Abstract:
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 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;   wherein 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 further wherein 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.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
     This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/939,615, filed May 22, 2007 by Tov Vestgaarden for PERCUTANEOUS SPINAL FACET FIXATION DEVICE FOR FACET FUSION, 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 cause 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. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel method and apparatus for effecting spinal facet fusion. More particularly, the present invention comprises the provision and use of a novel spinal facet fusion implant for disposition between the opposing articular surfaces of a facet joint, whereby to immobilize the facet joint and facilitate fusion between the opposing facets. 
     More particularly, in one form of the present 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 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; 
     wherein 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 further wherein 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. 
     In another form of the present invention, there is provided a method for fusing a spinal facet joint, the method comprising the steps of: 
     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 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;   wherein 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 further wherein 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;       

     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 the at least one stabilizer is positioned within the gap between the spinal facets; and 
     maintaining the spinal facet fusion implant in this position while fusion occurs. 
     In another form of the present 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 having a cross-sectional profile which is characterized by a primary axis and a secondary axis; 
     wherein 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 further wherein the cross-sectional profile is non-circular. 
     In yet another form of the present invention, there is provided a method for fusing a spinal facet joint, the method comprising the steps of: 
     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 having a cross-sectional profile which is characterized by a primary axis and a secondary axis;   wherein 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 further wherein the cross-sectional profile is non-circular;       

     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 this position while fusion occurs. 
     In still another form of the present invention, there is provided a joint 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 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; 
     wherein 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 further wherein 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. 
     In an additional form of the present invention, there is provided a method for fusing a joint, the method comprising the steps of: 
     providing a 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 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;   wherein 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 further wherein 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;       

     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 this position while fusion occurs. 
    
    
     
       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  illustrate fusion implants formed in accordance with the present invention; 
         FIGS. 4 and 5  illustrate a fusion implant being installed in a facet joint; 
         FIGS. 6-12  illustrate instrumentation which may be used to install a solid fusion implant in a facet joint; 
         FIGS. 13-26  illustrate a preferred method for installing a solid fusion implant in the facet joint; 
         FIGS. 27-28  illustrate instrumentation which may be used to install a hollow fusion implant in a facet joint; and 
         FIGS. 29-74  illustrate alternative fusion implants formed in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In General 
     Looking first at  FIG. 1 , there is shown a novel spinal facet fusion implant  5  formed in accordance with the present invention. Fusion implant  5  generally comprises a body  10  and at least one stabilizer  15 . 
     Body  10  comprises an elongated element having structural integrity. Preferably the distal end of body  10  (and the distal end of stabilizer  15  as well) is chamfered as shown at  20  to facilitate insertion of fusion implant  5  into the facet joint, as will hereinafter be discussed. Preferably, and as seen in  FIG. 1 , body  10  has a rounded rectangular cross-section, or an ovoid cross-section, a laterally-extended cross-section, or some other non-round cross-section, so as to inhibit rotation of body  10  about a longitudinal center axis. If desired, body  10  may include a plurality of barbs (i.e., forward biting teeth)  25  extending outwardly therefrom. Barbs  25  are designed to permit body  10  to be inserted into the facet joint and to impede retraction of body  10  out of the facet joint. 
     The at least one stabilizer  15  is intended to be received in the gap located between the opposing facet surfaces, whereby to prevent rotation of fusion implant  5  within the facet joint. In one preferred form of the present invention, two stabilizers  15  are provided, one disposed along the upper surface of body  10  and one disposed along the lower surface of body  10 . Stabilizers  15  preferably have a width just slightly larger than the gap between the opposing articular surfaces of a facet joint, so that the stabilizers can make a snug fit therebetween. 
     If desired, and looking now at  FIG. 2 , fusion implant  5  may also be configured so that its body  10  lacks barbs  25  on its outer surface. 
     Alternatively, if desired, and looking now at  FIG. 3 , fusion implant  5  may comprise a hollow body  10  having an internal cavity  30 . Hollow body  10  may also have a plurality of openings  35  extending through the side wall of body  10  and communicating with cavity  30 . Internal cavity  30  and openings  35  can facilitate facet fusion by permitting bone ingrowth into and/or through fusion implant  5 . 
     Fusion implant  5  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. 
     In use, and looking now at  FIG. 4 , an instrument is first used to determine the vertical plane  40  of the facet joint. Identifying the vertical plane of the facet joint is important, since this is used to identify the proper position for a cavity  45  which is to be formed in the facet joint to receive the fusion implant. 
     To this respect it should be appreciated that at least one of the instruments comprises a directional feature which is used to maintain the alignment of the instrumentation with the vertical plane of the facet joint. By way of example but not limitation, a directional cannula may comprise a flat portion and the remaining instruments may comprise a flat portion on an opposite portion of the instrument so that the instruments may only be inserted through the cannula at 0 degrees and/or 180 degrees. 
     After the proper position for cavity  45  has been identified, a drill (or reamer, punch, dremel, router, burr, etc.) is used to form the cavity  45  in the facet joint. Cavity  45  is formed across vertical plane  40  so that substantially one-half of cavity  45  is formed in a first facet  50 , and substantially one-half is formed in its opposing facet  55 . 
     After cavity  45  has been formed in (or, perhaps more literally, across) the facet joint, fusion implant  5  is inserted into cavity  45 . See  FIG. 5 . More particularly, fusion implant  5  is inserted into cavity  45  so that (i) body  10  spans the gap between opposing facets  50 ,  55 , and (ii) stabilizers  15  extend between the opposing facet surfaces. Preferably, fusion implant  5  is slightly oversized relative to cavity  45  so as to create a press fit. Fusion implant  5  provides the stability and strength needed to immobilize the facet joint while fusion occurs. Due to the positioning of stabilizers  15  between the opposing facet surfaces, and due to the non-circular cross-section of body  10 , fusion implant  5  will be held against rotation within cavity  45 , which will in turn hold facets  50 ,  55  stable relative to one another. 
     It should be appreciated that where the hollow fusion implant  10  of  FIG. 3  is used, and where the implant is formed out of a sufficiently strong and rigid material, cavity  45  need not be pre-formed in the opposing facets. In this case, the hollow fusion implant can be simply tapped into place, in much the same manner that a punch is used. 
     Thus it will be seen that the present invention provides a new and improved fusion implant for facilitating facet fusion. This new fusion implant is able to withstand greater forces, prohibit motion in all directions and drastically reduce 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. 
     It should be appreciated that the new fusion implant combines two unique “shapes” in one implant (i.e., the shape of body  10  and the shape of stabilizer  15 ) in order to limit motion in a multi-directional joint. More particularly, the shape of body  10  limits motion (e.g., in flexion/extension for the lumbar facets and in axial rotation for the cervical facets), while the shape of stabilizer  15  (i.e., the “keel”) rests between two bony structures (i.e., in the gap of the facet joint) and limits lateral bending. This construction eliminates the possibility of eccentric forces inducing motion in the facet joint. 
     Furthermore, it has been found that while the present invention effectively stabilizes the joint, it still allows the “micro motion” which is required for the fusion process to begin. 
     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  10 , stabilizers  15 , barbs  25 , openings  35 , etc. may all be varied without departing from the scope of the present invention. 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. 
     Detailed Surgical Technique 
     Solid Fusion Implant 
     A preferred surgical technique for utilizing a solid fusion implant  5  will now be described. The preferred surgical technique preferably uses a guide pin  100  ( FIG. 6 ) a facet distractor  105  ( FIG. 7 ), a directional cannula  110  ( FIG. 8 ), a drill guide  115  ( FIG. 9 ), a cavity cutter  117  ( FIG. 9A ), an implant loading block  120  ( FIG. 10 ), an implant holder  125  ( FIG. 11 ) and an implant tamp  130  ( FIG. 12 ). 
     First, the facet joint is localized indirectly by fluoroscopy, or directly by visualization during an open procedure. Next, a guide pin  100  ( FIG. 13 ) is inserted into the gap between the opposing facet surfaces. The position of guide pin  100  is verified by viewing the coronal and sagittal planes. Then guide pin  100  is lightly tapped so as to insert the guide pin approximately 5 mm into the facet joint, along vertical plane  40 . In this respect it will be appreciated that the inferior facet is curved medially and will help prevent the guide pin from damaging the nerve structures. 
     Next, a cannulated facet distractor  105  is slid over guide pin  100  ( FIG. 14 ) so that it is aligned with the vertical plane of the facet joint. Then facet distractor  105  is lightly tapped into the facet joint, along vertical plane  40  ( FIG. 15 ). 
     Next, a directional cannula  110  is placed over facet distractor  105  ( FIG. 16 ). Then the tip of directional cannula  110  is pushed into the facet joint ( FIG. 17 ). Once the tip of directional cannula  110  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  110  with the vertical plane of the facet joint. After verifying that directional cannula  110  has been inserted all the way into the facet joint and is stabilized in the joint, facet distractor  105  is removed ( FIG. 18 ). 
     Next, a drill guide  115  is inserted into directional cannula  110  ( FIG. 19 ). Drill guide  115  is advanced within directional cannula  110  until a drill guide stop is resting on directional cannula  110 . Then, with drill guide  115  in place, irrigation (e.g., a few drops of saline) is placed into drill guide. Next, a drill bit  135  is used to drill a cavity in the inferior facet ( FIG. 20 ). This is done by drilling until drill bit  135  reaches the mechanical stop on drill guide  115  ( FIG. 21 ). Then drill guide  115  and drill bit  135  are pulled out of directional cannula  110 , drill guide  115  is rotated 180 degrees, and then drill guide  115  is reinserted into directional cannula  110  in order to drill the superior facet. With drill guide  115  in place, irrigation (e.g., a few drops of saline) is placed into drill guide  115 , and then drill bit  135  is used to drill a cavity in the superior facet ( FIG. 22 ). Again, drilling occurs until drill bit  135  reaches the mechanical stop on drill guide  115 . Then drill bit  135  is removed ( FIG. 23 ). 
     A cavity cutter  117  is then used to make an opening having the perfect shape for fusion implant  5 . 
     Using implant loading block  120  shown in  FIG. 10 , fusion implant  5  is then inserted into implant holder  125 . Then implant holder  125 , with fusion implant  5  in place, is placed into directional cannula  110  ( FIG. 24 ). Next, implant holder  125  is lightly tapped so as to insert fusion implant  5  into the cavity created in the facet joint ( FIG. 25 ). Once the implant has been positioned in the cavity created in the facet joint, implant tamp  130  is inserted into implant holder  125 . Next, implant tamp  130  is lightly tapped so as to drive the implant into the cavity created in the facet joint ( FIG. 26 ). The implant is preferably countersunk 1-2 mm into the facet joint. 
     Then the foregoing steps are repeated for the contralateral facet joint. 
     Finally, implant tamp  130 , implant holder  125  and directional cannula  110  are removed from the surgical site and the incision is closed. 
     Detailed Surgical Technique 
     Hollow Fusion Implant 
     A preferred surgical technique for utilizing a hollow fusion implant  5  will now be described. The preferred surgical technique preferably uses guide pin  100  ( FIG. 6 ), facet distractor  105  ( FIG. 7 ), and an implant punch  140  ( FIG. 27 ). 
     First, the facet joint is localized indirectly by fluoroscopy or directly by visualization during an open procedure. Next, guide pin  100  is inserted in the gap between the opposing facet surfaces. The position of guide pin  100  is verified by viewing the coronal and sagittal planes. Then guide pin  100  is lightly tapped so as to insert guide pin  100  approximately 5 mm into the facet joint, along the vertical plane of the facet joint. In this respect it will be appreciated that inasmuch as the inferior facet curves medially, this will help prevent the guide pin from damaging the nerve structures. 
     Then the cannulated facet distractor  105  is slid over guide pin  100  so that it is aligned with the vertical plane of the facet joint. Facet distractor  105  is lightly tapped into the facet joint, along the vertical plane of the facet joint. 
     Next, implant punch  140  ( FIG. 27 ), with a hollow fusion implant  5  mounted thereto ( FIG. 28 ) is pushed (or hammered or otherwise advanced) downwards so as to drive hollow fusion implant  5  into the facet joint. 
     Finally, implant punch  140  and guide pin  100  are removed, leaving hollow fusion implant  5  in the facet joint, and the incision is closed. 
     Alternative Constructions 
     The configuration of fusion implant  5  may be varied without departing from the scope of the present invention. 
     In one configuration, and looking now at  FIGS. 29-31 , there is provided a fusion implant  5  comprising a rounded rectangular elongated body and two stabilizers. Preferably, the body comprises a groove extending circumferentially around the exterior surface of the body. 
     Looking next at  FIGS. 32-34 , there is shown a fusion implant  5  comprising a rounded elongated body, which is similar to the embodiment shown in  FIGS. 29-31 , however, the elongated body has a different aspect ratio and the elongated body is formed with a substantially smooth outer surface (e.g., without grooves or barbs). 
       FIGS. 35-37  illustrate a fusion implant  5  having an elongated body which is similar to the elongated body shown in  FIGS. 29-31 , but without a stabilizer and with an elongated body which is formed with a substantially smooth outer surface (e.g., without grooves or barbs). 
       FIGS. 38-40  illustrate a fusion implant  5  having an elongated body with a smaller radius on the rounded edges than the embodiment shown in  FIGS. 29-31 . Furthermore, the elongated body is formed with a smooth outer surface. 
       FIGS. 41-43  illustrate a fusion implant  5  which is similar to the implant of  FIGS. 29-31 , but with the main body having a substantially circular configuration. 
       FIGS. 44-47  illustrate a fusion implant  5  which is similar to the implant of  FIGS. 29-31  and further comprises a through-hole extending through the elongated body. The through-hole allows a bone growth promoter to be packed through and across the width of the fusion implant, thereby enabling rapid fusion through the implant. 
       FIGS. 48-50  illustrate a fusion implant  5  which is similar to the implant of  FIGS. 29-31 . However, in this embodiment, the grooves are replaced with barbs (i.e., forward biting teeth) extending around the surface of the body. 
       FIGS. 51-54  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 48-50 , however, the fusion implant comprises a hollow body having an internal cavity and plurality of openings extending through the side wall of the body and communicating with the cavity. 
       FIGS. 55-57  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 29-31 , however, the fusion implant comprises a hollow body having an internal cavity and plurality of openings extending through the side wall of the body and communicating with the cavity. 
       FIGS. 58-60  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 29-31  and further comprises a hole for attaching the implant to the facet joint. The attachment may be effected by K-Wire, suture, staple, screw or other fixation device. 
       FIGS. 61-64  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 29-31  and further comprises a hole for attaching the implant to the facet joint. The attachment may be effected by K-Wire, suture, staple, screw or other fixation device. Preferably, a screw is used to attach the implant to the facet joint. 
       FIGS. 65-68  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 29-31  and further comprises a hole for attaching the implant to the facet joint. The attachment may be effected by an integrated screw. Like  FIGS. 29-31 , this embodiment may also comprise grooves. 
       FIGS. 69-71  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 29-31  and further comprises rectangular, sharp spikes for attaching the implant to the facet joint. 
       FIGS. 72-74  illustrate a fusion implant  5  which is similar to the embodiment shown in  FIGS. 29-31  and further comprises round, sharp spikes for attaching the implant to the facet joint. 
     ADVANTAGES OF THE INVENTION 
     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. 
     Applications to Joints Other than Facet Joints 
     While fusion implant  5  has been discussed above in the context of fusing a facet joint, it should also be appreciated that fusion implant  5  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.