U.S. patent application number 11/477144 was filed with the patent office on 2007-01-18 for intervertebral disc replacement.
Invention is credited to Curtis A. Dickman.
Application Number | 20070016302 11/477144 |
Document ID | / |
Family ID | 36600390 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070016302 |
Kind Code |
A1 |
Dickman; Curtis A. |
January 18, 2007 |
Intervertebral disc replacement
Abstract
An intervertebral disc prosthesis in a preferred embodiment has
a matrix of bioincorporable fabric, and a nuclear core centrally
mixed into the matrix. The core is formed by impregnating the
fabric substrate centrally with a polymer, preferably of liquid
form that cures into a viscoelastic solid, in which each
component--polymer and fabric--reinforces the other against
tearing, shearing and weakening under stress. The core is a hybrid
composite adapted for elastic deformation centrally of the matrix,
in which the polymer is mixed with the fabric, and is surrounded by
the outer bioincorporable fabric margin of the matrix. In another
embodiment, the nuclear core is separated from an outer sheath of
the bioincorporable fabric by an intermediate ligament encasement
that surrounds the purely polymeric core In either embodiment, each
edge of the outer fabric that interfaces a vertebral end plate is
impregnated with an agent to stimulate osseus incorporation and
anchoring. An adjunct anchoring system with penetration of bone of
adjacent vertebra may be used to for attachment until and after
bioincorporation occurs.
Inventors: |
Dickman; Curtis A.;
(Phoenix, AZ) |
Correspondence
Address: |
DONALD R. GREENE
P.O. BOX 6238
GOODYEAR
AZ
85338
US
|
Family ID: |
36600390 |
Appl. No.: |
11/477144 |
Filed: |
June 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10185738 |
Jun 28, 2002 |
7066960 |
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11477144 |
Jun 27, 2006 |
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Current U.S.
Class: |
623/17.13 ;
623/17.16 |
Current CPC
Class: |
A61F 2210/0085 20130101;
A61F 2310/00976 20130101; A61F 2/08 20130101; A61F 2/441 20130101;
A61F 2230/0015 20130101; A61F 2310/00796 20130101; A61F 2/442
20130101; A61F 2/4425 20130101; A61F 2002/30133 20130101; A61F
2002/30578 20130101; A61F 2/30965 20130101; A61F 2310/00293
20130101; A61F 2002/30062 20130101; A61F 2310/00023 20130101; A61F
2002/2817 20130101; A61F 2210/0004 20130101; A61F 2220/0025
20130101; A61F 2310/00365 20130101; A61F 2002/30884 20130101; A61B
17/7062 20130101; A61F 2002/30383 20130101; A61F 2002/30583
20130101 |
Class at
Publication: |
623/017.13 ;
623/017.16 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Claims
1-29. (canceled)
30. In combination with an intervertebral disc prosthesis, said
prosthesis having an elastically deformable central core, and a
fabric matrix intimately surrounding said central core, said
prosthesis being adapted for replacement of a designated disc in
the vertebral column of a patient, an anchoring system comprising
fixation means adapted to cooperate with said fabric matrix of the
prosthesis for stabilizing said prosthesis relative to adjacent
vertebrae at either side thereof when the prosthesis is implanted
in the vertebral columnar space vacated by said designated disc
being replaced, said fixation means including at least one fastener
adapted for manual penetration of bone of an adjacent vertebra to
secure said fabric matrix and, thereby, said prosthesis to said
adjacent vertebra, so as to avoid interfering with elastic
deformation of said central core of the implanted prosthesis under
compressive force exerted thereon along the vertebral column.
31. The combination according to claim 30, wherein said fabric
matrix comprises a sheath surrounding the central core of said
intervertebral disc prosthesis, said sheath having integral fabric
extensions projecting from either side of the prosthesis adapted to
confront said adjacent vertebrae when the prosthesis is implanted,
each of said extensions constituting part of said fixation means
and adapted to be secured to bone of an adjacent vertebra by a
respective said at least one fastener.
32. The combination according to claim 30, wherein said fabric
matrix has fibers running through said central core and an integral
fabric margin of said matrix surrounding the core, said margin
having integral fabric extensions projecting from either side of
the prosthesis adapted to confront said adjacent vertebrae when the
prosthesis is implanted, each of said extensions constituting part
of said fixation means and adapted to be secured to bone of an
adjacent vertebra by a respective said at least one fastener.
33. The combination according to claim 30, wherein said fabric
matrix has a distinctive contour at either side of the
intervertebral disc prosthesis adapted to confront said adjacent
vertebrae end plates, and said fixation means includes at least two
anchoring plates, each having a distinctive contour adapted to mate
with the distinctive contour of the fabric matrix at a respective
side of said prosthesis, each of said plates having passages
therein to accommodate projections of said fabric matrix side
contour.
34. The combination according to claim 33, wherein said distinctive
contour of the sides of said fabric matrix includes at least one
semicircular hump spanning the prosthesis at each side of the
fabric matrix, with projections located on each respective hump
adapted to extend through respective passages of the associated
anchoring plate to promote osseus incorporation from marrow and
cancellous bone exposed at regions of said adjacent vertebrae
arranged to accommodate nesting of the hump and associated
anchoring plate upon implantation of said prosthesis.
35. The combination according to claim 33, wherein each of said
anchoring plates includes a fastener portion for securing the
respective plate to bone exposed at the adjacent vertebra.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to intervertebral
disc prostheses, and more particularly to improvements in and
anchoring systems for disc prostheses to assure operation, fixation
and stabilization of the prosthesis corresponding to a natural
disc.
[0002] The human backbone (the vertebral or spinal column) 10 (FIG.
1) consists of numerous longitudinally aligned vertebrae 11,
adjacent ones of which are separated by individual cartilaginous
intervertebral discs 12 and connected to one another by ligaments.
The vertebral column is the vertical axis of the skeleton,
extending from the skull (not show) at its proximal end to the
pelvis (not shown) at the distal end. It serves to support the head
and trunk of the body, and to protect the spinal cord (not shown)
that passes through the vertebral (spinal) canal formed by openings
in the vertebrae. Adjacent the distal end of the vertebral column,
the sacrum portion 13 of the pelvis is formed by several vertebrae
that are fused together and to which the coccyx (tailbone) 14 is
attached.
[0003] The flexibility of the overall vertebral column allows
movements of the trunk in flexion or bending forward, lateral
flexion or bending sideways, extension or bending backward,
rotation about its longitudinal axis, and circumduction, which is a
combination of the aforementioned movements. However, these
movements are attributable to the cumulative effect of the numerous
small movements that take place at the joints between the
vertebrae.
[0004] As shown in FIG. 1, the vertebral column 10 has an upper
cervical curvature 15, a middle thoracic curvature 16, a lower
lumbar curvature 17, and a lowest pelvic curvature 18. The cervical
region comprises seven vertebrae, the thoracic region twelve
vertebrae, and the lumbar region five vertebrae (for convenience,
all of the vertebrae are depicted by reference number 11), with
various common characteristics as well some different features
according to the functions the respective vertebrae serve. For
example, the first cervical vertebrae is configured to support and
balance the head, and the second is configured to pivot within the
first as the head is turned from side to side, respectively. These
vertebrae are atypical, possessing certain structural features not
found in the others. The thoracic vertebrae have larger bodies than
the cervical vertebrae to accommodate increased stress as a result
of their support of successively increasing body weight, and the
lumbar vertebrae still larger bodies for the same reason.
[0005] A typical vertebra 11 is depicted from above in FIG. 2,
which illustrates the bony vertebral body 20 anteriorly of the
intervertebral column 10, aligned along the respective curvatures
with the bodies of the other vertebrae of the column. A bony
vertebral arch is formed by a pair of posteriorly projecting
pedicles 21, 22, and laminae 23, 24 extending from the pedicles and
joined together as the spinous process 25, the arch surrounding a
vertebral foramen 26 aligned with corresponding foramen of the
other vertebrae to form a vertebral or spinal canal for passage of
the spinal cord (not shown). Transverse processes 28, 29 project
from between the pedicle 21 and the lamina 23 on one side, and 22
and 23 on the other, respectively. Superior 30, 31 and inferior
(hidden from view in FIG. 2) articular processes have cartilage
covered facets for joining adjacent vertebrae above and below to
the vertebra shown in FIG. 2. Notches on the lower portion of the
pedicles 21, 22 create intervertebral foramina openings (34, FIG.
1) through which spinal nerves (not shown) pass between adjacent
vertebrae to connect to the spinal cord.
[0006] The facet joints on each side between the articular
processes of adjacent vertebrae constitute two of the three
separate intervertebral joints between each vertebra and the
adjacent vertebra above (or below) it. The third is the anterior
joint formed between the bodies of adjacent vertebrae by the
intervertebral disc 12 which both unites them and allows movement
between them. The type and degree of movement of the vertebral
column acting as a unit is controlled by the actions of all three
intervertebral joints between the separate vertebrae, allowing, for
example, pure rotation of the column only in the thoracic
region.
[0007] The intervertebral discs 12 separating the vertebrae are
masses of fibrocartilage that cushion and soften forces arising
from movements such as walking and jumping, as well as providing
one of the joints between the adjacent vertebrae. The discs are
shaped according to their locations in the spine. Those in the
thoracic region are relatively thinner and flatter than the discs
in each of the cervical and lumbar regions, which are wedge-shaped
and relatively thicker. Anterior and posterior longitudinal bands
of ligamentous fibers extending along the length of the vertebral
column and attached to the bodies of the vertebrae serve in part to
reinforce the discs in front and behind.
[0008] Each disc 12 itself is composed of a tough outer layer of
the fibrocartilage and an elastic central region. Injury, exertion
or the aging process can produce changes in the discs, such as loss
of firmness of the central region and thinning, weakening and
cracking of the outer layer as a result of degenerative changes,
and breakage of the outer layer and squeezing out of the central
region as a result of injury from external pressure or heavy
lifting. Pressure on the spinal cord or individual nerves branching
from the cord caused by the ruptured or slipped disc often produces
back pain, numbness and loss of muscular function in the body parts
innervated by the affected spinal nerve(s).
[0009] More specifically, each disc has an annulus composed of
concentric rings of strong fibers (the annulus fibrosus, or
annulus) that surrounds a central gelatinous nucleus (the nucleus
pulposus, or nucleus). The annulus fibers are attached in an
oblique direction at top and bottom of the disc to the adjacent
vertebrae so that some of the fibers tighten when the related
vertebrae are rotated in one direction and the others tighten when
the rotation is in the opposite direction, to resist torsional
motion between vertebral segments and excessive movement in almost
any direction. Also, the fibers in adjacent rings are oriented at
right angles to enable strong bonding while allowing some movement
between the bones. The nucleus is not centered in the disc but
resides more toward the posterior, with the annulus thinner in that
sector and thicker at the anterior sector. The nucleus is designed
for deformation in response to exertion of pressure on the disc so
that the disc can change shape during movement of the vertebral
column. This accommodates bending of the vertebral column and
resulting displacement of confronting surfaces of the adjacent
vertebrae from a substantially parallel orientation. In concert,
the annulus undergoes stretching in the sector of wider
displacement and bulging in the sector of narrower displacement of
those opposing surfaces.
[0010] Vertebral end plates at opposite ends of the disc abut
against the bodies of the respective adjacent vertebrae above and
below, operating as a transition zone between the bony vertebrae
and the soft intervertebral disc. The disc itself is without blood
vessels, so it receives its nutrients for metabolism by diffusion
through the end plates.
[0011] The intervertebral disc can become herniated when the fibers
of the annulus weaken or tear as a result of abnormal or repeated
stress or because of degenerative processes with aging. The nucleus
then tends to become distended and unable to recover to its normal
position within the annulus. In such cases, nerve compression can
occur as the bulging disc penetrates the vertebral canal and begins
exerting ongoing pressure on the spinal cord or on individual
nerves that pass between adjacent vertebrae and connect to the
spinal cord. This occurs most often with discs located in the
lumbar region, where the greatest stresses attributable to weight
are present. The result is chronic lower back or leg pain, which
can be disabling.
[0012] A common procedure in such cases, typically after first
having attempted a conservative approach with treatment regimens of
anti-inflammatory drugs, patient rest, or physical therapy or a
combination thereof, without significant success, is to surgically
remove the defective disc, implant patient or donor bone, and/or
fuse the adjacent vertebrae, so as to alleviate the pain at least
to an extent.
[0013] While fusion enjoys success in alleviating symptoms and
stabilizing the joint at the previous vertebra-disc interface, it
decreases the range of motion of the vertebral column in the
portion of the region where the surgical procedure was performed.
Also, the biomechanical rigidity of the fused vertebrae may
exacerbate deterioration of adjacent portions of the region.
[0014] Artificial intervertebral discs or disc prostheses offer
replacement of the defective natural intervertebral disc with a
capability of performing many of the functions of the latter, at
least to an extent to reduce problems suffered as a result of the
defects, and an opportunity to avoid further degeneration of the
vertebral column.
[0015] Artificial intervertebral discs, partial disc prostheses and
techniques of fixation thereof have been proposed in several United
States patents, including the following.
[0016] U.S. Pat. No. 3,867,728 discloses a synthetic kidney shaped
prosthetic disc with a core (nucleus) composed of biocompatible
viscoelastic liquid or elastomer contained in a sealed chamber such
that the core resists deformation under compressive loading. The
walls of the chamber are surrounded by medical elastomer layers
reinforced with embedded fibrous material such as Dacron filaments.
The fibrous material is intended to act as an open-pore, tissue
ingrowth-receptive surface that abuts the exposed bony surface of
an adjacent vertebra when resident between the natural surfaces of
the vertebral cavity from which the replaced disc was excised. The
elastomeric core is reinforced with an annular ring of laminated
fibrous material embedded in the elastomer. The biocompatible
viscoelastic liquid core has reinforced side and end walls of
medical grade elastomer with embedded fibrous material to provide a
sealed chamber for the liquid, and an open pore tissue-ingrowth
receptive surface positioned to abut the exposed bony surface of an
adjacent vertebra.
[0017] U.S. Pat. No. 4,772,287 discloses a prosthetic disc capsule
for repairing a natural herniated disc. The prosthesis has an outer
layer composed of strong inert fibers that surrounds a bladder
containing a thixotropic gel having a viscosity and velocity shear
behavior imitating that of a natural spinal disc. One or more of
the capsules are inserted into bores formed in the natural
herniated disc under repair. The inert fibers of the outer layer
are composed of carbon or a polymer, including either natural or
synthetic polymers such as cold-drawn poly(ethylene terephthalate)
polyester fibers, or of bioresorbable material consisting of
polylactic or polyglycolic acid or collagen (e.g., semi-synthetic),
for replacement by tissue ingrowth for bonding to surrounding
tissue. The bladder is flexible, composed of oriented poly(ethylene
terephthalate), high-density polypropylene, silicone rubber, and
copolymers of silicone and carbonate. The thixotropic gel is a
mixture of an inorganic oil such as silicone or fluorocarbon, and a
gelling agent such as fumed silica. The viscosity of the gel is
selected to permit fast movement during bending at the
intervertebral space while restricting motion during slow postural
changes.
[0018] U.S. Pat. No. 4,946,378 discloses an artificial
intervertebral disc with a pair of metallic end bodies having 0.1
to 0.5 mm hydroxyapatite layer-coated outer surfaces and a
biocompatible synthetic elastic polymeric intermediate material of
silicone rubber, polyvinyl alcohol, polyurethyane resin, or the
like held between the end bodies through connecting members
composed of titanium, stainless steel, or the like.
[0019] U.S. Pat. No. 5,047,055 discloses a prosthetic lumbar disc
nucleus fabricated from synthetic hydrogels selected because of
their biocompatibility, characteristics of softness, hydration, low
friction, viscoelasticity, shape memory, and mechanical strength
which can aid the healing of a defective annulus of the
intervertebral disc. The prosthetic nucleus may purportedly be
implanted in the dehydrated state laterally to reduce the
complexity and risk of traditional intraspinal surgery, and allowed
to swell slowly in the body thereafter, with reduced incision area
on the annulus to aid healing of the annulus and prevent herniation
of the disc. The implanted hydrogel nucleus, after hydration, is
constrained tightly in the cavity formed by the excised natural
nucleus, by the restoring force of stretched fibers of the annulus
and the external force through the end plates.
[0020] U.S. Pat. No. 5,192,326, a continuation-in-part of the '055
patent, also discloses a prosthetic lumbar disc nucleus, but in
which the nucleus is composed of hydrogel beads.
[0021] U.S. Pat. No. 5,258,043 discloses a prosthetic disc composed
of a dry, porous, volume matrix of biocompatable and bioresorbable
fibers, a portion of which may be cross-linked, structured for
implantation to assume the form and role of a natural
intervertebral disc. The matrix may promote regrowth of
intervertebral fibrochondrocytes and provides a scaffold for the
regenerating intervertebral disc tissue. The prosthetic disc may
further include a mesh composed of a bioresorbable, biocompatible
material attached to lateral portions of the outer surface of the
matrix to aid disc implantation by providing a temporary anchoring
mechanism.
[0022] U.S. Pat. No. 5,458,643 discloses a prosthesis in the form
of an artificial intervertebral disc composed of polyvinyl alcohol
(PVA) hydrogel and a porous ceramic or metal. The PVA hydrogel is
said to enhance lubrication and shock absorbing functions, and the
porous body to allow the ingrowth and ossification of adjacent bone
tissue of the body in which the prosthesis is implanted.
[0023] U.S. Pat. No. 5,824,093, related to the '287 patent cited
above, discloses a prosthetic disc capsule to be implanted in pairs
side-by-side in a damaged natural intervertebral disc to maintain
both height and motion. Each capsule is an elongated, prosthetic
spinal disc nucleus body composed of a hydrogel core and a
surrounding constraining jacket that permits the hydrogel core to
deform and reform and to hydrate to a predetermined volume with
deformation and reformation in response to various loads placed
upon the spinal tract.
[0024] At least some of these artificial intervertebral discs
present the possibility of successful implants on a practical
scale. However, so far as is known to the applicant herein, the
prior art has not given promise of successful implementations and
methods for anchoring implanted artificial intervertebral discs in
the vertebral column to achieve relatively permanent fixation and
stabilization of the implant.
[0025] U.S. Pat. No. 5,562,738 discloses an intervertebral disc
arthroplasty device for replacing a degenerated or ruptured
intervertebral disc. The disc includes a first member with a socket
portion and a second member with a ball portion fitting in the
socket portion. The first member fits adjacent the first vertebrae
and the second member fits adjacent the second vertebrae so that
the ball portion fits in the socket portion, in the space vacated
by the excised disc. The members are anchored in place by base
plates with tabs fastened to the members, and by screws through the
tabs into the adjacent vertebrae. A second embodiment utilizes
metal insert cups fastened to the members, and another embodiment
contemplates bone ingrowth into ceramic members.
[0026] Prior art artificial intervertebral discs, and techniques of
fastening, anchoring and stabilizing implanted artificial discs do
not appear likely to yield satisfactory results.
[0027] It is a primary aim of the present invention to provide
improved intervertebral disc prostheses, and anchoring systems that
achieve reliable stabilization and relatively permanent fixation of
the improved intervertebral disc prosthesis in a vertebral
column.
[0028] Another aim of the invention is to provide methods of
fabricating intervertebral disc prostheses, and of anchoring the
prosthesis, for achieving those results.
SUMMARY OF THE INVENTION
[0029] In essence, the intervertebral disc is an anatomical spacer,
stabilizer, load dampener and shock absorber, positioned between
the vertebral end plates. The disc and surrounding ligaments also
stabilize the spine, connect adjacent vertebrae, and prevent
excessive rotation and subluxation of the vertebrae. As noted in
the background section above, intervertebral discs have some
desirable deformability attributable to their viscoelastic
properties. They also demonstrate hysteresis, and have a high
degree of hydration. The hysteresis is exhibited in response to the
mild deformability of the disk under compressive loading, when,
after unloading, the disc rebounds to its normal position. This
resiliency to compressive loading and unloading is a key function
of the disc. To yield suitable results, an artificial disc--a disc
prosthesis--must replicate all of these properties, characteristics
or functions of the natural intervertebral disc.
[0030] According to an important aspect of the invention, a disc
prosthesis is fabricated in a unified structure comprising a matrix
or substrate of bioincorporable continuous fabric and a nuclear
core or nucleus that is centrally impregnated into the substrate
and retained in place therein without a separate surrounding
sheath. Preferably, the substrate is woven collagen fabric, and the
nucleus is a hydrogel polymer centrally impregnated by injection or
soaking into the continuous substrate. The collagen fabric is woven
for tensile strength. The polymer is preferably an injectable,
curable liquid that, upon setting, forms a viscoelastic solid in
which each component reinforces the other. This makes it less
likely that the fabric component or the polymeric component will
tear, shear or weaken under stress; after the prosthesis is
implanted into a vertebral column and subjected to the normal
forces associated with spinal movements. Instead of a viscoelastic
solid, the polymer may be retained in a liquid or semi-liquid state
that impregnates the fabric at the core. In any event, the nuclear
core is a hybrid composite of the two primary materials. Outside
the core, the substrate defines an outer bioincorporable fabric
margin. The overall prosthesis is a deformable solid adapted for
elastic deformation as a result of its structure.
[0031] The intervertebral disc prosthesis of the invention
possesses characteristics of being compressible, mildly deformable,
resilient, hydrated, durable, nontoxic, nondegradable,
nonfragmenting, and viscoelastic, among other properties. The
prosthesis acts as a solid material to provide structural spacing
and load dampening effect between vertebral end plates. It is
suitable for replacement of a damaged natural disc in any of the
cervical, thoracic or lumber spinal regions of the vertebral
column.
[0032] In an alternative embodiment, the nuclear core is a separate
viscoelastic region surrounded by fabric encasement, centrally of
an outer bioincorporable fabric sheath.
[0033] It is equally important that an artificial intervertebral
disc, when implanted, be suitably and reliably anchored in place,
again mimicking the anchoring of a natural disc in the vertebral
column. The anchoring systems of the present invention target bone
ingrowth into the outer fabric margin or sheath of the disc
prosthesis as the primary mechanism or biomechanism for attachment
of the prosthesis, with subordinate or adjunct assistance of other
fasteners where appropriate. To that end, the outer edge of the
fabric matrix is impregnated or surface coated, at least at the
sides where the disc interfaces the vertebral end plates, with one
or more biological agents that promote growth and incorporation of
bone, fibrocartilage or fibrous tissue into the fabric, for osseus
incorporation and anchoring, to provide both near and long-term
post-implantation stability. Suitable agents include growth
factors, hydroxyapatite and BMP, for example.
[0034] A total or integrated anchoring system according to the
invention may comprise one or more anchorage mechanisms, at least
one of which is the above-stated bioincorporation of the disc
prosthesis structure to adjacent vertebrae. Adjunct anchoring
mechanisms may comprise mechanical and/or biological fasteners such
as titanium screws, bioincorporable cement, natural ligaments or
artificial ligamentous members, or platforms, may be used to
provide immediate stability of the implanted disc prosthesis until
osseus incorporation occurs. The immediate stability system may use
resorbable or nonresorbable materials for permanent reinforcement
of the anchorage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The above and still further aims, objectives, features,
aspects and attendant advantages of the present invention will
become apparent from the following detailed description of a
preferred embodiment of the interventional device and method of use
thereof, constituting the best mode presently contemplated of
practicing the invention, when taken in conjunction with the
accompanying drawings, in which:
[0036] FIG. 1 is a lateral side view of a human vertebral
column;
[0037] FIG. 2 is a top view of a typical vertebra in the vertebral
column of FIG. 1;
[0038] FIG. 3 is a simplified anterior side view of an embodiment
of an intervertebral disc prosthesis according to the invention,
positioned between adjacent vertebrae;
[0039] FIG. 4 is an axial cross-section view of the disc prosthesis
embodiment of FIG. 3, taken along the lines 4-4;
[0040] FIG. 5 is a simplified anterior (or lateral) side view,
partly in section, of an alternative embodiment of a disc
prosthesis;
[0041] FIG. 6 is a simplified lateral side view, partly in section,
of one embodiment of a fully integrated adjunct anchoring system
for the prosthesis, according to an aspect of the invention;
[0042] FIG. 7 is a simplified lateral side view, partly in section,
of another embodiment of a fully integrated adjunct anchoring
system;
[0043] FIG. 8 is a simplified lateral side view of an embodiment of
a separate adjunct anchoring system, according to another aspect of
the invention;
[0044] FIG. 9 is a simplified perspective view, partly in section,
of an embodiment of a partially integrated adjunct anchoring
system, according to yet another aspect of the invention;
[0045] FIGS. 10A and 10B are, respectively, a simplified anterior
side view and a top view of a second embodiment of a partially
integrated adjunct anchoring system; and
[0046] FIG. 11 is a simplified side view of a sliding rail
embodiment of a partially integrated adjunct anchoring system.
DESCRIPTION OF THE PRESENTLY CONTEMPLATED BEST MODE OF PRACTICING
THE INVENTION
[0047] Referring to FIGS. 3 and 4, a presently preferred embodiment
is a unified composite disc prosthesis 40, shown as being
positioned following implantation between the bodies of two
vertebrae 11-1 and 11-2 of the vertebral column, as viewed
anteriorly (FIG. 3). Prosthesis 40 includes a matrix 41 with a
substrate of bioincorporable continuous fabric. In its central
region, the substrate is impregnated with a liquid or semi-liquid
polymer--preferably a hydrogel polymer--that intermixes with the
substrate fabric in that region to form a nuclear core (nucleus)
42. This core is a hybrid composite of the two materials, a central
mix of polymer and fabric that is elastically deformable and
accurately mimics the nucleus pulposus of a native or natural
disc.
[0048] The polymer is preferably an injectable, curable liquid
that, after injection into the central region of the substrate,
sets to form a viscoelastic solid. Alternatively, the liquid may be
soaked into the fabric at the central region of the substrate. In
other embodiments, the core may retain the polymer in a liquid or
semi-liquid or gelatinous state intermixed with and impregnated
into the fabric of the substrate to form the hybrid composite. In
either of the latter embodiments, a ligamentous encasement may be
used to encompass the core, and the substrate fabric within and
outside the core may be separate.
[0049] The hybrid composite core constitutes a medium in which each
component--polymer and fabric--reinforces the other to deter damage
from occurrences such as tearing, shearing or weakening under the
types of stress to which the intervertebral disc prosthesis will be
subjected as an implant in the vertebral column of a normally
active person, even an elderly person.
[0050] Outside the core, the substrate fabric is relatively devoid
of the polymer, and constitutes a bioincorporable fabric margin
that runs to the edge of the disc prosthesis and surrounds the core
42.
[0051] The disc prosthesis 40 of the preferred embodiment is
relatively solid, but the structure of its matrix and core adapts
it to undergo elastic deformation when under compression or
subjected to other forces as a result of movement of or along the
vertebral column, according to the nature of the movement. The
original size and shape of the prosthesis are restored when the
pressure attributable to the movement is removed, by virtue of its
elastic deformation.
[0052] Matrix 41 may be fabric made from bioincorporable continuous
woven fibers of collagen, polyethylene, or other biocompatible or
bioincorporable ligamentous material. A woven collagen fabric is
preferred for its tensile strength. Preferably, the fibers are
woven to criss-cross each other in layers.
[0053] Viewed from the top as shown in FIG. 4, the typical disc
prosthesis 40 has the distinctive kidney shape of the natural disc
it is intended to replace. Viewed anteriorly or laterally (FIG. 3),
the geometry of the prosthesis is relatively flat and rectangular
in the unloaded (relatively uncompressed) state as shown, for
replacing a natural disc in the thoracic region 16 of the vertebral
column (FIG. 1). In that region, adjacent vertebral end plates are
substantially parallel. The geometry of the prosthesis is lordotic
with a trapezoidal configuration for disk replacements in the
lumbar or cervical regions 17 and 15, respectively, owing to the
normal curve of the spine. The fabrication of the intervertebral
disc prosthesis of the invention allows it to be used in any of the
lumbar, cervical and thoracic regions, subject only to those
differences in geometry.
[0054] At the ends 45, 46 of disc prosthesis 40 that interface the
upper and lower (viewed from the side with the vertebral column in
an upright orientation) vertebral end plates, the fabric margin of
matrix 41 that surrounds the core 42 is impregnated with one or
more biological agents that promote growth and incorporation of
bone, fibrocartilage or fibrous tissue into the fabric, for osseus
incorporation and anchoring of the disc. Preferably, one or more
agents such as growth factors, hydroxyapatite, BMP, or others that
will stimulate osseus incorporation into the matrix fabric, to
simulate natural ligament insertion in bone, are selected for
impregnation of the fabric margin at those surfaces. This ingrowth
into the fabric is the primary mechanism targeted by the invention
to achieve attachment and stabilization of the disc prosthesis 40
to the adjoining vertebrae, as illustrated at interfaces 47 and 48.
It is also possible to apply bioincorporable cement at those
interfaces with end plates of the vertebrae 11-1 and 11-2 before
the disc prosthesis is inserted into the space vacated by the
excised damaged disc.
[0055] Also, the nuclear core may be transformed into a
biologically active material by injecting transfected or
genetically engineered cells into the core to create fibrocartilage
that prompts the transformation.
[0056] With the construction of the disc prostheses of the
invention, the bone growth mechanism of attachment (anchorage) will
provide long-term fixation and stabilization of an implanted
prosthesis. Such anchorage is sufficient for the prosthesis to
withstand virtually continual stresses as the patient engages in
walking (and perhaps jumping and running as well), bending forward,
bending sideways, bending backward, and twisting of the trunk, in
normal everyday activity, not necessarily involving strenuous
exercise.
[0057] An alternative embodiment of the disc prosthesis is
illustrated, partly in section, in FIG. 5. Prosthesis 50 has an
inner nuclear core 52 of viscoelastic solid (or, alternatively,
liquid or semi-liquid) polymer, preferably hydrogel or hydrogel
composite material. In this embodiment, however, the core is a
separate component. In liquid or semi-liquid state, the core may be
encapsulated in an intermediate ligamentous encasement 55, such as
collagen fabric or other bioincorporable material. An outer
bioincorporable continuous fabric sheath 51 surrounds the core, and
encompasses the ligamentous encasement if used. The sheath may have
the same composition as that of matrix 41 in the preferred
embodiment of FIGS. 3 and 4, i.e., bioincorporable continuous woven
fibers of collagen, polyethylene, or other biocompatible or
bioincorporable ligamentous material.
[0058] As with the previous embodiment, disc prosthesis 50
substantially conforms to the shape of a natural intervertebral
disc according to whether the prosthesis is a replacement for a
damaged natural disc in the cervical, thoracic or lumber spinal
region of a human vertebral column.
[0059] Here again, to provide the primary anchoring mechanism of
ingrowth into the outer fabric of the prosthesis 50, the outer edge
(e.g., 56) of the sheath is impregnated or surface-coated with one
or more biological agents that stimulate growth and incorporation
of bone, fibrocartilage or fibrous tissue into the outer fabric
sheath, for osseus incorporation and anchoring, at least at its
ends where the prosthesis is to interface with vertebral end plates
when implanted. The selected agent(s) preferably include growth
factors, hydroxyapatite, BMP, or other agent for osseus
incorporation into the outer fabric sheath, to simulate natural
ligament insertion in bone. The resulting osseus incorporation is
illustrated at contact areas 57 and 58.
[0060] For some patients, additional fixation and stabilization may
be required. For that purpose, and also in instances where it may
be desired to provide an immediate attachment until or even after
the osseus incorporation occurs, the invention provides adjunct
anchoring mechanisms. These may comprise mechanical and/or
biological fasteners such as titanium screws, bioincorporable
cement, natural ligaments (e.g., cadaveric ligament transplants) or
artificial ligamentous members, or platform or plate attachment at
the interface areas. Resorbable or nonresorbable materials may be
used for permanent reinforcement of the anchorage.
[0061] The adjunct anchoring systems of the invention fall into
three distinct categories, each of which involves at least some
penetration of vertebral bone. One category involves an integration
of an anchoring mechanism into the disc prosthesis itself
(integrated adjunct anchoring system). A second category involves
an anchoring mechanism separate from the disc prosthesis (separate
adjunct anchoring system). And the third category involves an
anchoring mechanism partially integrated into and partially
separate from the disc prosthesis (partially integrated adjunct
anchoring system).
[0062] In one mechanism, extensions such as of the fabric matrix
integral with the disc prosthesis are employed for anchoring. In
another aspect, separate adjunct anchoring mechanisms including
ligaments fastened or bonded to the adjacent bone serve as tension
bands for the vertebrae adjacent to the implant and for confining
the disc in place. Additional spinal stability may be provided by
fastening to dorsal spinal elements. Another aspect resides in
partially integrated adjunct anchoring mechanisms comprising
biocompatible metal plates or platforms that affix the disc
prosthesis in place with novel configurations that capture the
prosthesis through its fabric sheath.
[0063] One embodiment of an integrated adjunct anchoring system is
illustrated in FIG. 6. In this embodiment, the intervertebral disc
prosthesis 60 has a nucleus 62 surrounded by a confining matrix or
sheath 61 in much the same way as the disk prostheses illustrated
in FIGS. 3-5. Here also, matrix 61 is impregnated at its ends 63,
64 with bone growth-promoting elements for rapid incorporation of
bone in areas 65, 66 from the surface regions of the adjacent
vertebral bodies 11-1, 11-2 between which the disc prosthesis is to
be inserted in the implant procedure. But in this embodiment, a
ligament or ligamentous member 67 is integrated longitudinally into
the disc prosthesis by surrounding the ligament with the matrix 61
along a sector of the side of the prosthesis. The ligamentous
member may be a natural ligament, taken from the patient or a
donor, or composed of an analog of the natural tissue, or of a
synthetic material that preserves its function as a flexible band
adapted to connect bones.
[0064] The principal criteria for selection of other than natural
material are that the ligamentous member must have the tensile
strength and biocompatibility of a natural ligament. The ligament
need not occupy a very large sector (using either an anterior or
lateral approach, with the ligament oriented accordingly) of the
disc prosthesis; indeed, it may be a relatively thin strip with
dimensions, for example, in a range from about 10 to about 25 mm
wide, from about 10 to about 40 mm long, and from about 3 to about
10 mm thick. The matrix 61 is one of the two disc prosthesis
embodiments, and thus impregnated with ingrowth agent(s) along its
inner surface 68 adapted to contact the surface of the vertebral
body 11-1 or 11-2, to promote ingrowth of bone, fibrous tissue or
fibrocartilage for anchorage in that area.
[0065] The adjunct anchoring mechanism comprising integrated
ligament 67 with its surrounding matrix 61 is secured to the
anterior or lateral side of the bone of each adjacent vertebrae by
a pair of biocompatible screws 69, preferably of titanium,
biocompatible alloys or bioresorbable material. The disc prosthesis
60 is thereby firmly affixed in the vertebral column, with the
primary anchorage mechanism providing the capability to respond to
forces exerted along the column in substantially the same manner as
a natural intervertebral disc. The disc prosthesis is not hindered
by the adjunct anchoring mechanism from spreading laterally at the
location of that anchoring mechanism. For example, if compressive
forces are exerted longitudinally at the anterior of the adjacent
vertebrae, the nucleus 62 will tend to compress and spread
outwardly in that sector, urging itself against the sheathed
ligament 67, which itself has tended to buckle anteriorly under the
same compressive forces. When the compressive force is removed,
both the nucleus and the sheathed ligament will return to their
original size and shape, the sheathed ligament being urged to do so
under forces exerted by the restoration of the nucleus from its
elastic deformation.
[0066] Another embodiment of an integrated adjunct anchoring system
is illustrated in FIG. 7. Here, the disc prosthesis 70 has a core
72 which is less expansive anteriorly than the confronting or
opposing surfaces of the vertebral bodies 11-1 and 11-2 between
which it is to be sandwiched on implant. This is to provide the
feature of an anchoring mechanism in which the fabric or fibrous
sheath 71 extends anteriorly beyond the core at its ends 73, 74.
The tabs or projections 75, 76 of the sheath define a channel or
opening 77 that allows a pair of biocompatible screws 78 to be
threaded into the bone of the respective vertebral bodies 11-1,
11-2 during the implant procedure, thereby securely fastening the
disc prosthesis 70 in place in the vertebral column. The channel 77
may be formed by a "scooping out" of the core 72 in that sector, so
that the size and shape of the core is only minimally reduced
relative to the confronting surfaces of bone. Here also, the sheath
71 is impregnated at its ends 73, 74 with bone growth-promoting
elements for relatively rapid incorporation of bone into the
prosthesis in the area of the confronting surface regions of the
adjacent vertebrae. This embodiment is less preferred than that of
FIG. 6 because the core has less opportunity to spread anteriorly
under longitudinal compressive force exerted along the anterior of
the vertebral column, since the sheath will tend to constrain such
movement as the projections 75, 76 are being squeezed together.
[0067] An embodiment of a separate adjunct anchoring system for a
disc prosthesis is illustrated in FIG. 8. The disc prosthesis 80 is
implanted between vertebral bodies 11-1 and 11-2 in the space from
which the natural disc has been removed. A ligamentous member 81 is
implanted and secured as a tension band longitudinally between the
vertebral bodies 11-1 and 11-2 by fasteners such as biocompatible
screws 82 and 83, respectively, at the anterior or lateral portions
of the vertebral column. Again, member 81 may be a natural
ligament, or analog or synthetic ligament such as a bioincorporable
fabric. Bone ingrowth factors are impregnated in portions 84, 85 of
the ligament that are to reside against the vertebral bone to
hasten bone incorporation and bonding therewith. The ligament
serves not only to maintain the maximum spacing and provide a
tension band between the related vertebrae, but to confine the disc
prosthesis in place.
[0068] The ligament 81 constitutes part of an adjunct anchoring
mechanism that is unattached to but in contact with the disc
prosthesis.
[0069] To add further spinal stability, the adjunct anchoring
system may employ a second ligamentous member 86 placed
longitudinally on dorsal spinal elements 87, 88 bounding the disc
prosthesis 80 at the posterior side of the vertebral column.
Ligament 86 may be secured to the dorsal elements by any suitable
fastener, such as screws, cables, sutures, wires, or cement, to
name a few.
[0070] An embodiment of a partially integrated adjunct anchoring
system for a disc prosthesis is illustrated in FIG. 9, partly in
section viewed from the anterior or lateral portion of the
vertebral column, depending on desired location. The disc
prosthesis 90 includes nucleus 92 encased within fabric sheath 91.
The opposite ends 93, 94 of the fabric sheath are formed with two
parallel spaced-apart semicircular humps 95-1 and 95-2 extending
parallel to the median or midsaggital plane or coronally oriented
if inserted laterally, and spanning the prosthesis at end 93, and a
corresponding pair of humps 96-1 and 96-2 at end 94. Except for
these differences, the nucleus and the fabric sheath may correspond
to those of the embodiments of the earlier description herein.
[0071] The contour of each end of the sheath 91 is matched by four
semicircular elongate anchoring plates 97 designed to mate with
respective ones of the humps on the two ends of the sheath. These
plates are preferably fabricated of titanium, but other
biocompatible, high strength materials may be used as an
alternative. Each anchoring plate has a pair of threaded or knurled
circular windows or openings 98 to expose marrow and cancellous
bone in the adjacent vertebral bodies 11-1, 11-2 between which the
disc prosthesis and the anchoring mechanism will be sandwiched
after implantation. The openings 98 are intended to accommodate
bone ingrowth into the fabric sheath from the exposed
marrow/cancellous bone of the adjacent vertebra. Preferably, the
sheath 91 has projecting bone, fibrous tissue, fibrocartilage
ingrowth agents (e.g., hydroxyapatite, having morphogenetic
proteins) fabric bumps 99 that mate with and project through the
openings 98, which are passages or through-holes in the respective
plates 97, to spur relatively rapid bone ingrowth. Corresponding
bone growth-inducing material is impregnated into the fabric at the
ends 93, 94 of the sheath in the spaces between the humps and
outside the humps that are destined to contact the bony surface of
bodies 11-1 and 11-2.
[0072] The contours of the ends of the sheath 91 and the need to
position the anchoring plates 97 thereon to fit in the vacated
space mandate removal of portions of the bone from the opposing
surface of the vertebral bodies 11-1, 11-2, to provide similarly
contoured surfaces into which the anchoring plates and the
underlying prosthesis will nest when the prosthesis with partially
integrated anchoring system is implanted, and to expose the
marrow/cancellous bone to the fabric at the openings 98.
[0073] The presence of the humps and mating semicircular anchoring
plates in the embodiment of FIG. 9 offers some further stability in
preventing the disc prosthesis from shifting laterally, anteriorly
or posteriorly in the acute period that biological incorporation
anchoring is taking place.
[0074] Another embodiment of a partially integrated adjunct
anchoring system for a disc prosthesis, that utilizes some of the
same concepts present in the embodiment of FIG. 9, is illustrated
in FIGS. 10A and 10B. The disc prosthesis 100 comprises a nucleus
within a bioincorporable fabric sheath or matrix, as in each of the
other embodiments. Here, however, the sheath has a pair of bumps or
projections 101 on each of its end surfaces, adapted to align with
and enter respective mating through-holes 102 in a pair of plates
or platform bases 103, 104 at top and bottom of the disc
prosthesis. The plates are adapted, that is, configured and
arranged, to confine the prosthesis when implanted along with
installation of the anchoring system.
[0075] Upper plate 103 has a flat planar projection 106 that spans
much of the prosthesis along the median plane, designed to be
received within a mating slot (not shown) formed in the bone of the
upper vertebral body 11-1. Lower plate 104 is adapted to be
fastened to the lower vertebral body 11-2 by one or more screw
fasteners 107.
[0076] The partially integrated adjunct anchoring mechanism of FIG.
11 illustrates an embodiment that uses a sliding rail system. In
this embodiment, a pair of parallel rails 111 is mounted into one
vertebral end plate and another pair of parallel rails 113 is
mounted in the adjoining vertebral end plate bounding the space
from which the damaged natural disc has been removed. The parallel
rails span the end plate on which they are mounted. In this
embodiment, each vertebral end plate has the rails oriented
perpendicularly in the same direction to prevent improper insertion
of the prosthesis. The prosthesis end 110 has an embedded platform
114 with grooves 115 to accept the projecting rails 111. Platform
114 is anchored in the prosthesis with woven fabric of the outer
matrix or sheath. The other end 112 of the prosthesis has parallel
grooves 116 that match the configuration of the oppositely oriented
rails 113, so that the latter are accepted by and slide within the
grooves.
[0077] In practice, the rails are first mounted into the vertebral
end plates, and the prosthesis with its embedded matching
configuration of grooves is inserted into the vacant space aligned
with the rails, to lock the prosthesis and secure it in position
anteriorly, laterally and posteriorly. The primary anchorage
mechanism anchors to the bone.
[0078] It will be observed that the adjunct anchoring mechanism in
each embodiment of the invention is arranged and configured with
respect to the disc prosthesis, so that when the prosthesis is
implanted and the adjunct anchoring mechanism is installed, there
is minimal if any impeding by the anchoring mechanism of elastic
deformation of the nucleus of the disc prosthesis under compressive
force exerted along the vertebral column.
[0079] Although certain preferred embodiments and methods of the
invention have been described herein, it will be apparent to those
skilled in the art from a consideration of the foregoing
disclosure, that variations and modifications of the described
embodiments may be made without departing from the spirit and scope
of the invention. Accordingly, it is intended that the invention
shall be limited only to the extent required by the appended claims
and the rules and principles of applicable law.
* * * * *