Abstract:
An apparatus and method for spinal interbody fusion is disclosed. This implant includes fasteners which firmly attach it to vertebrae adjacent to excised tissue so as to transmit tension and torsional loads to and from those vertebrae. The instruments and methods are particularly adapted for interbody fusion from a posterior approach to the spine. One instrument is a vertebral spreader that is able to create anterior lift to a fixed or variable angle. Another instrument is a tome for cutting rectangular grooves in bone and preparing end plate surfaces. A method contemplates the use of these instruments to prepare a disk space and the insertion of the implant.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of Invention  
           [0002]    This invention relates generally to the treatment of injured, degenerated, or diseased tissue in the human spine, for example, damaged intervertebral discs and vertebrae. It further relates to the removal of damaged tissue and to the stabilization of the remaining spine by fusion to one another of at least two vertebrae adjacent or nearly adjacent to the space left by the surgical removal of tissue. More particularly, this invention relates to the implantation of devices that can be inserted from the patient&#39;s posterior, that is, from the back, to take the structural place of removed discs and vertebrae during healing while simultaneously sharing compressive loads. This invention further relates to the implantation of devices that do not interfere with the natural lordosis of the spinal column. More particularly, while aspects of the present invention may have other applications, the invention also provides instruments and techniques especially suited for interbody fusion from a generally posterior approach to the spine.  
           [0003]    2. Background of the Invention  
           [0004]    For many years a treatment, often a treatment of last resort, for serious back problems has been spinal fusion surgery. Disc surgery, for example, typically requires removal of a portion or all of an intervertebral disc. Such removal, of course, necessitates replacement of the structural contribution of the removed disc. The most common sites for such surgery, namely those locations where body weight most concentrates its load, are the lumbar discs in the L 1 - 2 , L 2 - 3 , L 3 - 4 , L 4 - 5 , and L 5 -S 1  intervertebral spaces. In addition, other injuries and conditions, such as tumor of the spine, may require removal not only of the disc but of all or part of one or more vertebrae, creating an even greater need to replace the structural contribution of the removed tissue. Also, a number of degenerative diseases and other conditions such as scoliosis require correction of the relative orientation of vertebrae by surgery and fusion.  
           [0005]    In current day practice, a surgeon will use one or more procedures currently known in the art to fuse remaining adjacent spinal vertebrae together in order to replace the structural contribution of the affected segment of the disc-vertebrae system. In general for spinal fusions a significant portion of the intervertebral disk and, if necessary, portions of vertebrae are removed and a stabilizing element, frequently including or composed entirely of bone graft material, is packed in the intervertebral space. In parallel with the bone graft material, typically additional external stabilizing instrumentation and devices are applied, in one method a series of pedicle screws and conformable metal rods. The purpose of these devices, among other things, is to prevent shifting and impingement of the vertebrae on the spinal nerve column. These bone graft implants and pedicle screws and rods, however, often do not provide enough stability to restrict relative motion between the two vertebrae while the bone grows together to fuse the adjacent vertebrae.  
           [0006]    Various surgical methods have been devised for the implantation of fusion devices into the disk space. Both anterior and posterior approaches have been used for interbody fusions. The anterior approach requires the added costs and associated risks for a general surgeon and/or a vascular surgeon to open the patient&#39;s abdominal cavity in order for the back surgeon to operate on the spine from an anterior approach. As a result, many surgeons prefer a posterior approach.  
           [0007]    The posterior surgical approach to the spine has often been used in the past. The primary difficulty of the posterior approach is that the spine surgeon must navigate past the spinal cord and subsidiary nerve structures. Also, unprotected drilling or trephining for implantation of cylindrical bone dowels carries risks to the patient.  
           [0008]    U.S. Pat. No. 5,484,437 to Michelson discloses a technique and associated instrumentation for inserting a fusion device from a posterior surgical approach that provides greater protection for the surrounding tissues and neurological structures during the procedure. As described in more detail in the &#39;437 patent, the surgical technique involves the use of a distractor having a penetrating portion that urges the vertebral bodies apart to facilitate the introduction of the necessary surgical instrumentation. The &#39;437 patent also discloses a hollow sleeve having teeth at one end that are driven into the vertebrae adjacent the disc space created by the distractor. These teeth engage the vertebrae to maintain the disc space height during subsequent steps of the procedure following removal of the distractor. In accordance with one aspect of the &#39;437 patent, a drill is passed through the hollow sleeve to remove portions of the disc material and vertebral bone to produce a prepared bore for insertion of the fusion device. The drill is then removed from the sleeve and the fusion device is positioned within the disc space using an insertion tool.  
           [0009]    While the more recent techniques and instrumentation represent an advance over earlier surgical procedures for the preparation of the disc space and insertion of the fusion device, the need for improvement still remains. The present invention is directed to this need and provides convenient methods and instruments to insure safe and effective preparation of a disc space in conjunction with implant placement.  
           [0010]    The restoration of normal anatomy is a basic principle of all orthopedic reconstructive surgery. Lordosis, which results in a pronounced forward curvature of the lumbar region of the spine, is a factor that needs to be taken into account in designing lumbar implants.  
           [0011]    Therefore, there is a perceived need for a device which simultaneously and reliably attaches mechanically to the bony spinal segments on either side of the removed tissue so as to prevent relative motion in extension or torsion of the spinal segments during healing, provides spaces in which bone growth material can be placed to create or enhance fusion, and enables the new bony growth, and, in a gradually increasing manner if possible, shares the spinal compressive load with the bone growth material and the new growth so as to enhance bone growth and calcification. The needed device will usually require a modest taper to preserve natural lumbar spinal lordosis.  
           [0012]    Thus, it is an object of the current invention to provide a stabilizing device for insertion in spaces created between vertebrae during spinal surgery. It is a further object to create a device implantable from the patient&#39;s posterior for stabilizing the spine by preventing or severely limiting relative motion between the involved vertebrae in tension (extension) and torsion loading during healing. It is a further object to provide a device which promotes growth of bone between vertebrae adjacent to the space left by the excised material by progressive sharing of the compressive load to the bone graft inserted in the space between the vertebrae. It is yet a further object to provide mechanical stability between adjacent vertebrae while bone grows and at the same time not diminish the natural lordosis of the lumbar spine. It is a further object to provide instrumentation that provides adequate protection for the sensitive vessels and neurological structures adjacent to the operating field. It is a further object to provide instrumentation that create a significant bed of bleeding bone while also preserving endplate structure for strength. Another object of this invention is to provide a spreader instrument that provides sequential angular and translational distraction of the disk space from the posterior side to restore the natural height and angle of the disk space and to help facilitate insertion of the implant. It is yet another object of this invention for the implant to be capable of being fabricated from human bone allograft material.  
         SUMMARY OF THE INVENTION  
         [0013]    The invention disclosed here is a novel implant and associated instrumentation designed to achieve the foregoing objects. The design of the new implant for spinal surgery includes the possibility of fabricating the device from human bone allograft material and from biocompatible manmade materials. The design is also such that the implant seats firmly in and mechanically mates with and ultimately fastens to adjacent vertebrae and stabilizes the involved vertebrae in tension and in torsion. Either the implant can be tapered or the vertebrae can be cut so as to preserve the natural lordosis of the spine. This invention also includes instrumentation necessary to effectively and safely prepare the intervertebral space, angularly distract the vertebrae and insert the implants.  
           [0014]    The implants generally have a rectangular geometry, although in some embodiments an annular geometry is preferred, with a lordotic slope of approximately ten degrees, such that the opposing anterior side is taller than the posterior side. The slope can be reversed for use in other portions of the spine for lumbar use where the curvature is opposite The implant also incorporates an anti-expulsion feature, such as notches or teeth on the top and bottom surfaces of the stabilizing fins. The implant may also contain opposing slots on either side to facilitate gripping with a bone holder instrument.  
           [0015]    The attachment portions of the implant are stabilizing fins projecting inferiorly and superiorly from the central one third of the wedge shaped implant. These stabilizing fins help stabilize the disk space in torsion, and help maintain a stable host-graft interface for fusion. The top and bottom surfaces of the implants, including the fins themselves, have a typically ten-degree lordosis. Therefore the anterior portion of the implant is a taller dimension than the posterior portion. This creates a challenge in the placement of the implant from the posterior direction. Therefore, for the lumbar region, the implant has an aggressive lead chamfer designed to further distract the endplates, as necessary to facilitate placement.  
           [0016]    In its most general form, the invention is an implant for mechanically attaching to the ends of and promoting bony fusion of at least two vertebrae adjacent to a space left by surgically removed spinal tissue, comprising a load-sharing body; said load-sharing body further comprising opposing rectangular fins on the top and bottom surfaces of the implant capable of mechanically anchoring the device to said adjacent vertebrae and thereby transmitting tensile and torsional loads to and from said adjacent vertebrae. In another embodiment, the vertebrae may be cut at such an angle so as to preserve the natural lordosis of the spine, i.e. 0-18 degrees, once the implant is inserted.  
           [0017]    In another embodiment, the invention generally is an implant for mechanically attaching to the ends of and promoting bony fusion of at least two vertebrae adjacent to a space left by surgically removed spinal tissue, comprising a structure formed from a single piece of bone allograft material having a top and bottom, said top and bottom surfaces including a stabilizing fin for mechanically interlocking with channels cut into said adjacent vertebrae.  
           [0018]    An important aspect in the implant procedure is the preparation of the space to receive the implant and the grooves for the rectangular stabilizing fins. A spacer/osteotome guide system is used which distracts the vertebrae and stabilizes them during preparation and acts as a guide for precise cutting. Special tomes are designed to precisely cut the rectangular channels and prepare the end plate surface. The spacer/osteotome guide is designed to avoid the nerve root and limit the depth of the cut for safety. The tomes also have depth stops which limit the depth of the cut for safety.  
           [0019]    Another important aspect of the implant procedure is an instrument system to facilitate translational and angular distraction from within the disk space to achieve the quality of distraction currently only obtained by the anterior approach. This provides a highly significant benefit to the surgeon. A double action vertebral spreader is provided that will penetrate more deeply into the disk space to create anterior lift to a fixed or variable angle. The design will allow the surgeon to set the lordotic angle prior to distraction of the vertebral endplates. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1A is an oblique view of the front and side of the spacer/osteotome guide part of the present invention;  
         [0021]    [0021]FIG. 1B is an orthogonal view of the side of the spacer/osteotome guide;  
         [0022]    [0022]FIG. 2 shows the osteotome in oblique view;  
         [0023]    [0023]FIG. 3A is a top view of the implant;  
         [0024]    [0024]FIG. 3B is a side view of the implant;  
         [0025]    [0025]FIG. 3C is a posterior view of the implant;  
         [0026]    [0026]FIG. 4A shows a posterior view of two vertebrae;  
         [0027]    [0027]FIG. 4B shows a posterior view of two vertebrae with spacer/osteotome guides installed between the vertebrae;  
         [0028]    [0028]FIG. 5A is an oblique view of the rear and side of the spacer/osteotome guide;  
         [0029]    [0029]FIG. 5B is an oblique view of the front and side of the spacer/osteotome guide;  
         [0030]    [0030]FIG. 5C shows a side view of the spacer/osteotome guide between two vertebrae;  
         [0031]    [0031]FIG. 6A is a detailed oblique view of the dual box osteotome;  
         [0032]    [0032]FIG. 6B shows the relationship of the dual box osteotome and the spacer/osteotome guide;  
         [0033]    [0033]FIG. 7A shows a posterior view of two vertebrae in which channels have been cut in adjacent endplates;  
         [0034]    [0034]FIG. 7B shows a posterior view of two vertebrae in which the implants have been installed;  
         [0035]    [0035]FIG. 7C shows a side view of the two vertebrae in which an implant has been installed;  
         [0036]    [0036]FIG. 8 is an oblique view of the vertebral body spreader;  
         [0037]    [0037]FIG. 9A is a view of the vertebral body spreader in a closed mode;  
         [0038]    [0038]FIG. 9B is a view of the vertebral body spreader with its jaws open at the lordotic angle;  
         [0039]    [0039]FIG. 9C is a view of the vertebral body spreader in its fully open mode;  
         [0040]    [0040]FIG. 10A is an oblique view of the rear and side of the spacer/osteotome guide;  
         [0041]    [0041]FIG. 10B is an orthogonal view of the side of the spacer/osteotome guide;  
         [0042]    [0042]FIG. 10C is an oblique view of the front and side of the spacer/osteotome guide part of the present invention;  
         [0043]    [0043]FIG. 10D is a posterior view of the spacer/osteotome guide;  
         [0044]    [0044]FIG. 10E is an oblique view of the front and side of the spacer/osteotome guide part of the present invention;  
         [0045]    [0045]FIG. 10F is a posterior view of the spacer/osteotome guide;  
         [0046]    [0046]FIG. 11A is a side view of the implant.  
         [0047]    [0047]FIG. 11B is a side view of the implant. 
     
    
     IDENTIFICATION OF ITEMS IN THE FIGURES  
       [0048]    [0048]FIG. 1A 
         [0049]    [0049] 2 —spacer/osteotome guide  
         [0050]    [0050]FIG. 1B 
         [0051]    [0051] 2 —spacer/osteotome guide  
         [0052]    [0052]FIG. 2 
         [0053]    [0053] 4 —osteotome  
         [0054]    [0054] 6 —blades of osteotome  
         [0055]    [0055] 8 —handle of osteotome  
         [0056]    [0056] 15 —end of handle portion of dual box osteotome  
         [0057]    [0057]FIG. 3A 
         [0058]    [0058] 7   a , 7   b —curved sides of bone implant  
         [0059]    [0059] 9 —chamfer  
         [0060]    [0060] 10 —bone implant  
         [0061]    [0061] 11 —locking teeth  
         [0062]    [0062] 13 —fins on implant  
         [0063]    [0063]FIG. 3B 
         [0064]    [0064] 7   b —curved sides of bone implant  
         [0065]    [0065] 9 —chamfer  
         [0066]    [0066] 10 —bone implant  
         [0067]    [0067] 11 —locking teeth  
         [0068]    [0068] 13 —fins on implant  
         [0069]    [0069]FIG. 3C 
         [0070]    [0070] 7   b —curved sides of bone implant  
         [0071]    [0071] 7   c , 7   d —opposing slots to facilitate gripping by a holding instrument  
         [0072]    [0072] 10 —bone implant  
         [0073]    [0073] 11 —locking teeth  
         [0074]    [0074] 13 —fins on implant  
         [0075]    [0075]FIG. 4A 
         [0076]    [0076] 12 —upper vertebral body  
         [0077]    [0077] 14 —lower vertebral body  
         [0078]    [0078] 16 —spinal cord  
         [0079]    [0079] 18   a , 18   b , 18   c , 18   d —lateral nerves from spinal cord  
         [0080]    [0080] 20 —intervertebral space  
         [0081]    [0081] 21 —upper cortical endplate of lower vertebra  
         [0082]    [0082] 23 —lower cortical endplate of upper vertebra  
         [0083]    [0083]FIG. 4B 
         [0084]    [0084] 2 —spacer/osteotome guide  
         [0085]    [0085] 12 —upper vertebral body  
         [0086]    [0086] 14 —lower vertebral body  
         [0087]    [0087] 16 —spinal cord  
         [0088]    [0088] 18   a , 18   b , 18   c , 18   d —lateral nerves from spinal cord  
         [0089]    [0089] 20 ′—intervertebral space  
         [0090]    [0090] 21 —upper cortical endplate of lower vertebra  
         [0091]    [0091] 30 —stop taps on spacer/osteotome guide  
         [0092]    [0092]FIG. 5A 
         [0093]    [0093] 2 —spacer/osteotome guide  
         [0094]    [0094] 30 —stop tabs  
         [0095]    [0095] 32 —angled side of spacer/osteotome guide  
         [0096]    [0096] 34 —guide channels  
         [0097]    [0097] 36 —hole that receives insertion handle  
         [0098]    [0098]FIG. 5B 
         [0099]    [0099] 2 —spacer/osteotome guide  
         [0100]    [0100] 30 —stop tabs  
         [0101]    [0101] 32 —angled side of spacer/osteotome guide  
         [0102]    [0102] 34 —guide channels  
         [0103]    [0103]FIG. 5C 
         [0104]    [0104] 2 —spacer/osteotome guide  
         [0105]    [0105] 20 ″—intervertebral space  
         [0106]    [0106] 27 —upper vertebral body  
         [0107]    [0107] 28 —lower vertebral body  
         [0108]    [0108] 30 —stop tabs  
         [0109]    [0109]FIG. 6A 
         [0110]    [0110] 4 —dual box osteotome  
         [0111]    [0111] 6 —blades of osteotome  
         [0112]    [0112] 8 —handle of osteotome  
         [0113]    [0113] 15 —end of handle portion of dual box osteotome  
         [0114]    [0114] 40 —front sides of osteotome blades  
         [0115]    [0115] 42 —sharp cutting edges of osteotome  
         [0116]    [0116]FIG. 6B 
         [0117]    [0117] 2 —spacer/osteotome guide  
         [0118]    [0118] 4 —dual box osteotome  
         [0119]    [0119] 6 —blades of osteotome  
         [0120]    [0120] 8 —handle of osteotome  
         [0121]    [0121] 34 —osteotome guide channel  
         [0122]    [0122] 44 —hollow centers of osteotome blades  
         [0123]    [0123] 45 —arrow showing direction of cut  
         [0124]    [0124]FIG. 7A 
         [0125]    [0125] 12 —upper vertebra  
         [0126]    [0126] 14 —lower vertebra  
         [0127]    [0127] 16 —spinal cord  
         [0128]    [0128] 21 —upper cortical endplate of lower vertebra  
         [0129]    [0129] 23 —lower cortical endplate of upper vertebra  
         [0130]    [0130] 50 —cut channels that receive implants  
         [0131]    [0131]FIG. 7B 
         [0132]    [0132] 10 —bone implants  
         [0133]    [0133] 12 —upper vertebra  
         [0134]    [0134] 14 —lower vertebra  
         [0135]    [0135] 16 —spinal cord  
         [0136]    [0136] 21 —upper cortical endplate of lower vertebra  
         [0137]    [0137] 23 —lower cortical endplate of upper vertebra  
         [0138]    [0138] 50 —cut channels that receive implants  
         [0139]    [0139]FIG. 7C 
         [0140]    [0140] 10 —bone implants  
         [0141]    [0141] 12 —upper vertebra  
         [0142]    [0142] 14 —lower vertebra  
         [0143]    [0143] 20 —intervertebral space  
         [0144]    [0144] 50 —cut channels that receive implants  
         [0145]    [0145]FIG. 8 
         [0146]    [0146] 60 —vertebral body spreader tool  
         [0147]    [0147] 61 , 61 ′—pivot points, or hinge pins, of jaw actuation arms  
         [0148]    [0148] 62 , 62 ′—jaws  
         [0149]    [0149] 64 , 64 ′—jaw actuation arms  
         [0150]    [0150] 65 , 65 ′—catch mechanisms  
         [0151]    [0151] 66 , 66 ′—handles  
         [0152]    [0152] 68 , 68 ′—locking stops  
         [0153]    [0153] 70 , 70 ′—parallel rails  
         [0154]    [0154] 82 ,  82 ′—guide slots for sliding pivot arms  
         [0155]    [0155]FIG. 9A 
         [0156]    [0156] 55 —disc space  
         [0157]    [0157] 57 —upper vertebral body  
         [0158]    [0158] 59 —lower vertebral body  
         [0159]    [0159] 60 —vertebral body spreader tool  
         [0160]    [0160] 62 , 62 ′—jaws  
         [0161]    [0161] 64 , 64 ′—jaw actuation arms  
         [0162]    [0162] 65 , 65 ′—catch mechanisms  
         [0163]    [0163] 66 , 66 ′—handles  
         [0164]    [0164] 68 , 68 ′—locking stops  
         [0165]    [0165] 70 , 70 ′—parallel rails  
         [0166]    [0166] 76 , 76 ′—crossing slider mechanism  
         [0167]    [0167] 77 , 77 ′—sliding pivot points  
         [0168]    [0168] 78 —one arm of crossing slider  
         [0169]    [0169] 79 —one arm of crossing slider  
         [0170]    [0170] 80 —common pivot point of arms of crossing slider  
         [0171]    [0171]FIG. 9B 
         [0172]    [0172] 55 —disc space  
         [0173]    [0173] 57 —upper vertebral body  
         [0174]    [0174] 59 —lower vertebral body  
         [0175]    [0175] 60 —vertebral body spreader tool  
         [0176]    [0176] 62 , 62 ′—jaws  
         [0177]    [0177] 64 , 64 ′—jaw actuation arms  
         [0178]    [0178] 65 , 65 ′—catch mechanisms  
         [0179]    [0179] 66 , 66 ′—handles  
         [0180]    [0180] 68 , 68 ′—locking stops  
         [0181]    [0181] 70 , 70 ′—parallel rails  
         [0182]    [0182] 76 , 76 ′—crossing slider mechanism  
         [0183]    [0183] 77 , 77 ′—sliding pivot points  
         [0184]    [0184] 78 —one arm of crossing slider  
         [0185]    [0185] 79 —one arm of crossing slider  
         [0186]    [0186] 80 —common pivot point of arms of crossing slider  
         [0187]    [0187]FIG. 9C 
         [0188]    [0188] 55 —disc space  
         [0189]    [0189] 57 —upper vertebral body  
         [0190]    [0190] 59 —lower vertebral body  
         [0191]    [0191] 60 —vertebral body spreader tool  
         [0192]    [0192] 62 , 62 ′—jaws  
         [0193]    [0193] 64 , 64 ′—jaw actuation arms  
         [0194]    [0194] 65 , 65 ′—catch mechanisms  
         [0195]    [0195] 66 , 66 ′—handles  
         [0196]    [0196] 68 , 68 ′—locking stops  
         [0197]    [0197] 70 , 70 ′—parallel rails  
         [0198]    [0198] 76 , 76 ′—crossing slider mechanism  
         [0199]    [0199] 77 , 77 ′—sliding pivot points  
         [0200]    [0200] 78 —one arm of crossing slider  
         [0201]    [0201] 79 —one arm of crossing slider  
         [0202]    [0202] 80 —common pivot point of arms of crossing slider  
         [0203]    [0203]FIG. 10A 
         [0204]    [0204] 91 —spacer/osteotome guide  
         [0205]    [0205] 92 —stop tabs  
         [0206]    [0206] 93 —hole  
         [0207]    [0207]FIG. 10B 
         [0208]    [0208] 91 —spacer/osteotome guide  
         [0209]    [0209] 92 —stop tabs  
         [0210]    [0210]FIG. 10C 
         [0211]    [0211] 94 —spacer/osteotome guide  
         [0212]    [0212] 94 ′—blunt nose on anterior end of guide  
         [0213]    [0213]FIG. 10D 
         [0214]    [0214] 94 —spacer/osteotome guide  
         [0215]    [0215] 95 —hole  
         [0216]    [0216]FIG. 10E 
         [0217]    [0217] 96 —spacer/osteotome guide  
         [0218]    [0218] 96 ′—blunt nose on anterior end of guide  
         [0219]    [0219] 97 —main body of spacer  
         [0220]    [0220] 98 —box guide for osteotome  
         [0221]    [0221]FIG. 10F 
         [0222]    [0222] 96 —spacer/osteotome guide  
         [0223]    [0223] 97 —main body of spacer  
         [0224]    [0224] 98 —box guide for osteotome  
         [0225]    [0225] 99 —hole for detachable handle  
         [0226]    [0226]FIG. 11A 
         [0227]    [0227] 102 —implant  
         [0228]    [0228] 104 , 104 ′—shelves  
         [0229]    [0229] 105 , 105 ′—chamfers  
         [0230]    [0230] 106 , 106 ′—fins  
         [0231]    [0231] 108 —locking teeth  
         [0232]    [0232]FIG. 11B 
         [0233]    [0233] 110 —implant  
         [0234]    [0234] 111 —teeth  
         [0235]    [0235] 112 , 112 ′—fins  
         [0236]    [0236] 114 , 114 ′—shelves  
         [0237]    [0237] 115 , 115 ′—chamfers  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0238]    The implant itself is preferably allograft material but may also comprise a variety of presently acceptable biocompatible materials. The body of the implant may optionally have a modest taper to accommodate the natural lordosis of the lumbar spine. In a variant of one embodiment, locking notches or teeth may be located on the outer edge of both the stabilizing fins, to engage the cortical bone and prevent the implant from migrating out of the intervertebral space.  
         [0239]    As previously noted, any of the embodiments of the interlocking implant can be fabricated from cadaver bone which is processed to form bone allograft material. Tissue grafting of living tissue from the same patient, including bone grafting, is well known. Tissue such as bone is removed from one part of a body (the donor site) and inserted into tissue in another (the host site) part of the same (or another) body. With respect to living bone tissue, it has been desirable in the past to be able to remove a piece of living tissue graft material which is the exact size and shape needed for the host site where it will be implanted, but it has proved very difficult to achieve this goal.  
         [0240]    It is now possible to obtain allograft bone which has been processed to remove all living material which could present a tissue rejection problem or an infection problem. Such processed material retains much of the structural quality of the original living bone, rendering it osteoinductive. Moreover, it can be shaped according to known and new methods to attain enhanced structural behavior. In the present invention, allograft bone is reshaped into one of the spacer configurations for use as a spine implant.  
         [0241]    In the current invention, a blank is cut from cortical allograft bone, generally from long bones of the leg. The blank is machined by conventional milling to form the fins, grooves and outer surfaces. Such processes in general are able to maintain the biological and structural properties of the allograft material.  
         [0242]    [0242]FIGS. 3A, 3B,  3 C and  7 B depict the implant and its position once inserted within two vertebrae.  
         [0243]    [0243]FIGS. 1A, 1B,  2 ,  4 B, and  5 A- 6 B depict the surgical tools used to install the implant. This apparatus comprises a set of unique tools which will accurately cut the rectangular grooves in bone and prepare the endplate surfaces for the purpose of inserting an implant which locks adjacent vertebrae together.  
         [0244]    [0244]FIGS. 8 and 9A- 9 C depict the surgical tool used to facilitate translational and angular distraction from within the disk space to achieve the quality of distraction currently only obtained by the anterior approach. A double action vertebral spreader is used that will penetrate more deeply into the disk space to create anterior lift to a fixed or variable angle. The design will allow the surgeon to set the lordotic angle prior to distraction of the vertebral endplates.  
         [0245]    FIGS.  10 A-F and  11 A and B depict the various embodiments of the spacer/osteotome guide and of the implant.  
         [0246]    Overview of the Invention  
         [0247]    Referring to FIGS. 1A through 3C there are shown the three main components of the present invention. FIG. 1A is an oblique view of a spacer/osteotome guide  2 , two each of which are inserted between adjacent vertebra  12 ,  14  as shown in FIGS. 4A and 4B. FIG. 1B shows the angle α of the taper of the spacer, said taper corresponding to the desired lordosis of the two vertebrae being fused. FIG. 2 is an oblique view of a dual box osteotome  4  comprised of a two-part osteotome blade  6  and a driver handle  8 . As will be explained below, the osteotome blade  6  is guided by the guide  2  when it cuts channels in adjacent vertebral endplates so as to accommodate the insertion of a spinal fusion implant  10  as shown in the three orthogonal views, FIGS. 3A, 3B and  3 C. FIG. 3A shows curved sides  7   a ,  7   b  which correspond to the edges of the source bone from which the implant  10  has been machined, specifically from cortical bone of the femur or tibia. FIG. 3B is a side view showing the tapered side  7   b  (with corresponding taper on unshown side  7   a ), which has the same angle α that provides the desired angle or lordosis of the vertebrae being conjoined by the implants, specifically a lordotic angle of 2α or about 10 degrees. FIG. 3C shows the implant  10  from its posterior end, i.e., the end that, when installed corresponds to the posterior side of the spinal column. The serrations or locking teeth  11  provide a gripping effect when the implant has been installed in the channel that has been cut by the cutting tool  4 , said channels being visible in FIG. 7A. During the implantation process, two implants are installed in each intervertebral space, more or less symmetrically about the spinal cord, as will be described in more detail below.  
         [0248]    Use of the Invention  
         [0249]    [0249]FIG. 4A is a posterior view of two vertebra  12 ,  14  from which boney, muscle-supporting processes have been removed to expose the spinal cord  16 , the nerves  18   a ,  18   b ,  18   c  and  18   d  extending laterally outward therefrom, and the posterior portion of the intervertebral space  20 , within which two implants  10  are to be inserted, one on either side of spinal cord  16  within the specific locations occupied by the spacer/osteotome guides  2  in FIG. 4B. Note that the respective vertebrae  12 ,  14  have been separated in FIG. 4B compared to FIG. 4A to accommodate the installation of the spacer/osteotome guides  2 , or, more specifically, the space  20 ′ in FIG. 4B is larger than the corresponding space  20  in FIG. 4A.  
         [0250]    The Spacer/Osteotome Guide  
         [0251]    Referring to FIGS. 5A and 5B, the vertebral body spacer/osteotome guide  2  is shown in two oblique views, showing the length L, width W and height H dimensions. The spacer/osteotome guides  2  are made of stainless steel.  
         [0252]    The spacer/osteotome guides  2  measure approximately 20-30 mm long by 9-12 mm wide, and they have heights that vary from 6 mm to 14 mm. The anterior or front part of the spacer is chamfered or curved to facilitate introduction past bony landmarks. The posterior end has features to allow connection to a drive handle, which is easily removed after the spacer is fully inserted into the disc space. There are two centrally located coplanar slots  34  on the superior and inferior surface of the spacers which are approximately 1 mm to 3 mm deep, defining a guide channel. The posterior origin of these slots  34  is easily viewed, even when the spacer is fully inserted. The spacers further have tabs  30  extending superiorly and inferiorly that contact the vertebral body&#39;s posterior wall to prevent over insertion. In a second embodiment, the spacers may have a centrally located, hollow tab projecting 2 mm to 5 mm posteriorly so as to guide the bone cutting tome blade on both sides and also to provide additional protection to adjacent neural structures. This second embodiment also includes a flange projection on one lateral side that, when fully inserted, retracts the central dura. The combination of the hollow tab and flange provide full protection while allowing the safe subsequent passage of the sharp bone-cutting tome, and prevents over-insertion.  
         [0253]    [0253]FIGS. 5A and 5B show details of the spacer/osteotome guide  2  in two oblique views. FIG. 5A is a rear and side view of the spacer/osteotome guide  2 , showing the aforementioned stop tabs  30  which are contiguous with the main body  32 , and two osteotome guide channels  34 . The hole  36  receives the end of a detachable handle, not shown, which is used to insert the spacer/osteotome guide  2  between adjacent vertebrae  12 ,  14  as shown in FIG. 4B. FIG. 5B is a partial front and side view of the spacer/osteotome guide  2 . FIG. 5C is a schematic cross-sectional side view of a spacer/osteotome guide  2  within the vertebral space  20 ″ between two vertebrae  27 ,  28 . FIG. 5C is a side view of one of the installed spacers/osteotome guides  2  with stop tabs  30  abutting the posterior side of an upper vertebra  27  and a lower vertebra  28 . FIG. 5C complements FIG. 4B where the spacers/osteotome guides  2  are shown in posterior view between vertebrae designated as  12  and  14 . Note that, as shown in FIG. 5A, the spacer/osteotome guide  2  has only three stops  30 . The reason for only three stops  30  is evident in FIG. 4B where the nerves  18   a ,  18   b  are in proximity to where the missing fourth stop would otherwise be. Note yet further in FIG. 4B that the two spacers/osteotome guides  2  shown are not identical, but rather they are mirror images of each other with respect to the sagittal plane, or, in other words, in relation to the locations of the three respective stops tabs  30  on each spacer.  
         [0254]    The Osteotome  
         [0255]    The dual box osteotome  4 , i.e., the osteotome, is shown in oblique views in FIGS. 2, 6A and  6 B. The osteotome  4  is comprised of two parallel, hollow cutting blades  6  and a detachable handle  8 . The double blade portion is further connected to a male or female threaded boss to enable firm attachment to the handle  8 . Each box shape blade  6  is generally 4 mm wide by 4 mm tall on each side. Three sides of the box are sharpened and one side is blunt. The blunt side, generally the side closest to the central axis, may also protrude 1 mm to 3 mm from the sharp sides and may be chamfered. More specifically, as shown in FIG. 6A, the front sides  40  of the hollow cutting blades  6  have sharp cutting edges  42 . FIG. 6B shows in oblique view the way in which the osteotome and handle assembly  4  engages the channels  34  in the spacer/osteotome guide  2 . The arrow  45  shows the direction of the osteotome  4  when its blade portion  6  engages the spacer/osteotome guide  2  after the spacer/osteotome guide has been inserted between the vertebrae as shown in FIG. 4B. The cutting force to drive the cutting blade assembly  4  is applied by way of the handle  8 , through the use of a mallet tapping against the end  15  of the handle portion  8  of the osteotome assembly  4  shown in FIG. 6A.  
         [0256]    As the hollow cutting edges or blades  6  of the osteotome cut into the adjacent vertebral end plates  21 ,  23 , the pieces of cut bone accumulate inside the hollow spaces  44  shown in FIG. 6B. The open ended design of the cutting blade  6  facilitates removal of the bone chips and later cleaning of the instrument  4 .  
         [0257]    The depth of cut of the cutting blade into the vertebral endplates is intended to be sufficient to remove the hard cortical bone of the endplates  21 ,  23  shown in FIG. 7A of the vertebral bodies so as to expose blood-rich, underlying cancellous bone. FIG. 7A shows, in a posterior view, the channels  50  that have been cut by the cutting blade assembly  4 . The objective of the cutting process is to expose a significant bed of bleeding bone while maintaining a sufficient portion of strong cortical endplate bone.  
         [0258]    The Implants  
         [0259]    Two implants are used between each the vertebral bodies being fused. Each one is to provide structural support and stabilization to a lumbar spinal motion segment subsequent to removal of protruding or deranged intervertebral disc material, and also to provide a substrate for new bone growth accompanying successful fusion of two adjacent vertebral body segments.  
         [0260]    Referring to FIGS. 3A through 3C, the implant  10  has fins  13  projecting inferiorly and superiorly from the central ⅓ of a wedge shaped block. When viewed from behind, as in FIG. 3C, the geometry of thc bone spacer  10  resembles a “cross”. When viewed from the side (FIG. 3B), the implant  10  is wedge shaped. Such that sides  7   a ,  7   b , including the fins  13 , diverge from the posterior side  10   b  to the anterior side  10   a  about a line of symmetry. The outermost finned surface has a series of locking teeth  11 , or grooves or projections, that aid in anchoring the implant and its fins that engage the channels  50  shown in FIG. 7A. More specifically, the sharp, tooth—like projections  11  are about 1 mm tall, which is adequate to penetrate exposed cancellous bone after the vertebral endplate cortices have been cut to accommodate the fin portion  13  of the implant  10  thereby increasing interface friction and minimizing the potential for translation after implantation.  
         [0261]    The importance of achieving good fit of a spacer  10  within the disc space is essential. When the fit is maximized the surface area of contact and resultant friction at the interface is maximized. Accordingly, for lordotic disc spaces the anterior height of the spacer device  10  is taller than the posterior height. The anterior region  10   a  has a slope or chamfer  9  to aid in initial insertion between the vertebrae. Alternatively, for parallel shaped disc spaces, a non-lordotic or parallel spacer (FIGS. 10C-10F) may provide a preferred fit.  
         [0262]    The implant  10  shown in FIGS. 3A through 3C, is cut from human donor bone which accounts for the curved faces  7   a ,  7   b  which are most evident in the top view shown in FIG. 3A. More specifically, the implant  10  is cut from donor cortical bone of the femur or tibia. FIG. 7B shows two inserted implants  10  in posterior view between the vertebral bodies. FIG. 7C is a side view of an installed implant  10  within an intervertebral space  20  between two vertebrae  12 ,  14 . The respective posterior—to—anterior angles α are shown in FIG. 3B, while the corresponding lordosis angles α are shown in FIG. 7C.  
         [0263]    In FIG. 3B, the faces, or sides,  7   a  and  7   b  of the implant  10  are shown to be tapered at an angle α that corresponds to half of the desired lordosis of the vertebra. The angle α of the tapered side, shown in FIG. 3B corresponds to the angle α of the spacer/osteotome guide  2  shown in FIG. 1B. In FIG. 3C, the implant  10  has opposing slots  7   c ,  7   d  on either side to facilitate gripping with a holding instrument (not shown). The preferred configuration of the implant  10  is approximately 20 mm to 25 mm long by 9 mm to 12 mm wide by 6 mm to 14 mm high, as measured on the posterior region.  
         [0264]    The Vertebral Body Spreader  
         [0265]    [0265]FIG. 8 shows in oblique view the vertebral body spreader tool  60  used to separate or distract two adjacent vertebral bodies prior to insertion of the spacers  2 . The spreader  60  enables sequential angular and translation distraction of the disc space from the posterior side of the spine.  
         [0266]    More specifically, the spreader device  60  consists of two jaws  62 ,  62 ′ (which get inserted into an intervertebral disc space) connected to arms  64 ,  64 ′ each having a catch mechanism  65 ,  65 ′ that engages the respective locking stops  68 ,  68 ′. The arms  64 ,  64 ′ and the contiguous jaws  62 , 62 ′ pivot about the hinge pins  61 , 61 ′, respectively, so as to provide angular motion of the jaws. The handle grips  66 ,  66 ′ operate to displace the parallel rails  70 ,  70 ′ by way of the crossing slider mechanism  76 , the operation of which is shown in FIGS. 9A through 9C. The crossing slider  76  consists of two arms  78 ,  79 , which pivot about a common pivot point  80  when the handles  66 ,  66 ′ are squeezed together. Two ends of the crossing slider mechanism  76  engage respectively the handles  66 ,  66 ′ at the respective pivot points  77 ,  77 ′, which also slide forward (toward the jaws  62 ,  62 ′) inside of slots  82 ,  82 ′ in the rails  70 ,  70 ′ (visible in FIG. 8). The crossing slide mechanism maintains the rails  70 ,  70 ′ parallel with one another as they separate from one another when the handles  66 ,  66 ′ are squeezed together (FIG. 9C).  
         [0267]    A design criterion of the vertebral body spreader  60  is to take into account a common characteristic of the degenerated painful disc, namely loss of disc height and loss of lordotic orientation. The goal is to restore natural height and angle to a collapsed disc space. Since the greatest degree of angular collapse is anterior, it is particularly difficult to lift the anterior portion of the disc space with a device that is applied from the posterior direction.  
         [0268]    Spreaders of the sort typically used in posterior operations make contact only the posterior wall of the vertebral body and therefore provide only posterior lift. A consequence of posterior lift is anterior settling, resulting in a flattening of the disc space beyond anatomical norms. The spreader device  60  in FIG. 8 overcomes these disadvantages by way of two cooperating mechanisms that allow the following sequence of events: insertion, followed by angular distraction and then translational distraction. Referring now to FIGS. 9A through  9 C, insertion of two opposed jaws  62 ,  62 ′ into the disc space  55  between two adjacent vertebral bodies  57 ,  59  is achieved when the device  60  is in the fully closed position, as shown in FIG. 9A. The length of the jaws  62 ,  62 ′ is 24 mm, which is sufficient to ensure that the jaws make good contact with the anterior portion of the disc space. The locking pivot arms  64 ,  64 ′ are then engaged into the position shown in FIG. 9B with the catches  65 ,  65 ′ seated in the respective locking stops  68 ,  68 ′, so as to angulate the jaws  62 ,  62 ′ at an approximately 10 degree angular distraction within the disc space  55 . With the jaws locked at 10 degrees, a second translational motion is brought about by means of the handles  66 ,  66 ′ which cause parallel translational spreading of the disc space, as shown in FIG. 9C. This dual action ensures the disc space can be maintained at about 10 degrees while achieving maximum disc height restoration. Flattening of the disc height is prevented by maintaining of the 10 degree jaw position while distracting.  
         [0269]    The spreader tool  60  may also be used to help facilitate insertion of the graft in the final stages of the operation. The dual action spreader  60  may be placed on the contralateral side of the disc space or directly adjacent to the graft if space is available, and it may be used to create additional lift and angulation, as required, to lessen the force required to insert the bone graft.  
         [0270]    Summary of Operational Sequence  
         [0271]    Referring to FIGS. 4A and 4B as well as FIGS. 7A through 7C, the installation of the implants  10  can be described in a general, summarizing way. First, the posterior faces of the vertebrae  12 ,  14  are exposed and then the vertebrae are forced apart to accommodate the insertion of the two spacer/osteotome guides  2 , as shown in FIG. 4B. FIG. 6B shows the relationship of the spacer/osteotome guide  2  and the osteotome  4  when the cut is made to create each channel  50  as shown in FIG. 7A. The implants  10  are then installed as shown in FIGS. 7B and 7C.  
         [0272]    More specifically, prior to implantation, the disc material is removed from the intervertebral space  20 , shown in FIG. 4A, exposing the cortical endplates  21 ,  23  of the adjacent vertebral bodies  12 ,  14 . The vertebral bodies are displaced from one another by use of the spreader tool  60  so that the intervertebral space  20  can receive the two rigid spacers/osteotome guides  2 , shown in FIG. 4B, which are placed, one at a time, contralaterally in relation to the spinal cord  16 . Each spacer/osteotome guide  2 , upon being installed into the intervertebral space  20 , maintains contact with the strong, cortical, endplate bone.  
         [0273]    After the spacers/osteotome guides  2  are in place, the endplates  21 ,  23  are further prepared by inserting the dual box osteotome  4 , FIG. 6A, that simultaneously removes from the respective top and bottom vertebral bodies ( 12 ,  14 , respectively) a 4 mm wide by 4 mm deep portion of each endplate and vertebral body bone. The resultant channels  50  (FIG. 7A) in the respective vertebral bodies  12 ,  14  are parallel to each other in a plane that is parallel to the sagittal plane. The resultant channels  50  define a placement axis for the finned implant device  10 .  
         [0274]    The wedge-shaped implant  10 , as shown in FIG. 3B, does not precisely match the geometry of the respective prepared bone slot in the endplates  21 ,  23 . More specifically, the channels or slots  50  are cut such that they are parallel, not dependent on the planes of the respective vertebral endplates  21 ,  23 . The maximum height of the fins on the anterior side  10   a  (FIG. 3B) is greater than the distance between the prepared slots  50 . (As measured across the parallel bottoms of the slots.) Engagement of the leading chamfer  9  (FIG. 3B) into the prepared channels  50  locates the implant parallel to the sagittal plane. Subsequent impacting of the implant  10  causes the geometry of the initially parallel bottoms of the slots or channels  50  to assume the respective angles α of the implant fins  13  (carrying the teeth  11 ). Distraction of adjacent vertebral bodies through impaction upon the implant  10  is possible because the adjacent vertebral bodies  12 ,  14  are non-constrained. Their relative positions are controlled primarily by soft tissue structures (not shown) that can be non-destructively stretched or altered. A consequence of full device impaction is translational and angular distraction of the disc space  20  so as, in the end, to yield the lordosis angle 2α, shown in FIG. 7C/D.  
         [0275]    The two fins  13  of each implant  10  are also slightly wider than the prepared channel  50  shown in FIG. 7A, creating a press fit when impacted. The tight fit achieves increased biomechanical stability and reduces the likelihood of migration of the implant after installation. Since the channels  50  in the vertebral endplates are only 4 mm wide, the endplate on either side of the channel retains its strength for good structural support.  
         [0276]    Embodiments  
         [0277]    [0277]FIGS. 10A through 10F show three embodiments of the spacer/osteotome guide. FIGS. 11A and 11B show two embodiments of the implant.  
         [0278]    Spacer/Osteotome Guide—First Embodiment  
         [0279]    [0279]FIGS. 10A and 10B show two views of the first embodiment of the spacer/osteotome guide  91 . One of the distinguishing characteristics of this embodiment is the angle α made by the top T and bottom B relative to the central axis A-A′, the angle α being half the desired angle of lordosis, which is about 10 degrees. The other distinguishing feature is the stop tabs  92 , located on the posterior end of the spacer block  91 . The stop tabs  92  prohibit the spacer from moving to deep into the intervertebral space during insertion or during the bone cutting process. The hole  93  receives a detachable handle, used during insertion and removal of the spacer.  
         [0280]    Spacer/Osteotome Guide—Second Embodiment  
         [0281]    [0281]FIGS. 10C and 10D show two views of the second embodiment of the spacer/esteotome guide  94 . Unlike the first embodiment above, the top T and bottom B of this embodiment are parallel, and no stop tabs are used. The hole  95  receives a detachable handle, used during insertion of the spacer and removal of the spacer. The blunt nose  94 ′, located on the anterior end, aids in the spacer insertion process.  
         [0282]    Spacer/Osteotome Guide—Third Embodiment  
         [0283]    [0283]FIGS. 10E and 10F show two views of the third embodiment of the spacer/osteotome guide  96 . The main body  97  of the spacer is characterized by having a top T and bottom B that are parallel one another. The spacer is further characterized by an additional box guide  99  that receives the osteotome during bone cutting. The box guide  98  also serves to restrain the spacer block  96  from moving too deep into the intervertebral space during the bone cutting process. The hole  99  receives a detachable handle, used during insertion and removal of the spacer. The blunt nose  96 ′, located on the anterior end of the spacer, serves to distract the respective vertebral bodies during the spacer insertion process.  
         [0284]    Implant—First Embodiment  
         [0285]    [0285]FIG. 11A is an orthogonal side view of the first embodiment of the implant  102 . The characterizing feature of this first embodiment is the angle α′ which each shelf  104 ,  104 ′ (there are two shelves on each side, the second set is out of view in the FIGURE) makes relative to the main axis B-B′. Each angle α′ is half the desired angle of lordosis, which is about 10 degrees. Chamfers  105 ,  105 ′, which aid in the insertion process, are located on the anterior end of the implant. The two fins  106 ,  106 ′, respectively at the top T and the bottom B and having locking teeth  108 , each make an angle α″ relative to the axis B-B′. The angle α″ is essentially equal to the angle α′, both being about half the angle of lordosis, or about 10 degrees.  
         [0286]    Implant—Second Embodiment  
         [0287]    [0287]FIG. 11B is an orthogonal side view of the second embodiment of the implant  110 . This second embodiment is characterized relative to the first embodiment in that the sets of teeth  111  located on the tops of the respective fins  112 ,  112 ′ are parallel to the main axis C-C′, whereas each shelf  114 ,  114 ′ (there are two shelves on each side, the second set is out of view in the FIGURE) makes an angle α′″ relative to the main axis C-C′. Each angle α′″ is half the desired angle of lordosis, which is about 10 degrees. Chamfers  115 ,  115 ′, located on the anterior end of the implant, aid in the insertion process.  
         [0288]    While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.