 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to an artificial implant to be placed into
the intervertebral space left after the removal of a damaged spinal disc.
Specifically, the invention concerns an implant that facilitates
arthrodesis or fusion between adjacent vertebrae while also maintaining or
restoring the normal spinal anatomy at the particular vertebral level.
The number of spinal surgeries to correct the causes of low back pain has
steadily increased over the last several years. Most often, low back pain
originates from damage or defects in the spinal disc between adjacent
vertebrae. The disc can be herniated or can be suffering from a variety of
degenerative conditions, so that in either case the anatomical function of
the spinal disc is disrupted. The most prevalent surgical treatment for
these types of conditions has been to fuse the two vertebrae surrounding
the affected disc. In most cases, the entire disc will be removed, except
for the annulus, by way of a discectomy procedure. Since the damaged disc
material has been removed, something must be positioned within the
intra-discal space, otherwise the space may collapse resulting in damage
to the nerves extending along the spinal column.
In order to prevent this disc space collapse, the intra-discal space is
filled with bone or a bone substitute in order to fuse the two adjacent
vertebrae together. In early techniques, bone material was simply disposed
between the adjacent vertebrae, typically at the posterior aspect of the
vertebrae, and the spinal column was stabilized by way of a plate or a rod
spanning the affected vertebrae. With this technique once fusion occurred
the hardware used to maintain the stability of the segment became
superfluous. Moreover, the surgical procedures necessary to implant a rod
or plate to stabilize the level during fusion were frequently lengthy and
involved.
It was therefore determined that a more optimum solution to the
stabilization of an excised disc space is to fuse the vertebrae between
their respective end plates, most optimally without the need for anterior
or posterior plating. There have been an extensive number of attempts to
develop an acceptable intra-discal implant that could be used to replace a
damaged disc and yet maintain the stability of the disc interspace between
the adjacent vertebrae, at least until complete arthrodesis is achieved.
These "interbody fusion devices" have taken many forms. For example, one
of the more prevalent designs takes the form of a cylindrical implant.
These types of implants are represented by the patents to Bagby, U.S. Pat.
No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos.
4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In these
cylindrical implants, the exterior portion of the cylinder can be threaded
to facilitate insertion of the interbody fusion device, as represented by
the Ray, Brantigan and Michelson patents. In the alternative, some of the
fusion implants are designed to be pounded into the intra-discal space and
the vertebral end plates. These types of devices are represented by the
patents to Brantigan, U.S. Pat. Nos. 4,743,256; 4,834,757 and 5,192,327.
In each of the above listed patents, the transverse cross section of the
implant is constant throughout its length and is typically in the form of
a right circular cylinder. Other implants have been developed for
interbody fusion that do not have a constant cross section. For instance,
the patent to McKenna, U.S. Pat. No. 4,714,469 shows a hemispherical
implant with elongated protuberances that project into the vertebral end
plate. The patent to Kuntz, U.S. Pat. No. 4,714,469, shows a bullet shaped
prosthesis configured to optimize a friction fit between the prosthesis
and the adjacent vertebral bodies. Finally, the implant of Bagby, U.S.
Pat. No. 4,936,848 is in the form of a sphere which is preferably
positioned between the centrums of the adjacent vertebrae.
Interbody fusion devices can be generally divided into two basic
categories, namely solid implants and implants that are designed to permit
bone ingrowth. Solid implants are represented by U.S. Pat. Nos. 4,878,915;
4,743,256; 4,349,921 and 4,714,469. The remaining patents discussed above
include some aspect that permits bone to grow across the implant. It has
been found that devices that promote natural bone ingrowth achieve a more
rapid and stable arthrodesis. The device depicted in the Michelson patent
is representative of this type of hollow implant which is typically filled
with autologous bone prior to insertion into the intra-discal space. This
implant includes a plurality of circular apertures which communicate with
the hollow interior of the implant, thereby providing a path for tissue
growth between the vertebral end plates and the bone or bone substitute
within the implant. In preparing the intra-discal space, the end plates
are preferably reduced to bleeding bone to facilitate this tissue
ingrowth. During fusion, the metal structure provided by the Michelson
implant helps maintain the patency and stability of the motion segment to
be fused. In addition, once arthrodesis occurs, the implant itself serves
as a sort of anchor for the solid bony mass.
A number of difficulties still remain with the many interbody fusion
devices currently available. While it is recognized that hollow implants
that permit bone ingrowth into bone or bone substitute within the implant
is an optimum technique for achieving fusion, most of the prior art
devices have difficulty in achieving this fusion, at least without the aid
of some additional stabilizing device, such as a rod or plate. Moreover,
some of these devices are not structurally strong enough to support the
heavy loads and bending moments applied at the most frequently fused
vertebral levels, namely those in the lower lumbar spine.
There has been a need for providing a hollow interbody fusion device that
optimizes the bone ingrowth capabilities but is still strong enough to
support the spine segment until arthrodesis occurs. It has been found by
the present inventors that openings for bone ingrowth play an important
role in avoiding stress shielding of the autologous bone impacted within
the implant. In other words, if the ingrowth openings are improperly sized
or configured, the autologous bone will not endure the loading that is
typically found to be necessary to ensure rapid and complete fusion. In
this instance, the bone impacted within the implant may resorb or evolve
into simply fibrous tissue, rather than a bony fusion mass, which leads to
a generally unstable construction. On the other hand, the bone ingrowth
openings must not be so extensive that the cage provides insufficient
support to avoid subsidence into the adjacent vertebrae.
Another problem that is not addressed by the above prior devices concerns
maintaining or restoring the normal anatomy of the fused spinal segment.
Naturally, once the disc is removed, the normal lordotic or kyphotic
curvature of the spine is eliminated. With the prior devices, the need to
restore this curvature is neglected. For example, in one type of
commercial device, the BAK device of SpineTech, as represented by the
patent to Bagby, U.S. Pat. No. 4,501,269, the adjacent vertebral bodies
are reamed with a cylindrical reamer that fits the particular implant. In
some cases, the normal curvature is established prior to reaming and then
the implant inserted. This type of construct is illustrated in FIG. 1
which reveals the depth of penetration of the cylindrical implant into the
generally healthy vertebrae adjacent the instrumented discal space.
However, this over-reaming of the posterior portion is generally not well
accepted because of the removal of load bearing bone of the vertebrae, and
because it is typically difficult to ream through the posterior portion of
the lower lumbar segment where the lordosis is greatest. In most cases
using implants of this type, no effort is made to restore the lordotic
curvature, so that the cylindrical implant is likely to cause a kyphotic
deformity as the vertebra settles around the implant. This phenomenon can
often lead to revision surgeries because the spine becomes imbalanced.
In view of these limitations of the prior devices, there remains a need for
an interbody fusion device that can optimize bone ingrowth while still
maintaining its strength and stability. There is further a need for such
an implant that is capable of maintaining or restoring the normal spinal
anatomy at the instrumented segment. This implant must be strong enough to
support and withstand the heavy loads generated on the spine at the
instrumented level, while remaining stable throughout the duration.
SUMMARY OF THE INVENTION
In response to the needs still left unresolved by the prior devices, the
present invention contemplates a hollow threaded interbody fusion device
configured to restore the normal angular relation between adjacent
vertebrae. In particular, the device includes an elongated body, tapered
along substantially its entire length, defining a hollow interior and
having an outer diameter greater than the size of the space between the
adjacent vertebrae. The body includes an outer surface with opposite
tapered cylindrical portions and a pair of opposite flat tapered side
surfaces between the cylindrical portions. Thus, at an end view, the
fusion device gives the appearance of a cylindrical body in which the
sides of the body have been truncated along a chord of the body's outer
diameter. The cylindrical portions are threaded for controlled insertion
and engagement into the end plates of the adjacent vertebrae.
In another aspect of the invention, the outer surface is tapered along its
length at an angle corresponding, in one embodiment, to the normal
lordotic angle of lower lumbar vertebrae. The outer surface is also
provided with a number of vascularization openings defined in the flat
side surfaces, and a pair of elongated opposite bone ingrowth slots
defined in the cylindrical portions. The bone ingrowth slots have a
transverse width that is preferably about half of the effective width of
the cylindrical portions within which the slots are defined.
A driving tool is provided for inserting the fusion device within the
intra-discal space. In one feature, the driving tool includes a shaft
having a pair of opposite tapered tongs situated at one end. The tongs are
connected to the shaft by way of a hinge slot that biases the tongs apart
to receive a fusion device therebetween. The driving tool is further
provided with a sleeve concentrically disposed about the shaft and
configured to slide along the shaft and compress the hinge to push the
tongs together to grip the fusion device. Alternatively, an internal
expanding collet may be used to internally hold the fusion device securely
during insertion.
In one aspect of the driving tool, the tapered tongs have an outer surface
that tales on the form of the tapered cylindrical portions of the fusion
device. The tongs also have a flat inward facing surface to correspond to
the flat side surfaces of the fusion device. Thus, when the tongs are
compressed against the fusion device, the inward facing surfaces of the
tongs contact the flat sides of the fusion device and the outer surface of
the tongs complete the conical form of the fusion device to facilitate
screw-in insertion. The inward facing surface of the tongs may also be
provided with projections to engage openings in the fusion device to
permit driving and rotation of the device within the intra-discal space.
In another aspect of the invention, methods are provided for implanting the
fusion device between adjacent vertebrae. In one method, the approach is
anterior and includes the steps of dilating the disc space and drilling
the end plates of the adjacent vertebrae to the minor diameter of the
fusion device threads. A sleeve is inserted to provide a working channel
for the drilling step and the subsequent step of implanting the fusion
device. The implant is engaged with the driving tool, inserted through the
sleeve and threaded into the prepared bore. The depth of insertion of the
tapered fusion device determines the amount of angular separation achieved
for the adjacent vertebrae.
In another inventive method, the insertion site is prepared posteriorly,
namely the disc space is dilated and a minor diameter hole is drilled into
the vertebral end plates. A sleeve is also arranged to provide a working
channel for the drilling and insertion steps. The fusion device is
inserted into the drilled hole with the flat side walls facing the
adjacent vertebra. The device is then rotated so that the external threads
on the cylindrical portion cut into and engage the adjacent vertebrae. In
addition, since the fusion device is tapered, the tapered outer surface of
the device will angularly separate the adjacent vertebrae to restore the
normal anatomic lordosis.
DESCRIPTION OF THE FIGURES
FIG. 1 is a side-elevational view in the sagittal plane of a fusion device
of the prior art.
FIG. 2 is an enlarged perspective view of an interbody fusion device
according to one embodiment of the present invention.
FIG. 3 is a side cross-sectional view of the interbody fusion device shown
in FIG. 2, taken along line 3--3 as viewed in the direction of the arrows.
FIG. 4 is an end elevational view from the anterior end of the interbody
fusion device shown in FIG. 2.
FIG. 5 is a top-elevational view of the interbody fusion device shown in
FIG. 2.
FIG. 6 is an A-P lateral view from the anterior aspect of the spine showing
two interbody fusion devices according to FIG. 2 implanted within the
interbody space between L4 and L5.
FIG. 7 is a sagittal plane view of the interbody fusion device implanted
between L4 and L5 shown in FIG. 6.
FIG. 8 is a perspective view of an alternative embodiment of the interbody
fusion device according to the present invention. FIG. 9 is a
top-elevational view of an implant driver according to another aspect of
the present invention.
FIG. 10 is an enlarged perspective view of the end of the implant driver
engaged about an interbody fusion device, as depicted in FIG. 2.
FIG. 11 is an enlarged partial side cross-sectional view showing the
implant driver engaging the interbody fusion device, as shown in FIG. 10.
FIG. 12 is an enlarged partial side cross-sectional view showing an implant
driver of an alternative embodiment adapted for engaging the interbody
fusion device 10.
FIGS. 13(a)-12(d) show four steps of a method in accordance with one aspect
of the invention for implanting the interbody fusion device, such as the
device shown in FIG. 2.
FIGS. 14(a)-13(d) depict steps of an alternative method for implanting the
interbody fusion device, such as the device shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of promoting an understanding of the principles of the
invention, reference will now be made to the embodiments illustrated in
the drawings 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, such alterations and further modifications
in the illustrated device, and such further applications of the principles
of the invention as illustrated therein being contemplated as would
normally occur to one skilled in the art to which the invention relates.
An interbody fusion device 10 in accordance with one aspect of the present
invention is shown in FIGS. 2-5. The device is formed by a solid conical
body 11, that is preferably formed of a biocompatible or inert material.
For example, the body 11 can be made of a medical grade stainless steel or
titanium, or other suitable material having adequate strength
characteristics set forth herein. The device may also be composed of a
biocompatible porous material, such as porous tantalum provided by Implex
Corp. For purposes of reference, the device 10 has an anterior end 12 and
a posterior end 13, which correspond to the anatomic position of the
device 10 when implanted in the intra-discal space. The conical body 11
defines a hollow interior 15 which is bounded by a body wall 16 and closed
at the posterior end 13 by an end wall 17 (see FIG. 3). The hollow
interior 15 of the device 10 is configured to receive autograft bone or a
bone substitute material adapted to promote a solid fusion between
adjacent vertebrae and across the intra-discal space.
In accordance with the invention, the interbody fusion device 10 is a
threaded device configured to be screw threaded into the end plates of the
adjacent vertebrae. In one embodiment of the invention, the conical body
11 defines a series of interrupted external threads 18 and a complete
thread 19 at the leading end of the implant. The complete thread 19 serves
as a "starter" thread fur screwing the implant into the vertebral
endplates at the intra-discal space. The threads 18 and 19 can take
several forms known in the art for engagement into vertebral bone. For
instance, the threads can have a triangular cross-section or a truncated
triangular cross-section. Preferably, the threads have a height of 1.0 mm
(0.039 in) in order to provide adequate purchase in the vertebral bone so
that the fusion device 10 is not driven out of the intra-discal space by
the high loads experienced by the spine. The thread pitch in certain
specific embodiment can be 2.3 mm (0.091 in) or 3.0 mm (0.118 in),
depending upon the vertebral level at which the device 10 is to be
implanted and the amount of thread engagement necessary to hold the
implant in position.
In one aspect of the invention, the conical body 11, and particularly the
body wall 16, includes parallel truncated side walls 22, shown most
clearly in FIG. 4. The side walls are preferably flat to facilitate
insertion of the fusion device between the end plates of adjacent
vertebrae and provide area between for bony fusion. The truncated side
walls extend from the anterior end 12 of the device up to the complete
threads 19 at the posterior end 13. Thus, with the truncated side walls
22, the device 10 gives the appearance at its end view of an incomplete
circle in which the sides are cut across a chord of the circle. In one
specific example, the interbody fusion device 10 has a diameter at its
anterior end of 16.0 mm (0.630 in). In this specific embodiment, the
truncated side walls 22 are formed along parallel chord lines
approximately 12.0 mm (0.472 in) apart, so that the removed arc portion of
the circle roughly subtends 90.degree. at each side of the device. Other
benefits and advantages provided by the truncated side walls 22 of the
fusion device 10 will be described in more detail herein.
The conical body 11 of the device 10 includes a pair of vascularization
openings 24 and 25 defined through each of the truncated side walls 22.
These openings 24 and 25 are adapted to be oriented in a lateral direction
or facing the sagittal plane when the fusion device is implanted within
the intra-discal space. The openings are intended to provide a passageway
for vascularization to occur between the bone implant material within the
hollow interior 15 and the surrounding tissue. In addition, some bone
ingrowth may also occur through these openings. The openings 24 and 25
have been sized to provide optimum passage for vascularization to occur,
while still retaining a significant amount of structure in the conical
body 11 to support the high axial loads passing across the intra-discal
space between adjacent vertebrae.
The conical body 11 also defines opposite bone ingrowth slots 27, each of
which are oriented at 90.degree. to the truncated side walls 22.
Preferably, these slots 27 are directly adjacent the vertebral end plates
when the device 10 is implanted. More particularly, as the threads 18 and
19 of the device are screwed into the vertebral endplates, the vertebral
bone will extend partially into the slots 27 to contact bone implant
material contained within the hollow interior 15 of the device 10. As
shown more clearly in FIG. 5, the bone ingrowth slots 27 are configured to
provide maximum opening for bone ingrowth, in order to ensure complete
arthrodesis and a solid fusion. Preferably, the slots have a lateral width
that approximates the effective width of the threaded portions of the
body. It has been found that the prior devices which utilize a plurality
of small apertures do not promote a rapid and solid arthrodesis of the
bone material. Instead, the smaller apertures often lead to
pseudo-arthrosis and the generation of fibrous tissue. Since the bone
ingrowth slots 27 of the present invention are directly facing the
vertebrae, they are not situated in a portion of the device that must bear
high loads. Instead, the truncated side walls 22 will bear most of the
load passing between the vertebral end plates through the interrupted
threads 18 and across the intra-discal space.
In a further feature, the anterior end 12 of the body wall 16 can define a
pair of diametrically opposed notches 29, which are configured to engage
an implant driver tool as described herein. Moreover, the end wall 17 at
the posterior end 13 of the implant can be provided with a tool engagement
feature (not shown). For example, a hex recess can be provided to
accommodate a hex driver tool, as described further herein.
In one important feature of the interbody fusion device of the present
invention, the body 11 includes a tapered or conical form. In other words,
the outer diameter of the device at its anterior end 12 is larger than the
outer diameter at the posterior end 13. As depicted in FIG. 3, the body
wall 16 tapers at an angle A about the centerline CL of the device 10. The
taper of the body wall 16 is adapted to restore the normal relative angle
between adjacent vertebrae. For example, in the lumbar region, the angle A
is adapted to restore the normal lordotic angle and curvature of the spine
in that region. In one specific example, the angle A is 8.7940.degree.. It
is understood that the implant may have non-tapered portions, provided
that the portions do not otherwise interfere with the function of the
tapered body.
The taper angle A of the implant, coupled with the outer diameter at the
anterior and posterior ends of the fusion device 10, define the amount of
angular spreading that will occur between the adjacent vertebrae as the
implant is placed or screwed into position. This feature is depicted more
clearly in FIGS. 6 and 7 in which a preferred construct employing a pair
of fusion devices 10 is shown. In the depicted construct, the devices 10
are disposed between the lower lumbar vertebrae L4 and L5, with the
threads 18 and 19 threaded into the end plates E of the two vertebrae. As
shown in FIG. 7, as the device 10 is threaded into the end plates E, it
advances in the direction of the arrow I toward the pivot axis P of the
vertebral level. The pivot axis P is nominally the center of relative
rotation between the adjacent vertebrae of the motion segment. As the
tapered fusion device 10 is driven further in the direction of the arrow I
toward the pivot axis P, the adjacent vertebrae L4 and L5 are angularly
spread in the direction of the arrows S. Depth of insertion of the fusion
device 10 will determine the ultimate lordotic angle L achieved between
the two vertebrae.
In specific embodiments of the implant 10, the outer diameter or thread
crest diameter at the anterior end 12 can be 16, 18 or 20 mm, and the
overall length of the device 26 mm. The sizing of the device is driven by
the vertebral level into which the device is implanted and the amount of
angle that must be developed.
In another aspect of the invention, device 10 is sized so that two such
cylindrical bodies 11 can be implanted into a single disc space, as shown
in FIG. 6. This permits the placement of additional bone graft material
between and around the devices 10 in situ. This aspect further promotes
fusion across the intra-discal space and also serves to more firmly anchor
the devices between the adjacent vertebrae to prevent expulsion due to the
high axial loads at the particular vertebral level.
In one specific embodiment of the interbody fusion device 10, the
vascularization opening 24 is generally rectangular in shape having
dimensions of 6.0 mm (0.236 in) by 7.0 mm (0.276 in). Similarly, the
vascularization opening 25 is rectangular with dimensions of 4.0 mm (0.157
in) by 5.0 mm (0.197 in). Naturally, this opening is smaller because it is
disposed at the smaller posterior end 13 of the device 10. The bone
ingrowth slots 27 are also rectangular in shape with a long dimension of
20.0 mm (0.787 in) and a width of 6.0 mm (0.236 in). It has been found
that these dimensions of the vascularization openings 24, 25 and slots 27
provide optimum bone ingrowth and vascularization. In addition, these
openings are not so large that they compromise the structural integrity of
the device or that they permit the bone graft material contained within
the hollow interior 15 to be easily expelled during implantation.
As can be seen in FIG. 7, when the device is in position between the L4 and
L5 vertebrae, the vascularization openings 24 and 25 are side facing to
contact the highly vascularized tissue surrounding the vertebrae. In
addition, as can be seen in FIG. 6, the bone ingrowth slots 27 are axially
directed so that they contact the vertebral end plates E.
In an alternative embodiment of the invention, shown in FIG. 8, an
interbody fusion device 30 is formed of a conical body 31. The body wall
34 defines a hollow interior 33 as with the fusion device 10 of the
previous embodiment. However, in this embodiment the truncated side wall
38 does not include any vascularization openings. Moreover, the bone
ingrowth slots 39 on opposite sides of the device 30 are smaller. This
means that the interrupted threads 36 on the exterior of the device 30
extend a greater length around the implant. Such a design could be
utilized if a porous material (e.g., porous tantalum) were used to provide
additional surface area for tissue ingrowth and anchorage to the adjacent
bone. Also, this interbody fusion device 30 of the embodiment shown in
FIG. 8 can have application at certain vertebral levels where the risk of
expulsion of the device is greatest. Consequently, the amount of thread
contact is increased to prevent such expulsion. Prior to insertion, the
hollow interior 15 of the fusion device 10 is filled completely with bone
or substitute to facilitate this pre-loading.
The interbody fusion device 10 can be implanted using an implant driver 50,
shown in FIG. 9, according to one aspect of the invention. The implant
driver 50 is comprised of a shaft 51 and sl | | |
