U.S. patent application number 09/871298 was filed with the patent office on 2001-10-18 for dovetail tome for implanting spinal fusion devices.
Invention is credited to Nicholson, James E., Tromanhauser, Scott G., Whipple, Dale E..
Application Number | 20010031967 09/871298 |
Document ID | / |
Family ID | 26753728 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010031967 |
Kind Code |
A1 |
Nicholson, James E. ; et
al. |
October 18, 2001 |
Dovetail tome for implanting spinal fusion devices
Abstract
A tome for cutting at least one dovetail in bone is disclosed
wherein said tome comprises: A) a shaft having first and second
ends, said first end having attached thereto a blade shaped for
cutting said dovetail in said bone and said second end having
attached thereto an extension for engagement with mechanical energy
transmission devices, and B) a depth stop on said shaft between
said blade and said extension.
Inventors: |
Nicholson, James E.;
(Lincoln, MA) ; Tromanhauser, Scott G.;
(Marblehead, MA) ; Whipple, Dale E.; (East
Taunton, MA) |
Correspondence
Address: |
Williams & Associates
Suite 300
1030 Fifteenth Street, N.W.
Washington
DC
20005-1501
US
|
Family ID: |
26753728 |
Appl. No.: |
09/871298 |
Filed: |
May 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09871298 |
May 31, 2001 |
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09408762 |
Sep 30, 1999 |
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6241733 |
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09408762 |
Sep 30, 1999 |
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09248151 |
Feb 10, 1999 |
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6096080 |
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09248151 |
Feb 10, 1999 |
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09072777 |
May 6, 1998 |
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6241769 |
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Current U.S.
Class: |
606/84 |
Current CPC
Class: |
A61F 2002/30075
20130101; A61F 2/30965 20130101; A61B 17/1757 20130101; A61F 2/442
20130101; A61F 2002/305 20130101; A61F 2220/005 20130101; A61F
2230/0021 20130101; A61B 17/1604 20130101; A61F 2002/30604
20130101; A61F 2310/00023 20130101; A61F 2002/30154 20130101; A61F
2002/30883 20130101; A61F 2002/30975 20130101; A61B 17/1659
20130101; A61B 2017/924 20130101; A61F 2/28 20130101; A61F
2230/0082 20130101; A61B 2090/0801 20160201; A61F 2002/30004
20130101; A61F 2002/30177 20130101; A61F 2002/30329 20130101; A61F
2220/0025 20130101; A61B 2090/033 20160201; A61F 2/4603 20130101;
A61F 2002/4648 20130101; A61F 2002/30448 20130101; A61F 2002/30785
20130101; A61F 2002/286 20130101; A61F 2250/0014 20130101; A61B
2017/0256 20130101; A61F 2002/30593 20130101; A61F 2/4455 20130101;
A61F 2002/30131 20130101; A61F 2002/30261 20130101; A61F 2002/30565
20130101; A61F 2230/0013 20130101; A61F 2310/00293 20130101; A61B
2017/00544 20130101; A61F 2002/2835 20130101; A61F 2210/0061
20130101; A61B 17/0642 20130101; A61F 2230/0056 20130101; A61F
2/4611 20130101; A61F 2002/30579 20130101 |
Class at
Publication: |
606/84 |
International
Class: |
A61B 017/00 |
Claims
We claim:
31. A tome for precisely cutting in at least one bone segment at
least one groove having the shape of an intimately mating dovetail,
said tome comprising a shaft having first and second ends, said
first end having attached thereto a blade shaped for cutting said
dovetail groove in said bone and said second end having attached
thereto an extension for engagement with mechanical energy
transmission devices, said blade having a shape comprising a
plurality of substantially planar segments.
32. The tome of claim 31 wherein the blade having a shape
comprising a plurality of substantially planar segments further
comprises short radius curves connecting said plurality of
substantially planar segments into a single piece.
34. The tome of claim 33 wherein the blade configured to cut a
first dovetail groove in a first vertebra and a second dovetail
groove in a second vertebra simultaneously is replaced by an
assembly of blades configured to cut a first dovetail groove in a
first vertebra and a second dovetail groove in a second vertebra
simultaneously.
Description
[0001] This application is a continuation of application No. Ser.
09/408,762, filed Sept. 30, 1999 and incorporated herein by
reference, which application is a divisional of application Ser.
No. 09/248,151, filed Feb. 10, 1999, which in turn is a
continuation-in-part of application Ser. No. 09/072,777, filed May
6, 1998. This application claims the benefit of priority of those
applications.
BACKGROUND OF INVENTION
[0002] 1. Field Of Invention
[0003] This invention relates generally to the treatment of
injured, degenerated, or diseased tissue in the human spine, for
example, intervertebral discs and vertebrae themselves. It further
relates to the removal of damaged tissue and to the stabilization
of the remaining spine by fusion to one another of at least two
vertebrae adjacent or nearly adjacent to the space left by the
surgical removal of tissue. More particularly, this invention
relates to the implantation of devices which can be inserted to
take the structural place of removed discs and vertebrae during
healing while simultaneously sharing compressive load to facilitate
bony fusion by bone growth between adjacent vertebrae to replace
permanently the structural contribution of the removed tissue. This
invention further relates to the implantation of devices which do
not interfere with the natural lordosis of the spinal column. This
invention further relates to surgical instruments for preparing the
patient for recieving such implants.
[0004] 2. Background of the Invention
[0005] For many years a treatment, often a treatment of last
resort, for serious back problems has been spinal fusion surgery.
Disc surgery, for example, typically requires removal of a portion
or all of an intervertebral disc. Such removal, of course,
necessitates replacement of the structural contribution of the
removed disc. The most common sites for such surgery, namely those
locations where body weight most concentrates its load, are the
lumbar discs in the L1-2, L2-3, L3-4, L4-5, and L5-S1
intervertebral spaces. In addition, other injuries and conditions,
such as tumor of the spine, may require removal not only of the
disc but of all or part of one or more vertebrae, creating an even
greater need to replace the structural contribution of the removed
tissue. Also, a number of degenerative diseases and other
conditions such as scoliosis require correction of the relative
orientation of vertebrae by surgery and fusion.
[0006] In current day practice, a surgeon will use one or more
procedures currently known in the art to fuse remaining adjacent
spinal vertebrae together in order to replace the structural
contribution of the affected segment of the disc-vertebrae system.
In general for spinal fusions a significant portion of the
intervertebral disk is removed, and if necessary portions of
vertebrae, and a stabilizing element, frequently including bone
graft material, is packed in the intervertebral space. In parallel
with the bone graft material, typically additional external
stabilizing instrumentation and devices are applied, in one method
a series of pedicle screws and conformable metal rods. The purpose
of these devices, among other things, is to prevent shifting and
impingement of the vertebrae on the spinal nerve column. These bone
graft implants and pedicle screws and rods, however, often do not
provide enough stability to restrict relative motion between the
two vertebrae while the bone grows together to fuse the adjacent
vertebrae.
[0007] Results from conventional methods of attempting spinal
fusion have been distinctly mixed. For example, the posterior
surgical approach to the spine has often been used in the past for
conditions such as scoliosis, using Harrington rods and hooks to
align and stabilize the spinal column. In recent years many
surgeons have adopted anterior fusion because of the drawbacks of
the posterior approach, the primary problem being that in the
posterior approach the spine surgeon must navigate past the spinal
column and its nerve structure. However, results of anterior
surgery are variable and uncertain because constraining the
vertebrae from this side does not address the loads put on the
spine by hyperextension, such as from rocking the body in a
backwards direction.
[0008] Pedicle screws and rods, always implanted posteriorly, tend
to loosen either in the bone or at the screw-rod interface if
fusion is not obtained. Fusion rates for posterolateral
instrumented fusions range from 50% to 90%. It must be kept in mind
that plain x-rays are only 65-70% accurate in determining fusion
status and most studies use this inadequate method to determine
fusion status, suggesting that the non-union rate may be greater
than reported. It is also known that posterior pedicle screw
systems do not prevent all motion anteriorly, leading to the risk
of fatigue failure of the metal and screw breakage. This continued
motion may also lead to persistent pain, despite solid posterior
bony fusion, if the disc was the original pain generator. These
well documented failures of pedicle screws have given rise to
extensive litigation in the United States.
[0009] It is well established from the study of bone growth that a
bone which carries load, especially compressive load, tends to grow
and become stronger. Existing stabilizing implants, in particular
IBF's, do not share any of the compressive load with the new bone
growth, in fact possibly shielding new bone growth from load.
[0010] The biggest limitation in any method of fusion at the
present time is the nature of available devices for bridging the
space left by excision of diseased or damaged tissue. In
particular, interbody fusion (IBF) devices currently on the market
in the United States do not provide stability in all planes of
motion. There is very little evidence to support the biomechanical
stability of these devices. They are generally stable in
compression (forward flexion) unless the bone is osteoporotic,
which condition could lead to subsidence of the device into the
adjacent vertebral body with loss of disc space height. They may be
much less stable in torsion and certainly less so in extension
where there is no constraint to motion except by the diseased
annulus fibrosus which is kept intact to provide just such
constraint. It is doubtful that a degenerative annulus could
provide any long term "stiffness" and would most likely exhibit the
creep typically expected in such fibro-collagenous structures.
[0011] Accordingly, there is widespread recognition among spine
surgeons of the need for a flexible radiolucent implant device
which would replace removed degenerated tissue and be firmly
affixed mechanically to opposing vertebrae. Such a device would
dramatically increase the probability of successful fusion because
it a) would eliminate or significantly reduce relative movement of
the adjacent vertebrae and the intervertebral fixation device in
extension and torsion, b) would thereby reduce or eliminate the
need for supplemental external fixation, c) by compressive load
sharing would stimulate rapid growth of the bone elements packed
within the intervertebral device by causing osteoinduction within
the bone chips, thereby accelerating fusion, d) would allow
confirmation that fusion had taken place using standard CT or
possibly plain x-rays, and e) would have the potential to be
bioabsorbable, potentially being fabricated from such materials as
a D-LPLA polylactide or a remodelable type-two collagen so as to
leave in the long term no foreign matter in the intervertebral
space. In addition, a flexible implant device can be fabricated in
whole or in part from human bone autograft or from bone allograft
material which is sterilized and processed, automatically
approximately matching the elastic properties of the patient's
bone. The success rate of fusion using such an approach is
anticipated to exceed the success rate of the IBF devices or the
external fusion devices alone and at least equal the combined
success rate of the current combination IBF and posterior
instrumented technique.
[0012] Lordosis, which is a pronounced forward curvature of the
lumbar spine, is a factor which needs to be taken into account in
designing lumbar implants. It is known in the art that preserving
the natural curvature of the lumbar spine requires designing into a
new device such as the current invention a modest taper
approximately equivalent to the effective angularity of the removed
tissue. The restoration of normal anatomy is a basic principle of
all orthopedic reconstructive surgery.
[0013] Therefore there is a perceived need for a device which
simultaneously and reliably attaches mechanically to the bony
spinal segments on either side of the removed tissue so as to
prevent relative motion in extension (tension) of the spinal
segments during healing, provides spaces in which bone growth
material can be placed to create or enhance fusion, and enables the
new bony growth, and, in a gradually increasing manner if possible,
shares the spinal compressive load with the bone growth material
and the new growth so as to enhance bone growth and calcification.
The needed device will in some instances require a modest taper to
preserve natural lumbar spinal lordosis. It will also be extremely
useful if a new device minimizes interference with or obscuring of
x-ray and CT imaging of the fusing process. Based on such need,
there is also obviously a need for the surgical tools necessary to
prepare the intervertebral space and insert the type of implant
which is covered in the parent application.
[0014] Thus it is an object of the current invention to provide a
stabilizing device for insertion in spaces created between
vertebrae during spinal surgery. It is a further object to create
an implantable device for stabilizing the spine by preventing or
severely limiting relative motion between the involved vertebrae in
tension (extension) and torsion loading during healing. It is a
further object to provide a device which promotes growth of bone
between vertebrae adjacent to the space left by the excised
material by progressive sharing of the compressive load to the bone
graft inserted within the device. It is yet a further object to
provide mechanical stability between adjacent vertebrae while bone
grows through a lumen in the implant and at the same time not
diminish the natural lordosis of the lumbar spine. It is a further
object of the invention to provide a device which avoids or
minimizes interference with various imaging technologies. It is
another object of this invention to be capable of being fabricated
from human bone allograft material. It is yet a further object of
this invention to provide the surgical tools necessary for
implantation of such devices. In particular, it is an object of the
invention of this application to provide the surgical tools
necessary to prepare the intervertebral space and the mating
grooves for an interlocking implant.
SUMMARY OF THE INVENTION
[0015] The invention disclosed here is a cutting tool for preparing
dovetail grooves in adjacent vertebrae to receive the novel implant
of the parent application. The design of the new implant for spinal
surgery includes the possibility of fabricating the device out of
material which is elastic, especially in response to compressive
loads, preferably with a compressive elasticity closely matched to
that of human bone, preferably the patient's bone. In particular,
the design includes the capability to fabricate the device from
human bone allograft material. The design is also such that the
implant mechanically fastens or locks to adjacent vertebrae and
stabilizes the involved vertebrae in tension and in torsion while
transmitting a portion of the vertical compressive load to new bone
growth associated with the device. This feature of the invention
will cause osteoinduction within the bone chips loaded into the
implant and will share a sufficient portion of the load with
existing bone and with the new bone growth to promote further bone
growth and not interfere with bone fusion growth. This invention
can be tapered to preserve natural lordosis. This invention also
minimizes interference with x-ray imaging by virtue of being
fabricated in whole or in part from radiolucent materials.
[0016] The implant joins two vertebrae by means of a mechanical
fixation device which is hollow to allow bone growth matter to be
added to one or more spaces communicating with the top and bottom
surfaces for the purpose of promoting fusion. The attachment
portion of the mechanical fixation device is, in a first
embodiment, a tongue and groove mechanical fastening arrangement.
Other mechanical fasteners commonly used in the woodworking art,
such as tack and staple devices, can also be used. The mechanical
properties of the device are closely matched to the bone's modulus
of elasticity so as to promote osteoinduction and rapid bone
growth. The devices are generally transparent to existing
radiologic imaging techniques so as to allow follow up confirmation
of fusion of the adjacent vertebrae. The implant can also be
fabricated from bioabsorbable materials so as to leave no long term
foreign matter in the body. Human bone allograft material can also
be used as the material from which the implant device is
fabricated.
[0017] In its most general form, the implant mechanically attaches
to the ends of and promotes bony fusion between at least two
vertebrae adjacent to a space left by surgically removed spinal
tissue, comprising a load-sharing body comprising a structure
having a combination of structural elements fabricated from at
least one material having a greater than zero elastic compliance,
the combination comprising at least a top surface and a bottom
surface; said combination of structural elements establishing for
the structure as a whole a composite greater than zero elastic
compliance at least in compression in directions generally axial to
said top surface and said bottom surface; said combination of
structural elements further comprising at least one cavity
communicating with both said top surface and said bottom surface in
a configuration suitable as a receptacle for bone implant and
growth material; and on each of said top surface and said bottom
surface at least one fastener capable of mechanically anchoring the
body to said adjacent vertebrae and thereby transmitting tensile
and torsional loads to and from said adjacent vertebrae.
[0018] In another embodiment, the implant mechanically attaches to
the ends of and promotes bony fusion between at least two vertebrae
adjacent to a space left by surgically removed spinal tissue,
comprising a structure formed from a single piece of bone allograft
material and having a top and a bottom, said structure having an
internal cavity communicating with said top and said bottom for
receiving autograft bone implant material and bone growth factors,
said unitary structure having at least one dovetail tongue
protrusion on each of said top and said bottom for mechanically
interlocking with said adjacent vertebrae by forming a mechanical
tongue-and-groove joint.
[0019] In yet a third embodiment, the implant mechanically attaches
to the ends of and promotes bony fusion between at least two
vertebrae adjacent to a space left by surgically removed spinal
tissue, comprising a composite structure fabricated with at least
two separate portions of bone allograft material with different
structural properties and having a top and a bottom, said structure
having an internal cavity communicating with said top and said
bottom for receiving autograft bone implant material and bone
growth factors, said unitary structure having at least one dovetail
tongue on each of said top and said bottom for mechanically
interlocking with said adjacent vertebrae.
[0020] Perhaps the most important aspect of the implant procedure
is the preparation of the space to receive the implant and the
grooves for the dovetail fasteners. A cutting jig is used which
distracts the vertebrae and stabilizes them during preparation and
acts as a guide for precise cutting. The invention of this
application comprises a special tome specifically designed to
precisely cut a dovetail shaped groove in the adjacent vertebrae
and to prepare the end plate surfaces. The tome or combination of
tomes has an offset which provides for the implant to be sized to
slide through the jig but fit very tightly in the space cut into
the vertebrae so as to prevent backout of the implant. Once the
cutting jig is in place an x-ray is taken to show that the end of
the distraction tangs are clearing the spinal canal. The tomes have
depth stops which prevent cutting beyond the distraction tangs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a frontal view of an implant placed between
lumbar vertebrae.
[0022] FIG. 1B is a side view of the same implant.
[0023] FIG. 2 is a plan view of the same implant.
[0024] FIG. 3A is a plan view of an implant showing cavities
communicating with top and bottom surfaces into which bone growth
material is placed.
[0025] FIG. 3B is a frontal view of the same implant showing
cavities.
[0026] FIG. 3C is a side view of the same implant showing
cavities.
[0027] FIG. 4A shows a composite implant with inset titanium
endplates in plan view.
[0028] FIG. 4B is a frontal view of a composite implant with inset
titanium endplates.
[0029] FIG. 4C is a side view of a composite implant with inset
titanium endplates.
[0030] FIG. 5 is an isometric representation of the second
embodiment using a horseshoe shaped tongue and groove dovetail
fastener and showing the retaining barb.
[0031] FIG. 6 shows the implant of FIG. 5 inserted between adjacent
vertebrae.
[0032] FIG. 7 is an isometric view of a modular implant.
[0033] FIG. 8 is an isometric view of the same modular implant with
partial depiction of adjacent vertebrae.
[0034] FIG. 9 shows an implant with a retaining barb.
[0035] FIGS. 10A and 10B depict the handle of the emplacement
instruments for preparation of the implant site.
[0036] FIGS. 11A and 11B show further details of a cutting tool or
instrument for preparation of the implant site.
[0037] FIGS. 12A and 12B show the operation of the interlock
mechanism for the cutting instrument for preparation of the implant
site.
[0038] FIGS. 13A, 13B, and 13C show the cutting instrument for
preparation of the implant site with dovetail tome deployed.
[0039] FIGS. 14A and 14B display details of the dovetail tome.
[0040] FIG. 15 is an isometric view of the driver.
[0041] FIGS. 16A and 16B show detail of the placement
implement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] In the currently preferred embodiments of the implants,
torsional and tensional stability of the spine are provided by
fasteners comprising dovetail joints which engage grooves cut
during surgery in the vertebrae adjacent to the removed tissue such
that the implant and which has large surface contact areas. The
dovetails transfer extension and torsional loads between the two
vertebrae and the flat contact surface transmits the compressive
loads. The device further comprises one or more holes through
and/or cavities inside the implant such that the spaces created can
be filled with bone graft material which will grow into and attach
to the healthy vertebral bone. Optionally in all embodiments tapers
to accommodate natural lumbar lordosis can be incorporated as
necessary.
[0043] In this discussion, we use for convenience a definition of
"elastic compliance" as the elastic displacement per unit of
applied force, in other words the reciprocal of stiffness. The
composite elastic compliance of the device is selected at a value
which promotes sharing of compressive load with bone graft and
growth material and new bony growth. As discussed at greater length
below, in one embodiment, human bone allograft material is used to
fabricate the implant. The new fusion bone will gradually share an
increasing portion of the compressive loads experienced by the
spine because the implant is made of a material, such as a polymer,
which has a compressive modulus which works in conjunction with the
implant design to closely match the modulus of elasticity of bone
during deformation under load. The polymer, or in one embodiment
human bone allograft material, has the added advantage of being
transparent in x-ray imaging permitting, easy visualization of the
fusion process at the vertebral interface. In a variant of one
embodiment, metal retaining clips may be located in the implant
surface, both above and below the dovetails, to engage the cortical
bone and prevent the implant from migrating out of the
intervertebral space. The retainers will generally be metal in
order to benchmark x-ray imaging for locking engagement assessment.
In yet another variation, locking barbs will be included on the
implant top and bottom surfaces to assist in securing the implant
to adjacent bony surfaces to minimize pullout.
[0044] In a second embodiment of the implant, a plurality of
dovetail protrusions, or a compound dovetail protrusion in the
approximate layout of a horseshoe may be located on the outboard
portions of the implant, thereby utilizing the strength and
rigidity of the vertebrae to support the spinal column load. In
this case the device would contain a hollow central core which
would be filled with bone chip and biological medium to accelerate
the fusion in the intervertebral space.
[0045] In the first preferred embodiment, as shown in FIGS. 1A and
1B (elevation views), vertebrae L4 and L5 (or vertebrae L5 and S1)
are mechanically attached by the implant 3. The device 3 is held
mechanically to the adjacent vertebrae 1 and 2 by tongue and
groove, or dovetail, arrangements 4. As shown in FIG. 2 (plan
view), the implant 3 is sited so as to provide mechanical support
to the spine both in compression and in tension, but not so as to
intrude into the space 6 occupied by the spinal nerve bundle. In
this preferred embodiment, as shown in FIG.2, the implant 3 will
include penetrations or holes 7 the purpose of which is to contain
bone growth material to facilitate bony fusion of the adjacent
vertebrae. The implant itself may comprise a variety of presently
acceptable biocompatible materials such as Polyphenolsulfone,
Polyetheretherketone (PEEK), Polysulfone, Acetal (Delrin), UHMW
Polyethylene, and composites of these materials involving high
strength carbon fibers or REM glass filaments to add tensile and
shear strength. As discussed more extensively below, the implant
may also be fabricated from human bone allograft material,
autograft material, or bone substitute material, such as coral or
calcium phosphate. The body of the implant may optionally have a
modest taper to accommodate the natural lordosis of the lumbar
spine.
[0046] One possible problem with an implant with dovetail fasteners
fabricated from a material such as polysulfone is that torque on
one adjacent vertebra relative to the other may place large tension
stresses on the angular portions of the dovetail, thereby causing
breaking and crazing of the polysulfone. Thus a variation on this
embodiment comprises a composite implant fabricated from plastic
material such as polysulfone for the body and titanium for
endplates bearing the dovetail protrusions.
[0047] FIGS. 3A, 3B, and 3C show one possible arrangement of such a
composite structure, with a titanium endplate 8 set into the
plastic (and radiolucent) body 9. FIGS. 4A through 4C show a
variation on this arrangement with the endplate extending to the
shoulders of the plastic body of the implant 11. Both FIGS. 3 and 4
show a variation of this structure, with the titanium endplate 12
set into the plastic body of the implant 9 and 11 in a
configuration designed to provide through spaces or cavities 14 in
which to place bone growth material. In these latter
configurations, the polysulfone body is insert molded into the
titanium endplates. The titanium dovetail fasteners possess the
tensile strength necessary to avoid fracture or crazing, but the
body is still "see through" with respect to X-ray and other methods
of visualizing healing progress. In addition, holes in the titanium
endplates which are aligned with the bone growth material cavities
provides "see through" capability in the vertical direction for
assessing new bone growth.
[0048] A second major preferred embodiment, shown in isometric view
in FIG. 5, is inserted between two vertebrae, e.g., L4 and L5 or L5
and S1 and mechanically attached by two or more dovetail joints, or
by a compound horseshoe shaped dovetail, located on each of the top
and bottom surfaces of the implant to the adjacent remaining
vertebrae by a composite tongue and groove mechanism similar to but
larger than that used to secure the implant of the previous
embodiment. In this configuration, the implant comprises either a
horseshoe shaped dovetail tongue 33 which in effect creates two
dovetail joints per surface toward the outboard ends of the implant
top and bottom surfaces or simply two outboard dovetail tongues
without the horseshoe top closure. The horseshoe top closure may be
substantially curved or it may be substantially straight, with
relatively square corners where the dovetail tongue angles back
into the body of the vertebra. In a variation on this embodiment,
inside the horseshoe shaped dovetail tongue protrusion 33 the body
of the implant is hollow, that is, it contains an opening or cavity
34 communicating with both the top surface and the bottom surface
into which bone growth material is placed.
[0049] In this preferred embodiment, as further shown in the
isometric view of FIG. 6, the implant 35 with a relatively squared
off horseshoe top closure will have a surface approximately flush
with the exterior surface of the adjacent vertebrae and will appear
to create one very wide dovetail 37. This embodiment of the implant
will also include penetrations or holes in addition to or as an
alternative to that shown in FIG. 5, 34, the purpose of which is
also to contain bone growth material to facilitate bony fusion of
the adjacent vertebrae. As in the prior configuration, the implant
35 is sited so as to provide mechanical support both in compression
and in tension to the spinal column, but not so as to intrude into
the space 6 occupied by the spinal nerve bundle. The implant in
some cases is further inserted inside remaining segments of
intervertebral disc tissue 38. As shown in both FIGS. 5 and 6, an
optional feature of these embodiments is for the faces of the
implant to have locking barbs 36 to retain the implant in place
between the remaining vertebrae once it is inserted.
[0050] This implant, as in the prior embodiment, may itself
comprise a variety of presently acceptable implant materials such
as PEEK (Polyetheretherketone), acetal (DELRIN), polysulfone, Ultra
High Molecular Weight Polyethylene (UHMW Poly), and composites
involving high strength carbon fibers or glass filaments to add
tensile and shear strength. Again, as discussed at greater length
below, human bone allograft material may be used to fabricate this
device. This embodiment may also be fabricated with a modest taper
to accommodate natural lordosis.
[0051] A third preferred embodiment of the lumbar implant, shown in
isometric view in FIG. 7, comprises three elements, two modular
dovetail halves, 41 and 42, which are inserted between vertebrae L4
and L5 or L5 and S1 and mechanically attached by two dovetail
protrusions (similar to those fabricated for the second embodiment)
located on the top and bottom of the implant to the adjacent
vertebrae by a tongue and groove mechanism similar to but larger
than that used to secure previous embodiments of the implant. The
two modular dovetail halves are held together by a retainer 43. As
in the prior configuration, as shown in the isometric view of
FIG.8, the implant 35 is sited so as to provide mechanical support
both in compression and in tension to the spinal column, but not so
as to intrude into the space 8 occupied by the spinal nerve
bundle.
[0052] In this preferred embodiment, as shown in FIG.8, the implant
35 will include a cavity 39 the purpose of which is to contain bone
growth material to facilitate bony fusion of the adjacent
vertebrae. The open space 39 is packed with bone growth material
and then capped with a retainer, 43, designed to snap in place to
add stability to the implant and to retain the bone growth factor
to prevent it from migrating. This implant, as in the prior
embodiment, may itself comprise a variety of presently acceptable
implant materials such as PEEK (Polyesther Esther Ketone), Acetyl
(delrin), polysulphone, Ultra High Molecular Weight polyethylene
(UHMW Poly), and composites involving high strength carbon fibers
or glass filaments to add tensile and shear strength. Again the
modular dovetail halves may be tapered to accommodate lordosis.
[0053] Any of the foregoing embodiments can additionally have a
feature shown in FIGS. 5, 6, and 9, namely a retractable barb 36.
This barb comprises a spring wire which when deployed engages the
adjacent vertebrae to prevent the implant from dislodging. A
retraction tool may be inserted into the hole 39 to cause the
sigma-shaped barb to retract its probe-like end so that the implant
disengages from the adjacent vertebra.
[0054] As previously noted, any of the foregoing embodiments of the
Cor-Lok.TM. interlocking implant can be fabricated from cadaver
bone which is processed to form bone allograft material. Tissue
grafting of living tissue from the same patient, including bone
grafting, is well known. Tissue such as bone is removed from one
part of a body (the donor site) and inserted into tissue in another
(the host site) part of the same (or another) body. With respect to
living bone tissue, it has been desirable in the past to be able to
remove a piece of living tissue graft material which is the exact
size and shape needed for the host site where it will be implanted,
but it has proved very difficult to achieve this goal.
[0055] On the other hand, processing of bone material which does
not contain living tissue is becoming more and more important.
Non-living bone grafting techniques have been attempted both for
autografts and for allografts. For example, Nashef U.S. Pat. No.
4,678,470 discloses a method of creating bone graft material by
machining a block of bone to a particular shape or by pulverizing
and milling it. The graft material is then tanned with
glutaraldehyde to sterilize it. This process can produce bone plugs
of a desired shape.
[0056] In the Nashef process, the process of pulverizing or milling
the bone material destroys the structure of the bone tissue. The
step of tanning it with glutaraldehyde then renders the graft
material completely sterile.
[0057] In the prior art, inventors have believed that it is
desirable to maintain graft tissue in a living state during the
grafting process. There is no doubt that the use of living tissue
in a graft will promote bone healing, but much surgical experience
has shown that healing can be achieved with allografts of
non-living bone material which has been processed. In fact, spine
surgeons express a distinct preference for such materials, and at
least one supplier, the Musculoskeletal Transplant Foundation
(MTF), has introduced femoral ring allografts for spine
surgeries.
[0058] It is now possible to obtain allograft bone which has been
processed to remove all living material which could present a
tissue rejection problem or an infection problem. Such processed
material retains much of the structural quality of the original
living bone, rendering it osteoinductive. Moreover, it can be
shaped according to known and new methods to attain enhanced
structural behavior.
[0059] Research shows that such allografts are very favorable for
spinal surgery. According to Brantigan, J. W., Cunningham, B. W.,
Warden, K., McAfee, P. C., and Steffee, A. D., "Compression
Strength Of Donor Bone For Posterior Lumbar Interbody Fusion,"
Spine, Vol. 18, No. 9, pp. 12113-21 (July 1993):
[0060] Many authors have viewed donor bone as the equivalent of
autologous bone. Nasca et al . . . compared spinal fusions in 62
patients with autologous bone and 90 patients with cryopreserved
bone and found successful arthrodesis in 87% of autologous and
86.6% of allograft patients.
[0061] (Citations omitted.) Moreover, as previously noted, sources
of safely processed allograft material have recently become
available.
[0062] In the present invention, allograft bone is reshaped into
one of the Cor-Lok.TM. configurations for use as a spine implant.
Various methods, including that of Bonutti, U.S. Pat. Nos.
5,662,710 and 5,545,222, can be used to shape the allograft
material into the desired shape.
[0063] In the first sub-embodiment of this aspect of the current
invention, bone material which yields to compressive loads at the
exterior surfaces without significant degradation of the interior
structural properties, such as cancellous or trabecular bone, is
shaped. It is not unusual that reshaping of graft tissue is
necessary to obtain the best possible graft. In particular, bone
tissue may be stronger and better able to bear force when it is
denser and more compact.
[0064] Compression of allograft bone is desirable from general
considerations. Generally, bone samples are stronger when they are
more dense. Compressing allograft bone increases its density and
thus generally strengthens the allograft. The allograft bone also
stays together better. In addition, recent studies have indicated
that the shell of vertebral bone is very much like condensed
trabecular bone. Mosekilde, L., "Vertebral structure and strength
in vivo and in vitro," Calc. Tissue Int. 1993;53(Suppl): 121-6;
Silva, M. J., Wang, C., Keaveny, T. M., and Hayes, W. C., "Direct
and computed tomography thickness measurements of the human lumbar
vertebral shell and endplate," Bone 1994;15:409-14; Vesterby, A.,
Mosekilde, L., Gunderson, H. J. G., et al., "Biologically
meaningful determinants of the in vitro strength of lumbar
vertebrae," Bone 1991;12:219-24. Compressing bone allograft
material prior to implantation thus generally produces a stronger
graft.
[0065] Compression also allows conversion of larger irregular
shapes into the desirable smaller shape, thereby permitting more
disparate sources of allograft bone to be used. By compressing bone
to a given shape it is possible to configure the allograft to match
a preformed donee site prepared by using a shaped cutter to cut a
precisely matching cut space. In particular, this method of
formation facilitates the formation of dovetail tongue protrusions
on the upper and lower surfaces of the implant for the formation of
a tongue-and-groove mechanical joint with adjacent vertebrae.
[0066] In the current invention, a blank is cut from cancellous or
trabecular allograft bone and placed in a forming apparatus. The
forming apparatus compresses the sample into the desired shape. In
particular, this process forms the dovetail tongue protrusions on
the implant upper and lower surfaces for the tongue-and-groove
joint. The cancellous or trabecular material yields at the external
surface under the pressure to form a compacted layer around the
outside of the allograft form. This compacted layer is not
destroyed material but rather forms substantially a structure with
properties of the vertebral shell or of a monococque design,
including additional structural properties such as enhanced tensile
strength. This enhanced tensile strength enables the allograft
material to perform the same function in resisting torsion and
extension of the spine as does the synthetic materials previously
discussed. Such processes in general are able to maintain the
homologous property of the allograft material.
[0067] In the second embodiment of this aspect of the implant,
different types of allograft bone are formed into a composite
structure to provide the necessary structural properties. Both
cortical or shell and dense cancellous or trabecular bone may be
compacted into a unified structure. Fibrin "glue" is highly
suitable for use as an adhesive in such structures. Fibrin is a
blood component important in blood clotting. It can be separated or
centrifuged from blood and has the nature of an adhesive gel.
Fibrin can be used as an adhesive, either in a natural state or
after being compressed, to hold together material such as separate
tissue pieces pressed together in a tissue press. In particular,
cortical bone from the same source can be used as a shell to
provide needed additional structural properties, such as tensile
strength to a composite shape. Cortical bone can also be provided
in a shell, much like the known femoral ring implants, to provide
the needed structural properties. Moreover, a shell is not the only
structural element which can be added in this way. Buttresses,
gussets, cross-braces, and other structural elements can be
included in the same way. Using such materials, the homologous
property of the bone allograft material may be maintained.
[0068] In another sub-embodiment of the implant, a relatively thin
external shell of a synthetic material can be provided for
enclosing compressed allograft material and providing any needed
additional structural properties. After the graft is compressed,
the shell is placed around the graft. The shell may be made of a
material which expands after it is placed in the spine, thereby
supplementing the interlocking properties of the Cor-Lok.TM.
mechanical design by improving the fill between the allograft and
the donee site. There are a number of suitable materials which
expand when they come in contact with water or other fluids. One is
PEEK (polyether-etherketone)(water absorption about 1.5%). A
desiccated biodegradable material or suitable desiccated allograft
material may also be used.
[0069] The expansion can take place in one of two ways. First, the
retainer can itself be compressed, as with the tissue, then expand
when placed in the body. Second, the retainer can be made of a
material which expands when it comes in contact with water or other
bodily fluids.
[0070] It should be noted that the entire allograft implant can
itself be compressed so that it expands when contacted by water.
The expandable shell material can first be compressed with the
allograft material, which then expands when placed in the body.
[0071] It should further be understood that the graft can be
multiple tissue fragments rather than a composite material. The
compressing process can be used to compress multiple bone fragments
into one larger piece. It should also be understood that the
compression process can be used to add additional materials to an
allograft composite. For example, to bone tissue there can be added
tri-calcium phosphate, an antibiotic, hydroxyapatite, autografts,
or polymeric materials.
[0072] FIGS. 10A through 16B depict the surgical tools used to
install the implant. This apparatus comprises a set of unique tools
which will accurately cut a dovetail joint in bone for the purpose
of inserting an implant which locks adjacent vertebrae
together.
[0073] The guide 44, shown in FIGS. 10A and 10B, is a tubular tool
with tangs 45 extending from one end. The tangs, tapered 46 to
conform to natural lordosis, are inserted between the vertebrae 47
and distract them to a preferred dimension 48, as shown in FIG.
10B. The driver 68, shown in FIG. 15, can be used with a rod
extension guide adapter 70, also shown in FIG. 15, to drive the
guide 44 into place. This step establishes a fixed reference
relative to the two vertebrae 47 and secures the vertebrae from
moving. The length 49 of the tangs 45 is consistent with the other
tools in the set and establishes the extent 49 to which any tool
can penetrate. A lateral x-ray is used to assure that the extent of
penetration 49 is safely away from the spinal canal 50. All of the
other tools have positive stops which contact the guide depth stop
51 to control the depth of cut.
[0074] The end cut tool 52, shown in FIGS. 11A and 11B, is inserted
into the guide 44 to make an end-cut 25, shown in FIG. 11B, for the
dovetail. Once completely inserted to the depth stop 53, a single
piece interlock 54, shown in FIGS. 12A and 12B, which prevented
rotation of the blade 55 during insertion, is disengaged from the
shaft 56 and then prevents withdrawal of the end cut tool 52 from
the guide 44. As shown in FIGS. 12A and 12B, the interlock 54 is
held by spring 57 such that it engages the slot 58 in the shaft 56,
preventing rotation as shown in FIG. 12A. As the end cut tool 52 is
inserted into the guide 44 it pushes the interlock 54, rotating it
out of the slot 58 in the shaft 56 as shown in FIG. 12B. As the
interlock rotates, it engages the guide 44 as shown in FIG. 12B.
When the shaft 56 is rotated as shown in FIG. 12B the interlock 54
cannot return to its original position as shown in FIG. 12A, thus
securing the end cut tool 52 in the guide 44. The rotation
interlock protects the surgeon from the end cut blade 55 and the
withdrawal interlock holds the end cut tool 52 in the guide 44
while the blade 55 is exposed. The surgeon rotates the handle 59
one turn, causing the end cut blade 55 to make end-cuts 25 as shown
in FIG. 11B, in both vertebrae 47 simultaneously, and returns it to
the "zero" position at which the end cut tool 52 can be removed
from the guide 44.
[0075] The dovetail tome 60, shown in FIG. 13A, is inserted into
the guide 44 to the point where the blade 61 rests against the
vertebrae 47. As shown in FIG. 15, the driver 68 is placed on the
dovetail tome rod extension 62 and drives the dovetail tome 60,
cutting the vertebrae 47, until the depth stop 63 of the dovetail
tome contacts the stop 51 on the guide 44, stopping the blade 61 at
the end-cut 25, as shown in FIG. 13C. The dovetail tome blade 61,
as shown in FIG. 14A, has endplate breakers 64 which split the
endplates 65 of the vertebrae (see FIG. 13C) in two 66 as shown in
FIG. 14B, preventing them from jamming in the blade and preparing
them for later use. The dovetail tome 60 is removed and the bone 67
and the split vertebral end plate 66 contained in the blade 61 is
harvested for later use in the implant 33.
[0076] As shown in FIG. 15, the driver 68 is a pneumatic tool like
a miniature jackhammer. The driver 68 is powered by compressed gas
supplied through the input tube 69. The driver 68 receives the rod
extension from the guide adapter 70 or the rod extension of
dovetail tome 62 into a guide port 71. A piston 72, within the
driver 68, repeatedly impacts the guide adapter 70 or the dovetail
tome rod extension 62, driving the tool into place. The driver 68
is activated by the finger-actuated valve 73. Control of the force
and rate of the impacts is attained by modulating the valve 73. The
driver will deliver several thousand small impacts in place of a
few massive blows from a hammer.
[0077] The implant 33 of FIG. 5 is prepared for insertion by
filling the interior portion 34 with harvested bone 67 and the
split end plates 66 from the dovetail tome cuts and additional bone
and graft stock. The implant 33 is then slid down the guide 44
(FIG. 10) and driven into place by the insertion tool 74, shown in
FIGS. 16A and 16B. The insertion tool 74 has a positive stop 75
which contacts the depth stop 51 of the guide 44 and assures
correct placement of the implant 33, locking the vertebrae 47.
[0078] The above implant devices contain attachment means which are
well known in the woodworking industry, but are not used in
Orthopedic Spine Surgery. However, one skilled in the art of
intervertebral implants would readily be able to adapt other
fastening devices known in the woodworking art to spinal implant
devices. It should be readily apparent to anyone skilled in the art
that there are several available means to attach bone surfaces to
the adjacent implant surfaces, such as causing bone anchors to
protrude from the implant surface and impinge and attach the
adjacent vertebrae to the implant. Metal staple-like clips can be
driven between adjacent vertebrae to attach the edges of the
vertebrae. Tack and staple configurations can substitute for the
dovetail tongue and groove fasteners. Bone anchors can also be used
to attach natural tissue to adjacent vertebrae, creating an
artificial ligament which could scar down, thus retaining an
artificial implant within the disc space while osteoinduction takes
place and the vertebrae fuse.
* * * * *