An allogenic intervertebral implant for fusing vertebrae is disclosed. The implant is a piece of allogenic bone conforming in size and shape with a portion of an end plate of a vertebra. The implant has a wedge-shaped profile to restore disc height and the natural curvature of the spine. The top and bottom surfaces of the implant have a plurality of teeth to resist expulsion and provide initial stability. The implant according to the present invention provides initial stability need for fusion without stress shielding.

FIELD OF THE INVENTION
 The present invention is directed to an allogenic implant and, more
 particularly, to an allogenic intervertebral implant.
 BACKGROUND OF THE INVENTION
 A number of medical conditions such as compression of spinal cord nerve
 roots, degenerative disc disease, and spondylolisthesis can cause severe
 low back pain. Intervertebral fusion is a surgical method of alleviating
 low back pain. In posterior lumbar interbody fusion ("PLIF"), two adjacent
 vertebral bodies are fused together by removing the affected disc and
 inserting an implant that would allow for bone to grow between the two
 vertebral bodies to bridge the gap left by the disc removal.
 A number of different implants and implant materials have been used in PLIF
 with varying success. Current implants used for PLIF include threaded
 titanium cages and allografts. Threaded titanium cages suffer from the
 disadvantage of requiring drilling and tapping of the vertebral end plates
 for insertion. In addition, the incidence of subsidence in long term use
 is not known. Due to MRI incompatibility of titanium, determining fusion
 is problematic. Finally, restoration of lordosis, i.e., the natural
 curvature of the lumbar spine is very difficult when a cylindrical
 titanium cage is used.
 Allografts are sections of bone taken from a long bone of a donor. A cross
 section of the bone is taken and processed using known techniques to
 preserve the allograft until implantation and reduce the risk of an
 adverse immunological response when implanted. For example, U.S. Pat. No.
 4,678,470 discloses a method for processing a bone grafting material which
 uses glutaraldehyde tanning to produce a non-antigenic, biocompatible
 material. Allografts have mechanical properties which are similar to the
 mechanical properties of vertebrae even after processing. This prevents
 stress shielding that occurs with metallic implants. They are also MRI
 compatible so that fusion can be more accurately ascertained and promote
 the formation of bone, i.e., osteoconductive. Although the osteoconductive
 nature of the allograft provides a biological interlocking between the
 allograft and the vertebrae for long term mechanical strength, initial and
 short term mechanical strength of the interface between the allograft and
 the vertebrae are lacking as evidenced by the possibility of the allograft
 being expelled after implantation.
 Currently commercially available allografts are simply sections of bone not
 specifically designed for use in PLIF. As a result, the fusion of the
 vertebral bodies does not occur in optimal anatomical position. A surgeon
 may do some minimal intraoperative shaping and sizing to customize the
 allograft for the patient's spinal anatomy. However, significant shaping
 and sizing of the allograft is not possible due to the nature of the
 allograft. Even if extensive shaping and sizing were possible, a surgeon's
 ability to manually shape and size the allograft to the desired dimensions
 is severely limited.
 Most PLIF implants, whether threaded cages or allograft, are available in
 different sizes and have widths that vary with the implant height. For
 example, the width of a cylindrical cages will be substantially equivalent
 to the height. Although larger heights may be clinically indicated, wider
 implants are generally not desirable since increased width requires
 removal of more of the facet, which can lead to decreases stability, and
 more retraction of nerve roots, which can lead to temporary or permanent
 nerve damage.
 As the discussion above illustrates, there is a need for an improved
 implant for fusing vertebrae.
 SUMMARY OF THE INVENTION
 The present invention relates to an allogenic intervertebral implant for
 use when surgical fusion of vertebral bodies is indicated. The implant
 comprises a piece of allogenic bone conforming in size and shape with a
 portion of an end plates of the vertebrae and has a wedge-shaped profile
 with a plurality of teeth located on top and bottom surfaces. The top and
 bottom surfaces can be flat planar surfaces or curved surfaces to mimic
 the topography of the end plates. The implant has a channel on at least
 one side for receiving a surgical tool. This channel runs in the anterior
 direction to accommodate a variety of surgical approaches. A threaded hole
 on the anterior, posterior, posterior-lateral, or lateral side can be
 provided for receiving a threaded arm of an insertion tool.
 In another embodiment, the implant has an interior space for receiving an
 osteoconductive material to promote the formation of new bone.
 In another embodiment, the implant is made of a plurality of
 interconnecting sections with mating sections. Preferably, the implant is
 made in two halves: a top portion having a top connecting surface and a
 bottom portion having a bottom connecting surface. The top connecting
 surface mates with the bottom connecting surface when the top and bottom
 portions are joined. The top and bottom portions have holes that align for
 receiving a pin to secure the top and bottom portions together. The pin
 can be made of allogenic bone.
 In a different embodiment, the medial side of the implant has a scalloped
 edge such that when a first implant is implanted with a second implant
 with the medial sides facing each other, the scalloped edges define a
 cylindrical space.
 The present invention also relates to a discrete spacer used in conjunction
 with any of the other embodiments of the implant. The spacer comprises a
 piece of allogenic bone conforming in size and shape with a portion of an
 end plates of the vertebrae and has a wedge-shaped profile with
 substantially smooth top and bottom surfaces. The intersecting regions
 between the top and bottom surfaces and at least one of the lateral sides
 and the intersecting regions between the anterior and posterior sides and
 the same lateral side are curved surfaces to facilitate implantation of
 the spacer. Thus, the spacer can be implanted through an opening on one
 side of the spinal canal and moved with a surgical instrument to the
 contralateral side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows a top view of a first embodiment of intervertebral allograft
 spacer or implant 10 according to the present invention. Implant 10
 conforms in size and shape with a portion of end plates of the vertebrae
 between which implant 10 is to be implanted. Because implant 10 is an
 allograft, implant 10 promotes the formation of new bone to fuse the two
 vertebral bodies together. Although implant 10 will probably be
 predominantly used in the lumbar region of the spine, implant 10 can be
 configured for implantation in any region of the spine. Implant 10 has a
 plurality of teeth 12 on superior and inferior surfaces 14, 16 which
 provide a mechanical interlock between implant 10 and the end plates.
 Teeth 12 provide the mechanical interlock by penetrating the end plates.
 The initial mechanical stability afforded by teeth 12 minimizes the risk
 of post-operative expulsion of implant 10. Teeth 12 can be pyramid-shaped
 (FIG. 10A). Preferably, the angle formed from the tip to the base is
 approximately 60.degree.. Alternatively, teeth 12 have a saw tooth shape
 with the saw tooth running in the anterior-posterior direction (FIG. 10B).
 As shown in FIG. 2 and FIG. 3, a first lateral side 18 has a channel 20 and
 a second lateral side 22 also has a channel 20. Channels 20 are sized to
 receive a surgical instrument such as an inserter for implantation of
 implant 10. If the inserter has a threaded arm, implant 10 can be provided
 with a threaded hole 24. In FIG. 2, channel 20 is shown extended only
 partially along first lateral side 18. Channel 20 can extend along the
 entire length of first lateral side 18 as shown in the embodiment of FIG.
 5. In FIG. 3, channels 20 are shown on both first and second lateral sides
 18, 22. It should be noted that implant 10 can also have no channels or
 channels on one lateral side only as shown in the embodiment of FIG. 9.
 The dimensions of implant 10 can be varied to accommodate a patient's
 anatomy. Typically, implant 10 would have a width between 6-15 mm (in the
 medial-lateral direction), a length between 15-30 mm (in the
 anterior-posterior direction), and a height between 4-30 mm (maximum
 height in the superior-inferior direction). The size of implant 10 allows
 implant 10 to be implanted using conventional open surgical procedures or
 minimally invasive procedures, such as laparoscopic surgery. Additionally,
 because the width is kept to a restricted size range and does not
 necessarily increase with implant height, taller implants can be used
 without requiring wider implants. Thus, facet removal and retraction of
 nerve roots can remain minimal.
 In order to restore the natural curvature of the spine after the affected
 disc has been removed, implant 10 has a wedge-shaped profile. As shown in
 FIG. 2, this wedge shape results from a gradual decrease in height from an
 anterior side 26 to a posterior side 28. In anatomical terms, the natural
 curvature of the lumbar spine is referred to as lordosis. When implant 10
 is to be used in the lumbar region, the angle formed by the wedge should
 be approximately between 4.2.degree. and 15.degree. so that the wedge
 shape is a lordotic shape which mimics the anatomy of the lumbar spine.
 In order to facilitate insertion of implant 10, anterior side 26
 transitions to superior and inferior surfaces 14, 16 with rounded edges
 30. Rounded edges 30 enable implant 10 to slide between the end plates
 while minimizing the necessary distraction of the end plates.
 Although implant 10 is typically a solid piece of allogenic bone, implant
 10 can be provided with a hollow interior to form an interior space. This
 interior space can be filled with bone chips or any other osteoconductive
 material to further promote the formation of new bone.
 FIG. 4 shows a top view of a second embodiment of an implant 40 according
 to the present invention. In general, most of the structure of implant 40
 is like or comparable to the structure of implant 10. Accordingly,
 discussion of the like components is not believed necessary. The superior
 and inferior surfaces 14, 16 of implant 10 are flat planar surfaces. As
 seen best in FIG. 5, superior and inferior surfaces 14, 16 of implant 40
 are curved surfaces which still retain the wedge-shaped profile. The
 curved surfaces of superior and inferior surfaces 14, 16 of implant 40 are
 a mirror-image of the topography of the vertebral end plates. Thus, the
 curved surfaces conform to the contours of the end plates.
 FIG. 6 shows a top view of a third embodiment of an implant 50 according to
 the present invention. In general, most of the structure of implant 50 is
 like or comparable to the structure of implants 10, 40. Accordingly,
 discussion of the like components is not believed necessary. As best seen
 in FIG. 7, implant 50 comprises a top portion 52 joined to a bottom
 portion 54. As it may be difficult to obtain a single section of allogenic
 bone from which implant 50 is to be made, fabricating implant 50 in two
 pieces, i.e. top and bottom portions 52, 54, allows smaller sections of
 allogenic bone to be used. A top connecting surface 56 and a bottom
 connecting surface 58 define the interface between top and bottom portions
 52, 54. As shown in FIGS. 8A and 8B, top and bottom surfaces 56, 58 have
 ridges 60 that mate with grooves 62 to interlock top and bottom portions
 52, 54. Preferably, ridges 60 and grooves 62 are formed by milling top and
 bottom surfaces 56, 58 in a first direction and then milling a second time
 with top and bottom surfaces 56, 58 oriented 90.degree. with respect to
 the first direction.
 A pin 64 passing through aligned holes 66 in top and bottom portions 52, 54
 serves to retain top and bottom portions 52, 54 together. Although pin 64
 can be made of any biocompatible material, pin 64 is preferably made of
 allogenic bone. The number and orientation of pins 64 can be varied.
 FIG. 11 shows an embodiment of an implant 80 which, like implant 50, is
 made in multiple pieces. In general, most of the structure of implant 80
 is like or comparable to the structure of implants 10, 40, 50.
 Accordingly, discussion of the like components is not believed necessary.
 Implant 80 has a top portion 82, a middle portion 84, and a bottom portion
 86. As was the case for implant 80, the surfaces between the portions are
 mating surfaces with interlocking surface features, such as ridges and
 grooves. One or more pins preferably hold top, middle, and bottom portions
 82, 84, 86 together.
 FIG. 9 shows a perspective view of a fourth embodiment of a first implant
 70 according to the present invention. A second implant 70', which is
 substantially similar to first implant 70, is also shown. In general, most
 of the structure of first and second implants 70, 70' is like or
 comparable to the structure of implants 10, 40, 50. Accordingly,
 discussion of the like components is not believed necessary. First lateral
 sides 18 of first and second implants 70, 70' are scalloped to have a
 C-shape. When first and second implants 70, 70' are placed side by side
 with the first lateral sides 18 facing each other, a cylindrical space 72
 is formed. When first and second implants 70, 70' are implanted together,
 cylindrical space 72 can be filled with osteoconductive material to help
 promote the formation of new bone. First and second implants 70, 70' can
 be provided with locking pins 74 which engage apertures 76 to maintain the
 spatial relationship between first and second implants 70, 70'.
 The use of the implant according to the present invention will now be
 described with reference to FIGS. 12-14 and using posterior lumbar
 interbody fusion as an example. As the implant according to the present
 invention conforms in size and shape to a portion of end plates 100,
 preoperative planning is recommended for proper sizing. Determine the
 appropriate implant height by measuring adjacent intervertebral discs 102
 on a lateral radiograph. The implant must be seated firmly with a tight
 fit between end plates 100 when the segment is fully distracted. The
 tallest possible implant should be used to maximize segmental stability.
 Due to variability in degrees of magnification from radiographs, the
 measurements are only an estimate.
 With the patient in a prone position on a lumbar frame, radiographic
 equipment can assist in confirming the precise intraoperative position of
 the implant. The surgeon incises and dissects the skin from the midline
 laterally and locates spinous process 104, lamina 106, dura 108, and nerve
 roots of the appropriate level(s). As much as facets 110 as possible
 should be preserved to provide stability to the intervertebral segment.
 The surgeon performs a laminotomy to the medial aspect of facet 110 and
 reflects dura 108 to expose an approximately 13 mm window to the disc
 space. Disc 102 is removed through the window until only anterior 112 and
 lateral 114 annulus remain. The superficial layers of the entire
 cartilaginous end plates 100 are also removed to expose bleeding bone.
 Excessive removal of the subchondral bone may weaken the anterior column.
 Furthermore, if the entire end plate is removed, this may result in
 subsidence and a loss of segmental stability.
 Distraction can be done with either a surgical distractor or a trial spacer
 implant. In the first method, the distractor blades are placed into the
 disc space lateral to dura 108. The curve on the neck of the distractor
 should be oriented toward the midline. The distractor blades should be
 completely inserted into the disc space so that the ridges at the end of
 the blades rest on vertebral body 116. Fluoroscopy can assist in
 confirming that the distractor blades are parallel to end plates 100.
 Correct placement will angle the handles of the distractor cranially.
 particularly at L5-S1. The handle of the distractor is squeezed to
 distract the innerspace. The distraction is secured by tightening the
 speed nut on the handle.
 Using the preoperatively determined size, a trial spacer is inserted in the
 contralateral disc space with gentle impaction. Fluoroscopy and tactile
 judgement can assist in confirming the fit of the trial spacer until a
 secure fit is achieved. Using either the slots or threader hole on the
 implant, the selected implant is inserted in the contralateral disc space.
 Alternatively, the channels on the implant allow distraction and insertion
 to occur on the same side. Regardless of the side the implant is inserted
 in, autogenous cancellous bone or a bone substitute should be placed in
 the anterior and medial aspect of the vertebral disc space prior to
 placement of the second implant. The distractor is removed and a second
 implant of the same height as the first implant is inserted into the
 space, using gentle impaction as before. Preferably, the implants are
 recessed 2-4 mm beyond the posterior rim of the vertebral body.
 As previously noted, the implant according to the present invention can be
 inserted using minimally invasive procedures. In some of these procedures,
 only one side of the spinal cord needs to be approached. This minimizes
 muscle stripping, scar tissue in the canal, and nerve root retraction and
 handling. In clinical situations in which bilateral implant placement is
 required, proper implantation on the side opposite the incision can be
 difficult. FIG. 15 shows a beveled spacer 120 that facilitates placement
 on the side contralateral to the incision. In general and unless otherwise
 described, most of the structure of beveled spacer 120 is like or
 comparable to the structure of implants 10, 40, 50, and 80. Accordingly,
 discussion of the like components is not believed necessary. First lateral
 side 18 transitions to superior and inferior surfaces 14, 16 with rounded
 edges 30. First lateral side 18 also transitions to anterior and posterior
 sides 26, 28 with rounded edges 30. Additionally, spacer 120 has no teeth.
 The lack of teeth and rounded edges 30 enable spacer 120 to slide between
 the end plate and across the evacuated disc space (from one lateral
 annulus to the other) to the contralateral side. As first lateral side 18
 is the side that must promote movement of spacer 120, the use of rounded
 edges 30 on second lateral side 22 is optionally. Once spacer 120 has been
 placed on the side contralateral to the single incision using a surgical
 instrument to push spacer 120, bone graft or other osteoconductive
 material is packed in the disc space. Finally, an implant (any of implant
 10, 40, 50, 70, or 70' can be used) is implanted in the side proximal to
 the incision.
 While it is apparent that the illustrative embodiments of the invention
 herein disclosed fulfill the objectives stated above, it will be
 appreciated that numerous modifications and other embodiments may be
 devised by those skilled in the art. Therefore, it will be understood that
 the appended claims are intended to cover all such modifications and
 embodiments which come within the spirit and scope of the present
 invention.