Abstract:
A spinal facet fusion implant includes an elongated main body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end. The main body has a cross-sectional profile characterized by a primary axis and a secondary axis. At least one stabilizer extends radially outwardly from the main body in the secondary axis. The main body has a length along the primary axis that is less than the combined width of the spinal facets making up a facet joint. The stabilizer has a width that is sized to make a press fit into the gap between the spinal facets making up a facet joint.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation-in-part of and claims benefit of pending prior U.S. non-provisional patent application Ser. No. 12/154,372, filed May 22, 2008 by Tov Vestgaarden for Method and Apparatus for Spinal Facet Fusion, which claims priority to U.S. provisional patent application No. 60/939,615, filed May 22, 2007 by the same inventor for Percutaneous Spinal Facet Fixation Device for Facet Fusion. This application also claims priority to, and is a non-provisional of pending U.S. provisional patent application No. 61/394,419, filed Oct. 19, 2010 by the same inventor for Open, Minimally Invasive, Percutaneous, Arthroscopic Spinal Facet Fusion Device and Delivery Method, all of which applications are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to surgical methods and apparatus in general, and more particularly to surgical methods and apparatus for fusing spinal facets. 
         [0004]    2. Description of the Related Art 
         [0005]    Disc herniation is a condition where a spinal disc bulges from between two vertebral bodies and impinges on adjacent nerves, thereby causing pain. The current standard of care for surgically treating disc herniation in patients who have chronic pain and who have (or are likely to develop) associated spinal instability is spinal fixation. Spinal fixation procedures are intended to relieve the impingement on the nerves by removing the portion of the disc and/or bone responsible for compressing the neural structures and destabilizing the spine. The excised disc or bone is replaced with one or more intervertebral implants, or spacers, placed between the adjacent vertebral bodies. 
         [0006]    In some cases, the spinal fixation leaves the affected spinal segment unstable. In this case, the spinal facets (i.e., the bony fins extending upwardly and downwardly from the rear of each vertebral body) can disengage with one another. The disengagement of the spinal facets can cause substantial pain to the patient. Furthermore, when left untreated, such disengagement of the spinal facets can result in the degeneration of the cartilage located between opposing facet surfaces, ultimately resulting in osteoarthritis, which can in turn lead to worsening pain for the patient. 
         [0007]    Thus, where the patient suffers from spinal instability, it can be helpful to stabilize the facet joints as well as the vertebral bodies. The facet joints are frequently stabilized by fusing the spinal facets in position relative to one another. 
         [0008]    In addition to providing stability, fusing the spinal facets can also be beneficial in other situations as well. By way of example but not limitation, osteoarthritis (a condition involving the degeneration, or wearing away, of the cartilage at the end of bones) frequently occurs in the facet joints. The prescribed treatment for osteoarthritis disorders depends on the location, severity and duration of the disorder. In some cases, non-operative procedures (including bed rest, medication, lifestyle modifications, exercise, physical therapy, chiropractic care and steroid injections) may be satisfactory treatment. However, in other cases, surgical intervention may be necessary. In cases where surgical intervention is prescribed, spinal facet fusion may be desirable. 
         [0009]    A minimally-invasive, percutaneous approach for fusing spinal facets was proposed by Stein et al. (“Stein”) in 1993. The Stein approach involved using a conical plug, made from cortical bone and disposed in a hole formed intermediate the spinal facet joint, to facilitate the fusing of opposing facet surfaces. However, the clinical success of this approach was limited. This is believed to be because the Stein approach did not adequately restrict facet motion. In particular, it is believed that movement of Stein&#39;s conical plug within its hole permitted unwanted facet movement to occur, thereby undermining facet fusion. Furthermore, the Stein approach also suffered from plug failure and plug migration. 
         [0010]    Thus there is a need for a new and improved approach for effecting spinal facet fusion. 
         [0011]    However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome. 
       SUMMARY OF THE INVENTION 
       [0012]    The long-standing but heretofore unfulfilled need for improved devices and methods for effecting spinal facet fusion is now met by a new, useful, and nonobvious invention. 
         [0013]    The novel method and apparatus for effecting spinal facet fusion includes a novel spinal facet fusion implant for disposition between opposing articular surfaces of a facet joint to immobilize the facet joint and facilitate fusion between the opposing facets. 
         [0014]    More particularly, in one form of the present invention, there is provided a spinal facet fusion implant that includes an elongated body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end. The elongated body has a cross-sectional profile characterized by a primary axis and a secondary axis; and at least one stabilizer extends radially outwardly from the elongated body in the secondary axis. 
         [0015]    The elongated body has a length along the primary axis that is less than the combined width of the spinal facets making up a facet joint. 
         [0016]    The at least one stabilizer has a width which is sized to make a press fit into the gap between the spinal facets making up a facet joint. 
         [0017]    A method for fusing a spinal facet joint includes the steps of providing a spinal facet fusion implant having an elongated body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end. The method further includes the steps of providing the elongated body with a cross-sectional profile characterized by a primary axis and a secondary axis and providing at least one stabilizer that extends radially outwardly from the elongated body in the secondary axis. 
         [0018]    The method steps further include the steps of forming the elongated body so that it has a length along the primary axis which is less than the combined width of the spinal facets making up a facet joint and forming the at least one stabilizer so that it has a width which is sized to make a press fit into the gap between the spinal facets making up a facet joint. 
         [0019]    Further method steps include the steps of deploying the spinal facet fusion implant in the facet joint so that the elongated body is simultaneously positioned within both of the facets of the facet joint and so that the at least one stabilizer is positioned within the gap between the spinal facets and maintaining the spinal facet fusion implant in such position while fusion occurs. 
         [0020]    In another embodiment, a spinal facet fusion implant includes an elongated body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end, the elongated body having a cross-sectional profile which is characterized by a primary axis and a secondary axis. 
         [0021]    The elongated body has a length along the primary axis which is less than the combined width of the spinal facets making up a facet joint and the cross-sectional profile is non-circular. 
         [0022]    In yet another embodiment, a method for fusing a spinal facet joint includes the steps of providing a spinal facet fusion implant having an elongated body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end, the elongated body having a cross-sectional profile which is characterized by a primary axis and a secondary axis and forming the elongated body so that it has a length along the primary axis which is less than the combined width of the spinal facets making up a facet joint and further providing a non-circular cross-sectional profile. 
         [0023]    Further steps include deploying the spinal facet fusion implant in the facet joint so that the elongated body is simultaneously positioned within both of the facets of the facet joint and maintaining the spinal facet fusion implant in such position while fusion occurs. 
         [0024]    In still another embodiment, a joint fusion implant includes an elongated body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end, the elongated body having a cross-sectional profile characterized by a primary axis and a secondary axis and at least one stabilizer extending radially outwardly from the elongated body in the secondary axis. 
         [0025]    The elongated body has a length along the primary axis which is less than the combined width of the bones making up the joint and the at least one stabilizer has a width which is sized to make a press fit into the gap between the bones making up the joint. 
         [0026]    In another embodiment, a method for fusing a joint includes the steps of providing a fusion implant that includes an elongated body having a distal end, a proximal end and a longitudinal axis extending between the distal end and the proximal end, the elongated body having a cross-sectional profile characterized by a primary axis and a secondary axis and at least one stabilizer extending radially outwardly from the elongated body in the secondary axis. 
         [0027]    Further steps include forming the elongated body so that it has a length along the primary axis which is less than the combined width of the bones making up the joint and forming the at least one stabilizer so that it has a width which is sized to make a press fit into the gap between the bones making up the joint. 
         [0028]    Still further steps include deploying the fusion implant in the joint so that the elongated body is simultaneously positioned within both of the bones of the joint and the at least one stabilizer is positioned within the gap between the bones and maintaining the fusion implant in such position while fusion occurs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
           [0030]      FIG. 1A  is a perspective view of a fusion implant having a stepped main body and fins; 
           [0031]      FIG. 1B  is a top plan view thereof; 
           [0032]      FIG. 1C  is a side elevational view thereof; 
           [0033]      FIG. 2A  is a perspective view of a fusion implant main body having fins and a greater thickness on the distal end relative to the proximal end; 
           [0034]      FIG. 2B  is a top plan view thereof; 
           [0035]      FIG. 2C  is a side elevational view thereof; 
           [0036]      FIG. 3A  is a perspective view of a fusion implant having a main body with bulbous parts and fins; 
           [0037]      FIG. 3B  is a top plan view thereof; 
           [0038]      FIG. 3C  is an end elevational view thereof; 
           [0039]      FIG. 4A  is a perspective view of an implant having a hex shape and at least one stabilizing fin; 
           [0040]      FIG. 4B  is an end elevational view thereof; 
           [0041]      FIG. 4C  is a top plan view thereof; 
           [0042]      FIG. 4D  is a side elevational view thereof; 
           [0043]      FIG. 5A  is a perspective view of an implant having a hex shape and no fins; 
           [0044]      FIG. 5B  is a side elevational view thereof; 
           [0045]      FIG. 5C  is a top plan view thereof; 
           [0046]      FIG. 6A  is a perspective view of an implant having a polygonal shape in transverse section and at least one stabilizing fin; 
           [0047]      FIG. 6B  is a side elevational view thereof; 
           [0048]      FIG. 6C  is a top plan view thereof; 
           [0049]      FIG. 6D  is an end elevational view thereof; 
           [0050]      FIG. 7A  is a perspective view of an implant having an octagonal main body and no fins; 
           [0051]      FIG. 7B  is a side elevational view thereof; 
           [0052]      FIG. 8A  is a diagrammatic top plan view of a superior and an inferior facet joint and a drilled bore or cavity formed in said facet joints, said cavity receiving a tapered implant; 
           [0053]      FIG. 8B  is a diagrammatic perspective view of said facet joints and said cavity; 
           [0054]      FIG. 8C  is a diagrammatic front view of said facet joints and said cavity; 
           [0055]      FIG. 8D  is a perspective view of a superior and inferior facet joint; 
           [0056]      FIG. 9  is a perspective view of the novel implant loading block; 
           [0057]      FIG. 10  is a perspective view of the novel implant holder; 
           [0058]      FIG. 11  is a perspective view of the novel directional cannula; 
           [0059]      FIG. 12  is a perspective view of the novel facet distractor; 
           [0060]      FIG. 13  is a perspective view of the novel guide pin; 
           [0061]      FIG. 14A  is a first perspective view of the drill guide and blade; 
           [0062]      FIG. 14B  is a side elevational view thereof; 
           [0063]      FIG. 14C  is a top plan view thereof; 
           [0064]      FIG. 14D  is a second perspective view thereof; 
           [0065]      FIG. 15A  is a perspective view of a tapping cap; 
           [0066]      FIG. 15B  is a perspective view thereof as in  FIG. 15A  and further including dotted lines to indicate hidden structure; 
           [0067]      FIG. 15C  is a longitudinal sectional view of the structure depicted in  FIGS. 15A and 15B ; 
           [0068]      FIG. 16A  is a perspective view of the one step facet distractor and implant holder with the facet distractor ensleeved within the lumen of the implant holder; 
           [0069]      FIG. 16B  is an enlarged view of the distal end of the facet distractor and implant holder of  FIG. 16A ; 
           [0070]      FIG. 17  is a perspective view of the one step sleeve or implant tamp; 
           [0071]      FIG. 18  is a perspective view of the one step handle; 
           [0072]      FIG. 19  is an exploded perspective view of the one step assembly with implant before the implant is loaded onto the facet distractor; 
           [0073]      FIG. 20A  is a perspective view of the one step assembly with the implant loaded onto the implant holder; 
           [0074]      FIG. 20B  is a longitudinal sectional view of the structure depicted in  FIG. 20A ; 
           [0075]      FIG. 20C  is a top plan view of the structure depicted in  FIG. 20A ; 
           [0076]      FIG. 20D  is a perspective view of the one step assembly with the sleeve and implant in the final or ejected position; 
           [0077]      FIG. 20E  is a longitudinal sectional view of the structure depicted in  FIG. 20D ; 
           [0078]      FIG. 21A  is a perspective view of the drill guide with blade on tip to stabilize the drill bit in the joint; 
           [0079]      FIG. 21B  is a top plan view thereof; 
           [0080]      FIG. 21C  is a side elevational view thereof; 
           [0081]      FIG. 22A  is a second embodiment of the implant tamp or implant positioner where the shaft is shaped like the main body of the implant and the shaft is cannulated to allow injection of bone growth stimulation product; 
           [0082]      FIG. 22B  is an end view of the structure depicted in  FIG. 22A ; 
           [0083]      FIG. 23A  is a perspective view of a second embodiment of the directional cannula; 
           [0084]      FIG. 23B  is a transverse cross-sectional view of said second embodiment of said directional cannula; 
           [0085]      FIG. 24  is a perspective view of a second embodiment of the facet distractor; 
           [0086]      FIG. 25  is a perspective view of a drill bit; and 
           [0087]      FIG. 26  is a perspective view of a cavity cutter. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0088]    Referring now to  FIGS. 1A-C , the novel spinal facet fusion implant is denoted  10  as a whole. Fusion implant  10  includes main body  12  and at least one stabilizer fin  14 . The illustrated embodiment includes first stabilizer fin  14   a  and second stabilizer fin  14   b.    
         [0089]    Body  10  is an elongated element having structural integrity. The distal end of main body  12  and the distal end of stabilizers  14   a ,  14   b  are chamfered as at  16  to facilitate insertion of fusion implant  10  into the facet joint as disclosed hereinafter. Body  12  preferably has a rounded rectangular cross-section, an ovoid cross-section, a laterally-extended cross-section, or some other non-round cross-section to inhibit rotation of main body  12  about a longitudinal center axis. 
         [0090]    Fusion implant  10  is intended to be inserted into a facet joint using a posterior approach. The posterior approach is familiar to spine surgeons, thereby providing an increased level of comfort for the surgeon, and also minimizing the possibility of damage to the spinal cord during fusion implant insertion. 
         [0091]    Stabilizer fins  14   a ,  14   b  are received in a gap located between opposing facet surfaces to prevent rotation of fusion implant  10  within the facet joint. Stabilizers  14   a  is formed in and extends along the upper surface of main body  12  and stabilizer  14   b  is formed in and extends along the lower surface of main body  12 . Stabilizers  14   a ,  14   b  preferably have a width just slightly larger than the gap between the opposing articular surfaces of a facet joint so that the stabilizers fit snugly therebetween. 
         [0092]    The distal end  12   a  of main body  12   a  has a greater thickness than proximal end  12   b  of said main body, there being transversely disposed step  12   c  therebetween. The greater thickness of said distal end supports the load for a long period of time. If said distal end  12   a  of main body  12  is eventually crushed, it becomes flush with proximal end  12   b  and fusion implant  10  continues to perform its function. 
         [0093]    The embodiment of  FIGS. 2A-C  has an inverse taper formed in main body  12  and in stabilizer fins  14   a ,  14   b  to prevent migration of implant  10 . As perhaps best understood in connection with  FIG. 2B , fin  14   a  is wider at its distal end than at its proximal end; fin  14   b  has the same structure. This wedge shape prevents distal-to-proximal travel of implant  10 . This eliminates the need for teeth that perform the same function. 
         [0094]    The embodiment of  FIG. 3A  differs from the embodiment of  FIGS. 2A-C  in that main body  12  is bulbous on its left and right sides as depicted. It is sometimes referred to as a figure eight main body in view of said bulbosities. The bulbosities are denoted  12   c  and  12   d . They serve the same function as raised area  12   a  in the embodiment of  FIGS. 1A-C  in that if they are crushed over time until they are flush with the non-bulbous central region of main body  12 , said main body will still remain firmly and functionally positioned in the facet joint. Without the raised area or the bulbosities, crushing of main body  12  over time would loosen it relative to its facet joint. 
         [0095]      FIGS. 4A-D  depict an embodiment characterized by main body  12  that is hexagonal in transverse section as depicted. Stabilizer fins  14 ,  14   b  may also be shorter in radial extent in this embodiment. This shape helps prevent rotation of implant  10 . 
         [0096]    The embodiment of  FIGS. 5A-C  differs from the embodiment of  FIGS. 4A-D  in that the embodiment of  FIGS. 5A-C  is not provided with stabilizer fins  14   a ,  14   b.    
         [0097]      FIGS. 6A-D  depict an embodiment of implant  10  having a polygonal main body  12  and stabilizer fins  14   a ,  14   b  of truncate radial extent. 
         [0098]    The embodiment of  FIGS. 7A-B  has main body  12  of polygonal configuration and no stabilizer fins. 
         [0099]    Referring now to  FIGS. 8A-D , an instrument is first used to determine the vertical plane  18  of the facet joint. Identifying the vertical plane of the facet joint is important because said plane is used to identify the proper position for cavity  20  which is to be formed in the facet joint to receive fusion implant  10 . The superior facet is denoted  22   a  in  FIGS. 8-D  and the inferior facet is denoted  22   b . The inverted tapered cavity depicted in  FIGS. 8A-D  is intended for use with the inverted tapered implant of  FIGS. 2A-C . 
         [0100]    A disclosure of the novel tools used with implant  10  follows. 
         [0101]    Implant loading block  24  having bores  24   a ,  24   b  for slideably receiving implants  10  is depicted in  FIG. 9 . 
         [0102]    Implant holder  26  is depicted in  FIG. 10 . Leading end  28  includes a plurality of flexible arms  28  that engage an implant  10  to lift it from bore  24   a  or  24   b  of implant loading block  24 . Implant holder  26  does not have alignment pins. It has flats that align inside directional cannula  30 . Drill guide  36 , disclosed hereinafter, also has such flats. 
         [0103]    Directional cannula  30  having diametrically opposed arms  30   a ,  30   b  at its leading or distal end is depicted in  FIG. 11 . Arms  30   a ,  30   b  maintain the direction of the joint to guide the other instruments, and also maintain the distraction of the joint. 
         [0104]      FIG. 12  depicts facet distractor  32  having leading end  32   a  adapted to engage into the facet joint to find the direction of the plane of the facet joint. 
         [0105]    Guide pin  34  is depicted in  FIG. 13 . Its use is optional. 
         [0106]      FIGS. 14A-D  depict drill guide  36  having blade  36   a , positive stop  36   b , and alignment flats  36   c .  FIG. 14A  is a first perspective view,  FIG. 14B  provides a side elevational view,  FIG. 14C  provides a top plan view and  FIG. 14D  provides a second perspective view. Drill guide  36  stabilizes the drill bit during the drilling procedure. 
         [0107]      FIGS. 15A-C  respectively depict tapping cap  38  in perspective, perspective with dotted lines to indicate hidden structure, and in longitudinal section to also reveal hidden structure. Distal bore  38   a  of tapping cap  38  is used to tap directional cannula  30  into its functional position and proximal bore  38   c  is used to tap facet distractor  32  into its functional positional. The diameter of distal bore  38   a  reduces down to medial bore  38   b  and proximal bore  38   c  has the same diameter as distal bore  38   a . Medial bore  38   b  allows guide pin  34  to slide through. 
         [0108]      FIGS. 16A and 16B  depict one step facet distractor  32  when it is received within the lumen of implant holder  40  which is a second embodiment of implant holder  26 .  FIG. 16B  depicts the tip of facet distractor  32  and implant holder  40  in enlarged detail. Implant Holder  40  does not need alignment pins  40   a  because the orientation is fixed relative to facet distractor  32 . 
         [0109]      FIG. 17  depicts implant tamp  44  that is used to drive a hollow implant  10  into its functional position. Slots  44   a  allow implant tamp  44  to slide a predetermined distance as disclosed hereinafter 
         [0110]      FIG. 18  depicts one step holder  46  having openings collectively denoted  46   a  that holds the complete instrument assembly while implant  10  is being tapped into its functional position 
         [0111]    The four pins, collectively denoted  41  in  FIG. 16A , are used in the assembly of implant tamp  44  and handle  46 . The instrument as assembled includes facet distractor  32  which is ensleeved within the lumen of implant holder  26 , which is in turn ensleeved within the lumen of implant tamp  44 , which is in turn ensleeved within the lumen of handle  46 . More particularly, pins  26   a  extend sequentially through their associated slots  44   a  and into their associated opening  46   a  formed in handle  46 . 
         [0112]      FIG. 19  depicts facet distractor  32 , implant holder  26 , implant tamp  44  and handle  46  in their assembled configuration. 
         [0113]      FIG. 20A  is similar to  FIG. 19  but it depicts implant  10  engaged to the distal end of implant holder  26 . 
         [0114]      FIG. 20B  is a longitudinal sectional view of the structure depicted in  FIG. 20A . 
         [0115]      FIG. 20C  depicts implant tamp  44  flush with handle  46 . Implant  10  is depicted ejected over facet distractor  32 . 
         [0116]      FIG. 20D  is a longitudinal sectional view of the structure depicted in  FIG. 20E ; 
         [0117]      FIGS. 21A-C  depict an alternative embodiment of drill guide  36  depicted in  FIGS. 14A-D . This embodiment is denoted  48  and has blade  48   a . Two opposed alignment pins  50   a ,  50   b  are formed in drill guide  48  near handle  50 . The alignment pins allow insertion into directional cannula  30  at zero degrees (0°) or one hundred eighty degrees) (180°). 
         [0118]      FIG. 22A  depicts a second embodiment, denoted  52 , of implant tamp  44 . The shaft of implant tamp  52  shaft conforms to main body  12  and said shaft is cannulated as at  52   a  to enable injection of growth stimulation product. The handle of implant tamp  52  is denoted  54 . 
         [0119]      FIG. 22B  provides an end view of the structure depicted in  FIG. 22A . 
         [0120]    A second embodiment of directional cannula  30  is depicted in  FIG. 23  and is denoted  54 . Transversely opposed distraction blades  54   a ,  54   b  are formed in its distal end and handle  56  is mounted thereto at its proximal end. The transverse cross-sectional shape of directional cannula  30  matches the transverse cross-sectional shape of implant  10 . 
         [0121]    A second embodiment of facet distractor  32  is depicted in  FIG. 24  and is denoted  56 . It includes blade  56   a  and positive stop  56   b.    
         [0122]    Drill bit  58  having positive stop  58   a  is depicted in  FIG. 25 . 
         [0123]    At least one of the instruments includes a directional feature that is used to maintain the alignment of the instrumentation with vertical plane  18  of the facet joint. By way of example but not limitation, directional cannula  30  may include a flat portion and the remaining instruments may include a flat portion on an opposite portion of the instrument so that the instruments may only be inserted through said directional cannula at zero degrees (0°) or one hundred eighty degrees)(180°). 
         [0124]    After the proper position for cavity  20  has been identified, a drill (or reamer, punch, dremel, router, burr, etc.) is used to form cavity  20  in the facet joint. Cavity  20  is formed across vertical plane  18  so that substantially one-half of cavity  20  is formed in a first facet  22   a , and substantially one-half is formed in opposing facet  22   b.    
         [0125]    After cavity  20  has been formed in (or, perhaps more literally, across) the facet joint, fusion implant  10  is inserted into cavity  20  as perhaps best understood in connection with  FIG. 8D . More particularly, fusion implant  10  is inserted into cavity  20  so that main body  12  spans the gap between opposing facets  22   a ,  22   b , and so that stabilizers  14   a ,  14   b  extend between the opposing facet surfaces. Preferably, fusion implant  10  is slightly oversized relative to cavity  20  to create a press fit. 
         [0126]    Fusion implant  10  provides the stability and strength needed to immobilize the facet joint while fusion occurs. Due to the positioning of stabilizers  14   a ,  14   b  between the opposing facet surfaces, and due to the non-circular cross-section of main body  12 , fusion implant  10  will be held against rotation within cavity  20 , which will in turn hold facets  22   a ,  22   b  stable relative to one another. 
         [0127]    When a hollow fusion implant is used, and where the implant is formed of a sufficiently strong and rigid material, cavity  20  need not be pre-formed in the opposing facets. The hollow fusion implant can be simply tapped into place, in much the same manner that a punch is used. 
         [0128]    The novel structure provides a new and improved fusion implant for facilitating facet fusion. This novel fusion implant withstands greater forces, prohibits motion in all directions and substantially reduces the risk of implant failure. The new fusion implant also eliminates the possibility of slippage during spinal motion, greatly improves facet stability and promotes better facet fusion. 
         [0129]    It should be appreciated that the new fusion implant combines two unique “shapes” in one implant (i.e., the shape of main body  12  and the shape of stabilizers  14   a ,  14   b ) in order to limit motion in a multidirectional joint. More particularly, the shape of main body  12  limits motion (e.g., in flexion/extension for the lumbar facets and in axial rotation for the cervical facets), while the shape of stabilizers  14   a ,  14   b  (i.e., the “keel”) rests between two bony structures (i.e., in the gap of the facet joint) and limits lateral bending. This novel construction eliminates the possibility of eccentric forces inducing motion in the facet joint. 
         [0130]    Moreover, although the novel structure effectively stabilizes the joint, it still allows the “micro motion” which is required for the fusion process to begin. 
         [0131]    It should be appreciated that the novel fusion implant may be manufactured in a wide range of different sizes in order to accommodate any size of facet joint. Furthermore, the scale and aspect ratio of main body  12 , stabilizers  14   a ,  14   b , may be varied without departing from the scope of the present invention. Additionally, the novel fusion implant may be constructed out of any substantially biocompatible material which has properties consistent with the present invention including, but not limited to, allograft, autograft, synthetic bone, simulated bone material, biocomposites, ceramics, PEEK, stainless steel and titanium. Thus, the novel structure permits a surgeon to select a fusion implant having the appropriate size and composition for a given facet fusion. 
         [0132]    Detailed Surgical Technique (Solid Fusion Implant) 
         [0133]    A preferred surgical technique for using a solid fusion implant  10  will now be disclosed. The preferred surgical technique preferably uses guide pin  34  ( FIG. 13 ) facet distractor  32  ( FIG. 12 ), directional cannula  30  ( FIG. 11 ), drill guide  36  ( FIGS. 14A-D ), implant loading block  24  ( FIG. 9 ), implant holder  26  ( FIG. 10 ) implant tamp  44  ( FIG. 17 ), and tapping cap  38  ( FIGS. 15A-C ). 
         [0134]    First, the facet joint is localized indirectly by fluoroscopy, or directly by visualization during an open procedure. Next, guide pin  34  ( FIG. 13 ) is inserted into the gap between the opposing facet surfaces. The position of guide pin  34  is verified by viewing the coronal and sagittal planes. Then guide pin  34  is lightly tapped to insert the guide pin approximately five millimeters (5 mm) into the facet joint, along vertical plane  18 . The inferior facet is curved medially and helps prevent guide pin  34  from damaging nerve structures. 
         [0135]    Cannulated facet distractor  32  is then slid over guide pin  34  so that it is aligned with the vertical plane of the facet joint. Then facet distractor  32  is lightly tapped into the facet joint, along vertical plane  18 . 
         [0136]    Next, directional cannula  30  is placed over facet distractor  32  (Fig. ?) and the tip of directional cannula  30  is pushed into the facet joint (Fig. ?). Once the tip of directional cannula  30  has entered the facet joint, the directional cannula is lightly tapped so as to seat the cannula in the facet joint. This aligns directional cannula  30  with the vertical plane of the facet joint. After verifying that directional cannula  30  has been inserted all the way into the facet joint and is stabilized in the joint, facet distractor  32  is removed. 
         [0137]    Drill guide  36  is then inserted into directional cannula  30 . Drill guide  36  is advanced within directional cannula  30  until a drill guide stop is resting on directional cannula  30 . Then, with drill guide  36  in place, irrigation (e.g., a few drops of saline) is placed into drill guide. Next, drill bit  58  is used to drill a cavity  20 . This is done by drilling until drill bit  58  reaches the mechanical stop on drill guide  36  ( FIGS. 21A-B ). Drill guide  36  and drill bit  58  are then pulled out of directional cannula  30 , drill guide  36  is rotated 180 degrees, and drill guide  36  is reinserted into directional cannula  30  in order to drill the superior facet. With drill guide  36  in place, irrigation (e.g., a few drops of saline) is placed into said drill guide, and drill bit  58  is used to drill cavity  20  in the superior facet. Again, drilling occurs until drill bit  58  reaches the mechanical stop on drill guide  36 . Drill bit  58  is then removed. 
         [0138]    Cavity cutter  60 , depicted in  FIG. 26 , may replace drill guide  36  and drill bit  58  to make an opening having the perfect shape for fusion implant  10 . 
         [0139]    Using implant loading block  24  depicted in  FIG. 9 , fusion implant  10  is then inserted into implant holder  26 . Implant holder  26 , with fusion implant  10  in place, is then placed into directional cannula  30 . Next, implant holder  26  is lightly tapped so as to insert fusion implant  10  into cavity  20  created in the facet joint. Once the implant has been positioned in cavity  20 , implant tamp  44  is inserted into implant holder  26 . Next, implant tamp  44  is lightly tapped so as to drive the implant into cavity  20 . The implant is preferably countersunk 1-2 mm into the facet joint. 
         [0140]    Implant tamp  44 , implant holder  26  and directional cannula  30  are removed from the surgical site and the incision is closed to conclude the procedure. 
         [0141]    Detailed Surgical Technique (Hollow Fusion Implant) 
         [0142]    A preferred surgical technique for using a hollow fusion implant  10  will now be disclosed. The preferred surgical technique preferably uses guide pin  34  (optional) ( FIG. 13 ), one step facet distractor and implant holder  40  ( FIG. 16A ), implant tamp  44  ( FIG. 17 ), and handle  46  ( FIG. 17 ). 
         [0143]    First, the facet joint is localized indirectly by fluoroscopy or directly by visualization during an open procedure. The following step involving use of guide pin  34  is entirely optional. If used, guide pin  34  is inserted in the gap between the opposing facet surfaces. The position of guide pin  34  is verified by viewing the coronal and sagittal planes. Guide pin  34  is then lightly tapped so as to insert said guide pin approximately five millimeters (5 mm) into the facet joint, along the vertical plane of the facet joint. The inferior facet curves medially and helps prevent the guide pin from damaging nerve structures. 
         [0144]    One step facet distractor with implant holder  40 , which may be cannulated or not cannulated, is then slid over guide pin  34 , if used, so that it is aligned with the vertical plane of the facet joint. Facet distractor  32  is lightly tapped into the facet joint, along the vertical plane of the facet joint. This step may be accomplished without use of guide pin  34 . 
         [0145]    Next, facet distractor with implant holder  40 , implant tamp  44 , handle  46  assembly, with hollow fusion implant  10  mounted thereto ( FIG. 20A ) is pushed, hammered, or otherwise advanced downwards to drive hollow fusion implant  10  into the facet joint. 
         [0146]    Finally, the facet distractor/implant tamp assembly is removed, leaving hollow fusion implant  10  in the facet joint, and the incision is closed. 
         [0147]    The following procedure applies to both solid or hollow implants. 
         [0148]    Performing posterior facet fusion with the novel tools is a nine step procedure. 
         [0149]    In the first step, the facet joint is localized either indirectly using fluoroscopy or directly by visualization during an open procedure. Facet distractor  56  is then inserted into the plane of the facet joint. Placement is verified by viewing in the coronal and sagittal plane. The inferior facet curves medially and should prevent guide pin  34  from being advanced into nerve structures. Tapping cap  38  is then tapped lightly so that blade  56   a  of facet distractor  56   a  enters into the plane of the facet joint. If necessary, the shallow end  38   c  of the tapping cap can be used to seat the facet distractor. Positive stop  56   b  is formed in facet distractor  56  to prevent it from being advanced into the nerve structures. 
         [0150]    In step three, directional cannula  54  is placed over facet distractor  56 . Tip  56   a  of facet distractor  56  is aligned with tips  54   a ,  54   b  of directional cannula  54  and is lightly pushed into the facet joint. After tips  54   a ,  54   b  have entered into the facet joint, directional cannula  54  is lightly tapped to fully seat it. If necessary, the deep end  38   a  of tapping cap  38   a  can be used to seat the directional cannula. 
         [0151]    The insertion of directional cannula  54  all the way into the facet joint is then verified. Facet distractor  56  is removed after such positioning is verified. 
         [0152]    In step four, drill guide  36  is inserted into the lumen of directional cannula  54 , aligning pins  50   a ,  50   b  into slots  54   a ,  54   b  formed in directional cannula  54 . The insertion continues until drill guide positive stop  36   b  abuts directional cannula  54  and blade  48   a  is in the facet joint. 
         [0153]    Step five is the drilling of the inferior facet. With drill guide  36  in place upon the completion of step four. Cavity  20  is then drilled by drill bit  58  into the inferior facet. Drilling continues until drill bit  58  abuts positive stop  58   a . Drill guide  36  is held down when drill bit  58  is removed and said drill bit is not removed until it has stopped rotating. 
         [0154]    Drill guide  36  and drill bit  58  are pulled from directional cannula  54  in step six and it is cleaned to remove tissue. It is then rotated one hundred eighty degrees (180°) and re-inserted into directional cannula  30 . 
         [0155]    Cavity  20  is drilled into the superior facet in step seven. A few drops of irrigation (saline) are placed into the drill guide. Said cavity is then drilled by drill bit  58  into the superior facet. Drilling continues until drill bit  58  abuts positive stop  58   a . Drill guide  36  is held down when drill bit  58  is removed and said drill bit is not removed until it has stopped rotating. 
         [0156]    In step eight, an implant is loaded into directional cannula  54  with the chamfer  16  pointed downward. Implant tamp  52  is inserted into the lumen of directional cannula  54 . Implant tamp  52  is lightly tapped until it reaches positive stop  52   b  to fully seat implant  10  in cavity  20 . Implant tamp  52  and directional cannula  54  are then removed. 
         [0157]    Numerous advantages are achieved by the present invention. Among other things, the present invention provides a fast, simple, minimally-invasive and easily reproduced approach for effecting facet fusion. 
         [0158]    While fusion implant  10  has been disclosed in the context of fusing a facet joint, it should also be appreciated that fusion implant  10  may be used to stabilize and fuse any joint having anatomy similar to the facet joint, i.e., a pair of opposing bony surfaces defining a gap therebetween, with the stabilizer of the fusion implant being sized to be positioned within the gap. By way of example but not limitation, the fusion implant may be used in small joints such as the fingers, toes, etc. 
         [0159]    Many additional changes in the details, materials, steps and arrangements of parts, which have been herein disclosed in order to explain the nature of the present invention, may be made by those skilled in the art while still remaining within the principles and scope of the invention. 
         [0160]    It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.