Abstract:
An implant is insertable onto a vertebral facet joint includes a body with an inner surface that mates with one or both articular processes of the facet joint. The inner surface may define an interior cavity that accepts at least a portion of an inferior articular process and a superior articular process forming the facet joint. The body may be constructed of a bone growth material that fuses with the articular processes. The implant may help retain bone growth promoting substance that is inserted between or on articulating surfaces of the facet joint. A receiving portion of an exterior surface of one or both articular process may be prepared to stimulate bone growth. The implant may be placed onto the receiving portion to cover an exterior junction between or on the articulating surfaces of the facet joint. The implant may be secured to one or both of the articular processes.

Description:
BACKGROUND  
       [0001]     The human spine serves many functions. The vertebral members of the spinal column protect the spinal cord. The spinal column also supports other portions of the human body. Furthermore, moveable facet joints and resilient discs disposed between the vertebral members permit motion between individual vertebral members. Each vertebrae includes an anterior body and a posterior arch. The posterior arch includes two pedicles and two laminae that join together to form the spinous process. A transverse process is laterally positioned at the transition from the pedicles to the laminae. Both the spinous process and transverse process provide for attachment of fibrous tissue, including muscle. Two inferior articular processes extend downward from the junction of the laminae and the transverse process. Further, two superior articular processes extend upward from the junction. The articular processes of adjacent vertebrae form the facet joints. The inferior articular process of one vertebra articulates with the superior articular process of the vertebra below. The facet joints are gliding joints because the articular surfaces glide over each other.  
         [0002]     Vertebral implants are often used in the surgical treatment of spinal disorders such as degenerative disc disease, disc herniations, curvature abnormalities, and trauma. Many different types of treatments are used. In some cases, spinal fusion is indicated to inhibit relative motion between vertebral bodies. Spinal fusion often involves the removal of the vertebral disc and insertion of an interbody implant to create a fused junction between a pair of vertebral bodies. Furthermore, the facet joints may be fused to complete the fusion between vertebral pairs. Facet fusion often involves destruction of the facet by decorticating the opposing articulating surfaces and packing bone growth promoting substances such as grafts or synthetic materials into the space between the articular processes. The facet joints are generally small as compared to the intervertebral space. Consequently, limited amounts of bone-growth promoting substances may be inserted into the joint. Some of the bone-growth promoting substances tend to disperse post-operatively resulting in a less robust fusion. Furthermore, the overlying fibrous tissue may further disperse the bone-growth promoting substances as a result of contact, friction, and/or the ingrowth of fibrous mass. These and other factors may result in pseudarthrosis or inadequate fusion.  
       SUMMARY  
       [0003]     Illustrative embodiments disclosed herein are directed to an implant that is insertable onto a vertebral facet joint. The implant includes a body with an inner bone-contact surface that is configured to fuse with the articular processes. The inner surface may define an interior cavity that is configured to accept at least a portion of an inferior articular process and a superior articular process forming the facet joint. The inner surface may include curved and/or planar portions. The body may be constructed of a bone growth material that fuses with the articular processes. The implant body may be constructed from manufactured materials that include bone growth promoting materials and/or bone ingrowth promoting features.  
         [0004]     The implant may be used in conjunction with bone growth promoting materials that are packed into the facet joint. The implant may help retain the bone growth promoting substance between the articulating surfaces of the facet joint. The implant may also protect the bone growth promoting substance from adjacent muscle compression and fibrous tissue invasion and may improve bone fusion rate and size of the fusion mass. A receiving portion of an exterior surface of one or both articular process may be prepared to stimulate bone growth. The implant may be placed onto the receiving portion to cover an exterior junction between the articulating surfaces of the facet joint. The implant may be secured to one or both of the articular processes. For example, the implant may be staked, pinned, screwed, or adhered to the exterior of one or both articular processes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a lateral view of a facet implant according to one embodiment shown relative to vertebral bodies;  
         [0006]      FIG. 2  is a section view according to the section lines in  FIG. 1 ;  
         [0007]      FIG. 3  is a detail view of a facet joint showing one embodiment of a facet implant;  
         [0008]      FIG. 4  is a perspective view of a facet implant according to one embodiment;  
         [0009]      FIG. 5  is a detail view of a facet joint showing the preparation of a receiving area about the exterior of the articular processes according to one embodiment;  
         [0010]      FIG. 6  is a detail view of a facet joint showing one embodiment of a facet implant;  
         [0011]      FIG. 7  is a detail view of a facet joint showing one embodiment of a facet implant;  
         [0012]      FIG. 8  is a perspective view of a facet implant according to one embodiment;  
         [0013]      FIG. 9  is a detail view of a facet joint showing the preparation of a receiving area about the exterior of the articular processes according to one embodiment;  
         [0014]      FIG. 10  is a detail view of a facet joint showing one embodiment of a facet implant;  
         [0015]      FIG. 11  is a perspective view of a facet implant according to one embodiment;  
         [0016]      FIG. 12  is a detail view of a facet joint showing the preparation of a receiving area about the exterior of the articular processes according to one embodiment;  
         [0017]      FIG. 13  is a detail view of a facet joint showing one embodiment of a facet implant;  
         [0018]      FIG. 14  is a perspective view of a facet implant according to one embodiment;  
         [0019]      FIG. 15  is a detail view of a facet joint showing the preparation of a receiving area about the exterior of the articular processes according to one embodiment;  
         [0020]      FIG. 16  is a detail view of a facet joint showing one embodiment of a facet implant;  
         [0021]      FIG. 17  is a perspective view of a facet implant according to one embodiment;  
         [0022]      FIG. 18  is a perspective view of a facet implant according to one embodiment;  
         [0023]      FIG. 19  is a detail view of a facet joint showing the preparation of a receiving area about the exterior of the articular processes according to one embodiment;  
         [0024]      FIG. 20  is a detail view of a facet joint showing one embodiment of a facet implant; and  
         [0025]      FIG. 21  is a detail view of a facet joint showing one embodiment of a facet implant. 
     
    
     DETAILED DESCRIPTION  
       [0026]     The various embodiments disclosed herein relate to an implant in which a graft plate or cap is disposed over the exterior of a facet joint to promote fusion of the facet in spinal fusion surgery.  FIG. 1  illustrates one embodiment of an implant  10  installed in this manner. Specifically,  FIG. 1  shows a lateral view of two vertebrae V 1 , V 2  and an intervertebral disc D disposed therebetween. During fusion surgery, some or all of the disc D is removed and may be replaced with an implant or graft that ultimately fuses to the vertebrae V 1 , V 2 . In addition, a surgeon may elect to fuse the facet joints J that are formed between the inferior articular process IP of the superior vertebra V 1  and the superior articular process SP of the inferior vertebra V 2 . To that end, the implant  10  may be attached to the articular processes IP, SP as illustrated and disclosed herein. In one or more embodiments, the implant  10  may be pinned or screwed to the articular processes IP, SP using a fastener  12 . The fastener  12  may be implemented using a pin, a nail, a screw, a staple, a wedge, or other feature that secures the implant  10  to the facet joint J until fusion occurs. Other embodiments disclosed herein may be implemented without a fastener  12 .  
         [0027]     In one or more embodiments, the implant  10  is disposed over the exterior of the facet joint J in a manner that physically separates the facet joint J from fibrous tissue (e.g., muscle and ligaments) that is depicted generally by the dashed lines and the letter F in  FIG. 1 . Further, the exemplary implant  10  serves as a cap that covers the posterior junction between the inferior articular process IP of the superior vertebra V 1  and the superior articular process SP of the inferior vertebra V 2 . The implant  10  and fastener  12  may be constructed of biocompatible materials, including metals, such as titanium or stainless steel, non-metals, such as PEEK or UHMWPE. The implant  10  and fastener  12  may be constructed of a graft material, which is interpreted herein to include implants constructed from natural or synthetic bone materials including, but not limited to autograft, allograft, xenograft, or calcium phosphate. In embodiments where the implant  10  is constructed from synthetic or manufactured materials, the implant  10  may be coated or textured to improve the likelihood of bony ingrowth into the implant. Similarly, the implant may be impregnated, packed, or filled with bone growth promoting substances such as Bone Morphogenetic Protein (BMP), demineralized bone matrix (DBM), allograft, autograft, xenograft, or other osteoinductive growth factors. For example, the implant  10  may have a porous structure with open portions of the implant  10  packed with the bone growth promoting substance. In certain implementations, the implant  10  may osseointegrate or become part of the fusion mass at the facet joint J to increase the size and stability of the fusion mass. In one embodiment, the fastener  12  may be constructed from a bioabsorbable material that begins to dissolve after the implant  10  has begun to fuse to the facet joint J.  
         [0028]     The section view shown in  FIG. 2  is depicted according to the section line labeled II-II in  FIG. 1 . The exemplary implant  10  illustrated in  FIG. 1  is also presented in  FIG. 2 . As described above, this embodiment of the implant  10  is disposed so that it covers the posterior side of the facet joint J and forms a barrier between joint J and the fibrous tissue F that covers the facet joint J. In other embodiments, the implant  10  may cover other portions of the facet joint, including lateral or anterior junctions between the inferior articular process IP of the superior vertebra V 1  and the superior articular process SP of the inferior vertebra V 2 . Notably,  FIG. 2  includes a “DETAIL” callout that identifies the view illustrated in various other Figures. The detail views provide a magnified representation of the facet joint and various embodiments of the implant.  
         [0029]      FIG. 3  shows a detail view of the same embodiment of the implant  10  depicted in  FIGS. 1 and 2 .  FIG. 3  further illustrates bone growth promoting material  100  that is packed between the articular processes IP, SP that form the facet joint J. These bone growth promoting materials  100  are often used in an attempt to fuse the facet joint J and stabilize a motion segment of the spine. The fusion process may involve some destruction of the facet, including removing cartilage at the interface surfaces  26 ,  28 , decorticating each interface surface  26 ,  28 , and packing the bone growth promoting material  100  into the joint J in an attempt to promote new bone growth that will ultimately fuse the facet joint J. Some non-limiting examples of bone growth promoting substances that may be inserted into the facet joint J include Bone Morphogenetic Protein (BMP), demineralized bone matrix (DBM), allograft, autograft, xenograft, or other osteoinductive growth factors to facilitate fusion of the facet joint J.  
         [0030]     An inner surface  14  of the implant  10  is positioned so that it contacts outer cortical surfaces  19 ,  20  of the inferior articular process IP and the superior articular process SP, respectively. In one embodiment, the implant  10  is generally curved to match the anatomy of the outer, cortical surfaces  19 ,  20 . As such, the implant  10  may include an elongated, curved shape as illustrated in the perspective view in  FIG. 4 . Other shapes are possible as the various embodiments described below bear out.  
         [0031]     The exemplary implant  10  covers the facet joint J by an amount that helps prevent the bone growth promoting substance  100  from evacuating the facet joint J. Further, the exemplary implant  10  may act as a barrier to prevent the migration of fibroblasts or other ingrowth of fibrous matter F (see  FIGS. 1 and 2 ) that lies over or adjacent the facet joint J. The implant  10  may be secured as described above with a fastener  12  that extends through the implant  10 , from the outer surface  16 , and engages one or both of the articular processes IP, SP. In one embodiment, the implant  10  includes an aperture  18  through which the fastener  12  may pass. Additional apertures  18  may be included in the implant  10 . The fastener  12  includes an enlarged head  22  and elongated body  24 . The length of the illustrated body  24  is sufficient to engage both of the articular processes IP, SP. That is, the fastener  12  passes through the implant  10 , through the superior articular process SP, across the interface surfaces  26 ,  28 , and into the inferior articular process IP. In the embodiment shown, the aperture  18  is disposed in proximity to the superior articular process SP so that the fastener  12  initially engages the superior articular process SP of the inferior vertebra V 2 . In one embodiment, the aperture  18  is disposed so that the fastener  12  initially engages an inferior articular process IP of a superior vertebra V 1 . Certainly, the length of the elongated body  24  may be shortened so that it engages only one of the articular processes IP, SP that form the facet joint J.  
         [0032]     The fastener  12  may require a pilot hole for attachment of the implant  10  over the facet joint J. Thus, in one implementation, the implant may be positioned as desired. Then, a pilot hole may be drilled into the articular processes IP and/or SP using the aperture  18  as a guide. Then, if necessary, the implant may be removed and bone growth promoting material  100  inserted into the facet joint J, between the interface surfaces  26 ,  28 , and under the implant  10  as desired. The implant  10  is then secured to the articular processes IP, SP by inserting the fastener  12 .  
         [0033]     The illustrated embodiment of the implant  10  may be attached to the facet joint J without any preparation of the exterior surfaces  19 ,  20  of the articular processes IP, SP. In another embodiment, the exterior surfaces  19 ,  20  of the articular processes IP, SP are prepared for the implant  10 . As used herein, the term “preparing” is intended to encompass such actions as abrading, ablating, roughening, or scouring such as with an abrading tool (not shown). Alternatively, preparing the exterior surfaces  19 ,  20  may comprise contouring or decorticating, with bony material removed in preparation for receiving the implant  10 . For example,  FIG. 5  shows a facet joint J as previously described with a dashed line  30  representing a region of the outer surfaces  19 ,  20  that are removed using conventionally known techniques. The dashed line  30  represents a receiving portion of the articular processes IP, SP that will receive the implant  10 .  
         [0034]      FIG. 6  shows the previously described implant  10  inserted into the receiving portion  30  formerly occupied by the bony material that is removed according to  FIG. 5 . As above, the implant  10  is retained with a fastener  12 .  FIG. 7  shows an embodiment in which a similar implant  110  is inserted into a similar receiving portion  30  as in  FIGS. 5 and 6 . However, in this implementation, a fastener  12  is not used. The overlying fibrous tissue F may retain the implant  110  in the position shown. Further, the implant  110  may be adhered to the articular processes IP, SP with a biocompatible adhesive. Suitable adhesives may include protein derived, aldehyde based adhesive materials, albumin/glutaraldehyde materials, and cyanoacrylate-based materials.  
         [0035]     The previously described implants  10 ,  110  included a substantially curved construction similar to that shown in  FIG. 4 . In one embodiment shown in  FIG. 8 , the implant  210  includes an inner surface  214  and outer surface  216  that are comprised of three substantially planar portions  214   a ,  214   b ,  214   c  oriented at angles relative to one another. In other embodiments, the implant  210  may include a single substantially planar portion or include two planar portions oriented at an angle with respect to each other. In other embodiments, the implant  210  may include four or more substantially planar portions oriented at angles with respect to one another. The angles between the planar portions  14   a - c  may be configured to substantially match the contour of the exterior surfaces  19 ,  20  of the facet joint J. The amount of bony material that is removed from the outer surfaces  19 ,  20  of the facet joint J may be minimized to the extent the implant  210  matches the contour of the exterior surfaces  19 ,  20 . In other implementations, the implant  210  may be attached to a facet joint J where the exterior surfaces  19 ,  20  are not abraded or decorticated.  
         [0036]     In implementations where the outer cortical surfaces  19 ,  20  of the facet joint J are decorticated in anticipation of receiving the implant  210 , substantially planar faces may be formed in the articular processes IP, SP that substantially match the configuration of the implant  210 . For example,  FIG. 9  illustrates substantially planar cuts identified by the dashed line  32 .  FIG. 10  illustrates the exemplary implant  210  inserted against the prepared articular processes IP, SP. In the illustrated embodiment, the implant  210  is secured to the inferior articular process IP with a threaded fastener  212 . The threaded fastener  212  includes an enlarged head  222  and a threaded shank  224  that passes through the aperture  218  and into the inferior articular process IP. The threaded fastener  212  may include a drive feature (not shown) disposed in the head  222 , such as a hex recess, slotted recess, cross recess, or other driving feature that permits insertion into the facet joint J. In other embodiments, the threaded fastener  212  may engage both articular processes IP, SP. In yet other embodiments, multiple threaded fasteners  212  may be installed to secure the implant  210  to the articular processes IP, SP.  
         [0037]     In an embodiment shown in  FIG. 11 , the implant  310  includes a cover portion  312  and a wedge portion  314  joined together to form a substantially T-shaped implant  310 . The cover portion  312  serves a similar purpose to embodiments described above. That is, the cover portion  312  covers the posterior junction between the inferior articular process IP of the superior vertebra V 1  and the superior articular process SP of the inferior vertebra V 2 . Accordingly, the cover portion  312  may include a substantially planar configuration as shown in  FIG. 11  or include curved or partially curved configurations similar to previously described embodiments. In contrast with embodiments described above, this implant  310  includes a wedge portion  314  that is positionable within the facet joint J, between the interface surfaces  26 ,  28  of the articular processes IP, SP. Consequently, in addition to preparing the outer surfaces  19 ,  20  of the facet joint J to receive the implant  310 , the fusion site may benefit from decorticating the interface surfaces  26 ,  28  as depicted by the dashed lines  36  in  FIG. 12 . Once the facet joint J is prepared by removing cortical bone as shown in  FIG. 12 , the implant  310  may be inserted as shown in  FIG. 13  and secured with a fastener  12 .  
         [0038]     In an embodiment shown in  FIGS. 14, 15 , and  16 , the implant  410  includes an elongated stake  414  and an enlarged head  412 . The stake portion  414  is similar to the retainer  12  used in other embodiments in that it includes a pointed insertion tip  420  that can be driven through one or both of the articular processes IP, SP. The head  412 , being wider than the stake  414 , limits the depth to which the stake  414  is inserted. In one implementation, the outer cortical surfaces  19 ,  20  of the articular processes IP, SP may be prepared to receive the head  412 . In one embodiment shown in  FIG. 15 , only one of the articular processes SP is decorticated as indicated by dashed line  38  to receive the head  412 . In other embodiments, neither or both articular processes IP, SP may be prepared by removing cortical bone.  
         [0039]     The enlarged head  412  extends laterally from the stake  414  at a first end  416  and curves along an arcuate path towards a second end  418 . The length and curvature of the head  412  may be configured so that the head covers the posterior junction between the inferior articular process IP and the superior articular process SP as shown in  FIG. 16 . In other embodiments, the head  412  may be formed similar to the head  22  of the retainer  12  shown in other embodiments. That is, the head portion  412  may be configured with a substantially cylindrical or disc shape.  
         [0040]      FIGS. 17 and 18  depict implant embodiments  510 ,  610 , that are similar to one another in that lateral sides  514 ,  614  of the implants  510 ,  610  extend substantially perpendicular to a substantially planar top surface  512 ,  612 . In each implant  510 ,  610 , the top surface  512 ,  612  and lateral sides  514 ,  614  form an open bottom end that leads into an interior cavity  520 ,  620 . In one embodiment, the intersection of the top surface  612  and side surface  614  may be rounded such that the implant  610  includes a substantially continuous outer surface forming a dome shaped structure. The facet joint J may be prepared to receive the implants  510 ,  610  by removing cortical bone from the exterior surfaces  19 ,  20  of the articular processes IP, SP as shown in  FIG. 19 . The dashed line  40  illustrates an exemplary cutting path to accommodate the lateral sides  514 ,  614  of the implants  510 ,  610 . Dashed line  42  represents a cut that reduces the height of the facet joint J and may be optional depending on the implementation. The cut may traverse a substantially cylindrical path or a square path depending on the shape of the side walls  514 ,  614  of the implant  510 ,  610 . The prepared facet joint J is inserted into the interior cavity  520 ,  620  as shown in  FIGS. 20 and 21 .  
         [0041]      FIG. 17  shows that the implant  510  includes an aperture  516  in the top surface  512  through which bone growth promoting material  100  may be inserted. For example,  FIG. 20  shows the implant  510  inserted onto the facet joint J and the aperture  516  exposing the interface surfaces  26 ,  28  to permit packing of the bone growth promoting material  100  between the articular processes IP, SP. Notably, the top surface  512  of the implant  510  is disposed a distance H above the top surface of the articular processes IP, SP, which may advantageously provide a buffer between the packed bone growth promoting material  100  and the overlying fibrous tissue. In  FIG. 17 , this distance H is relative to an outer surface  19 ,  20  that is cut according to dashed line  42  in  FIG. 19 . In other implementations, the height of the articular processes IP, SP may be retained by omitting the cut identified by dashed line  42  in  FIG. 19 . In either case, the implant  510  can be configured to provide the buffer represented by dimension H.  
         [0042]      FIG. 17  also shows that the implant  510  includes two side surfaces  514 . Thus, the implant  510  includes a generally inverted U-shaped structure. In other embodiments, the implant  510  may include three or four side surfaces  514  to form a box structure. The implant  510  also includes one or more apertures  518  that are sized to accept one of the aforementioned retainers  12 ,  212 . Alternatively, the implant  510  may be attached to the facet joint J without a retainer  12 ,  212  as shown in  FIG. 20  and using one of the aforementioned adhering techniques.  
         [0043]      FIG. 18  also shows that the implant  610  includes one or more apertures  618  that are sized to accept a retainer  12 ,  212 .  FIG. 21  illustrates that apertures  618  may be disposed opposite one another so that a retainer  12  can be driven through opposite sides of the side wall  614  and both articular processes IP, SP. Implant  510  may be secured to the facet joint J using a similar configuration.  
         [0044]     Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc and are also not intended to be limiting. Like terms refer to like elements throughout the description.  
         [0045]     As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.  
         [0046]     The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For instance, while only one of the two facet joints are depicted in the various detailed views provided according to the “DETAIL” callout in  FIG. 2 , a similar configuration may exist at the facet joint located on the opposite lateral side of the spine. The descriptions disclosed herein are not intended to be limited to facet joints on a single side of the spine. Those skilled in the art will comprehend the symmetry and applicability of the various embodiments disclosed herein. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.