U.S. patent application number 12/548225 was filed with the patent office on 2010-03-04 for rail-based modular disc nucleus prosthesis.
Invention is credited to Stephen H. Crosbie, Jeffrey C. Felt, Mark A. Rydell, John E. Sherman.
Application Number | 20100057144 12/548225 |
Document ID | / |
Family ID | 46062833 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100057144 |
Kind Code |
A1 |
Felt; Jeffrey C. ; et
al. |
March 4, 2010 |
RAIL-BASED MODULAR DISC NUCLEUS PROSTHESIS
Abstract
A method and apparatus for repairing a damaged intervertebral
disc nucleus in a minimally invasive manner utilizes a modular disc
prosthesis preferably comprised of at least three modular segments
and at least two rails that operably connect adjacent modular
segments. In one embodiment, each modular segment includes a harder
inner portion and a softer outer portion. Preferably, the rails
operate to slidably connect and interlock adjacent modular
segments. A stem portion of the rails that extends outside the
periphery of the body of the prosthesis is removable after
implantation such that the modular segments form an implanted
unitary device that closely mimics the geometry of the disc nucleus
cavity.
Inventors: |
Felt; Jeffrey C.;
(Minnetonka, MN) ; Rydell; Mark A.; (Minnetonka,
MN) ; Crosbie; Stephen H.; (Minnetonka, MN) ;
Sherman; John E.; (Wayzata, MN) |
Correspondence
Address: |
PATTERSON, THUENTE, SKAAR & CHRISTENSEN, P.A.
4800 IDS CENTER, 80 SOUTH 8TH STREET
MINNEAPOLIS
MN
55402-2100
US
|
Family ID: |
46062833 |
Appl. No.: |
12/548225 |
Filed: |
August 26, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11372477 |
Mar 9, 2006 |
7591853 |
|
|
12548225 |
|
|
|
|
60685332 |
May 24, 2005 |
|
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60700459 |
Jul 19, 2005 |
|
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|
60660107 |
Mar 9, 2005 |
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Current U.S.
Class: |
606/86R |
Current CPC
Class: |
A61F 2002/30563
20130101; A61F 2230/0069 20130101; A61F 2002/30014 20130101; A61F
2/4455 20130101; A61F 2002/30166 20130101; A61F 2210/0061 20130101;
A61F 2250/0019 20130101; A61F 2/4611 20130101; A61F 2002/30383
20130101; A61F 2/3094 20130101; A61F 2220/0075 20130101; A61F
2002/30971 20130101; A61F 2002/3052 20130101; A61F 2002/4627
20130101; A61F 2002/30075 20130101; A61F 2230/0028 20130101; A61F
2002/30387 20130101; A61F 2002/30462 20130101; A61F 2002/30235
20130101; A61F 2002/30561 20130101; A61F 2002/30594 20130101; A61F
2002/30677 20130101; A61F 2/442 20130101; A61F 2002/30016 20130101;
A61F 2250/0018 20130101; A61F 2002/30604 20130101; A61F 2002/444
20130101; A61F 2220/0025 20130101 |
Class at
Publication: |
606/86.R |
International
Class: |
A61B 17/56 20060101
A61B017/56 |
Claims
1. A minimally invasive method of implanting a modular disc
prosthesis into an evacuated disc nucleus space, the method
comprising: inserting a first modular segment into the disc nucleus
space through an opening, the first modular segment having a width
with a first rail that extends out of the disc nucleus space when
the first modular segment is located within the disc nucleus space;
sliding a second modular segment along the first rail into the disc
nucleus space through the opening until the second modular segment
interlocks with the first modular segment, the second modular
segment having a width with a second rail that extends out of the
disc nucleus space when the second modular segment is located
within the disc nucleus space; removing a portion of the first rail
that extends from the interlocked first and second modular
segments; sliding a third modular segment along the second rail
into the disc nucleus space through the opening until the third
modular segment interlocks with the second modular segment, the
third modular having a width; and removing a portion of the second
rail that extends from the interlocked second and third modular
segments to form an implanted modular disc prosthesis having a
generally continuous periphery that corresponds generally to the
evacuated nucleus disc space and having a total width at least
equal to the widths of the first, second and third modular segment
and greater than a width of the opening that is substantially only
the width of a largest of the widths of any one of the modular
segments.
2. The minimally invasive method of implanting a modular disc
prosthesis of claim 1, the third modular segment having a third
rail that extends out of the disc nucleus space when the third
modular segment is located within the disc nucleus space, the
method further comprising: sliding a fourth modular segment along
the third rail into the disc nucleus space through the opening
until the fourth modular segment interlocks with the third modular
segment, the fourth modular segment having a width; and removing
the portion of the third rail that extends from the interlocked
third and fourth modular segments.
3. The minimally invasive method of implanting a modular disc
prosthesis of claim 2, the fourth modular segment having a fourth
rail that extends out of the disc nucleus space when the fourth
modular segment is located within the disc nucleus space, the
method further comprising: sliding a fifth modular segment along
the fourth rail into the disc nucleus space through the opening
until the fifth modular segment interlocks with the fourth modular
segment, the fifth modular segment having a width; and removing the
portion of the fourth rail that extends from the interlocked fourth
and fifth modular segments.
4. The minimally invasive method of implanting a modular disc
prosthesis of claim 1, wherein the steps of sliding the second and
third modular segments are performed using an insertion tool and
the steps of removing the first and second rails is accomplished by
using a cutting mechanism provided at the distal end of the
insertion tool.
Description
RELATED APPLICATIONS
[0001] The present invention is a divisional application of U.S.
patent application Ser. No. 11/372,477, filed Mar. 9, 2006, which
claims priority to U.S. Provisional Patent Application No.
60/685,332, filed May 24, 2005, U.S. Provisional Patent Application
No. 60/700,459, filed Jul. 19, 2005, and U.S. Provisional Patent
Application No. 60/660,107, filed Mar. 29, 2005, the disclosures of
which are hereby incorporated by reference. The present invention
is also related to the U.S. application Ser. No. 11/372,357, filed
Mar. 9, 2006, now U.S. Pat. No. 7,267,690 which is hereby
incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to an implantable
prosthesis for repairing damaged intervertebral discs. More
particularly, the present invention relates to a rail-based modular
disc nucleus prosthesis of predetermined size and shape.
BACKGROUND OF THE INVENTION
[0003] The spinal motion segment consists of a unit of spinal
anatomy bounded by two vertebral bodies, including the two
vertebral bodies, the interposed intervertebral disc, as well as
the attached ligaments, muscles, and the facet joints. The disc
consists of the end plates at the top and bottom of the vertebral
bones, the soft inner core, called the nucleus and the annulus
fibrosus running circumferentially around the nucleus. In normal
discs, the nucleus cushions applied loads, thus protecting the
other elements of the spinal motion segment. A normal disc responds
to compression forces by bulging outward against the vertebral end
plates and the annulus fibrosus. The annulus consists of collagen
fibers and a smaller amount of elastic fibers, both of which are
effective in resisting tension forces. However, the annulus on its
own is not very effective in withstanding compression and shear
forces.
[0004] As people age the intervertebral discs often degenerate
naturally. Degeneration of the intervertebral discs may also occur
in people as a result of degenerative disc disease. Degenerative
disc disease of the spine is one of the most common conditions
causing pain and disability in our population. When a disc
degenerates, the nucleus dehydrates. When a nucleus dehydrates, its
ability to act as a cushion is reduced. Because the dehydrated
nucleus is no longer able to bear loads, the loads are transferred
to the annulus and to the facet joints. The annulus and facet
joints are not capable of withstanding their increased share of the
applied compression and torsional loads, and as such, they
gradually deteriorate. As the annulus and facet joints deteriorate,
many other effects ensue, including the narrowing of the
interspace, bony spur formation, fragmentation of the annulus,
fracture and deterioration of the cartilaginous end plates, and
deterioration of the cartilage of the facet joints. The annulus and
facet joints lose their structural stability and subtle but
pathologic motions occur between the spinal bones.
[0005] As the annulus loses stability it tends to bulge outward and
may develop a tear allowing nucleus material to extrude. Breakdown
products of the disc, including macroscopic debris, microscopic
particles, and noxious biochemical substances build up. The
particles and debris may produce sciatica and the noxious
biochemical substances can irritate sensitive nerve endings in and
around the disc and produce low back pain. Affected individuals
experience muscle spasms, reduced flexibility of the low back, and
pain when ordinary movements of the trunk are attempted.
[0006] Degeneration of a disc is irreversible. In some cases, the
body will eventually stiffen the joints of the motion segment,
effectively re-stabilizing the discs. Even in the cases where
re-stabilization occurs, the process can take many years and
patients often continue to experience disabling pain. Extended
painful episodes of longer than three months often leads patients
to seek a surgical solution for their pain.
[0007] Several methods have been devised to attempt to stabilize
the spinal motion segment. Some of these methods include: heating
the annular region to destroy nerve endings and strengthen the
annulus; applying rigid or semi-rigid support members on the sides
of the motion segment or within the disc space; removing and
replacing the entire disc with a generally rigid plastic,
articulating artificial device; removing and replacing the nucleus;
and spinal fusion involving permanently fusing the vertebrae
adjacent the affected disc.
[0008] Until recently, spinal fusion has generally been regarded as
the most effective surgical treatment to alleviate back pain due to
degeneration of a disc. While this treatment is often effective at
relieving back pain, all discal motion is lost in the fused spinal
motion segment. The loss of motion in the affected spinal segment
necessarily limits the overall spinal mobility of the patient.
Ultimately, the spinal fusion places greater stress on the discs
adjacent the fused segment as these segments attempt to compensate
for lack of motion in the fused segment, often leading to early
degeneration of these adjacent spinal segments.
[0009] Current developments are focusing on treatments that can
preserve some or all of the motion of the affected spinal segment.
One of these methods to stabilize the spinal motion segment without
the disadvantages of spinal fusion is total disc replacement. Total
disc replacement is a highly invasive and technically demanding
procedure which accesses the disc from an anterior or frontal
approach and includes dividing the anterior longitudinal ligament,
removing the cartilaginous end plates between the vertebral bone
and the disc, large portions of the outer annulus and the complete
inner nucleus. Then an artificial total disc replacement is
carefully placed in the evacuated disc space. Many of the
artificial total disc replacements currently available consist of a
generally rigid plastic such as ultra high molecular weight
polyethylene ("UHMWPE") as the nucleus that is interposed between
two metal plates that are anchored or attached to the vertebral
endplates. A summary of the history of early development and
designs of artificial discs is set forth in Ray, "The Artificial
Disc: Introduction, History and Socioeconomics," Chpt. 21, Clinical
Efficacy and Outcome in the Diagnosis of Low Back Pain, pgs.
205-225, Raven Press (1992). Examples of these layered total disc
replacement devices are shown, for example, in U.S. Pat. Nos.
4,911,718, 5,458,643, 5,545,229 and 6,533,818.
[0010] These types of artificial total discs have several
disadvantages. First, because the artificial disc replacements are
relatively large, they require relatively large surgical exposures
to accommodate their insertion. The larger the surgical exposure,
the higher the chance of infection, hemorrhage or even morbidity.
Also, in order to implant the prosthesis, a large portion of the
annulus must be removed. Removing a large portion of the annulus
reduces the stability of the motion segment, at least until healing
occurs around the artificial disc. Further, because the devices are
constructed from rigid materials, they can cause serious damage if
they were to displace from the disc space and contact local nerve
or vascular tissues. Another disadvantage is that rigid artificial
disc replacements do not reproduce natural disc mechanics.
[0011] An alternative to total disc replacement is nucleus
replacement. Like an artificial disc prosthesis, these nucleus
replacements are also inert, non-rigid, non-biological implants.
The procedure for implanting a nucleus replacement is less invasive
than the procedure for a total disc replacement and generally
includes the removal of only the nucleus and replacement of the
nucleus with a prosthesis that may be elastically compressible and
provide cushioning that mimics a natural disc nucleus. Examples of
implants used for nucleus replacement include: U.S. Pat. Nos.
4,772,287, 4,904,260, 5,192,326, 5,919,236 and 6,726,721.
[0012] Nucleus replacements are intended to more closely mimic
natural disc mechanics. To that end, some nucleus replacements
utilize hydrogels because of their water imbibing properties that
enable these replacements to expand in situ to permit a more
complete filling of the evacuated nucleus cavity. However, there is
usually a trade-off in that the more expansion the hydrogel
achieves, the less structural support the end product can provide.
As a result, many hydrogel nucleus disc replacements have generally
adopted the use of some form of a jacket or fabric to constrain the
hydrogel material. For example, the implant described in U.S. Pat.
Nos. 4,772,287 and 4,904,260 consists of a block of hydrogel
encased in a plastic fabric casing. The implant described in U.S.
Pat. No. 5,192,326 consists of hydrogel beads enclosed by a fabric
shell. Without the jacket or other form of constraint, the hydrogel
is susceptible to displacement because of the slippery nature of
the hydrogel. Unfortunately, the jacket or fabric shell will be
subject to long term abrasive wear issues that could result in
failure of the jacket or shell's ability to constrain the hydrogel
and thus the hydrogel may be subject to displacement.
[0013] Another approach to nucleus replacement involves
implantation of a balloon or other container into the nucleus,
which is then filled with a biocompatible material that hardens in
situ. Examples of this in situ approach to nucleus replacement
include U.S. Pat. Nos. 6,443,988 and 7,001,431. One of the problems
with this approach is that the chemical hardening process is
exothermic and can generate significant amounts of heat that may
cause tissue damage. In addition, there is a possibility that the
balloon may rupture during expansion, causing leakage of material
into the disc cavity and surrounding tissues, which may cause
undesirable complications.
[0014] Another technique for nucleus replacement involves
implanting a multiplicity of individual support members, such as
beads, one at a time in the evacuated disc nucleus cavity until the
cavity is full. Examples of this approach include U.S. Pat. Nos.
5,702,454 and 5,755,797. Because each of the individual support
members or beads is relatively small, there is a possibility that
one or more of the individual support members or beads may extrude
out of the evacuated disc nucleus cavity. From a mechanical
perspective, this technique is limited in the ability to produce
consistent and reproducible results because the location and
interaction of the multiplicity of beads or support members is not
controlled and the beads or support members can shift during and
after implantation.
[0015] Accordingly, there is a need for a nucleus prosthesis that
may be inserted using a minimally invasive procedure and that
mimics the characteristics of a natural disc.
SUMMARY OF THE INVENTION
[0016] The present invention provides a method and apparatus for
repairing a damaged intervertebral disc nucleus in a minimally
invasive manner with a modular disc prosthesis. The modular disc
prosthesis preferably comprises at least three modular segments and
at least two rails that operably connect adjacent modular segments.
This configuration allows the prosthesis to be adapted for
implantation through various surgical approaches, although the
preferred method is the posterolateral ("posterior") approach where
the disc is accessed through the patient's back. In one embodiment,
each modular segment includes a harder inner portion and a softer
outer portion. Preferably, the rails operate with a sliding
mechanism to connect and interlock adjacent modular segments. A
stem portion of the rails that extends outside the periphery of the
body of the prosthesis is removable after implantation such that
the modular segments form an implanted unitary device that closely
mimics the geometry of the disc nucleus cavity.
[0017] In one embodiment, a modular disc prosthesis that is adapted
to be implanted in an evacuated disc nucleus cavity includes at
least three modular segments each having a width. The first modular
segment has a first rail extending at least partially along one
side of the width and beyond a periphery of the first modular
segment. The second modular segment is slidably connected to the
first rail on one side of the width and has a second rail extending
at least partially along another side of the width and beyond a
periphery of the second modular segment. The third modular segment
is slidably connected to the second rail on one side of the width.
The prosthesis has an expanded position in which the modular
segments are extended along the first and second rails and
positioned in a generally end to end configuration spaced apart by
the rails prior to implantation. The prosthesis also has an
implanted position in which the modular segments are positioned in
a generally side by side configuration that defines a unitary body
having a generally continuous periphery that generally corresponds
to the evacuated disc nucleus cavity with at least a portion of the
rails extending beyond the periphery of the body.
[0018] Preferably, each modular segment comprises an inner portion
and an outer portion. The inner portion includes structure that
mates with one of the rails. The outer portion substantially
surrounds the inner portion, except for the side having one of the
rails and the side having structure that mates with one of the
rails. In one embodiment, the inner portion of each modular segment
and the outer portion of each modular segment are made of polymers
of different durometers. Preferably, the inner portion of each
modular segment has a compressive modulus from about 70-100 MPa and
the outer portion of each modular segment has a compressive modulus
from about 6-20 MPa. The use of a harder inner portion and softer
outer portion as part of an integrated unitary implanted device
permits the modular prosthesis of the present invention to more
closely mimic the stress response of a biological disc nucleus
while simultaneously permitting effective operation of the slidable
relationship between adjacent modular segments.
[0019] In one embodiment, locking features are provided to ensure
that the modular disc prosthesis is a unitary device both before
and after insertion. To prevent the device from being separated
prior to insertion, locking features may be provided on the rigid
rails to prevent modular segments from being slid back off of the
rails. This ensures that each modular segment is connected in its
proper position and in the proper order. In addition, locking
features may be provided on the modular segments to lock them
together upon insertion. This prevents individual segments from
dislocating from the assembled prosthesis and migrating outside of
the annulus.
[0020] Another aspect of the present invention comprises a method
for implanting a modular disc prosthesis. Because the modular disc
prosthesis may be implanted one segment at a time, a hole made in
the annulus for implantation of the prosthesis may be a fraction of
the size of the device in its final assembled form. The first
modular segment is inserted into the disc nucleus space through the
small hole in the annulus. The second modular segment is then slid
up the first rigid rail and into the disc nucleus space until the
second modular segment interlocks with the first modular segment.
The tail stem of the first rigid rail is then severed from the
device. Subsequent modular segments are slid up the adjoining rigid
rail into the disc nucleus space and then interlocked with the
previously inserted modular segment in a similar manner. Once all
of the modular segments have been inserted and all of the tail
stems severed, the modular disc prosthesis is fully inserted into
the patient's disc nucleus space.
[0021] Another aspect of the present invention provides an
insertion tool that may be used to aid in the insertion,
positioning, and rail removal of the modular prosthesis. The
proximal end of the tool has a handle with an enclosed ratchet or
roller mechanism attached to and in line with the inner lumen of an
elongated tube at the distal end of the tool through which a rail
may be inserted. The elongated tube may have a slit or other
openings along the length of the tube to aid in threading the rails
into the tube. Insertion tool may be provided with a cutting
mechanism for removing the rails from the modular segments once
they are fully inserted.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0023] FIG. 1 is a cross-sectional view of a modular disc
prosthesis according to the preferred embodiment of the present
invention in its inserted configuration.
[0024] FIG. 2 is a top view of a modular disc prosthesis according
to the preferred embodiment of the present invention prior to
insertion.
[0025] FIG. 3 is a perspective view of a modular disc prosthesis
according to an alternate embodiment of the present invention prior
to insertion.
[0026] FIG. 4 is a perspective view of a modular disc prosthesis
according to an alternate embodiment of the present invention at a
first stage of insertion.
[0027] FIG. 5 is a perspective a view of a modular disc prosthesis
according to an alternate embodiment of the present invention at a
second stage of insertion.
[0028] FIG. 6 is a perspective view of a modular disc prosthesis
according to an alternate embodiment of the present invention at a
final state of insertion.
[0029] FIG. 7 is a partial perspective view of a portion of a
modular disc prosthesis according to an embodiment of the present
invention.
[0030] FIG. 8 is a view of an insertion tool for use in accordance
with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0031] Referring to FIG. 1, there can be seen a cross-sectional
view of a modular disc prosthesis 100 according to the preferred
embodiment of the present invention as configured once inserted
into the body. In this embodiment, modular disc prosthesis 100
comprises first 102, second 104, third 106, fourth 108, and fifth
109 modular segments. Preferably, each modular segment 102, 104,
106, 108, 109 comprises a soft outer portion 102a, 104a, 106a,
108a, 109a and a hard inner portion 102b, 104b, 106b, 108b,
109b.
[0032] In a preferred embodiment, hard inner portions 104b, 106b
and 108b have an I-beam cross-sectional shape that optimizes
flexibility and strength of the hard inner portions. Alternatively,
hard inner portions 104b, 106b, 108b, can have a uniformly shaped
cross-sectional area to reduce any differences in compressibility
of the modular disc prosthesis 100 across the surface area in order
to minimize the potential for stress risers to be created in the
interface between the outer surface of the modular disc prosthesis
100 and the inner surfaces of the disc space cavity. It will be
recognized that various cross-sectional shapes of hard inner
portions 102b, 104b, 106b, 108b and 109b can be utilized in
accordance with the present invention and that the cross-sectional
shapes of the hard inner portions do not need to be symmetric.
[0033] Hard inner portion 102b of first modular segment 102
includes first segment interlocking portion 116. Hard inner portion
104b of second modular segment 104 includes second segment
interlocking portion 118 and a first slot 128. Hard inner portion
106b of third modular segment 106 includes third segment
interlocking portion 120 and a second slot 130. Hard inner portion
108b of fourth modular segment 108 includes fourth segment
interlocking portion 121 and a third slot 132. Hard inner portion
109b of fifth modular segment 109 includes a fourth slot 133.
[0034] In the preferred embodiment, as shown in FIG. 2, rails 110,
112, 114, 115 have a noncircular cross-sectional shape, although it
will be understood that other cross-sectional shapes could be
utilized and that there is no requirement that all of the rails
have similar cross-sectional shapes. It has been found that the
noncircular cross-sectional shape as shown (corresponding mating C
and sideways T cross-sectional shapes) provides for better
alignment of the modular segments and supports larger insertion
forces along the axis of the rail.
[0035] It will be understood that in the preferred embodiment, the
rails 110, 112, 114, 115 of the present invention have a
non-uniform cross-sectional aspect ratio in terms of the height and
width of the rail. Preferably, the rails have a relative rigidity
along a longitudinal axis of the rail in a dimension of the height
of the rail that is greater than a width of the rail, whereas in a
dimension transverse to the width of the rail the relative rigidity
of the rail permits a greater degree of flexibility such that
succeeding modular segments can be moved laterally with respect to
one another in the expanded position without deforming the rails.
Preferably, the height of the rail is in the range of about 1 mm to
6 mm and the width of the rail is in the range of about 0.5 mm to 4
mm. This differential rigidity in the two dimensions transverse to
the longitudinal axis of the rail is important in permitting
effective and efficient sliding operation of the adjacent modular
segments.
[0036] Referring to FIG. 2, there can be seen a portion of a
modular disc prosthesis 100 according to the preferred embodiment
of the present invention prior to insertion into the evacuated disc
nucleus cavity. Note that in FIG. 2, modular disc prosthesis 100 is
depicted showing only the hard inner portion 102b, 104b, 106b,
108b, 109b of each modular segment 102, 104, 106, 108, 109 for
convenience of illustration. However, in the preferred embodiment
of the invention each modular segment would also have a soft outer
portion as described above and shown in FIG. 1.
[0037] In alternate embodiments, the modular disc prosthesis may
comprise greater or fewer numbers of modular segments and rails, as
long as there are at least three modular segments and two rails.
For example, FIG. 3 depicts a modular disc prosthesis 200 having
four modular segments and three rails.
[0038] Prior to insertion, modular disc prosthesis 100 further
includes first 110, second 112, third 114, and fourth 115 rails.
First modular segment 102 is rigidly attached to first rail 110 at
first segment interlocking portion 116. Second modular segment 104
is slidably attached to first rail 110 at first slot 128 and
rigidly attached to second rail 112 at second segment interlocking
portion 118. Third modular segment 106 is slidably attached to
second rail 112 at second slot 130 and rigidly attached to third
rail 114 at third segment interlocking portion 120. Fourth modular
segment 108 is slidably attached to third rail 114 at third slot
132 and rigidly attached to fourth rail 115 at fourth segment
interlocking portion 121. Fifth modular segment 109 is slidably
attached to fourth rail 115 at fourth slot 133.
[0039] As shown in FIG. 2 and FIG. 3, each rail 110, 112, 114 and
115 or 210, 212 and 214 includes a stem portion that extends beyond
a periphery of the body of the prosthesis 100, 200, respectively.
Preferably these stem portions are long enough to permit access
into the evacuated disc nucleus space such that one modular segment
can be positioned inside the evacuated disc nucleus cavity while
the next modular segment on the rail is still outside of the body.
In an exemplary embodiment, the length of the stem portions ranges
between 6 cm-20 cm.
[0040] As shown in the alternate embodiment of FIG. 3, each rail
210, 212, 214 may further include a retaining portion 222, 224, 226
to keep the device from being separated prior to insertion. The
retaining portions 222, 224 and 226 are configured to prevent the
corresponding modular segments 204, 206 and 206 from sliding off
the rails. The retaining portions may be molded into the rails or
may be separate pieces or deformations of the rails added during
the manufacture of the device.
[0041] The preferred embodiment is a unitary prosthesis that is
packaged, sterile, and ready for implantation at the surgical site.
Since the device is fully preformed and delivered as a unitary
implant, the device is under direct surgeon control until the disc
nucleus prosthesis is completely formed. This unitary design
reduces the need for the surgeon to determine how to configure the
prosthesis to allow for the most efficacious filling of the
evacuated disc nucleus cavity and assures that the components'
order of insertion and connection are properly achieved. The
ability to predetermine the size of the modular disc prosthesis
also allows for the evacuated disc nucleus cavity to be more
completely filled and provides a greater degree of control over the
uniformity of the stress response of the implant as compared to
other kinds of minimally invasive implants. In this regard, it will
be understood that the modular disc prosthesis 100 of the present
invention may be provided in a variety of different final assembled
volumes and shapes to correspond to different sizes and shapes of
different evacuated disc nucleus cavities.
[0042] Modular disc prosthesis 100 may be introduced through an
access tube that is inserted partially into the disc nucleus space.
Access tube is at least 3 inches long and preferably about 6 inches
long. An insertion tool 400 as shown in FIG. 8 may be used to aid
in the insertion, positioning, and rail removal of the modular
prosthesis. The proximal end of the tool 400 has a handle 402 with
an enclosed ratchet or roller mechanism attached to and in line
with the inner lumen of an elongated tube 404 at the distal end of
the tool through which a rail 406 may be inserted. The elongated
tube 404 may have a slit or other openings along the length of the
tube 404 to aid in threading the rails 406 into the tube. Insertion
tool 400 can then be used to guide modular segments 408 into the
disc space. The insertion tool 400 may be made out of any
combination of plastics, metals, ceramics, or the like.
[0043] It should be noted that although the insertion of modular
disc prosthesis 100 is described in relation to a preferred
five-segment embodiment or an alternate four-segment embodiment,
embodiments having any other number of segments would be inserted
in a similar fashion.
[0044] Referring again to FIG. 3, there can be seen a modular disc
prosthesis 200 prior to insertion into the body. Upon inserting the
access tube into the disc nucleus space, the first rail 210 is
threaded through the lumen of the elongated tube 404 of the
insertion tool 400. The insertion tool 400 is then used to push
first modular segment 202 through the tube and into the disc space.
Upon complete insertion of first modular segment 202, modular disc
prosthesis 200 is moved centrally and the insertion tool 400 is
repositioned onto the first rail 210 proximal to second modular
segment 204 which is slid along the first rail 210 into the
evacuated disc nucleus space onto first segment interlocking
portion 216 until it is flush with first modular segment 202. This
stage of insertion is depicted in FIG. 4. A stem portion of first
rail 210 is then removed and modular disc prosthesis 200 is moved
centrally again.
[0045] Second rail 212 is then threaded through insertion tool 400
and third modular segment 206 is slid down second rail 212 and into
the disc nucleus space onto second segment interlocking portion 218
until it is flush with second modular segment 204. This
configuration is shown in FIG. 5. A stem portion of second rail 212
is then removed and modular disc prosthesis 200 is moved centrally.
Third rail 214 is then threaded through insertion tool and fourth
modular segment 208 is slid along third rail 214 into the disc
nucleus space and onto third segment interlocking portion 220 until
it is flush with the other modular segments 202, 204, 206. Finally,
a stem portion of third rail 214 is removed. This final implanted
configuration of modular disc prosthesis 200 with all modular
segments aligned and locked together is shown in FIG. 6. Modular
disc prosthesis 200 is sized and shaped to conform to the geometry
of the evacuated disc nucleus cavity.
[0046] In an alternate embodiment, a keystone approach can be used
to insert the modular disc prosthesis such that the last modular
segment inserted into the disc nucleus space is not one of the
outside segments. Instead, the outside segments can be the first
two segments inserted. This creates a bilateral expansion force as
the remaining segments are inserted between the two outside
segments. This helps make a tighter fit within the evacuated disc
nucleus cavity than does the asymmetric lateral force imparted when
the segments are implanted sequentially.
[0047] The stem portions of rails 110, 112, 114, 115 that extend
beyond the periphery of the body of the modular disc prosthesis 100
can be removed by many different techniques. Insertion tool 400 may
be provided with a cutting mechanism that can remove the stem
portions of the rails. The cutting mechanism may be a pair of fixed
blades located on the distal end of the pushing tool. In this
embodiment, the cutting blades would act as a cutting wheel in
which a turning of the handle connected to the blades causes the
blades to circumscribe the rail. Alternatively, the cutting
mechanism can be a clamping means that removes the rails through
twisting or pinching. Stem rails may also be cut off with any other
sharp instrument.
[0048] In another embodiment, the stem portions of the rails may be
provided with a perforation at the junction with each modular
segment such that they can be torn, broken, twisted, or more easily
cut off. Cutting may also be accomplished with a wire loop provided
to the part. Additionally, heat, laser, or any other local energy
source can be used to accomplish the separation. One of skill in
the art will recognize that numerous alternative means exist
whereby stem rails can be severed from the modular disc
prosthesis.
[0049] Alternatively, the modular disc prosthesis may be implanted
using an anterior lateral approach. An anterior lateral approach
allows for a larger insertion opening to be used while still being
minimally invasive. In this approach, the disc is accessed from the
patient's side through the psoas muscle, which avoids major nerve
and vascular tissues, and may be used in the presence of medical
conditions mitigating against the posterior approach. This approach
is essentially oriented 90.degree. from the posterior approach. In
this embodiment, it may be acceptable to have only two modular
segments that comprise the prosthesis with the side-by-side
orientation of the segments being generally medial-to-lateral
instead of posterior-to-anterior.
[0050] During insertion, slots 128, 130, 132, 133 slide along the
stem portions of rails 110, 112, 114, 115 and onto segment
interlocking portions 116, 118, 120, 121. Slots 128, 130, 132, 133
and segment interlocking portions 116, 118, 120, 121 may be
provided with locking features to prevent separation of modular
segments 102, 104, 106, 108, 109. Locking features, such as a barb
or stud or a series of barbs or studs, may be provided such that
once a slot is slid onto a segment interlocking portion, it cannot
be slid back off of it. A ratchet and pawl may also be used to lock
modular segments together. A ratchet release tool may also be
provided in case separation of modular segments is desired once
they are locked together.
[0051] One example of these locking features is depicted in FIG. 7.
Hard inner portion 304b of each modular segment 304 is provided
with a pair of depressible projections 334 on segment interlocking
portion 318 and a complementary pair of apertures 336 on slot 328.
When slot 328 of a first modular segment 304 is slid onto segment
interlocking portion 318 of a second modular segment, projections
are depressed. When apertures of the first modular segment are
positioned over projections of the second modular segment, the
projections pop through apertures 336, locking the modular segments
relative to one another. Modular segments may be separated by
depressing the projections and sliding the first modular segment
back off of the second modular segment.
[0052] Alternatively, free movement of modular segments 102, 104,
106, 108, 109 along rails 110, 112, 114, 115 may be allowed until
insertion in the body. It will be understood that, depending upon
the material configuration of the modular prosthesis 100 and the
interface fit, segment interlocking portions 116, 118, 120, 121 may
swell due to hydration to lock in the final configuration. This
feature may be used alone or in combination with a mechanical
locking feature. Alternative methods of locking modular segments
together will be appreciated by those skilled in the art.
[0053] In the preferred embodiment, modular disc prosthesis 100 is
molded from elastomeric biomaterials, preferably polyurethane. Stem
rails 110, 112, 114, 115 and hard inner portions 102b, 104b, 106b,
108b, 109b are made from a hard durometer polyurethane, such as a
polyurethane with a Shore D hardness of about 45 or above and
compressive modulus in the range of about 70 to 100 MPa. Soft outer
portions 102a, 104a, 106a, 108a, 109a are made from a softer
durometer polyurethane, such as a polyurethane with a Shore A
hardness ranging from about 40 to 80 and a compressive modulus in
the range of about 6 to 20 MPa.
[0054] In the preferred embodiment, the two different durometer
polyurethanes may be co-polymerized to create a chemical bond
between the two portions of each modular segment 102, 104, 106,
108, 109. In alternate embodiments, other polymers such as PEEK,
polyethylene, silicones, acrylates, nylon, polyacetyls, and other
similar biocompatible polymers may be used for the hard inner
portions or the soft outer portions.
[0055] In an alternate embodiment, the stem of the tails may be
molded from a harder durometer material than soft outer portion and
hard inner portion of modular segments. Utilizing this approach
allows the rails to be extruded, rather than molded as part of the
modular segments. A bond joint can then be made with the hard inner
portion external to the periphery of the modular segments to form
the unitary design. Extruding the stem portions of the tails makes
the modular disc prosthesis easier and less expensive to
manufacture than a completely molded product.
[0056] In the preferred embodiment, the modular disc prosthesis is
deformable in response to normal physiological forces of 30 to 300
pounds. Because of this deformability, the prosthesis produces a
physiologically appropriate amount of loading on the end plates of
the intervertebral disc. As a result, the end plates will not
excessively deform over time and ultimately conform to the contours
of the implant as is the case with many more rigid disc nucleus
replacement implants.
[0057] In an alternate embodiment, the outer shell of the modular
disc nucleus prosthesis may be modified to provide for elution of
medicants. Such medicants may include analgesics, antibiotics,
antineoplastics or bioosteologics such as bone growth agents. While
motion preservation is generally a principle goal in nucleus
replacement, in certain indications it may be desirable to promote
some bony fusion. Such indications may include nucleus replacements
in the cervical spine.
[0058] The solid polymer outer shell of the modular disc nucleus
prosthesis may provide for better and more controllable elution
rates than some hydrogel materials. In an alternate embodiment, the
modular disc nucleus prosthesis may include different elution rates
for each polymer material. This would allow for varying elution
rates for different medicants.
[0059] Various modifications to the disclosed apparatuses and
methods may be apparent to one of skill in the art upon reading
this disclosure. The above is not contemplated to limit the scope
of the present invention, which is limited only by the claims
below.
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