervertebral spacer and
a cushion, and potentially restores the normal fluid pumping action
of the disc space 160 (FIG. 11). Function of the prosthetic nuclei
is described below with reference to the rectangular prosthetic
spinal disc nucleus 20 of FIGS. 1A-1D, implanted between the L-3
vertebrae 175 and the L-4 vertebrae 176 shown in FIG. 11. It should
be understood, however, that the tapered prosthetic spinal disc
nucleus 70, the angled prosthetic spinal disc nucleus 100 and the
reverse angle prosthetic spinal disc nucleus 130 function in an
identical manner. Following implant, the hydrogel core 22 imbibes
fluids. In this regard, the constraining jacket 24 has sufficient
flexibility to allow the hydrogel core 22 to expand. As the
hydrogel core 22 hydrates, its volume increases significantly. Due
to the preshaping and shape memory of the hydrogel core 22, the
hydrogel core 22 will expand from the dehydrated, streamlined shape
(FIG. 1A) to the hydrated, rectangular shape (FIG. 1D). Because the
constraining jacket 24 is flexible, it will conform to the
preferred, predetermined shape of the hydrogel core 22, as shown in
FIG. 1D. At a certain, predetermined hydration point, the hydrogel
core 22 reaches a horizontal expansion limit (x-y plane of FIG. 1A)
of the constraining jacket 24, which becomes tight. The
constraining jacket 24 has a relatively fixed maximum volume so
that the constraining jacket 24 forces the hydrogel core 22 to
increase mainly in height (z-axis in FIG. 1B) as more fluids are
imbibed. In other words, once the hydrogel core 22 expands to the
length (y-axis in FIG. 1C) and width (x-axis in FIGS. 1B and 1C)
limits of the constraining jacket 24, the constraining jacket 24
forces further expansion to occur solely in height (z-axis in FIG.
1B). Thus, the constraining jacket 24 works in concert with the
hydrogel core 22 to control expansion of the prosthetic spinal disc
nucleus 20 after implant. With reference to the implanted position
of the rectangular prosthetic spinal disc nucleus 20 shown in FIG.
11, this controlled swelling pushes apart or further separates the
vertebrae 175, 176 adjacent the disc space 160, as would a normal
nucleus. Importantly, the limitation on expansion of the hydrogel
core 22 occurs independent of the anulus 164. In other words, the
constraining jacket 24 prevents the hydrogel core 22 from expanding
to a point at which it would engage and conform to an inner surface
of the anulus 164. Once hydrated, the prosthetic spinal disc
nucleus 20 will still have a rectangular cross-section, but may be
slightly circular. The prosthetic spinal disc nucleus 20 will not
expand to a completely circular cross-section due to the forces
imparted by the vertebral end plates, conditioning of the hydrogel
core 22 prior to implant, and the volume limits of the constraining
jacket 24.
[0078] Following implant and hydration, the prosthetic spinal disc
nucleus 20 will deform and reform in response to the placement and
removal of loads on the disc space 160 (FIG. 11). The prosthetic
spinal disc nucleus 20 flattens in response to placement of
physiological loads on the spine, thus assuming a more flattened
shape, and acts as a cushion against various loads placed upon it.
As these loads are decreased (e.g., when the patient reclines), the
hydrogel core 22 reforms back in a predetermined fashion to its
original, hydrated shape, due to the conditioning process described
above. To prevent the hydrogel core 22 from escaping, the
constraining jacket 24 ideally has a burst strength that is greater
than the swelling pressure of the hydrogel core 22 when fully
hydrated.
[0079] The prosthetic spinal disc nucleus 20 also restores the
natural fluid pumping action of the disc space. This relationship
is best described with reference to FIG. 10, which depicts the
tapered prosthetic spinal disc nucleus 70 and the angled prosthetic
spinal disc nucleus 100 implanted within the nucleus cavity 166 of
the disc space 160. The hydrated prosthetic spinal disc nuclei 70,
100 occupy a certain percentage, but not all of, the nucleus cavity
166. As loads upon the disc space 160 increase, the prosthetic
spinal disc nuclei 70, 100 cushion the vertebral end plates (not
shown) and slowly deform. As a result, the volume within the
nucleus cavity 166 decreases. Notably, because the prosthetic
spinal disc nuclei 70, 100 do not occupy the entire nucleus cavity
166, there is room for the prosthetic spinal disc nuclei 70, 100 to
deform, and the reduction in volume of the nucleus cavity 166 is
allowed to take place as would otherwise occur with a normal
nucleus. In this regard, the respective hydrogel cores 72, 102
(FIGS. 2A and 3A) will flatten or deform as a whole, but not
decrease in volume in response to the load so that the prosthetic
spinal disc nuclei 70, 100 now occupy a larger percentage of the
nucleus cavity 166. As a result of the reduction in space, fluids
otherwise found within the nucleus cavity 166 are forced out of the
disc space 160, thus flushing out the accumulated acids or
autotoxins contained therein.
[0080] Conversely, when the load is removed or decreased, the
prosthetic spinal disc nuclei 70, 100 reform back to a more
circular (but wedge-shaped) cross-sectional shape. This entails an
increase in the vertical direction (relative to the spine in an
upright position), causing the vertebral end plates (not shown) to
separate, creating an increased volume in the nucleus cavity 166.
It will be remembered that the respective hydrogel cores 72, 102
(FIGS. 2A and 3A) do not increase in volume, but simply reform. As
a result, bodily fluid, containing beneficial nutrients, fills the
now-increased volume of the nucleus cavity 166, revitalizing the
overall disc space 160. Thus, the prosthetic spinal disc nuclei 20,
70, 100 or 130 act in concert with the natural disc space 160 to
restore the natural pumping action of the disc space.
[0081] Notably, the prosthetic spinal disc nucleus 20, 70, 100 or
130 of the present invention independently absorbs the
force/pressure placed upon the disc space 160. Thus, the anulus 164
is not required to support the force/pressure generated by swelling
of the hydrogel core 22, 72, 102 or 132 during hydration. The
anulus 164 does not provide any circumferential support to the
prosthetic spinal disc nucleus 20, 70, 100 or 130.
[0082] The prosthetic spinal disc nucleus of the present invention:
(a) restores and maintains the height of the damaged disc space;
(b) restores and tightens the natural anulus to stop further
degeneration and permit its healing; (c) restores the normal
load-unload cycling and thus flushes out toxic by-products,
bringing in fresh nutrients to the disc space; (d) allows a
near-normal range of motion; (e) relieves the movement-induced
discogenic pain of the vertebral segment; and (f) allows the use of
a minimal, posterior surgical procedure that provides both cost and
medical benefits. In short, the prosthetic spinal disc nucleus of
the present invention has the ability to elevate the disc space
from the inside, as does the normal, highly hygroscopic nucleus. It
will tighten the ligamentous anulus and therefore promote the
health and repairability of anular fibers. Beyond these functions,
the prosthetic spinal disc nucleus of the present is configured to
have a pre-implant dehydrated shape that facilitates implantation.
Subsequently, upon hydration, the prosthetic spinal disc nucleus of
the present invention transitions to a hydrated shape corresponding
generally to an anatomical shape of at least a portion of a disc
space.
[0083] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention. For example,
other methods of sealing the ends of the constraining jacket exist
such as heat, ultrasound,
[0084] crimp ring seals or spin entanglement. Additionally, more
than a single layer of material may be used to maintain the
integrity of the hydrogel core. In other words, a plurality of
jackets can surround the hydrogel core. With respect to
implantation of the prosthesis of the present invention, it has
been preferably described that the prosthetic spinal disc nucleus
be implanted without the assistance of implant tools.
Alternatively, however, the shape change characteristic can be used
to facilitate insertion via a tubed projection device, such as a
cannula. By imparting a streamlined pre-implant shape into the
prosthesis, the prosthesis will easily pass through a cannula into
the disc space.
[0085] The hydrogel itself can have an outer "skin" formed by ion
implantation which causes outer layer cross linking and functions
as the constraining jacket or as an interposed membrane between the
gel mass and the constraining jacket. Alternatively, expansion and
contraction of the hydrogel core can be achieved via the use of a
hydrogel that readily expels fluid. Further, other means exist for
limiting expansion and contraction in height of the hydrogel core
without the use of a separate jacket.
* * * * *