Patent Publication Number: US-2020281738-A1

Title: Method and apparatus for spinal facet fusion

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
     This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 62/353,809, filed Jun. 23, 2016 by VGI Medical, LLC and Tov Vestgaarden et al. for METHOD AND APPARATUS FOR SPINAL FACET FUSION (Attorney&#39;s Docket No. VG-5 PROV), which patent application is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to surgical methods and apparatus in general, and more particularly to surgical methods and apparatus for fusing spinal facets. 
     BACKGROUND OF THE INVENTION 
     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. 
     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 misengage with one another. The misengagement of the spinal facets can result in substantial pain to the patient. Furthermore, when left untreated, such misengagement 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. 
     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. 
     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. 
     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. 
     Thus there is a need for a new and improved approach for effecting spinal facet fusion. 
     In addition to the foregoing, it should be appreciated that the spine comprises various regions having differing characteristics. More particularly, the first seven vertebrae (C1-C7) are the so-called cervical vertebrae, the next twelve vertebrae (T1-T12) are the so-called thoracic vertebrae, and the next five vertebrae (L1-L5) are the so-called lumbar vertebrae. Beneath the lumbar vertebrae are the five fused vertebrae of the sacrum, and then the four fused vertebrae of the coccyx (or tailbone). The facet joints in the cervical vertebrae can differ somewhat from the facet joints in the thoracic vertebrae and lumbar vertebrae, e.g., the facet joints in the cervical vertebrae are typically oriented generally horizontal to the longitudinal axis of the spine, whereas the facet joints of the thoracic vertebrae and the facet joints of the lumbar vertebrae are typically oriented generally vertical to the longitudinal axis of the spine. It has been found that it can be significantly more difficult to successfully effect spinal facet fusion in the cervical vertebrae than in the thoracic vertebrae and in the lumbar vertebrae. 
     Thus there is a need for a new and improved approach for effecting spinal facet fusion in the cervical vertebrae. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel method and apparatus for effecting spinal facet fusion, and is particularly advantageous for use in effecting spinal facet fusion in the cervical vertebrae. More particularly, the present invention comprises the provision and use of a novel spinal facet fusion implant (sometimes hereinafter referred to as a “novel fusion implant”) for disposition between the opposing articular surfaces of a facet joint, including the facet joint of a cervical vertebrae, whereby to immobilize the facet joint and facilitate fusion between the opposing facets. The present invention also comprises the provision and use of novel instrumentation for installing the novel spinal facet fusion implant in a facet joint. 
     In one preferred form of the invention, there is provided a spinal facet fusion implant comprising: 
     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 being characterized by a superior body surface and an inferior body surface; 
     a superior stabilizer extending outwardly from the superior body surface, the superior stabilizer being characterized by a superior stabilizer surface; and 
     an inferior stabilizer extending outwardly from the inferior body surface, the inferior stabilizer being characterized by an inferior stabilizer surface; 
     wherein (i) the superior body surface and the inferior body surface are tapered relative to one another, and/or (ii) the superior stabilizer surface and the inferior stabilizer surface are tapered relative to one another. 
     In another preferred form of the invention, there is provided a system for effecting spinal facet fusion, the system comprising: 
     a spinal facet fusion implant comprising:
         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 being characterized by a superior body surface and an inferior body surface;   a superior stabilizer extending outwardly from the superior body surface, the superior stabilizer being characterized by a superior stabilizer surface; and   an inferior stabilizer extending outwardly from the inferior body surface, the inferior stabilizer being characterized by an inferior stabilizer surface;   wherein (i) the superior body surface and the inferior body surface are tapered relative to one another, and/or (ii) the superior stabilizer surface and the inferior stabilizer surface are tapered relative to one another;       

     a drill guide/cannula for preparing the anatomy to receive the spinal facet fusion implant and for delivering the spinal facet fusion implant to the anatomy, the drill guide/cannula comprising:
         a body having a distal end, a proximal end and a longitudinal axis extending therebetween, the distal end of the body being configured for engaging the gap between a descending facet of a first vertebra and an ascending facet of a second vertebra;   a central lumen extending between the distal end of the body and the proximal end of the body, the central lumen having a cross-sectional profile which matches the cross-sectional profile of the spinal facet fusion implant such that the spinal facet fusion implant can be introduced into the proximal end of the central lumen, advanced distally along the central lumen, and advanced distally out of the distal end of the central lumen and into the gap between the descending facet of the first vertebra and the ascending facet of the second vertebra;   a first drill guide angled relative to the longitudinal axis of the central lumen, the first drill guide being configured to receive a drill therein so as to drill a first seat in the descending facet of the first vertebra;   a second drill guide angled relative to the longitudinal axis of the central lumen, the second drill guide being configured to receive a drill therein so as to drill a second seat in the ascending facet of the second vertebra;   wherein the first seat in the descending facet of the first vertebra is sized and angled so as to receive the superior stabilizer of the spinal facet fusion implant when the spinal facet fusion implant is advanced into the gap between the first vertebra and the second vertebra; and   wherein the second seat in the ascending facet of the second vertebra is sized and angled so as to receive the inferior stabilizer of the spinal facet fusion implant when the spinal facet fusion implant is advanced into the gap between the first vertebra and the second vertebra.       

     In another preferred form of the invention, there is provided a system for effecting spinal facet fusion, the system comprising:
         a spinal facet fusion implant, the spinal facet fusion implant comprising a taper; and       

     a drill guide/cannula for preparing the anatomy to receive the spinal facet fusion implant and for delivering the spinal facet fusion implant to the anatomy, the drill guide/cannula comprising:
         a body having a distal end, a proximal end and a longitudinal axis extending therebetween, the distal end of the body being configured for engaging the gap between a descending facet of a first vertebra and an ascending facet of a second vertebra;   a central lumen extending between the distal end of the body and the proximal end of the body, the central lumen having a cross-sectional profile which matches the cross-sectional profile of the spinal facet fusion implant such that the spinal facet fusion implant can be introduced into the proximal end of the central lumen, advanced distally along the central lumen, and advanced distally out of the distal end of the central lumen and into the gap between the descending facet of the first vertebra and the ascending facet of the second vertebra;   a first drill guide angled relative to the longitudinal axis of the central lumen, the first drill guide being configured to receive a drill therein so as to drill a first seat in the descending facet of the first vertebra;   a second drill guide angled relative to the longitudinal axis of the central lumen, the second drill guide being configured to receive a drill therein so as to drill a second seat in the ascending facet of the second vertebra;   wherein the first seat in the descending facet of the first vertebra and the second seat in the ascending facet of the second vertebra are sized and angled so as to receive the spinal facet fusion implant therein when the spinal facet fusion implant is advanced into the gap between the first vertebra and the second vertebra.       

     In another preferred form of the invention, there is provided a method for effecting spinal facet fusion, the method comprising: 
     providing a spinal facet fusion implant comprising:
         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 being characterized by a superior body surface and an inferior body surface;   a superior stabilizer extending outwardly from the superior body surface, the superior stabilizer being characterized by a superior stabilizer surface; and   an inferior stabilizer extending outwardly from the inferior body surface, the inferior stabilizer being characterized by an inferior stabilizer surface;   wherein (i) the superior body surface and the inferior body surface are tapered relative to one another, and/or (ii) the superior stabilizer surface and the inferior stabilizer surface are tapered relative to one another;       

     providing a drill guide/cannula for preparing the anatomy to receive the spinal facet fusion implant and for delivering the spinal facet fusion implant to the anatomy, the drill guide/cannula comprising:
         a body having a distal end, a proximal end and a longitudinal axis extending therebetween, the distal end of the body being configured for engaging the gap between a descending facet of a first vertebra and an ascending facet of a second vertebra;   a central lumen extending between the distal end of the body and the proximal end of the body, the central lumen having a cross-sectional profile which matches the cross-sectional profile of the spinal facet fusion implant such that the spinal facet fusion implant can be introduced into the proximal end of the central lumen, advanced distally along the central lumen, and advanced distally out of the distal end of the central lumen and into the gap between the descending facet of the first vertebra and the ascending facet of the second vertebra;   a first drill guide angled relative to the longitudinal axis of the central lumen, the first drill guide being configured to receive a drill therein;   a second drill guide angled relative to the longitudinal axis of the central lumen, the second drill guide being configured to receive a drill therein;       

     advancing a drill into the first drill guide so as to form a first seat in the descending facet of the first vertebra;
         advancing a drill into the second drill guide so as to form a second seat in the ascending facet of the second vertebra;       

     advancing the spinal facet fusion implant into the gap between the first vertebra and the second vertebra so that (i) the superior stabilizer is disposed in the first seat of the descending facet of the first vertebra, and (ii) the inferior stabilizer is disposed in the second seat of the ascending facet of the second vertebra. 
     In another preferred form of the invention, there is provided a method for effecting spinal facet fusion, the method comprising: 
     providing a spinal facet fusion implant, the spinal facet fusion implant comprising a taper; 
     forming a first seat in the descending facet of a first vertebra, and forming a second seat in the ascending facet of a second vertebra, wherein at least one of the first seat and the second seat comprises a taper; and 
     advancing the spinal facet fusion implant into the gap between the first vertebra and the second vertebra so that the spinal facet fusion implant is disposed in the first seat of the descending facet and the second seat of the ascending facet of the second vertebra. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS. 1-3  are schematic views showing the seven cervical vertebrae (C1-C7); 
         FIG. 4  is a schematic view showing a facet joint between the C3 cervical vertebra and the C4 cervical vertebra; 
         FIGS. 5-10  are schematic views showing a novel fusion implant formed in accordance with the present invention; 
         FIGS. 11 and 12  are schematic views showing a joint locator which may be used to deploy the novel fusion implant of  FIGS. 5-10 ; 
         FIGS. 13 and 14  are schematic views showing a joint decorticator which may be used to decorticate bone to promote fusion; 
         FIGS. 15-17  are schematic views showing a drill guide/cannula which may be used to prepare bone to receive the novel fusion implant of  FIGS. 5-10  and which may be used to deploy the novel fusion implant of  FIGS. 5-10 ; 
         FIGS. 18 and 19  are schematic views showing a tamp which may be used to deploy the novel fusion implant of  FIGS. 5-10 ; 
         FIGS. 20, 21, 21A, 22, 23, 23A, 24-26, 26A, 27-32, 32A, 33-36  are schematic views showing the novel fusion implant of  FIGS. 5-10  being installed in a facet joint; 
         FIG. 36A  is a schematic view showing two novel fusion implants of  FIGS. 5-10  installed in a pair of facet joints; 
         FIG. 37  is a schematic view showing how the novel fusion implant of  FIGS. 5-10  interacts with the opposing facets of two adjacent cervical vertebrae so as to immobilize the facet joint and facilitate fusion between the opposing facets; and 
         FIGS. 38-42  are schematic views showing additional novel fusion implants formed in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The Cervical Vertebrae in General 
     Looking first at  FIGS. 1-3 , there is shown the seven cervical vertebrae (C1-C7) which are located at the top of the spine.  FIGS. 1 and 2  show the seven cervical vertebrae being oriented vertically (e.g., such as when the patient is sitting or standing), and  FIG. 3  shows the seven cervical vertebrae being oriented horizontally (e.g., such as when the patient is lying down).  FIG. 4  shows an exemplary facet joint  5  located between a descending facet  10  of a C3 cervical vertebra  15  and an ascending facet  20  of a C4 cervical vertebra  25 . In facet joint  5 , face  30  of descending facet  10  of C3 cervical vertebra  15  engages face  35  of ascending facet  20  of C4 cervical vertebra  25 . 
     The Novel Fusion Implant 
     Looking next at  FIGS. 5-10 , there is shown a novel spinal facet fusion implant  105  formed in accordance with the present invention. Fusion implant  105  generally comprises a body  110 , a superior stabilizer  115  and an inferior stabilizer  120 . As used herein, the term “superior stabilizer” is intended to identify the stabilizer which engages the descending facet of a facet joint (e.g., descending facet  10  of C3 cervical vertebra  15 ), and the term “inferior stabilizer” is intended to identify the stabilizer which engages the ascending facet of a facet joint (e.g., ascending facet  20  of a C4 cervical vertebra  25 ). 
     Body  110  comprises an elongated element having structural integrity. More particularly, body  110  generally comprises a distal end surface  125 , a proximal end surface  130 , a superior surface  135  extending distally from proximal end surface  130 , and an inferior surface  140  extending distally from proximal end surface  130 . Superior surface  135  and inferior surface  140  diverge as they extend distally from proximal end surface  130 , i.e., so that the “height” of the distal end of body  110  is greater than the “height” of the proximal end of body  110 . A superior beveled surface  145  connects the distal end of superior surface  135  with distal end surface  125 , and an inferior beveled surface  150  connects the distal end of inferior surface  140  with distal end surface  125 . Body  110  further comprises a medial side surface  155  and a lateral side surface  160 , each of which is bounded by the aforementioned distal end surface  125 , proximal end surface  130 , superior surface  135 , inferior surface  140 , superior beveled surface  145 , and inferior beveled surface  150 . 
     In one preferred form of the invention, the “height” of proximal end surface  130  is sized to be approximately the width of the gap between the two facets of a facet joint. 
     Superior stabilizer  115  generally comprises a distal end surface  165 , a proximal end surface  170 , a rounded superior surface  175  extending distally from proximal end surface  170 , and a superior beveled surface  180  which connects distal end surface  165  with generally rounded superior surface  175 . Superior stabilizer  115  further comprises a medial side surface  185  and a lateral side surface  190 , each of which is bounded by the aforementioned distal end surface  165 , proximal end surface  170 , rounded superior surface  175  and superior beveled surface  180 . 
     Inferior stabilizer  120  generally comprises a distal end surface  195 , a proximal end surface  200 , a rounded inferior surface  205  extending distally from proximal end surface  200 , and an inferior beveled surface  210  which connects distal end surface  195  with generally rounded inferior surface  205 . Inferior stabilizer  120  further comprises a medial side surface  215  and a lateral side surface  220 , each of which is bounded by the aforementioned distal end surface  195 , proximal end surface  200 , rounded inferior surface  205  and inferior beveled surface  210 . 
     In one preferred form of the invention, and as seen in  FIGS. 5-10 , proximal end surfaces  130 ,  170  and  200  are substantially co-planar. 
     In one preferred form of the invention, the tangent line  225  ( FIG. 6 ) of rounded superior surface  175  of superior stabilizer  115  extends at an angle α (e.g., of approximately 10 degrees) relative to the longitudinal axis  230  of body  110 , and the tangent line  235  of rounded inferior surface  205  of inferior stabilizer  120  extends at an angle β (of approximately 15 degrees) relative to the longitudinal axis  230  of body  110 . 
     In one preferred form of the invention, distal end surface  165  of superior stabilizer  115  is disposed distal to distal end surface  195  of inferior stabilizer  120 . 
     Thus it will be seen that, in one preferred form of the invention, superior stabilizer  115  and inferior stabilizer  120  have different configurations. 
     As will hereinafter be discussed in further detail, body  110  of novel fusion implant  105  is intended to be disposed the space between the descending facet of a facet joint and the opposing ascending facet of a facet joint (e.g., in the space between face  30  of descending facet  10  of C3 cervical vertebra  15  and face  35  of ascending facet  20  of C4 cervical vertebra  25 ); superior stabilizer  115  is intended to be disposed in a seat formed in the descending facet of the facet joint (e.g., a seat formed in descending facet  10  of C3 cervical vertebra  15 ); and inferior stabilizer  120  is intended to be disposed in a seat formed in the ascending facet of the facet joint (e.g., a seat formed in ascending facet  20  of C4 cervical vertebra  25 ), whereby to immobilize the facet joint and facilitate fusing of the facet joint. In this respect it should be appreciated that, and as will hereinafter be discussed, fusion implant  105  preferably makes a “friction fit” with the two facets of the facet joint, and superior stabilizer  115  and inferior stabilizer  120  are seated in descending facet  10  and ascending facet  20 , respectively, so as to lock the facets against movement relative to one another. Significantly, the tapered configuration of novel fusion implant  105 , when seated in an appropriately-configured pocket formed in the descending facet  10  and ascending facet  20  of a facet joint, locks the fusion implant against anterior or posterior movement. The tapered configuration of novel fusion implant  105  also helps create/restore lordosis. 
     It should be appreciated that novel fusion implant  105  is particularly well suited for use in fusing a cervical facet joint. 
     And as will hereinafter be discussed in further detail, novel fusion implant  105  is intended to be inserted into a facet joint using a posterior approach. Such a posterior approach is familiar to spine surgeons (thereby providing an increased level of comfort for the surgeon), and also minimizes the possibility of damage to the spinal cord during fusion implant insertion. 
     Preferred Instrumentation for Installing the Novel Fusion Implant 
     A preferred method for installing novel fusion implant  105  will hereinafter be described. The preferred method for installing novel fusion implant  105  preferably utilizes a joint locator  305  ( FIGS. 11 and 12 ), a joint decorticator  405  ( FIGS. 13 and 14 ), a drill guide/cannula  505  ( FIGS. 15-17 ), and a tamp  605  ( FIGS. 18 and 19 ). 
     More particularly, joint locator  305  ( FIGS. 11 and 12 ) generally comprises a shaft  310  having a distal end  315  and a proximal end  320 . A finger  325  extends distally from distal end  315  of shaft  310 . 
     Joint decorticator  405  ( FIGS. 13 and 14 ) generally comprises a hollow tubular structure  410  having a distal end  415  and a proximal end  420 . A cutting element  425  is disposed at distal end  415  of hollow tubular structure  410 , and a handle  430  is disposed at proximal end  420  of hollow tubular structure  410 . 
     Drill guide/cannula  505  ( FIGS. 15-17 ) generally comprises a body  510  having a distal end  515  and a proximal end  520 . A central lumen  525  extends from distal end  515  to proximal end  520 . Central lumen  525  has a cross-sectional profile generally matching a longitudinal projection of the maximum profile of novel fusion implant  105 , such that fusion implant  105  can be advanced along the length of central lumen  525  in a controlled sliding motion. More particularly, the cross-sectional profile of central lumen  525  comprises a generally rectangular portion  530  for accommodating body  110  of fusion implant  105 , a first generally hemispherical portion  535  for accommodating superior stabilizer  115  of fusion implant  105 , and a second generally hemispherical portion  540  for accommodating inferior stabilizer  120  of fusion implant  105 . 
     Drill guide/cannula  505  also comprises a first drill guide lumen  545 . First drill guide lumen  545  is oriented at an angle to the longitudinal axis of central lumen  525 . Note that the angle at which first drill guide lumen  545  is oriented relative to the longitudinal axis of central lumen  525  is the same angle β at which tangent line  235  of inferior stabilizer  120  extends to longitudinal axis  230  of body  110  of fusion implant  105 , such that first drill guide lumen  545  can be used to prepare a seat in the ascending facet of a facet joint (e.g., ascending facet  20  of C4 cervical vertebra  25 ), as will hereinafter be discussed in further detail. 
     Drill guide/cannula  505  also comprises a second drill guide lumen  550 . Second drill guide lumen  550  is oriented at an angle to the longitudinal axis of central lumen  525 . Note that the angle at which second drill guide lumen  550  is oriented relative to the longitudinal axis of central lumen  525  is the same angle α at which tangent line  225  of superior stabilizer  115  extends to longitudinal axis  230  of body  110  of fusion implant  105 , such that second drill guide lumen  550  can be used to prepare a seat in the descending facet of a facet joint (e.g., descending facet  10  of C3 cervical vertebra  15 ), as will hereinafter be discussed in further detail. 
     Drill guide/cannula  505  also comprises a pair of fingers  555  extending distally from its distal end  515 . Note that fingers  555  are aligned in a first plane  560  ( FIG. 15 ) which extends perpendicular to a second plane  565  ( FIG. 17 ) which passes through first drill guide lumen  545  and second drill guide lumen  550  so that, when fingers  555  are inserted into the gap in the facet joint (see below), first drill guide lumen  545  and second drill guide lumen  550  may be used to prepare seats in the descending facet of a facet joint, and the ascending facet of a facet joint, respectively (as will hereinafter be discussed). Note also that the aforementioned second plane  565  bisects first generally hemispherical portion  535  of central lumen  525  and second generally hemispherical portion  540  of central lumen  525  so that, when fingers  555  are inserted into the gap in the facet joint (see below), first generally hemispherical portion  535  and second generally hemispherical portion  540  may be used to orient fusion implant  105  relative to the seats previously prepared in the descending facet of a facet joint, and the ascending facet of a facet joint, respectively (as will hereinafter be discussed). 
     Tamp  605  ( FIGS. 18 and 19 ) comprises a shaft  610  having a distal end  615  and a proximal end  620 . Shaft  610  preferably has a rectangular cross-section similar to generally rectangular portion  530  of central lumen  525  of drill guide/cannula  505 . A finger  625  extends distally from distal end  615  of shaft  610 . An impactor extension  630  is mountable to proximal end  620  of shaft  610 , such that hammering on the proximal end of impactor extension  630  will transmit distally-directed force to shaft  610 . A superior projection  635  is disposed on shaft  610  just proximal to finger  625 . Superior projection  635  has a hemispherical cross-section similar to first generally hemispherical portion  535  of central lumen  525  of drill guide/cannula  505 . As a result of this construction, tamp  605  can make a close sliding fit within central lumen  525  of drill guide/cannula  505 , whereby to enable tamp  605  to advance fusion implant  105  along the length of central lumen  525 , as will hereinafter be discussed. In addition, when impactor extension  630  is mounted to proximal end  620  of shaft  610 , hammering on the proximal end of impactor extension  630  will transmit distally-directed force to shaft  610  of tamp  605 , whereby to transmit distally-directed force to fusion implant  105 , as will hereinafter be discussed. 
     Preferred Method for Installing the Novel Fusion Implant 
     First, the facet joint is visualized by the surgeon, either indirectly by imaging the patient (e.g., by fluoroscopy) or directly by visualization during an open procedure. 
     Next, and looking now at  FIGS. 20, 21 and 21A , joint locator  305  is inserted into the gap between the opposing facet surfaces (e.g., between face  30  of descending facet  10  of C3 cervical vertebrae  15  and face  35  of ascending facet  20  of C4 cervical vertebrae  25 ). More particularly, finger  325  of joint locator  305  is advanced into the gap between the opposing facet surfaces and then the position of joint locator  305  is verified, e.g., by viewing along the coronal and sagittal planes of the patient. After verification is complete, the proximal end of joint locator  305  is lightly tapped so as to advance finger  325  of joint locator  305  further into the facet joint, until distal end  315  of shaft  310  engages the outer surfaces of the facets. At this point, joint locator  305  is essentially fixed to the facet joint and provides a “track” to the facet joint. 
     Next, and looking now at  FIGS. 22, 23 and 23A , joint decorticator  405  is slid over joint locator  305  so that the joint decorticator is aligned with the gap between the opposing facet surfaces (e.g., between face  30  of descending facet  10  of C3 cervical vertebrae  15  and face  35  of ascending facet  20  of C4 cervical vertebrae  25 ). Then joint decorticator  405  is used to decorticate the ends of the opposing facets. After decortication is completed, joint decorticator  405  is removed from joint locator  305 . 
     Then, and looking now at  FIG. 24 , drill guide/cannula  505  is advanced over joint locator  305  (i.e., by fitting the distal end of central lumen  525  of drill guide/cannula  505  over proximal end  320  of joint locator  305 , and then advancing the drill guide/cannula distally along joint locator  305 ). Drill guide/cannula  505  is advanced distally along joint locator  305  until fingers  555  of drill guide/cannula  505  enter the gap between the opposing facets. Then fingers  555  of drill guide/cannula  505  are driven into the gap between the opposing facets, whereby to secure drill guide/cannula  505  to the facet joint. 
     Next, and looking now at  FIGS. 25, 26 and 26A , joint locator  305  is removed, e.g., by pulling the joint locator proximally out of central lumen  525  of the facet joint and out of drill guide/cannula  505 . 
     At this point, and looking now at  FIGS. 27 and 28 , a drill  705  is inserted into first drill guide lumen  545  of drill guide/cannula  505  and advanced into the ascending facet of the facet joint (e.g., ascending facet  20  of C4 cervical vertebrae  25 ) so as to create a recess  710  in the ascending facet of the facet joint. Recess  710  will subsequently receive inferior stabilizer  120  of fusion implant  105 , as will hereinafter be discussed. Then drill  705  is removed from first drill guide lumen  545 . 
     Next, and looking now at  FIGS. 29 and 30 , a drill  715  (which may or may not be the same as the aforementioned drill  705 ) is inserted into second drill guide lumen  550  of drill guide/cannula  505  and advanced into the descending facet of the facet joint (e.g., descending facet  10  of C3 cervical vertebrae  15 ) so as to create a recess  720  in the descending facet of the facet joint. Recess  720  will subsequently receive superior stabilizer  115  of fusion implant  105 , as will hereinafter be discussed. Then drill  715  is removed from second drill guide lumen  550 . 
     At this point, and looking now at  FIG. 31 , a pocket  725  will have been created in the facet joint, i.e., by forming recess  710  in the ascending facet of the facet joint and forming recess  720  in the descending facet of the facet joint. Note that pocket  725  is generally characterized by a superior anterior stop surface  730 , an inferior anterior stop surface  735 , a superior posterior stop surface  740  and an inferior posterior stop surface  745 . 
     Next, and looking now at  FIGS. 32-36 , fusion implant  105  is installed into the facet joint using drill guide/cannula  505  and tamp  605 . 
     More particularly, fusion implant  105  is inserted into the proximal end of central lumen  525  of drill guide/cannula  505  ( FIGS. 31 and 32 ) so that body  110  of fusion implant  105  is received in generally rectangular portion  530  of central lumen  525 , superior stabilizer  115  of fusion implant  105  is received in first generally hemispherical portion  535  of central lumen  525 , and inferior stabilizer  120  is received in second generally hemispherical portion  540  of central lumen  525 . 
     Then tamp  605  is used to advance fusion implant  105  along central lumen  525  of drill guide/cannula  505 , and then tamp  605  and impactor extension  630  are used to hammer fusion implant  105  into the aforementioned pocket  725  formed in the facet joint ( FIGS. 33-36 ), with body  110  of fusion implant  105  spanning the gap between the descending facet of the facet joint and the ascending facet of the facet joint, and with superior stabilizer  115  of fusion implant  105  being received in recess  720  in the descending facet of the facet joint, and with inferior stabilizer  120  of fusion implant  105  being received in recess  710  in the ascending facet of the facet joint, whereby to immobilize the facet joint and facilitate fusing of the facet joint. After fusion implant  105  is installed in pocket  725  in the facet joint, tamp  605  and drill guide/cannula  505  are removed from the surgical site, thereby completing the installation of fusion implant  105  into the facet joint. 
       FIG. 32A  shows two fusion implants  105  disposed in two facet joints in a spine. 
     Significantly, and looking now at  FIG. 37 , inasmuch as fusion implant  105  is configured to be “taller” at its distal end than at its proximal end, and inasmuch as fusion implant  105  is configured to have a “height” at its proximal end which is approximately the same as the “height” of the gap between the two facets of the facet joint, the two facets are distracted somewhat during insertion of the fusion implant into the aforementioned pocket  725 . As fusion implant  105  settles into pocket  725 , the facets are able to return toward their undistracted condition and compress back against the fusion implant, essentially creating something of a “friction fit” between fusion implant  105  and the two facets of facet joint  5 . By forming fusion implant  105  with angled surfaces, and by forming the aforementioned pocket  725  in the facets with angled surfaces, the fusion implant is prevented from moving anteriorly or posteriorly once the facets have compressed back into their normal disposition. More particularly, movement of fusion implant  105  in the anterior direction is prevented by engagement of superior stabilizer  115  and inferior stabilizer  120  with superior anterior stop surface  730  of pocket  725  and inferior anterior stop surface  735  of pocket  725 , respectively. Movement of fusion implant  105  in the posterior direction is prevented by engagement of superior stabilizer  115  and inferior stabilizer  120  with superior posterior stop surface  740  of pocket  725  and inferior posterior stop surface  745  of pocket  725 , respectively. Thus it will be appreciated that, in order for superior stabilizer  115  and inferior stabilizer  120  to pass by superior posterior stop surface  740  and inferior posterior stop surface  745 , respectively, the facet joint would have to over-distract, which is inhibited by the surrounding soft tissue structure. 
     By way of example but not limitation, where fusion implant  105  is to be seated between the C3 cervical vertebra and the C4 cervical vertebra ( FIG. 37 ), as fusion implant  105  is advanced into the gap between face  30  of descending facet  10  of C3 cervical vertebra  15  and face  35  of ascending facet  20  of C4 cervical vertebra  25 , superior beveled surface  145  of fusion implant  105  and inferior beveled surface  150  of fusion implant  105  will engage descending facet  10  of C3 cervical vertebra  15  and ascending facet  20  of C4 cervical vertebra  25 , respectively, and cam the two facets apart. Further insertion of fusion implant  105  into the facet joint causes superior beveled surface  180  of superior stabilizer  115  and inferior beveled surface  210  of inferior stabilizer  120  to engage descending facet  10  of C3 cervical vertebra  15  and ascending facet  20  of C4 cervical vertebra  25 , respectively, and cam the two facets further apart. As fusion implant  105  settles into pocket  725 , face  30  of descending facet  10  of C3 cervical vertebra  15  settles against superior surface  135  of body  110 , face  35  of ascending facet  20  of C4 cervical vertebra  25  settles against inferior surface  140  of body  110 , superior anterior stop surface  730  of pocket  725  settles against distal end surface  165  of superior stabilizer  115 , inferior anterior stop surface  735  of pocket  725  settles against distal end surface  195  of inferior stabilizer  120 , superior posterior stop surface  740  of pocket  725  settles against rounded superior surface  175  of superior stabilizer  115  and inferior posterior stop surface  745  of pocket  725  settles against rounded inferior surface  205  of inferior stabilizer  120 , whereby to lock fusion implant  105  against anterior or posterior movement. 
     Additionally, the wedge-like construction of fusion implant  105  creates/restores lordosis. 
     In the preferred form of the invention, descending facet  10  contacts, and is supported by, superior beveled surface  145 , superior surface  135 , distal end surface  165 , superior beveled surface  180  and rounded superior surface  175  of fusion implant  105 ; and ascending facet  20  contacts, and is supported by, inferior beveled surface  150 , inferior surface  140 , distal end surface  195 , inferior beveled surface  210  and rounded inferior surface  205  of fusion implant  105 . 
     However, if desired, in another form of the invention, descending facet  10  contacts, and is supported by, less than all of the aforementioned superior beveled surface  145 , superior surface  135 , distal end surface  165 , superior beveled surface  180  and rounded superior surface  175  of fusion implant  105 ; and ascending facet  20  contacts, and is supported by, less than all of the aforementioned inferior beveled surface  150 , inferior surface  140 , distal end surface  195 , inferior beveled surface  210  and rounded inferior surface  205  of fusion implant  105 . By way of example but not limitation, in another form of the invention, descending facet  10  contacts, and is supported by, only superior surface  135  of fusion implant  105  and ascending facet  20  contacts, and is supported by, only inferior surface  140  of fusion implant  105 . By way of further example but not limitation, in another form of the invention, descending facet  10  contacts, and is supported by, only rounded superior surface  175  of fusion implant  105  and ascending facet  20  contacts, and is supported by, only rounded inferior surface  205  of fusion implant  105 . 
     Alternative Constructions 
     If desired, fusion implant  105  may comprise visual markers to facilitate proper orientation of the fusion implant when fusion implant  105  is inserted into the proximal end of central lumen  525  of drill guide/cannula  505 . By way of example but not limitation, and looking now at  FIG. 38 , fusion implant  105  may comprise a black dot  750  on one or both of inferior stabilizer  120  (e.g., to identify the inferior side of fusion implant  105 ) and/or distal end surface  125  of body  110  (e.g., to identify the distal end of fusion implant  105 ). 
     The configuration of fusion implant  105  may be varied without departing from the scope of the present invention. 
     By way of example but not limitation, it should be appreciated that the new 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 body  110 , superior stabilizer  115  and inferior stabilizer  120  may all be varied without departing from the scope of the present invention. 
     See, for example,  FIG. 39 , which shows a fusion implant  105  wherein the configurations of superior stabilizer  115  and inferior stabilizer  120  differ from the configurations of superior stabilizer  115  and inferior stabilizer  120  in the fusion implant  105  shown in  FIGS. 5-10 . 
     By way of further example but not limitation, fusion implant  105  may be formed so that superior stabilizer  115  and inferior stabilizer  120  have identical configurations. 
     See, for example,  FIG. 40 , where superior stabilizer  115  and inferior stabilizer  120  have identical configurations (and where those configurations vary from the configurations of superior stabilizer  115  and inferior stabilizer  120  in the fusion implant  105  shown in  FIGS. 5-10 ). 
     See also, for example,  FIG. 41 , where the apexes of rounded superior surface  175  of superior stabilizer  115  and rounded inferior surface  205  of inferior stabilizer  120  extend substantially parallel to one another. 
     And see, for example,  FIG. 42 , where superior surface  135  and inferior surface  140  of body  110  extend substantially parallel to one another. 
     It will, of course, be appreciated that the configuration of drill guide/cannula  505  may change as the configuration of fusion implant  105  changes in order to (i) enable drill guide/cannula  505  to create an appropriate pocket  725  in the facet joint, and (ii) enable drill guide/cannula  505  to provide an appropriate channel for advancing fusion implant  105  into pocket  725  in the facet joint. 
     Additionally, the new 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 present invention permits the surgeon to select a fusion implant having the appropriate size and composition for a given facet fusion. 
     Advantages of the Invention 
     Numerous advantages are achieved by the present invention. Among other things, the present invention provides a fast, simple, minimally-invasive, easily reproduced and highly effective approach for effecting facet fusion, particularly with cervical facet joints. The fusion implant is able to withstand greater forces, prohibit motion in all directions and drastically reduce the risk of implant failure. The fusion implant also eliminates the possibility of slippage during spinal motion, greatly improves facet stability and promotes better facet fusion. 
     Applications to Joints Other than Facet Joints 
     While fusion implant  105  has been discussed above in the context of fusing a facet joint, it should also be appreciated that fusion implant  105  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. 
     Modifications of the Preferred Embodiments 
     It should be understood that many additional changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated 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.