Patent Publication Number: US-6986788-B2

Title: Intervertebral allograft spacer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 09/363,844, filed Jul. 30, 1999, now U.S. Pat. No. 6,258,125, which claims the benefit under 35 U.S.C. § 119(e) of Provisional Application No. 60/095,209, filed Aug. 3, 1998, and is a continuation-in-part of U.S. patent application Ser. No. 09/219,439, filed Dec. 23, 1998, now U.S. Pat. No. 6,143,033, which in turn claims the benefit under 35 U.S.C. § 119(e) of Provisional Application No. 60/073,271, filed on Jan. 30, 1998 and Provisional Application No. 60/095,425, filed on Aug. 5, 1998. The entire content of each of these applications is expressly incorporated herein by reference thereto. 
    
    
     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&#39;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&#39;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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a first embodiment of the implant according to the present invention; 
         FIG. 2  is a side view of the implant of  FIG. 1 ; 
         FIG. 3  is a back view of the implant of  FIG. 1 ; 
         FIG. 4  is a top view of a second embodiment of the implant; 
         FIG. 5  is a side view of the implant of  FIG. 4 ; 
         FIG. 6  is a top view of a third embodiment of the implant; 
         FIG. 7  is a side view of the implant of  FIG. 6 ; 
         FIG. 8A  is a top view of a top connecting surface of a top portion of the implant of  FIG. 6 ; 
         FIG. 8B  is a top view of a bottom connecting surface of a bottom portion of the implant of  FIG. 6 ; 
         FIG. 9  is a perspective view of a fourth embodiment of the implant; 
         FIG. 10A  is a side view of one embodiment of the teeth on the implant; 
         FIG. 10B  is a side view of a second embodiment of the teeth of the implant; 
         FIG. 11  is a side view of an embodiment of the implant similar to the embodiment of  FIGS. 6-8 ; 
         FIG. 12  is a top view of a vertebral bone characteristic of those of the cervical, thoracic, and lumbar spine; 
         FIG. 13  is a side view of sequentially aligned vertebral bones, such as are found in the cervical, thoracic, or lumbar spine; 
         FIG. 14  is a posterior view of a sequence of vertebrae; and 
         FIG. 15  is an end view of another embodiment of the implant. 
     
    
    
     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.  10 A). Preferably, the angle formed from the tip to the base is approximately 60°. Alternatively, teeth  12  have a saw tooth shape with the saw tooth running in the anterior-posterior direction (FIG.  10 B). 
     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&#39;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° and 15° 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° 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 fulfil 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.