Note: Descriptions are shown in the official language in which they were submitted.
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VERTEBRAL IMPLANT FOR PROMOTING ARTHRODESIS OF THE SPINE
TECHNICAL FIELD
This invention relates to an intervertebral spacer for treatment of spinal
deformities. More preferably, this invention is directed to a metallic or
synthetic,
intervertebral spacer for implantation into a prepared disc space to
facilitate spinal
p
fusion, maintain desired disc space height, and/or spinal orientation.
BACKGROUND
For degenerated, diseased or otherwise damaged spinal columns and
vertebrae, it is known to treat these defects by removal of all or a portion
of the
vertebral disk and inserting an implant such as a spinal spacer to restore
normal
disk height and spine orientation, and repair the spinal defects. When
desired,
osteogenic material also can be implanted into the intervertebral space to
enhance
arthrodesis, or spinal fusion between the two vertebrae adjacent to the
intervertebral space. Selected spacers are formed to provide a cavity for
receipt of
the osteogenic material.
The spinal column can exert tremendous force on the individual vertebrae,
and consequently also on any implant implanted in between the vertebrae. Often
for defective or diseased vertebrae the bone tissue in the center of the
endplate,
where the vertebral body is normally only covered by a thin cortical bone
layer, is
weakened. The strength and integrity of the endplate may be compromised.
Spacers inserted in between these weakened bone tissue can subside or sink
into
the vertebral body. This results in a failure to maintain the desired disc
space
height and causes tremendous pain to the patient.
Additionally, arthrodesis or fusion of the vertebrae adjacent is
recommended to treat a damaged disc or diseased vertebra. Spinal implants
typically are formed of a metal such as titanium or surgical steel. While the.
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selection of the implant configuration and composition can depend upon a
variety
of considerations, for arthrodesis it is often desirable to select a material
that does
not stress shield the bone ingrowth. Titanium and surgical steel provide the
requisite strength to maintain correct disk space height and orientation;
however,
these materials have been shown to stress shield the bone. Bone and bone
derived
material can provide an acceptable material having the similar strength and
compressibility as living bone tissue. However, suitable donor bone is scarce.
Further extensive screening and sterilization must be strictly observed to
minimize
any risk either real or perceived for the transmission of infections from the
donor
to the recipient.
The following are representative of the current state of the art for the
relevant technology.
A vertebral plant is described in WO-95/08306 issued to Beckers (U.S.
5,888,224). Intervertebral implant comprises an elongated body formed of a
titanium or titanium alloy material and having a shape that is basically lens-
shaped
with a width less than its height and provided with or without an internal
cavity.
Implantation of the implant requires distraction of the adjacent vertebral
bodies,
insertion of the implant, which is then rotated about its longitudinal axis.
Another vertebral implant is described in U.S. 4,834,757 issued to
Brantigan. This vertebral implant has a parallelepiped shape and comprises an
outer surface completely covered with nubs or barbs that are embedded into the
channel cut into the endplates.
An intervertebral implant is described in WO 96/27348 (U.S. 6,059,829)
issued to Schlapfer et al.; the implant consists essentially of a frame about
an
internal cavity and includes longitudinal sidewalls having perforations
therethrough. The frame is open without restriction on the top and bottom. The
upper and lower surfaces are convex and join the longitudinal sidewalls and
the
two endwalls of the frame at sharp edges.
Another implant that is described in FR 7/10664 issued to Liu et al. This
metallic implant has upper and lower surfaces that include paired projections
extending vertically from these surfaces for cutting into and piercing the
bone
tissue in opposing intervertebral bodies. The upper and lower surfaces also
include
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pairs of opposed bearing surfaces to contact the cortical bone portion of
vertebral
bodies.
In light of the above described problems for treating spinal defects, there is
a continuing need for advancements in the relative field, including treatment
of
damaged or diseased spinal columns, improved implants, selection of suitable
materials from which the implants can be formed and methods of promoting bone
fusion between the adjacent vertebra. The present invention is such an
advancement and provides a wide variety of benefits and advantages.
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DISCLOSURE OF THE INVENTION
The present invention relates to intervertebral
spacers, the manufacture and use thereof. Various aspects
of the invention are novel, nonobvious, and provide various
advantages. While the actual nature of the invention
covered herein can only be determined with reference to the
claims appended hereto, certain forms and features, which
are characteristic of the preferred embodiment disclosed
herein, are described briefly as follows.
According to one broad aspect of the present
invention, there is provided a vertebral implant for
installation in a disc space comprising a box-shaped,
elongated basis body defining a longitudinal axis, which is
provided with a cavity extending transverse to the
longitudinal axis and through upper and lower bearing
surfaces, which cavity is bordered by two planar walls
located opposite one another and extending along said
longitudinal axis and between said upper and lower bearing
surfaces, each of said planar walls having substantially
smooth portions extending around said cavity and said planar
walls extend between two frontal walls located opposite one
another on opposite sides of said cavity, whose crosswise-
extending edge surfaces serve as contact surfaces between
the vertebrae and the vertebral implant, wherein the two
frontal walls of the cavity are formed thicker than both of
the longitudinal walls, thereby widening their crosswise-
extending edge surfaces, and wherein the implant is made of
synthetic material and a groove is formed in each of the
crosswise-extending edge surfaces of both of the frontal
walls, which groove extends transverse to the longitudinal
axis of the basis body, wherein the basis body has receiving
means on one frontal end portion for receiving a hand tool
such that, with this, a rotational moment about the
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longitudinal axis of the basis body can be exerted on the
basis body and the receiving means has an outwardly open
hole formed in the middle of the frontal wall of the basis
body and two grooves formed to the side of the hole in the
frontal wall of the basis body, which hole is open toward
the outside.
An embodiment of the invention provides an
intervertebral implant or spacer for implantation into a
prepared disc space. The intervertebral spacer can be made
of a metallic or a synthetic, non-metallic material, such
as, a polymeric material, a ceramic material, or a
reinforced composite. In one form, this intervertebral
spacer comprises an elongate body having an internal cavity.
The cavity can serve as a depot for osteogenic material or
spongioseum bone material to facilitate the spinal fusion of
the vertebral bodies adjacent to the disc space. The spacer
body is bordered by two longitudinal walls or sidewalls and
by two frontal or endwalls located opposite one another.
Upper and lower surfaces extend laterally between the
longitudinal walls. The upper and lower surfaces include
openings into the inner cavity. Crosswise-extending edge
surfaces of the endwalls and longitudinal walls surface
define contact surfaces or bearing surfaces.
In one embodiment the opposite frontal walls or
endwalls are formed to be thicker than the pair of opposite
longitudinal walls, thereby widening the cross-edge
extending surfaces. The cross-edge surfaces are preferably
formed as integral parts of the opposing endwalls.
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ln another form the present invention provides an intervertebral spacer
provided for implantation into a disc space between adjacent vertebra. The
spacer
comprises an elongate body defining a longitudinal axis and at least one
tissue-
receiving groove extending transverse to the longitudinal axis. The spacer
comprises: also includes: a cavity bounded by a first endwall and an opposite
second endwall the first endwall defining a first bearing surface and an
opposite
second bearing surface and the second endwall defining a third bearing surface
and
an opposite fourth bearing surface; an upper surface extending between the
first
endwall and the second endwall, the upper surface having an arcuate portion
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adapted to bear against an inferior endplate of a first vertebra; and an
opposite
lower surface extending between the first endwall and the second endwall, the
lower surface having an arcuate portion adapted to bear against a superior
endplate
of a second vertebrae. In preferred embodiments the first and second endwall
had
5 a first thickness measured along the longitudinal axis that is thicker than
either the
thickness of the longitudinal walls or the thickness of the upper and lower
surfaces.
In another form, the present invention provides a spacer for promoting
fusion between adjacent vertebra. The spacer comprises: an elongate body
defining a longitudinal axis and having an opening extending therethrough
transverse to the longitudinal axis. The body comprises: a first supporting
endwall
terminating the body on a first end; an opposite second supporting endwall
terminating the body on a second end, the first and second supporting endwalls
positioned substantially transverse to the longitudinal axis and adapted to
bear
against cortical bone tissue in opposing endplates of the adjacent vertebrae,
a first
sidewall and an opposite second sidewall interconnecting the first supporting
wall
and the second supporting wall, wherein the body includes at least one tissue
receiving groove extending from the first sidewall to the second sidewall.
The preferred embodiments the intervertebral spacer includes endwalls that
are thicker than the longitudinal walls. The endwalls terminate in cross-edge
or
peripheral surfaces extending transverse to the longitudinal walls. The
peripheral
surfaces are wider than the cross-edge surfaces of the longitudinal walls. The
peripheral surfaces provide wide contact or supporting surfaces for bearing
against
the cortical portions of the adjacent vertebral bodies such as found in and
about the
cortical ring or the apophyseal ring structure. Cortical bone tissue is either
harder
and/or denser than the cancelleous bone or spongioseum tissue that is found in
the
interior of the vertebrae. The harder cortical bone tissue provides sufficient
strength to transmit the biomechanical forces exerted on the spinal column to
the
spacer. The wide bearing surfaces or contact surfaces of the spacer can
withstand
the biomechanical forces and inhibit subsidence of the implant into the
vertebral
body. This provides a spacer that can safely and durably support the spinal
column
during normal and/or recommended patient activity.
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The longitudinal walls of the spacer can be narrow in a cross-sectional
dimension, measured transverse to the longitudinal axis. The spacer can be
fabricated to minimize the thickness of the longitudinal walls, yet still
provide
requisite compressive strength to maintain desired disc space height and
orientation. This provides a spacer having an enlarged cavity compared to
implant
having thicker longitudinal walls. The larger cavity is capable of receiving a
greater amount of osteogenic material. This, in turn, provides increased
success
rate for spinal fusion and ultimately provides a stronger more stable bone
bridge
between the adjacent vertebra.
Since the wide, edge contact or bearing surface of the spacer support the
majority of the biochemical forces exerted by the spinal column, it is
possible to
remove a portion of the cortical tissue from the endplates of the vertebra to
reveal
the cancelleous bone tissue or spongioseum tissue. The implant can be placed
within the disc space and provide intimate contact of the osteogenic material
within the cavity and the exposed bone tissue of the opposing vertebrae. This
provides the advantages of promoting arthrodesis of the vertebrae.
Further, an integral implant design can be provided with minimal or no
projections or protrusions extending from its exterior surfaces. In one
embodiment, the outer surfaces of the endwalls are provided with a rounded
shape
or rounded-over edges. Additionally, the edges where the longitudinal walls
meet
the upper and lower surfaces can be chamfered, beveled or rounded-over. These
edges provide advantages in that the resulting spacer can be simply installed
into
the prepared disc space without danger of any projections, quarters, edges or
the
like tearing or gouging the surrounding tissue and therefore minimizes
unintentional injuries of the adjacent tissue.
The spacer has an interior cavity to serve as a depot for osteogenic material.
It is desirable to provide a large internal cavity to obtain a large bone
bridge or new
bone growth between the adjacent vertebra. In one form the cavity can be
provided to have a shape that generally corresponds to that of the external
surface
of the spacer. Thus in one form, the basic interior shape of the cavity is
also a box-
shaped and its dimensions are similar to that of the outer dimensions of the
body.
Alternatively, the cavity has a height that varies along the longitudinal
axis. For
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wedge shaped spacers, the internal dimensions of the cavity can also provide a
wedge shaped hollow interior. Similarly, when the exterior profile of the
spacer is
generally lens-shaped the interior chamber can be lens-shaped as well. The
interior
cavity can be made larger in the present invention by decreasing the thickness
of
either or both of the longitudinal walls and the upper and lower surface
walls. This
can be accomplished by providing the endwalls of substantial thickness to
support
the biomechanical stress exerted by the spinal column. In another form, the
height
of the cavity is substantially consistent along the longitudinal direction to
facilitate
uniform loading of an osteogenic material in the cavity. In still another
form, the
cross-sectional area of the cavity, measured in a plane lying substantially
transverse to the longitudinal walls is equal to or greater than the cross-
sectional
area of either of the openings in the upper and lower surfaces of the spacer.
The spacer can also be provided efficiently, economically and readily mass
produced while maintaining high quality assurance over very specification
tolerances for the outside dimensions, and compressive and elastic moduli. The
number of steps necessary for the production of the spacer are significantly
reduced--particularly the machining processes and milling procedures are
reduced.
The compact design of the implant often makes it possible to provide the
spacer
body in a variety of materials including metallic materials, synthetic
materials,
polymeric materials, ceramic materials, and composite materials including
reinforced materials i.e. glass, fiber, and/or carbon fiber reinforced
materials
(CFRP). These preferred materials for fabricating spacers in the present
invention
reduce costs, increase service life and provide excellent physiological
compatibility. The material can be selected to be either a substantially
permanent
material, a biodegradable material or a bioerodable material. Further, the
spacer
material can be provided to be radio-opaque to facilitate monitoring of bone
ingrowth both into the implant and between the opposing endplates of the
adjacent
vertebrae.
The basis body includes at least one groove. Preferably at least one groove
is formed on each of the cross-wise extending surfaces of the endwalls. The
groove can extend transverse across the entire width direction of the spacer
body.
The groove can be readily formed as an integral feature on the implant body.
The
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groove can provide sufficient resistance to expulsion and/or migration in the
disc
space. Further, the groove does not obstruct impaction of the spacer into the
disc
space. The groove including its upper edges lies either coplanar with, or
below the
exterior surface(s) of the spacer and extends inwardly. After implantation of
the
spacer into the prepared a disc space, the spacer can be maintained in the
desired
position by the pressure exerted on it by the spinal column. Tissue is forced
into
the grooves. The bone material fills the grooves of the spacer and helps
secure the
spacer's position in the disc space. The grooves can also engage bony tissue
proximal to the apophyseal ring under the compressive biomechanical force
exerted by the spinal column on the contact surfaces of the implant.
Additionally, the outer surface of the spacer can, but is not required to
include anti-expulsion structures or features. Such features include ridges,
and the
like. These features can be provided either through a milling and/or machining
process or through the molding of the spacer. The anti-expulsion features
inhibit
rejection of the implant from the disc space and/or inhibit undesirable
migration
within the disc space. The outer surface of the implant as a whole can be
roughened, by which the surrounding tissue interengages or mechanically
interlocks the roughen outer surface of the spacer.
The longitudinal walls of the spacer are preferably formed to be
substantially planar and parallel to each other. This provides significant
advantages regarding the installations of the implant into the disc space as
will be
discusses below. The thickness of the longitudinal walls can be selected to be
substantially less than the thickness of the endwalls. For example the
thickness of
the longitudinal walls measure transverse to the longitudinal axis can be one-
half
the thickness of the endwalls measured substantially parallel to the
longitudinal
axis. More preferably the thickness of the longitudinal walls is between about
0.5
and 0.4 times as thick as the thickness of the endwalls. The ratio of the
thickness
of the other longitudinal walls to the endwalls provides a spacer having an
ample
interior cavity to receive the osteogenic material and yet provides a spacer
that is
able to support the mechanical load exerted from the spinal column.
In selected the embodiments, the spacer can be provided to have a generally
convex shape. This can be accomplished by providing the longitudinal walls
with
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upper and lower edges having a convex shape. The upper and lower surfaces of
the longitudinal walls can have a maximum height that is positioned between
the
first and second endwalls. In one form, the convex surface portion can be
provided
to engage in the natural concavity surface portions of the opposing endplates.
This
shape has the advantage that the spacer can be readily fitted to the
respective
vertebral endplates without any previous machining or cutting process on the
endplates.
In other embodiments the longitudinal walls define a lordotic profile. In
this configuration the longitudinal walls are shaped to provide a spacer that
conforms to the desired lordosis or natural curvature of the spine.
Both the upper and lower surfaces define openings into the inner cavity.
These openings ensure that the osteogenic material in the internal cavity
contacts
the bone tissue in the opposing vertebral endplates. Preferably the upper and
lower
surfaces provide an opening that is substantially the same or equivalent to
the
cross-sectional area of the internal cavity. This provides the greatest amount
of
contact between the included osteogenic material and the opposing endplates.
Preferably an opening is formed in each of the two longitudinal walls to
provide access to the interior cavity. This also allows blood and nutrients to
infuse
laterally into the cavity containing an osteogenic material. Further the
lateral
openings allow the osteogenic material to enhance bone growth around the
spacer
between pairs of adjacent spacers in the disc space to facilitate spine fusion
from
all sides, laterally and vertically. In preferred embodiments the longitudinal
walls
each have at least one large opening into the interior cavity. It is
understood that a
plurality of smaller openings each opening providing access to the interior
chamber
can also be formed in the longitudinal walls. Preferably the each of small
openings
have a diameter, which in comparison with the height of the longitudinal
walls, is
small. Preferably a maximum of two large through holes are formed to each of
the
longitudinal wails. The diameter of each of the two large through holes
corresponds to approximately to half of the height of the longitudinal walls.
The spacer body preferably includes on its rear endwall, a tool-engaging
portion, for example provided on an end of a spacer holder. Preferably the
tool-
engaging portion comprises an outwardly opening or a bore formed in the middle
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of the rear endwall of the body. The opening can, but is not required to,
include a
threaded interior to receive a correspondingly threaded pin or stud on the
tool. The
tool-engaging portion can also include a pair of opposing grooves formed
laterally
besides the opening. The opening is positioned in line with the laterally
extended
5 grooves. In preferred forms, the rear wall is substantially free of any
further
projections and or shoulders. The counterparts portions on the tool can also
include a mid-position pin as well as outwardly extending blades to engage the
laterally grooves formed in the endwall of the spacer. The pin can engages
within
the opening in the endwall and can be used to fixedly secure the spacer to the
tool
10 and facilitate alignment of the spacer during insertion. Alternatively, the
tool can
include a pair of opposing arms that can open and close to clamp or grip the
spacer
either along a portion of the opposing longitudinal walls or along the upper
and
lower surfaces. Preferably the tool engaging portion is formed such that the
spacer
can be tightly attached to the tool in a relatively simple manner so that the
tool can
be used both as spacer holding tool as well as impacting spacer into the disc
space.
The vertebral implant according to the present invention preferably is
provided with the following mechanical properties:
- static compressive resistance in height direction, transverse to the
longitudinal axis: greater than or equal to about 15,000N
- fatigue strength corresponding to this compressive resistance: greater than
or equal to about 5,000 N
- torsional resistance (torque in(around) longitudinal direction of the
implant:
greater than or equal to about 4Nm
The above-described spacers can be prepared of a wide variety of materials
including metallic materials, synthetic, organic materials, composites,
ceramic, and
metal. Preferably the implants are formed of a synthetic, non-metallic
material.
The implants of the present invention can be either essentially permanent
implants,
which do not readily biodegrade. These implants can remain in the
intervertebral
space and often are incorporated into the bony tissue. Alternatively, the
implant
can biodegrade and are substantially replaced by bone tissue.
Examples of non-biodegradable polymeric or oligomeric materials include
the, polyacrylates, polyethers, polyketones, polyurethanes, and copolymers,
alloys
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and blends thereof. Use of the term co-polymers is intended to include within
the
scope of the invention polymers formed of two or more unique monomeric
repeating units. Such co-polymers can include random copolymers, graft
copolymers, block copolymers, radial block, diblock, triblock copolymers,
alternating co-polymers, and periodic co-polymers. Specific examples of non-
biodegradable polymeric materials include: poly(vinyl chloride) (PVC);
poly(methyl (meth)acrylate); acrylics; polyamides; polycarbonates; polyesters;
polyethylene terephthalate; polysulfones; polyolefins, i.e. polyethylene,
polypropylene, and UHMWPE (ultra high molecular weight polyethylene);
polyurethane; polyethers, i.e., epoxides; poly(ether ketones) (PEK),
poly(ether
ether ketones) (PEEK), poly(aryl ether ketones) (PAEK), and poly(ether ether
ketone ether ketone) (PEEKEK). A wide variety of suitable poly(ether-co-
ketone)
materials are commercially available.
Alternatively, implants of this invention can be made of a material that
either biodegrades or is bioabsorbed. Typically, biodegradable material is a
polymeric material or oligomeric material and often the monomers are joined
via
an amide linkage such as is observed in poly(amino acids). When the implant is
formed of material that biodegrades, it is desirable to provide a
biodegradable
material that degrades at a rate comparable to the bony ingrowth
characteristic of
bone fusion--often referred to as creeping substitution. It is still more
preferred to
select the biodegradable material to remain in situ and capable of providing
sufficient biomechanical support for the spine even after a bone bridge has
grown
and formed through the through-holes of the implant. Selecting an appropriate
synthetic material can vary the biodegradation rate of the implant. The
degradation
rate of the selected material can be further modified, for example, increasing
the
degree of polymerization and/or increasing the amount of crosslinking between
the
polymer chains can decrease the degradation rate. Further, it is not intended
to
limit the preferred materials to substances that are partly or totally
reabsorbed
within the body. Rather substances that can be broken down and eventually
flushed from the body are also intended to come within the scope of this
invention.
Examples of biodegradable polymers for use with this invention include
poly(amino acids), polyanhydrides, polycaprolactones, polyorthoesters
polylactic
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acid, poly(lactide-co-glycolide), i.e., copolymers of lactic acid and glycolic
acid,
including either D, L and D/L isomers of these components. One example of a
preferred biodegradable polymer for use with this invention is a copolymer of
70:30 poly(L, DL) lactate commercially available from Boehringer Ingelheim.
A particularly advantageous benefit provided by this invention is the ease of
manufacturing suitable synthetic spacer. Spacers formed of polymeric,
oligomeric
and composite material can be manufactured using known fabricating techniques,
including extrusion, injection and blow molding processes. In addition,
selected
polymeric materials are provided by suppliers in a form that can readily
formed,
and/or molded, usually at an elevated temperature. A copolymer of D/L lactate
is
one specific example. This material can be obtained in a wide variety of forms
including pellets or granules, sheets, ingots. The material can be molded at a
temperature of about 55 C or higher to provide a desired shaped and sized
implant.
The material can be repeatedly heated and contoured without any significant
change in its material or chemical properties. In addition, material is
readily cut
using a cautery to readily conform the spacer to the configuration of the bone
surfaces. The lower cautery temperature even permits cutting or shaping of the
material during the operation.
Examples of metallic materials include any of the metals and metals alloys
known to be suitable for implant in animals, including humans. Specific
examples
include titanium, titanium alloys, and surgical steel.
Specific examples of ceramic materials for use with this invention include
glass, calcium phosphate, alumina, zirconia, apatite, hydroxyapatite and
mixtures
of these materials.
Composites are also useful with this invention. Composites can combine
two or more of the desired materials to form a spacer body for implantation.
Examples of composites include reinforced ceramic, glass or polymeric
materials.
Preferred composites include a fiber-reinforced material such as a glass or
carbon
fiber reinforced organic polymer.
Carbon fiber vertebral spacer of this invention can be prepared according to
the following method. A fiber composite material such as a glass or a carbon
fiber
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composite material (GFRP or CFRP, respectively) is first soaked with a liquid
plastic, in particular a resin material such as epoxy resin. The soaked fibers
are
wound around the winding mandrel as bunched fibers, in particular, using a
filament winding method. Subsequently the plastic is cured; this is best done
by a
controlled temperature treatment. The winding mandrel is advantageously formed
to have a simple rod-like shape, and, consequently, the resulting elongated
basis
body provided with a cavity having dimensions and configuration corresponding
to
the exterior configuration of the winding mandrel. Since the end measurements
of
the cavity are already formed by the winding mandrel, little if any machining
is
required on the inner surfaces of the limiting walls surrounding the cavity.
The
soaked fiber material is wound around the winding mandrel until the resulting
body has external dimensions that are about the same or slightly smaller than
the
desired final dimensions. Preferably the soaked fibers are wound about the
mandrel until the resulting body has reached at least a selected minimum wall
thickness for the thickest wall, i.e., the endwalls to minimize subsequent
machining
steps.
The wound spacer body is inachined according to following steps. The
winding mandrel is replaced with a receiving mandrel to center the basis body
into
a chucking device for accurate machining. The exterior of the basis body is
machined to an intermediate configuration that has exterior dimensions smaller
than the desired final dimensions. Preferably each surface of the intermediate
configuration is smaller than the final configuration by approximately the
same
amount. That is, the basis body is machined to have an intermediate
configuration
that substantially corresponds to the final configuration, only as a scaled
down
version. The intermediate configure is a scaled down version so that as little
material as possible needs to be machined during the final machining step.
Preferably, the winding/machining steps provide an intermediate spacer that
has
external dimensions of about 90-98%, more preferably 95%, of the outer
measurements of the final basis body.
It should be understood that since in the final configuration some walls are
thicker than other walls, the basis body is not be wound to have the final
measurements on every exterior surface. Selected surfaces require that
additional
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plastic material be wound about to the mandrel. Milling, grinding and/or
polishing
methods are possible methods of machining. During the prior machining steps,
the
different wall thickness of the longitudinal and the frontal walls are preset
into the
spacer body by corresponding machining from the selected outer sides of the
basis
body.
In the next step, additional fibers are soaked with resin and wound around
the machined, intermediate basis body to build the sides to the desired
thickness.
The resulting basis body approaches the desired exterior dimensions. That is,
fiber
and resin are material is wound around the basis body until the latter has
reached at
least the exterior measurements for each surface dimensions of the implant are
reached. The second winding step is performed after the winding mandrel has
been re-inserted. In the last step the basis body is wherein it reaches its
end outer
dimensions. With respect to performance material, fibers and resin, this
second
machining step corresponds to the first machining step.
This method provides a closed course of fibers at both the inner and outer
surfaces of the spacer, and takiilg full advantage of the strength increasing
properties of the fiber material. A completely closed, unadulterated course of
fibers is achieved at the inner surfaces defining the cavity, since these
surfaces are
not machined after the winding process. Since two winding steps are use to
fabricate the basis body, only a little amount of material remains to be
removed
from the basis body after the second machining step. In this final machining
step
very few, if any, of the fibers in the outer fiber winding are cut. The
integrity of
the fiber windings is maintained largely intact
Optionally, the basis body is, in addition to the cavity, is provided with
holes and grooves by corresponding machining steps after the second winding
procedure and after the plastic has been cured. In the vertebral implant
according
to the present invention these are the through holes in the longitudinal and
frontal
walls as well as the grooves of the receiving means and the grooves in the
crosswise extending free edge surfaces of the frontal walls.
This method provides a highly stable vertebral implant from fiber-reinforced
material at low costs. In addition, the fibers can be orientated to wind in a
single
direction or optionally in varying directions.
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Alternatively, the implant formed of a fiber composite material can be
prepared using a pultrusion method by saturating individual fibers or bundles
of
fibers with a resin, for example one the polymeric materials described above,
and
pulling the resin saturated fibers through a die to provide the profile of the
desired
5 implant. The resulting implant can be machined as described above to provide
the
final configuration including a threaded exterior, chamfer surfaces and
openings.
Implants prepared according the pultrusion method generally have fibers
orientated
in the same direction, for example either in an axial direction or
longitudinal
direction.
10 In yet another method the fiber reinforced composite can be prepared using
chopped fibers (or short fibers) that have been embedded within a curable
resin, for
example one or more of the polymers described above. The chopped fiber
reinforced material can be cured, molded and/or extruded according to
techniques
known in the art.
15 The osteogenic compositions used in this invention can be harvested from
other locations in the patent, for example from the cancelleous bone in the
vertebrae or from other bone structures, such as the iliac crest. In other
embodiments, the osteogenic material can comprise a therapeutically effective
amount of a bone morphogenetic protein in a pharmaceutically acceptable
carrier.
The preferred osteoinductive factors include, but are not limited to, the
recombinant human bone morphogenic proteins (rhBMPs) because they are
available in unlimited supply and do not transmit infectious diseases. Most
preferably, the bone morphogenetic protein is a rhBMP-2, rhBMP-4 or
heterodimers thereof. The concentration of rhBMP-2 is generally between about
0.4 mg/ml to about 1.5 mg/ml, preferably near 1.5 mg/ml. However, any bone
morphogenetic protein is contemplated including bone morphogenetic proteins
designated as BMP-1 through BMP-13. BMPs are available from Genetics
Institute, Inc., Cambridge, Massachusetts and may also be prepared by one
skilled
in the art as described in U.S. Patent Nos. 5,187,076 to Wozney et al.;
5,366,875 to
Wozney et al.; 4,877,864 to Wang et al.; 5,108,922 to Wang et al.; 5,116,738
to
Wang et al.; 5,013,649 to Wang et al.; 5,106,748 to Wozney et al.; and PCT
Patent
Nos. W093/00432 to Wozney et al.; W094/26893 to Celeste et al.; and
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W094/26892 to Celeste et al. All osteoinductive factors are contemplated
whether
obtained as above or isolated from bone. Methods for isolating bone
morphogenic
protein from bone are described in U.S. Patent No. 4,294,753 to Urist and
Urist et
al., 81 PNAS 371, 1984.
The choice of carrier material for the osteogenic material is based on the
application desired, biocompatibility, biodegradability, and interface
properties.
The bone growth inducing composition can be introduced into the pores of the
bone material in any suitable manner. For exainple, the composition may be
injected into the spacer cavity. The osteogenic factor, preferably a BMP, may
be
provided in freeze-dried form and reconstituted in a pharmaceutically
acceptable
liquid or gel carrier such as sterile water, physiological saline or any other
suitable
carrier. The carrier may be any suitable medium capable of delivering the
proteins
to the implant. Preferably the medium is supplemented with a buffer solution
as is
known in the art. In one specific embodiment of the invention, rhBMP-2 is
suspended or admixed in a carrier, such as, water, saline, liquid collagen or
injectable bicalcium phosphate. In a most preferred embodiment, BMP is applied
to the pores of the graft and then lypholized or freeze-dried. The graft-BMP
composition can then be frozen for storage and transport. Alternatively, the
osteoinductive protein can be added at the time of surgery.
Other osteoinductive protein carriers are available to deliver proteins.
Potential carriers include calcium sulphates, polylactic acids,
polyanhydrides,
collagen, calcium phosphates, polymeric acrylic esters and demineralized bone.
The carrier may be any suitable carrier capable of delivering the proteins.
Most
preferably, the carrier is capable of being eventually resorbed into the body.
One
preferred carrier is an absorbable collagen sponge marketed by Integra
LifeSciences Corporation under the trade name Helistat Absorbable Collagen
Hemostatic Agent. Another preferred carrier is an open cell polylactic acid
polymer (OPLA). Other potential matrices for the compositions may be
biodegradable and chemically defined calcium sulfates, calcium phosphates such
as tricalcium phosphate (TCP) and hydroxyapatite (HA) and including injectable
bicalcium phosphates (BCP), and polyanhydrides. Other potential materials are
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biodegradable and biologically derived, such as bone or dermal collagen.
Further
matrices are comprised of pure proteins or extracellular matrix components.
The
osteoinductive material may also be an admixture of BMP and a polymeric
acrylic
ester carrier, such as polymethylmethacrylic.
One carrier is a biphasic calcium phosphate ceramic.
Hydroxyapatite/tricalcium phosphate ceramics are preferred because of their
desirable bioactive properties and degradation rates in vivo. The preferred
ratio of
hydroxyapatite to tricalcium phosphate is between about 1:99 and about 65:35.
Any size or shape ceramic carrier, which will fit into the cavity defined in
the
spacer is contemplated. Ceramic blocks are commercially available from Sofamor
Danek Group, B. P. 4-62180 Rang-du-Fliers, France and Bioland, 132 Route
d:Espagne, 31100 Toulouse, France. Of course, rectangular and other suitable
shapes are contemplated. The osteoinductive factor is introduced into the
carrier in
any suitable manner. For example, the carrier may be soaked in a solution
containing the factor.
In order to prevent a sliding of the hand tool off the spacer, the receiving
means on the rear wall is advantageously foimed such that it also serves as a
holding means. The hand tool, can for instance, be provided with a clamp, by
means of which the spacer can be held tight, e.g. by engagement into the
grooves,
so that the spacer is connected with the hand tool as a joined unit and can be
readily manipulated by the surgeon to precisely place the spacer in the
desired
position in the disc space. In order to determine the position of the spacer
with
respect to the hand tool even more precisely, the opening in the spacer can
extend
through the rear wall and aligned with a second through hole formed through
the
opposite frontal wall on the opposite end of the spacer. A corresponding pin
or
shaft on the hand tool can be extended through the first through hole and into
the
second though hole thereby centering the spacer on the hand tool. The frontal
side
through hole enhances access to the spacer cavity to facilitate ingrowth of
tissue
into the spacer from its front side.
The intervertebral spacer according to the present invention can be
implanted into prepared disc space from a variety of orientations or
directions
including posteriorly, lateral posteriorly, anteriorly, and lateral
anteriorly. The disc
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space is prepared prior to implantation of the space. The typically a partial
or full
discectomy is performed. The endplates are preferentially cut to expose the
cancelleous bony tissue. Preferably only portions of the endplates that will
be
directly opposite the openings in the implanted spacers are cut or scraped to
expose
the underlying cancelleous bone tissue. This preserves as much integrity of
the
endplates as possible to minimize the implanted spacer's subsidence into the
endplate tissue. (Scrapers/chisels suitable for use in this invention are
discussed in U.S. Patent Serial No. 6,610,089 entitled "Spinal Implant
Method and Cutting Tool Preparation Accessory For Mounting the
Implant".) The upper and lower surfaces of each of the longitudinal
walls can engage the uncut portions of the endplates while the bearing
surfaces of the spacer bear against the thicker cortical bone tissue
proximal to the apophyseal ring. The preparation can, but is
not required to, include cutting the cortical bone in the endplates to provide
an
opening into the interior of the disc space, as well as, cutting or removing
portions
of the cortical bone tissue of the endplates to expose the cancelleous bone
tissue.
More preferably the spacer is inserted into the disc space without trimming
portions of the cortical rim. If needed the vertebrae are distracted to
provide
sufficient clearance between the opposing cortical rims of the adjacent
vertebrae
for insertion of the spacers. Thereafter the intervertebral spacer is inserted
into the
prepared disc space such that the upper and lower surfaces contact the
respective
opposing endplates while the openings in the upper and lower surfaces are
opposite
the cut portions ofthe endplates. The bearing surfaces are adjacent to the
interior
surfaces of the cortical rim around the vertebral bodies. After the spacers
have
been inserted in the disc space, if need or desired, the vertebral bodies can
be
compressed toward each other to decrease the disc space. The disc space
compression can embed one or more of the spacer's surfaces into the cortical
bone
of the endplate. In one embodiment, the upper and lower surfaces of the spacer
are
embedded in the previously uncut portions of the endplate. Additionally
pedical
screws, plates and/or spinal rods or any other known fixation devices and
techniques can be used to maintain disc space separation and spinal
orientation.
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The present invention contemplates modifications as would occur to those
skilled in the art. It is also contemplated that processes embodied in the
present
invention can be altered, rearranged, substituted, deleted, duplicated,
combined, or
added to other processes as would occur to those sk.illed in the art without
departing from the spirit of the present invention. In addition, the various
stages,
steps, procedures, techniques, phases, and operations within these processes
may
be altered, rearranged, substituted, deleted, duplicated, or combined as would
occur
to those skilled in the art.
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BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is perspective view of one embodiment of an intervertebral spacer
according to the present invention.
Figure 2 is an elevated side view of the spacer of Figure 1.
5 Figure 3 is an elevated end view of the intervertebral spacer of Figure 1.
Figure 4 is a cross-sectional view of the intervertebral spacer of Figure 1
taking along intersection line designated 4-4 in Figure 2.
Figure 5 is a cross-sectional view of the intervertebral spacer of Figure 1
taken along intersection line designated 5-5 in Figure 4.
10 Figure 6 is an elevated second end view of the intervertebral spacer of
Figure 1.
Figure 7 is perspective view of an alternative embodiment of a spacer
according to the present invention.
Figure 8 is a perspective view of a spacer holder for use with the present
15 invention.
Figure 9 is perspective view of one embodiment of a cutting tool according
to the present invention.
Figure 10 is a partial perspective view of the head of the cutting tool
illustrated in Figure 9.
20 Figure 11 is an elevated side view in partial section illustrating a
portion of
the cutter of Figure 9 received within a disc space.
Figure 12 is an elevated side view in partial section illustrating a portion
of
the cutter of Figure 9 rotated 90 within the disc space of Figure 11.
Figure 13 is a stylized cross-sectional lateral view of a portion of the
spinal
column with an intervertebral spacer positioned between an adjacent pair of
vertebra.
Figure 14 is a top view of the superior endplate of a lumbar vertebra
illustration the bi-lateral placement of a pair of spacers according to the
present
invention.
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BEST MODE FOR CARRYING OUT THE INVENTION
For the purposes of promoting an understanding of the principles of the
invention, reference will now be made to the embodiments illustrated herein
and
specific language will be used to describe the same. It will nevertheless be
understood that no limitation of the scope of the invention is thereby
intended. Any
alterations and further modifications in the described processes, systems or
devices,
and any further applications of the principles of the invention as described
herein, are
contemplated as would normally occur to one skilled in the art to which the
invention
relates.
Figures 1-5 illustrate intervertebral spacer 1 according to the present
invention. Spacer 1 is formed by an elongated body 2 defining a longitudinal
axis.
Cavity 3 is formed in body 2. Cavity 3, which is surrounded by two
longitudinal
walls 4, 5 and two endwalls 6, 7 (rear wall 6 and a front wall 7). Cavity 3 is
foimed to extend through in the height direction H of the body 2 and is
provided to
have in a plan projection the shape of an elongated rectangle with semi-
circles
being flushly added at its frontal sides. The cavity 3 has the same plan
projection
throughout the entire height of the implant and consequently is formed in the
basis
body 2 without any undercut regions. The endwalls 6, 7 are, i.e. throughout
their
respective entire wall surface, formed to be thicker than the longitudinal
walls 4, 5.
In the illustrated embodiment, the walls 6, 7 are provided to have about 2.5
times
the thickness of the longitudinal limiting walls 4, 5 about cavity 3; herein
the
thickness in the respective center of the wall is used as the relevant wall
thickness.
Body 2 comprises bearing surfaces 8, 8', 9, 9' formed by the cross-edge
surfaces of the endwalls 6, 7, which edge surfaces 8, 8', 9, 9' have a larger
width
than the respective cross-edge surfaces 10 and 11 formed by the transverse
edge
surfaces of the longitudinal walls 4, 5.
The surfaces 8, 8', 9, 9' extend in a continuous way and such that they are
substantially smooth surfaces, free of steps and protrusions. As is evident
from
Figure 13 surfaces 8, 8', 9, 9' serve as contact and/or support surfaces to
the
vertebral bodies W and almost completely take up the compressive forces
occurring between the adjacent vertebrae W. For this, the fact that the
vertebral
bodies W comprise a harder, cortical bone material K in their outer regions,
near
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22
the cortical rim structure whereas a softer, cancelleous or spongiose bone
material,
S, exists inside the vertebral bodies W, is taken advantage of. Herein, the
circumferential shell or cortical rim of the vertebral bone W consisting of a
cortical
bony tissue and to which the walls 6, 7 of the spacer 1 are applied via the
bearing
surfaces 8, 8', 9, 9' is sufficiently stable to take up the compressive force
along the
longitudinal direction of the of the spinal column and to transmit the
compressive
force to the spacer 1 proximate to bearing surfaces 8, 8', 9, 9'.
Additionally, Figure 2 illustrates body 2, viewed in its widthwise direction
B (or a side view), i.e. parallel to the bearing surfaces 8, 8', 9, 9'. Upper
surface
24 and lower surface 26 have a bi-convex shape in the longitudinal direction
with a
maximum height H provided between first end 20 and second end 22. In the
illustrated embodiment, the maximum height H of body 2 is located proximate to
the front edge 31 of opening 32 in upper surface 24. (See Figure 1.) The
resulting
lens-shaped design of the body 2 allows it to be placed inside the disc space
and
restore and/or maintain a desired disc space height. This configuration
maintains
upper and lower surfaces 24, and 26 in contact with the exposed endplates of
the
vertebrae. Consequently the osteogenic material in cavity 3 is pressed against
the
cancelleous or spongiseum bone tissue of the vertebrae and facilitates
arthrodesis.
Additionally since body 2 matingly engages with the natural concavity of the
endplates, the potential for retropulsion is minimized.
Figure 4 is a cross-sectional view along intersection line 4-4, i.e., viewed
perpendicular to the bearing surfaces 8, 8', 9, 9'. Spacer 1 is provided as a
substantially elongate rectangle having substantially planar longitudinaf
walls.
Corners 12 on first end 20 of this rectangular shape are rounded to provide
body 2
with a generally tapered profile on first end 20. Corners 12 have radii of
about 1/3
of the width of the spacer 1. As can be observed from Figures 1 and 3, the
longitudinal and frontal edges of the implant 1 are rounded resulting in a
streamlined front face to ease insertion of spacer 1 into the disc space.
Consequently, spacer I can be inserted into disc space, with minimal risk of
unintended tissue injury occurring.
In an alternative embodiment the peripheral edge of the opening 32 into of
the cavity 3 through upper surface 24 is not provided with a rounded over edge
and
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forms a sharp edge. This sharp edge poses minimal risk of tissue injury during
installing the spacer exists. In contrast to the longitudinal and frontal
spacer edges,
peripheral edge of opening 32 it is located further inwards towards the
central
longitudinal axis because of the overall lens-shape of the spacer 1 tapers
from a
maximum height proximate to front edge 31 of opening 32 to a smaller height
proximate to second end 22. Bearing surfaces 9, 9' of which, if viewed in the
sideward direction of the spacer 1, then extend almost parallel to the
longitudinal
central axis of the spacer 1.
A grooves 13 and 13' extend transverse to the longitudinal axis in the
widthwise direction B of the spacer 1. Grooves 13/13' are formed in each of
the
contact surfaces 8, 8', 9, 9'. Grooves 13/13' are provided to be a triangular
groove
or trough, the side flanks of which form an angle 28, 30 of about 90 with
respect
to each other; however, other shapes for the groove, such as a rectangular or
a U-
shape are also possible. The compressive pressure forces from the two
vertebrae,
W, press tissue into grooves 13/13' and provide additional anchoring of the
spacer
1 in the disc space.
A receiving means 14 for a hand or an operation tool is provided at the rear
end of the basis body 2 (depicted as the right end Figures 1, 2, 4, 5 and 7).
The
receiving means 14 comprises a through hole 15 formed in the middle of the
rear
wall 7. Through hole 15 can be provided to engage a corresponding portion on
the
hand tool. Grooves 16, 16' in second end 22 extend laterally from through hole
15
to engage in blades on the hand tool.
A second through hole 17 aligned to the hole 15 is formed in the middle of
the first end 20 of the basis body 2. A corresponding counterpart pen
extension or
stud of the hand tool can be engaged into (and through) through hole 17 for
centering the spacer 1 with respect to the tool. The through hole 17 and the
hole 15
subsequently enable growth of bone material into the. spacer from the front
and rear
sides. -
Two apertures 18, 19 are formed in each of the longitudinal walls 4, 5 of
the cavity 3 in the embodiment according to Figure 1. In alternative
embodiments,
two or more openings are formed in each longitudinal wall 4, 5. The diameter
of
each of the through holes 18, 19 corresponds to about half of the maximum
height
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H of spacer 1. Bone material can grow in a lateral direction into through
holes 18,
19 to anchor spacer 1 in the disc space.
Figure 7 is a perspective view of another embodiment of a spacer 80 for use
in the present invention. Spacer 80, similar to spacer 1 comprises an elongate
basis
body 82 defining a longitudinal axis 84. Basis body 82 has a generally bi-
convex ,
lens-shape profile defined by lower surface 86 and upper surface 87. Upper and
lower surfaces 86, 87 have a plurality of grooves 104. Additionally, basis
body 82
includes an internal cavity 88 surrounded by first and second longitudinal
walls 90
and 92, respectively, and endwalls 94 and 96. Endwalls 94 and 96 have a
thickness measured generally along the longitudinal axis that is greater than
the
thickness of either first or second longitudinal wall 90, 92. Further endwalls
94
and 96 each included cross-edge surfaces defining bearing surfaces 98, 98'
100,
and 100'.
A plurality of grooves 104 extend across bearing surfaces 98, 98' 100 and
100'. Additionally, but not required, selected grooves can extend across upper
and
lower surfaces 86, 87 orthogonal to longitudinal axis. The selected grooves
are
interrupted by the peripheral edge 108 of opening 110 into cavity 88.
Otherwise
grooves 104 extend laterally across basis body 82 from longitudinal wall 90 to
longitudinal wal192. Grooves 104 can be provided as swales cut in to lower and
upper surfaces 86 and 87. Additionally grooves 104 define a uniform curvature
cut
in to the upper and lower surfaces 86 and 87. Grooves 104 are provided to be
substantially wider between lands 106 than the depth of the groove below
surfaces
86 or 87.
In the illustrated embodiment, pairs or adjacent grooves 107 and 109 are
separated by lands 106. Lands 106 are provided to be substantially co-planar
with
the upper surface 86 and lower surface 87.
Basis body 80 also includes a smooth bore 112 formed in endwa1196.
Smooth bore 112 can use to locate an insertion instrument prior to grasping
longitudinal wall 90 and 92.
Figure 8 is a perspective view of one embodiment of a spacer holder 150
for use in this invention. Spacer holder 150 includes an elongate shaft 152
having
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a first end 154 adapted to receive a handle and an opposite second end 156
provided with a head 158 for securing a vertebral spacer.
Head 158 includes a spacer-securing portion 160 to secure an end of a
spacer (not shown). Securing portion 160 is a generally concave surface or a U-
5 shaped surface. A bottom portion 162 is provided as either a planar surface
or
slightly convex surface to bear against an endwall of the spacer. Projecting
longitudinally from bottom portion 162 are a pair of opposing wings 164 and
166,
spaced from each other a distance selected to engage opposite longitudinal
wall of
an included spacer. Bottom portion 162 and wings 164, 166 define a U-shaped
10 cavity 167 adapted to matingly engage a first end and a portion or the
lateral sides
of a spacer. Bottom portion 162 in combination with wings 164, 166 engage a
spacer on three sides to cradle the spacer in head 158 and control lateral the
lateral
motion of the spacer during implantation into the disc space.
A pair of blades 168, 170 extend inwardly into cavity 167. One blade
15 168/170 is provided to protrude radially internally from each wing 164,
166.
Blades 168 and 170 are provided to engage in grooves 16, 16' of spacer 1. It
will
be understood that blades 168 and 170 can be eliminated from portion 160 to
secure alternative embodiments of spacers according to the present invention.
Centering pin 172 projects into U-shaped cavity 167 from bottom portion
20 162. In the illustrated embodiment, centering pin 172 includes a tube 174
having a
movable shaft extension 176 received therein. Either or both tube 174 and
shaft
extension 176 can be provided with external threads. When provided shaft
extension 176 is rotatable received within outer shaft 152 toward first end
154
where it connects or engages with a thumb screw or wheel to allow rotation to
25 either withdraw or extend shaft extension 176 through tube 174. In one
form, shaft
152 includes internal threads while shaft extension includes external thread
to
provide longitudinal movement of shaft 152.
Head 158 also includes a depth stop 178 projecting laterally on at least one
side. Depth stop 178 is provided to contact the cortical rim of a vertebra
adjacent
to the disc space and arrest further movement of the attached spacer into the
disc
space. A surgeon can secure spacer with spacer holder 150, impact it into a
disc
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26
space, and once inside the space, position the secured spacer to a desired
location
in the disc space--all while the spacer remains secured to the spacer holder.
Spacers according to this invention may be inserted into an intervertebral
space
after preparation of the endplates of adjacent vertebrae using cutting tool
180, which
will now be described with reference to Figures 9-12. Cutting tool 180
includes a
cutting head 182, shaft 184 defining a longitudinal axis 186, and handle-
engaging
portion 188.
Cutting head 182 is attached to the distal end of shaft 184. Cutting head 182
includes a first arm 190 and a second arm 192 extending generally parallel to
longitudinal axis 186. Opposed first arm 190 and second arm 192 include two
generally smooth, longitudinal faces 202 and 204. Faces 202 and 204 are
configured
to facilitate insertion of cutting head 182 into the intervertebral space, and
are
generally separated from each other by a distance D. Distance D is selected to
be
substantially the same as the width of opening 3 in spacer 1 or opening 88 in
spacer
80 measured transverse to the longitudinal axis of the respective spacers.
First and second arms 190 and 192 each include first arcuate cutting edge 194
and a second opposite arcuate edge 196. Thus, cutting head 182 includes a
total of
four cutting edges. First cutting and second cutting edges 194 and 196,
respectively,
are provided in a configuration to substantially conform to arcuate upper and
lower
surfaces of spacers 1 and 80. Further, first and second arms 190 and 192 and
their
included first and second cutting edges 194 and 196 are adapted to cut and
remove a
portion of cortical bone tissue on opposing endplates of adjacent vertebrae V1
and
V2, while substantially retaining the natural concave curvature of the
endplates. The
cutting edges 194 and 196 have a length L selected to avoid cutting cortical
rims and
preferably the anterior and posterior portions of the endplates proximal to
the
apophyseal ring. The cavity thus prepared with cutting tool 180 provides
contact with
the graft material in implant 110 and the spongy bone of the two vertebrae.
The
bearing surfaces of implant 110 are disposed adjacent the edges of the
openings of the
cortical endplates and bear against the remaining portions of the endplates to
establish
a strong load bearing relationship.
First arm 190 and second arm 192 are generally opposed and define a cavity
198 therebetween for receipt of bony debris generated during the cutting
operation.
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The bony debris collected from the cutting operation can be saved and packed
in the
opening 3 or 88 of spacers 1 and 80, respectively, to promote arthrodesis.
Proximal
end of first arm 190 and second arm 192 attach to the distal terminus of shaft
184.
Opposite ends of first arm 190 and second arm 192 attach to non-cutting
portion 200.
Non-cutting portion 200 of cutting head 182 is fixed to the distal end of
first
arm 190 and second arm 192. Preferably, non-cutting portion 200 has a first
dimension transverse to the longitudinal axis substantially the same as
distance D to
be generally co-extensive with faces 202 and 204 of arms 194 and 196. Non-
cutting
portion 200 also is adapted to align faces 202 and 204 an equal distance from
opposed
endplate surfaces of adjacent vertebrae to facilitate removal of equal amounts
of
cortical bone tissue from adjacent vertebrae. Further, non-cutting portion 200
is
adapted to inhibit removal of cortical bone from the anterior cortical bone
surfaces of
adjacent vertebrae. While the non-cutting portion is depicted as a cylindrical
abutment, it is understood that alternative configurations are also included
within this
invention. Such alternative configurations include spherical, semispherical,
frustoconical and the like.
Shaft 184 is rotatably received within sleeve 206. Sleeve 206 includes stop
208
adapted to bear against a vertebral body when the cutting edge is inserted
into the
intervertebral space. Preferably, stop 208 is adapted to inhibit interference
with the
inter-spinal processes and associated nerve bodies. In one embodiment, stop
208 is
adapted to engage a single vertebral body.
Handle-engaging portion 188 is attached to the proximate end of shaft 184.
Handle-engaging portion 188 is adapted to releasably engage a variety of
handles
known in the art (not shown) to facilitate rotation of shaft 184 and cutting
head
182. Alternatively, it is understood that cutting tool 180 can include a
handle
fixedly attached to the proximal end of shaft 184.
Various non-limiting embodiments of a spinal fixation or fusion procedure of
the present invention are next described with reference to Figures 11 and 12.
One
procedure is characterized by: (a) Cutting the vertebrae V 1 and V2 with tool
180 to
prepare for implantation of spacers 1 and/or 80, and (b) Inserting spacers 1
and/or 80
between vertebral bodies V 1' and V2'. Another more detailed procedure for
fusing
two vertebrae together is described in terms of the following procedural.
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The surgeon reveals the vertebrae in need of fusion using known surgical
techniques. The surgeon then separates the dural sleeve forming an extension
of the
bone marrow if the procedure is in the lumbar region and then carries out a
discectomy to provide a space for the spacers in the disc space. If the
spacers are to
be inserted posteriorly, the surgeon inserts between the two vertebral bodies
V 1, V2
from the rear (posterior), two distracters known in the art. Distracters may
be inserted
laterally with respect to the cavity provided by the discectomy and then
turned 90 so
as to spread apart the vertebral bodies and to restore disc height. If a
lordotic angle is
intended, the distracters may include tapered surfaces intended to establish
the desired
angulation. Next, one of the distracters is removed. The surgeon then inserts
cutting
tool 180 between vertebral bodies V 1 and V2 so that the faces 202 and 204 are
in
contact with the vertebral endplates as shown in Figure 11. When the cutting
head
182 is correctly positioned in the central region of the cortical endplates,
stop 208
abuts the outer surface of V1 or V2, and non-cutting portion 200 is proximal
to the
interior cortical bone wall of V 1 and V2. Next, the surgeon rotates handle
188,
causing cutting head 182 to rotate about longitudinal axis 186. Typically, the
surgeon
rotates handle 188 through only a partial rotation to engage cutting edges 194
and 196
with the cortical bone of the adjacent endplates and then changes direction to
generate
an oscillating cutting action. Cutting action continues until the desired or
proper
amount of vertebral endplate is removed. When non-cutting portion 200 is
correctly
positioned between interior cortical bone portions of adjacent vertebrae V1
and V2,
first cutting edge 194 and second cutting edge 196 cut equally through
endplates 244
and 246. This cuts an opening into both vertebral endplates 244 and 246
gouging out
a depression that is a concave both in the anterior to posterior direction and
in the
lateral direction. In preferred embodiments, the maximum lateral dimensions of
the
opening is selected to be equal to the opening 3 in spacer 1 or opening 88 in
spacer
80. Remaining portions of endplates 246 and 248 bear against non-cutting
portion
200 and non-rotating shaft 206. Bony debris generated by the cutting of
cortical bone
is received in cavity 198 between first arm 190 and second arm 192. Then, the
surgeon withdraws cutting tool 180 from the intervertebral space. Bony debris
residing in cavity 198 can then be collected and packed inside spacer 1 or 80.
The
surgeon then implants spacer 1(or spacer 80), previously filled with either
osteogenic
CA 02402654 2002-09-10
WO 01/68005 PCT/US01/08073
29
material or bony debris, between endplates 244 and 246 from the posterior of
vertebral bodies V 1 and V2. Spacer 1 is positioned such that arcuate upper
surface 24
and lower surface 26 engage adjacent to cut portions of endplates 244 and 246,
while
remaining uncut portions adjacent to the cortical rim of endplates 244 and 246
bear
against bearing surfaces 8, 8', 9 and 9'. The surgeon then removes the second
distracter and repeats the preceding sequences to mount a second spacer 1 (or
80) by
placing it in position generally parallel to the first spacer 1.
Figure 13 illustrates a cross-section lateral view of a portion of the spinal
column with spacer 1 positioned between adjacent vertebrae W2 and W3. It can
be
observed from the Figure that spacer 1 snuggly fits inside the disc space.
Upper
surface 24 and lower surface 26 contact the opposing endplates substantially
along
their entire longitudinal length. Bearing surfaces 8, 8', 9 and 9' bear
against and
support the apophyseal ring structure of the individual vertebrae.
Figure 14 illustrates the bi-lateral placement of a pair of spacers 1 and 1'
on
a profile of a superior endplate of a lumbar vertebra 220. The longitudinal
dimension illustrated by reference line 222 of spacer 1 is selected to provide
a
space having a sufficient length to extend across the endplate and position
bearing
surfaces 8, 8' 9 and 9' opposite the apophyseal ring 226.
In other embodiments, it is envisioned that the described stages may be
altered, deleted, combined, repeated, or re-sequenced, as would occur to those
skilled in the art. By way of a non-limiting example, the procedure according
to
the present invention may utilize one or more different tools to prepare the
spine
for fixation by the implantation of the present invention. In another example,
the
tools of the present invention may be utilized to prepare a surgical site for
an
implant.
While this invention has been illustrated and described in details and
drawings and foregoing description, the same is considered to be a list and
not
restrictive in character, it is understood that only the preferred embodiments
have
been shown and described and in all changes and modifications that come within
the spirit of the invention or desire to be protected.
