Abstract:
Devices and methods for altering the spacing and motion at the facet joints of the vertebral column are provided. One embodiment of the invention comprises a prosthesis with surfaces configured to articulate with the facets of the facet joint. A retaining member for anchoring the prosthesis within the facet joint is optionally included. Methods for surgically and less invasively implanting the prosthesis and securing the prosthesis to the articular processes or surrounding soft tissue are also provided.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 10/865,073, filed Jun. 10, 2004, which claims the priority benefit under 35 U.S.C. § 119(e) to a) U.S. Provisional Patent Application No. 60/542,351, filed Feb. 6, 2004, b) U.S. Provisional Patent Application No. 60/542,769, filed Feb. 6, 2004, and c) U.S. Provisional Patent Application No. 60/542,350, filed Feb. 6, 2004, the disclosures of which are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to devices for augmentation and restoration of vertebral facet joints affected by degeneration and the surgical method of implanting these devices in the spine. 
       BACKGROUND OF THE INVENTION 
       [0003]    Traumatic, inflammatory, and degenerative disorders of the spine can lead to severe pain and loss of mobility. According to some studies, back and spinal musculoskeletal impairments are the leading causes of lost work productivity in the United States. Pain as a result of some type of spinal impairment may have its source in a variety of pathologies or clinical conditions. 
         [0004]    One source for back and spine pain is related to degeneration of the facets of the spine or facet arthritis. Bony contact or grinding of degenerated facet joint surfaces may play a role in some pain syndromes. While many technological advances have focused on the spinal disc and artificial replacement or repair of the disc, little advancement in facet repair has been made. Facet joint and disc degeneration frequently occur together. Thus, there is a need to address the clinical concerns raised by degenerative facet joints. 
         [0005]    The current standard of care to address the degenerative problems with the facet joints is to fuse the two adjacent vertebrae together. By performing this surgical procedure, the relative motion between the two adjacent vertebrae is stopped, thus stopping motion of the facets and any potential pain generated as a result thereof. This surgical procedure has a high rate of morbidity and can potentially lead to further clinical complications such as adjacent segment disorders. This procedure is also not reversible. Therefore, if the patient has an unsatisfactory result, they maybe subject to additional surgical fusion procedures. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention aims at addressing the clinical condition of the patient while allowing the patient to maintain mobility not common with fusion procedures. The device and procedure allow the restoration of the relative spacing between the facets within the facet joint, alleviating the bone on bone contact that is common in degenerative facet joints and often the source of pain generation, while allowing relative motion between the facets to continue post-operatively. 
         [0007]    While other implants have been proposed with the objective of addressing facet degeneration by restoring motion, the subject device offers the benefit of requiring little to no bony resection in order for it to be placed within the spine. This advantage provides the opportunity for the patient to rely more on those anatomical structures unaffected by degeneration while providing for very little morbidity in the surgical procedure. 
         [0008]    One embodiment of the invention comprises a device for treating spinal disorders while preserving movement at a facet joint. The device comprises a prosthesis having a first face and a second face, where the first face is adapted to be secured to the adjacent articular surface of a facet and the second surface is configured for sliding contact with an adjacent structure. In one embodiment, the device is dimensioned to substantially fit within a joint capsule of the facet joint and has a thickness generally equal to the normal anatomic spacing between the two facets of the facet joint. In some embodiments, the device has a curve adapted to match the natural shape of a facet and a size adapted to fit substantially within a joint capsule of the facet joint. The device may comprise at least one material selected from the group consisting of polymers, polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyethylene, fluoropolymers, hydrogels, elastomers, ceramics, zirconia, alumina, silicon nitride; metal(s), titanium, titanium alloy, cobalt chromium, stainless steel, and combinations of these materials. In one embodiment, the second face of the device comprises a highly polished surface. In one embodiment, the first face may comprise a roughened surface or a porous surface. In some embodiments, at least one face of the device is sufficiently malleable to be capable of generally conforming to the shape of an adjacent surface or structure under normal anatomical loads. 
         [0009]    In one embodiment of the invention, a device for treating spinal disorders while preserving movement at a facet joint is provided. The device may comprise a prosthesis having a first face and a second face, where the first face is adapted for sliding contact with a first articular process of a facet joint and the second surface is configured for sliding contact with a second articular process of the facet joint. In one embodiment, the device is dimensioned to substantially fit within a joint capsule of the facet joint and has a thickness generally equal to the normal anatomic spacing between the two facets of a facet joint. In one embodiment, the device has a curve adapted to match the natural shape of a facet and a size adapted to fit substantially within a joint capsule of the facet joint. The device has a thickness approximately equal to the normal anatomic spacing between the two facets of the facet joint. In one embodiment, the device has an average thickness within the range of about 0.5 mm to about 3 mm. In one embodiment, the device has an average thickness within the range of about 1 mm to about 2 mm. In another embodiment, the device has a diameter within the range of about 5 mm to about 25 mm. In another embodiment, the device has a size within the range of about 10 mm to about 20 mm in diameter. In one embodiment, at least one face of the device has a bone contacting surface area of about 25 mm 2  to about 700 mm 2 . In another embodiment, at least one face of the device has a bone contacting surface area of about 20 mm 2  to about 400 mm 2 . In still another embodiment of the device, at least one face of the device has a bone contacting surface area of about 20 mm 2  to about 100 mm 2 . In one embodiment, the device has at least one face comprising a highly polished surface. In some embodiments, at least one face of the device is sufficiently malleable to be capable of generally conforming to the shape of at least a portion of an articular process under normal anatomical conditions. 
         [0010]    The prosthesis may further comprise an anchoring assembly configured to generally maintain at least a portion of the prosthesis between the first articular process and the second articular process of the facet joint. The anchoring assembly may comprise an elongate member and at least one retaining member. In one embodiment, the elongate member comprises a wire or cable. In another embodiment, the elongate member comprises a solid wire or cable. In still another embodiment, the elongate member comprises a braided cable. The retaining member may comprise a set screw retaining ring. In one embodiment, at least one end of the device comprises a threaded interface. In one embodiment, the retaining member comprises a threaded retainer. In some embodiments, the retaining member is integrally formed with one end of the elongate member. 
         [0011]    In another embodiment of the invention, the device for treating facet joint dysfunction is provided. The device comprises a body with a first face and a second face adapted to contact the bony or cartilaginous articular surfaces of the facets of adjacent vertebrae. The device has at least one retaining interface capable of accepting an elongate retainer through it. An elongate retainer is adapted for generally maintaining the location of the body with respect to the facet joint. The retainer has a first portion adapted to engage a first facet of the facet joint and a second portion adapted to engage a second facet of the facet joint. In some embodiments of the invention, the device has a generally circular cross-section and a diameter adapted to fit substantially within a joint capsule of the facet joint. The device has a thickness generally equal to the normal anatomic spacing between the two facets of the facet joint. In still other embodiments of the device, the device has a curve adapted to match the natural shape of the facet and a size adapted to substantially fit within a joint capsule of the facet. The device may comprise at least one material selected from the group consisting of polymers, polyetheretherketone, polyetherketoneketone, polyethylene, fluoropolymers, hydrogels, elastomers, ceramics, zirconia, alumina, silicon nitride; metal(s), titanium, titanium alloy, cobalt chromium, stainless steel, and combinations of these materials. The elongate retainer may comprise a braided polymer, a braided metal, or a solid structure. In some embodiments of the invention, the elongate retainer comprises a flexibility sufficient to tie a knot in the elongate retainer. In another embodiment, at least one end of the elongate retainer has a threaded metal section adapted to accept a threaded knot. A threaded knot is provided to retain the elongate retainer against an articular process. In one embodiment of the invention, the threaded section is pressed or crimped onto the elongate retainer. The threaded section and knot may comprise titanium, titanium alloy, cobalt chromium or stainless steel. In some embodiments of the invention, the device comprises at least one face of the highly polished surface. In some embodiments, the elongate member may comprise at least one element with an enlarged cross-sectional area. The elongate member may comprise at least one end of with a bulbous retainer, a flared retainer, a T-bar retainer or an integral ring retainer. In some embodiments, at least one face of the device is sufficiently malleable to be capable of generally conforming to the shape of at least a portion of an articular surface. 
         [0012]    In one embodiment of the invention, a prosthesis for treating facet joint dysfunction is provided. The prosthesis comprises a body with a first face and a second face, where at least one face adapted for sliding contact with the bony or cartilaginous articular surfaces of the facets of adjacent vertebrae or the prosthesis has at least one retaining interface capable of accepting a retainer member. The retaining member is adapted for securing the location of the body with respect to at least of the articular surfaces. The retaining member may comprise a first portion adapted to engage the retaining interface of the body and a second portion adapted to engage a first facet of the facet joint. The retainer may further comprise a third portion adapted to engage a second facet of the facet joint. In one embodiment, the retainer comprises a threaded shaft and a retaining interface of the body comprises a threaded hole with an opening on one face of the body. The retaining member may also comprise a projection extending from the body. In still another embodiment, the retaining member comprises a longitudinal member adapted to engage the retaining interface of the body and at least one retainer being capable of engaging the longitudinal member. The retaining ring may comprise a set screw retaining ring. The set screw of the retaining member may have a blunted tip, curved tip, or piercing tip. Alternatively, at least one of the retaining rings may be a friction fit retaining ring. The body of the prosthesis may be curved. The prosthesis may comprise at least one material selected from the group consisting of polymers, polyetheretherketone, polyetherketoneketone, polyethylene, fluoropolymers, hydrogels, elastomers, ceramics, zirconia, alumina, silicon nitride; metal(s), titanium, titanium alloy, cobalt chromium, stainless steel, and combinations of these materials. In some embodiments, at least one face of the prosthesis is sufficiently malleable to be capable of generally conforming to the shape of at least a portion of an articular surface. 
         [0013]    In one embodiment, a prosthesis for treating facet joint dysfunction is provided. The prosthesis comprises a first body with a first face and a second face and a second body within a first face and a second face. The first face of each body is adapted to articulate with the first face of the other body and the second face of each body is adapted to engage a facet of a facet joint. The prosthesis may further comprise a retaining member adapted for securing a location of at least one body. In some embodiments, at least one face of the prosthesis is sufficiently malleable to be capable of generally conforming to the shape of at least a portion of an articular surface. 
         [0014]    In another embodiment of the invention, a method for treating vertebral dysfunction is provided. This method comprises opening a facet joint capsule between two facets of adjacent vertebral bodies, distracting the adjacent vertebral bodies from a first spacing to a second spacing and placing the spacer between the two facets to maintain the second spacing. The method may further comprise the steps of securing the spacer to one facet of the facet joint. The method may also comprise securing the spacer in the facet joint capsule. The step of securing the spacer may comprise drilling a hole through each facet, threading a retainer through the hole of the first facet, threading the retainer through the hole in the spacer, threading the retainer through the hole of the second facet, and tying a knot in at least one end of the retainer. The method may further comprise the steps of drilling a hole through a first facet and a second facet, advancing the retainer through the hole of the first facet, advancing the retainer through the hole in the spacer, threading the retainer through the hole of the second facet and threadably engaging an anchor to at least one end of the retainer. The step of securing the spacer may further comprise providing a spacer with a retaining member and advancing the retaining member at least partially into a facet to engage the facet. The method may also further comprise the step of conforming the shape of at least a portion of the spacer to at least a portion of a facet of the facet joint. In a further embodiment, the conforming step is performed after the placing step. In another embodiment, the conforming step is performed while the spacer is generally located between the facets of the facet joint. 
         [0015]    In another embodiment of the invention, a method of treating the facet joint is provided. The method comprises providing a prosthesis dimension to fit within a facet joint capsule, accessing a facet joint capsule between two articular prosthesis of two vertebrae, inserting a prosthesis generally within the joint capsule and maintaining the prosthesis generally between the two articular prosthesis without penetrating the surface of a vertebrae. Maintaining the prosthesis may comprise anchoring the prosthesis to the joint capsule tissue, or generally closing the joint capsule over the prosthesis. The prosthesis can also be maintained between the articular prosthesis by suturing the prosthesis to the surrounding soft tissue. The method may also further comprise the step of conforming the shape of at least a portion of the prosthesis to at least a portion of a facet of the facet joint. In a further embodiment, the conforming step is performed after the inserting step. In another embodiment, the conforming step is performed while the prosthesis is generally located between the facets of the facet joint. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The structure and operation of the invention will be better understood with the following detailed description of embodiments of the invention, along with the accompanying illustrations, in which: 
           [0017]      FIG. 1  is a lateral elevational view of a portion of the vertebral column; 
           [0018]      FIGS. 2A and 2B  are schematic superior and side views of an isolated thoracic vertebra; 
           [0019]      FIGS. 3A and 3B  are schematic posterior and posterior-oblique elevational views of a portion of the vertebral column; 
           [0020]      FIGS. 4A and 4B  are schematic side and superior views of a facet joint in the cervical vertebrae; 
           [0021]      FIGS. 5A and 5B  are schematic side and superior views of a facet joint in the thoracic vertebrae; 
           [0022]      FIGS. 6A and 6B  are schematic side and superior views of a facet joint in the lumbar vertebrae; 
           [0023]      FIGS. 7A and 7B  are schematic views of one embodiment of a facet joint prosthesis comprising a circular disc; 
           [0024]      FIG. 8  is a schematic view of the prosthesis from  FIG. 7A  implanted in a facet joint; 
           [0025]      FIGS. 9A and 9B  are schematic views of one embodiment of a facet joint prosthesis comprising an octagonal disc; 
           [0026]      FIGS. 10A and 10B  are schematic views of one embodiment of a facet joint prosthesis comprising a biconcave disc; 
           [0027]      FIGS. 11A and 11B  are schematic views of one embodiment of a facet joint prosthesis comprising a single-face variable thickness disc; 
           [0028]      FIGS. 12A and 12B  are schematic views of one embodiment of a facet joint prosthesis comprising a curved disc; 
           [0029]      FIG. 13  is a schematic view of the prosthesis from  FIG. 12A  implanted in a facet joint; 
           [0030]      FIGS. 14A and 14B  are schematic views of one embodiment of a facet joint prosthesis comprising a disc with a roughened surface on one face; 
           [0031]      FIGS. 15A and 15B  are schematic views of one embodiment of a facet joint prosthesis comprising a disc with a porous surface on one face; 
           [0032]      FIGS. 16A and 16B  are schematic views of one embodiment of a facet joint prosthesis comprising a bent disc with a roughened surface on the greater face; 
           [0033]      FIG. 17  is a schematic view of the prosthesis from  FIG. 16A  implanted in a facet joint; 
           [0034]      FIGS. 18A and 18B  are schematic views of one embodiment of a facet joint prosthesis comprising two discs, each with a roughened surface on one face; 
           [0035]      FIG. 19  is a schematic view of the prosthesis from  FIG. 18A  implanted in a facet joint; 
           [0036]      FIG. 20  is a schematic view of a retaining member comprising a braided cable; 
           [0037]      FIGS. 21A and 21B  are schematic views of one embodiment of a facet joint prosthesis with a retaining interface comprising a centrally located hole; 
           [0038]      FIGS. 22A and 22B  are schematic views of one embodiment of a facet joint prosthesis with a retaining interface comprising an eccentrically located hole; 
           [0039]      FIGS. 23A and 23B  are schematic views of one embodiment of a facet joint prosthesis with a retaining interface comprising an edge contiguous hole; 
           [0040]      FIGS. 24A and 24B  are schematic views of one embodiment of a facet joint prosthesis comprising two discs, each with an eccentrically located hole; 
           [0041]      FIGS. 25A and 25B  are schematic views of one embodiment of a facet joint prosthesis comprising a curved disc with a retaining interface; 
           [0042]      FIG. 26  depicts one embodiment of the invention where the cable is engaged to the articular processes using knots in the cable; 
           [0043]      FIGS. 27A and 27B  depict another embodiment of the retaining member comprising a braided cable with threaded ends adapted to accept threaded nuts; 
           [0044]      FIG. 28  depicts one embodiment of the invention where a cable is engaged to the articular processes using nuts threaded onto the cable; 
           [0045]      FIG. 29  depicts a preferred embodiment of the invention comprising a curved prosthesis, cable and two set-screw retaining rings; 
           [0046]      FIGS. 30A and 30B  are elevational and cross-sectional views of one embodiment of the set-screw retaining rings, respectively; 
           [0047]      FIGS. 31 through 33  are elevational views of various embodiments of the screw in the set-screw retaining rings; 
           [0048]      FIGS. 34A to 35B  are one embodiment of the invention comprising friction fit retaining rings.  FIGS. 34A and 34B  depict the retaining rings in their reduced state and  FIGS. 35A and 35B  depict the retaining rings in their expanded state; 
           [0049]      FIGS. 36A to 36C  illustrate embodiments of the invention comprising a prosthesis with a close-ended threaded retaining interface and a threaded retaining member; 
           [0050]      FIGS. 36B and 36C  depict a threaded retaining member with a pivotable washer; 
           [0051]      FIG. 37A  is a cross sectional view of the prosthesis in  FIG. 36A  implanted in a facet joint;  FIG. 37B  is a cross sectional view of the prosthesis in  FIG. 36B  implanted in a facet joint; 
           [0052]      FIG. 38  is a cross sectional view of a two-part prosthesis comprising flat discs implanted into a facet joint; 
           [0053]      FIG. 39  is a cross sectional view of a two-part prosthesis comprising curved discs implanted into a facet joint; 
           [0054]      FIGS. 40A and 40B  are schematic views of one embodiment of a facet joint prosthesis with an integral retaining member comprising a centrally located barbed spike; 
           [0055]      FIGS. 41A and 41B  are schematic views of one embodiment of a facet joint prosthesis with an integral retaining member comprising an eccentrically located barbed spike; 
           [0056]      FIG. 42  depicts the prosthesis of  FIG. 38A  implanted into a facet joint; 
           [0057]      FIG. 43  illustrates a two-part prosthesis implanted into a facet joint; 
           [0058]      FIG. 44  shows one embodiment of the invention comprising a prosthesis with multiple anchoring projections; 
           [0059]      FIG. 45  shows the prosthesis of  FIG. 44  implanted into a facet joint; 
           [0060]      FIGS. 46A and 46B  depict one embodiment of the invention comprising a prosthesis with a rigid soft tissue side anchor; 
           [0061]      FIGS. 47A and 47B  depict one embodiment of the invention comprising a prosthesis with an embedded flexible soft tissue side anchor; 
           [0062]      FIG. 48  depicts one embodiment of the invention depicting a posterior surgical approach for implanting a prosthesis in the cervical vertebrae; 
           [0063]      FIG. 49  depicts one embodiment of the invention depicting the cross-sectional surgical approach for implanting a prosthesis in the cervical vertebrae; 
           [0064]      FIG. 50  depicts one embodiment of the invention depicting a posterior surgical approach for implanting a prosthesis in the thoracic vertebrae; and 
           [0065]      FIGS. 51A to 51E  depicts one embodiment of the invention depicting a posterior surgical approach for implanting a prosthesis in the lumbar vertebrae;  FIGS. 51A to 51C  are posterior views of the surgical procedure and  FIGS. 51D and 51E  are cross sectional views of the surgical procedure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A. Anatomy of the Spine 
       [0066]    As shown in  FIG. 1 , the vertebral column  2  comprises a series of alternating vertebrae  4  and fibrous discs  6  that provide axial support and movement to the upper portions of the body. The vertebral column  2  typically comprises thirty-three vertebrae  4 , with seven cervical (C 1 -C 7 ), twelve thoracic (T 1 -T 12 ), five lumbar (L 1 - 15 ), five fused sacral (S 1 -S 5 ) and four fused coccygeal vertebrae.  FIGS. 2A and 2B  depict a typical thoracic vertebra. Each vertebra includes an anterior body  8  with a posterior arch  10 . The posterior arch  10  comprises two pedicles  12  and two laminae  14  that join posteriorly to form a spinous process  16 . Projecting from each side of the posterior arch  10  is a transverse  18 , superior  20  and inferior articular process  22 . The facets  24 ,  26  of the superior  20  and inferior articular processes  22  form facet joints  28  with the articular processes of the adjacent vertebrae. See  FIGS. 3A and 3B . The facet joints are true synovial joints with cartilaginous surfaces and a joint capsule. 
         [0067]    The orientation of the facet joints vary, depending on the level of the vertebral column. In the C 1  and C 2  vertebrae, the facet joints are parallel to the transverse plane.  FIGS. 4A to 6B  depict the orientations of the facet joints at different levels of the vertebral column. In the C 3  to C 7  vertebrae shown in  FIGS. 4A and 4B , the facets are oriented at a 45-degree angle to the transverse plane  30  and parallel to the frontal plane  32 , respectively. This orientation allows the facet joints of the cervical vertebrae to flex, extend, lateral flex and rotate. At a 45-degree angle in the transverse plane  30 , the facet joints of the cervical spine can guide, but do not limit, the movement of the cervical vertebrae.  FIGS. 5A and 5B  depict the thoracic vertebrae, where the facets are oriented at a 60-degree angle to the transverse plane  30  and a 20-degree angle to the frontal plane  32 , respectively. This orientation is capable of providing lateral flexion and rotation, but only limited flexion and extension.  FIGS. 6A and 6B  illustrate the lumbar region, where the facet joints are oriented at 90-degree angles to the transverse plane  30  and a 45-degree angle to the frontal plane  32 , respectively. The lumbar vertebrae are capable of flexion, extension and lateral flexion, but little, if any, rotation because of the 90-degree orientation of the facet joints in the transverse plane. The actual range of motion along the vertebral column can vary considerably with each individual vertebra. 
         [0068]    In addition to guiding movement of the vertebrae, the facet joints also contribute to the load-bearing ability of the vertebral column. One study by King et al.  Mechanism of Spinal Injury Due to Caudocephalad Acceleration, Orthop. Clin. North Am.,  6:19 1975, found facet joint load-bearing as high as 30% in some positions of the vertebral column. The facet joints may also play a role in resisting shear stresses between the vertebrae. Over time, these forces acting on the facet joints can cause degeneration and arthritis. 
       B. Joint Prosthesis 
       [0069]    In one embodiment of the invention, a device for restoring the spacing between two facets of a facet joint is provided. As shown in  FIGS. 7A and 7B , the device comprises a prosthesis  34  with a least two faces, a first face  36  adapted to contact the articular surface of one facet of the facet joint and a second face  38  adapted to contact the articular surface of the other facet. In one embodiment, the prosthesis  34  has a generally circular profile and is sized to fit generally within the joint capsule of the facet joint  28 .  FIG. 8  illustrates the prosthesis  34  of  FIGS. 7A and 7B  positioned in a facet joint. In other embodiment of the invention, the prosthesis can have any of a variety of profiles, including but not limited to square, rectangle, oval, star, polygon or combination thereof. An octagonal prosthesis is shown in  FIGS. 9A and 9B . In one embodiment of the invention, a prosthesis having the desired shape is selected from an array of prostheses after radiographic visualization of the articular processes and/or by radio-contract injection into the facet joint to visualize the joint capsule. In one embodiment, the prosthesis has a diameter of about 4 mm to about 30 mm. In another embodiment, the prosthesis has a diameter of about 5 mm to about 25 mm. In still another embodiment, the prosthesis has a diameter of about 10 mm to about 20 mm. In one embodiment, the prosthesis has a cross-sectional area of about 10 mm 2  to about 700 mm 2 . In another embodiment, the prosthesis has a cross-sectional area of about 25 mm 2  to about 500 mm 2 . In still another embodiment, the prosthesis has a cross-sectional area of about 20 mm 2  to about 400 mm 2 , and preferably about 25 mm 2  to about 100 mm 2 . 
         [0070]    The prosthesis has a thickness generally equal to about the anatomic spacing between two facets of a facet joint. The prosthesis generally has a thickness within the range of about 0.5 mm to about 3.0 mm. In certain embodiments, the prosthesis has a thickness of about 1 mm to about 2 mm. In one preferred embodiment, the prosthesis has a thickness of about 0.5 mm to about 1.5 mm. In one embodiment, the thickness of the prosthesis is nonuniform within the same prosthesis. For example, in  FIGS. 10A and 10B , the thickness of the prosthesis  42  is increased around the entire outer edge  44 , along at least one and, as illustrated, both faces  46 ,  48 . In  FIGS. 11A and 11B , only a portion of the edge  44  on one face  46  of the prosthesis  42  has a thickness that is greater than the thickness of a central region, and, optionally, also thicker than the typical anatomic spacing between two facets of a facet joint. An increased edge thickness may resist lateral displacement of the prosthesis out of the facet joint. 
         [0071]    In some embodiments of the invention, the prosthesis is configured to provide an improved fit with the articular process and/or joint capsule. For example, in  FIGS. 12A and 12B , the prosthesis  49  has a bend, angle or curve  50  to generally match the natural shape of an articular facet.  FIG. 13  depicts the prosthesis of  FIGS. 12A and 12B  positioned in a facet joint. The prosthesis may be rigid with a preformed bend. Alternatively, the prosthesis may be sufficiently malleable that it will conform post implantation to the unique configuration of the adjacent facet face. Certain embodiments of the invention, such as those depicted in  FIG. 8  and  FIG. 13 , the prosthesis is configured to be implanted between the articular processes and/or within the joint capsule of the facet joint, without securing of the prosthesis to any bony structures. Such embodiments can thus be used without invasion or disruption of the vertebral bone and/or structure, thereby maintaining the integrity of the vertebral bone and/or structure. 
         [0072]    In one embodiment, at least a portion of one surface of the prosthesis is highly polished. A highly polished portion of the prosthesis may reduce the surface friction and/or wear in that portion of the prosthesis as it contacts bone, cartilage or another surface of the prosthesis. A highly polished surface on the prosthesis may also decrease the risk of the prosthesis wedging between the articular surfaces of the facet joint, which can cause pain and locking of the facet joint. 
         [0073]    In one embodiment, shown in  FIGS. 14A and 14B , at least a portion of one surface of the prosthesis  50  has a roughened surface  52 . A roughened surface may be advantageous when in contact with a bone or tissue surface because it may prevent slippage of the prosthesis  50  against the bone and aid in maintaining the prosthesis  50  in the joint. In one embodiment of the invention, shown in  FIGS. 15A and 15B , at least a portion of one surface of the prosthesis  50  has a porous surface  54 . A porous surface  54  can be created in any a variety of ways known in the art, such as by applying sintered beads or spraying plasma onto the prosthesis surface. A porous surface  54  can allow bone to grow into or attach to the surface of the prosthesis  50 , thus securing the prosthesis  50  to the bone. In one embodiment, an adhesive or sealant, such as a cyanoacrylate, polymethylmethacrylate, or other adhesive known in the art, is used to bond one face of the prosthesis to an articular surface. 
         [0074]    In one embodiment of the invention, one surface of the prosthesis is roughened or porous and a second surface that is highly polished. The first surface contacts or engages one facet of the facet joint and aids in maintaining the prosthesis between the articular surfaces. The second surface of the prosthesis is highly polished and contacts the other facet of the facet joint to provide movement at that facet joint.  FIGS. 16A and 16B  represent one embodiment of the prosthesis comprising a curved or bent disc  56  with a roughened surface  52  on the greater face  58  of the disc and a highly polished surface  60  on the lesser face  62 .  FIG. 17  depicts the prosthesis of  FIGS. 16A and 16B  positioned in a facet joint. The prosthesis generally maintains a fixed position relative to the facet contacting the roughened surface while the movement of the facet joint is preserved between the other facet and the highly polished lesser face of the prosthesis. 
         [0075]      FIGS. 18A and 18B  show one embodiment of the invention, where the prosthesis  64  comprises two separate discs  66 , each disc comprising a first face  68  that articulates with the complementary first face  68  of the other disc, and a second face  70  adapted to secure the disc to the adjacent bone or cartilage of one facet of the facet joint  28 . In one embodiment of the invention, the thickness of one disc will generally be about half of the anatomic spacing between two facets of the facet joint. In other embodiments of the invention, the prosthesis comprises three or more discs. In one embodiment the total thickness of all the discs is generally about 25% to about 300% of the anatomic spacing between the two facets. In another embodiment, the total thickness of the discs is generally about 50% to about 150% of the anatomic spacing. In still another embodiment, the total thickness of the discs is about 75% to about 125% of the anatomic spacing. Each disc of the two-part prosthesis can otherwise also have features similar to those of a single-disc prosthesis, including but not limited to curved or bent configurations, highly polished or roughened surfaces, and other feature mentioned below. The two discs need not have the same size, thickness, configuration or features.  FIG. 19  depicts one embodiment of a two-part prosthesis  64  positioned within a facet joint  28 . 
         [0076]    The prosthesis can be manufactured from any of a variety of materials known in the art, including but not limited to a polymer such as polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyethylene, fluoropolymer, hydrogel, or elastomer; a ceramic such as zirconia, alumina, or silicon nitride; a metal such as titanium, titanium alloy, cobalt chromium or stainless steel; or any combination of the above materials. 
         [0000]    C. Prosthesis with a Retaining Configuration 
         [0077]    In one embodiment of the invention, the prosthesis is maintained between the two facets of the facet joint by taking advantage of the joint capsule and/or other body tissue surrounding the facet joint to limit the migration of the prosthesis out of the facet joint. In some embodiments of the invention, the shape of the prosthesis itself is capable of resisting displacement of the prosthesis from its position generally between the facet joint surfaces. In one embodiment, a concave or biconcave configuration resists displacement of the prosthesis by providing an increased thickness at the periphery of the prosthesis that requires a larger force and/or greater distraction of facet joint surfaces in order to cause displacement. In other embodiments, surface treatments or texturing are used to maintain the prosthesis against a facet of the facet joint, as described previously. In some embodiments, a combination of disc configuration, surface texturing and existing body tissue or structures are used to maintain the position of the prosthesis. 
         [0078]    Bone growth facilitators, electrical current, or other known techniques may be used to accelerate osteoincorporation of textured or microporous anchoring surfaces. 
         [0000]    D. Prosthesis with a Retaining Member 
         [0079]    The prosthesis may be configured with a retaining interface to engage a retaining member that facilitates retention of the prosthesis within the joint capsule of the facet joint. Use of a retaining member may be advantageous for preventing migration of the prosthesis over time use or with the extreme ranges of vertebral movement that may distract the articular surfaces sufficiently to allow the prosthesis to slip out. 
         [0080]    1. Wire/Cable Retaining Member 
         [0081]    In one embodiment of the invention, shown in  FIGS. 20 to 21B , the retaining member comprises a wire or cable  72  with a portion  74  that engages the prosthesis  76  at a retaining interface  78 , and at least one other portion  80  that engages or anchors to the bone or soft tissue surrounding the facet joint. The wire or cable may be solid, braided or multi-filamented. The retaining member in this embodiment will be described primarily as a cable or wire, but it is to be understood that any of a variety of elongate structures capable of extending through a central aperture will also work, including pins, screws, and single strand or multistrand polymeric strings or weaves, polymeric meshes and fabric and other structures that will be apparent to those of skill in the art in view of the disclosure herein. 
         [0082]    The cross-sectional shape of the retaining member can be any of a variety of shapes, including but not limited to circles, ovals, squares, rectangles, other polygons or any other shape. The wire or cable generally has a diameter of about 0.5 mm to about 2 mm and a length of about 5 mm to about 60 mm. In another embodiment, wire or cable has a diameter of about 0.25 mm to about 1 mm, and preferably about 0.75 mm to about 1.25 mm. The diameter of the wire or cable may vary along the length of the wire or cable. In one embodiment, the wire or cable has a length of about 10 mm to about 40 mm. In another embodiment, the wire or cable has a length of about 20 mm to about 30 mm. 
         [0083]    In one embodiment, shown in  FIGS. 21A and 21B , the retaining interface  78  of the prosthesis  76  is a conduit between the two faces  82 ,  84  of the prosthesis  76 , forming an aperture  78 . In one embodiment, the aperture  78  has a diameter larger than the diameter of the wire or cable  72 , to provide the prosthesis  76  with a range of motion as the facet joint moves. The aperture  78  inside diameter may be at least about 110%, often at least about 150% and in certain embodiments at least about 200% or 300% or greater of the outside diameter or corresponding dimension of the retaining member in the vicinity of the engagement portion  78 . The cross-sectional shape of the aperture  78  can match or not match the cross sectional shape of the wire or cable used. 
         [0084]    In another embodiment, the retaining interface  78  extends only partially through the prosthesis  72 . The retaining interface  78  may be located generally in the center of the prosthesis, or it may be located eccentrically, as depicted in  FIGS. 22A and 22B . In one embodiment, shown in  FIGS. 23A and 23B , the retaining interface  78  is located at the edge  86  of the prosthesis  76  such that the interior surface of the hole  78  is contiguous with the outer edge of the prosthesis. This configuration of the retaining interface  78  does not require the cable  72  to be threaded through the retaining interface  78  and may facilitate engagement of the retaining member with the prosthesis.  FIGS. 24A and 24B  depict an embodiment of the invention comprising a two-part prosthesis  88 . Either a single cable or two separate cables may be used retain both discs within the facet joint.  FIGS. 25A and 25B  depict another embodiment of the invention comprising a curved prosthesis  90  with a retaining interface  78  adapted to accept a cable. 
         [0085]    In  FIG. 26 , the wire or cable  72  is secured to the articular processes  20 ,  22  by tying one or more knots  92  in the cable  72  that can resist pulling of the wire or cable through the articular process. In another embodiment, one or both ends of the wire or cable are provided with an anchor to resist migration of the implants. As shown in  FIGS. 27A and 27B , one or both ends of the wire or cable  72  may be threaded such that a nut  94  can be tightened on the wire or cable  72  to secure the wire or cable to the articular processes  20 ,  22 .  FIG. 28  depicts the attachment of a nut onto a threaded end of a cable. The threaded portion  96  of the wire or cable can be secured to the cable by pressing, crimping or twisting the threaded  96  portion onto the cable  72 . In one embodiment, the threaded portion  96  is made from titanium, titanium alloy, cobalt chromium, stainless steel, or any combination thereof. 
         [0086]    In one embodiment, the wire or cable has two threaded ends  96  for engaging the bony or cartilaginous tissue, one portion for each facet of the facet joint. 
         [0087]    In another embodiment, shown in  FIG. 29 , the wire or cable is secured to the articular process with retaining rings  98 . As depicted in  FIGS. 30A and 30B , the retaining rings  98  comprise a ring  100  with a central lumen  102  and a locking element to facilitate locking the ring  100  to a retaining member. The central lumen  102  is adapted to accept insertion of a wire or cable through it. The illustrated locking element is in the form of a side lumen  104  which is threaded and configured to accept a rotatable screw  106  with a proximal end  108 , a threaded body  110  and a distal end  112 . The threaded body  110  is complementary to the threads of the side lumen  104  so that when the screw  106  is rotated at its distal end  112 , the proximal end  108  of the screw  106  moves further into the central lumen  102  and is capable of applying increasing force to a wire or cable inserted through the central lumen  102 . In one embodiment, the force on the wire or cable is capable of creating a friction fit or a mechanical interfit to resist movement between the wire or cable and the retaining ring  98 , thereby securing the wire or cable to the articular process  20  or  22 . As shown in  FIGS. 31 to 33 , the distal end  112  of the screw  106  can be configured to engage the wire or cable in any of a variety designs, including but no limited to a blunt tip  114 , curved tip  116  and piercing tip  118 . 
         [0088]    In another embodiment, depicted in  FIGS. 34A and 34B , the wire or cable is securable to the articular process with a retaining ring  120  have radially inward biased projections  122  defining a central lumen  124 . The central lumen has a cross-sectional shape smaller than that of the wire or cable but is capable of enlargement when the inward projections  122  are bent away, as shown in  FIGS. 35A and 35B . The inward projections  122  apply increasing force to the wire or cable within the central lumen  124  as the projections  122  are bent, thereby creating a friction fit. 
         [0089]    In one embodiment of the invention, one end of the wire or cable retaining member is preformed with a retainer for engaging the articular process. The retainer may be a preformed ring, bulb, flared end, T-bar end, or any of a variety of shapes having a greater cross sectional area than the other portions of the wire or cable retaining member. This configuration of the wire or cable retaining member is adapted to engage an articular process by passing the free end of a wire or cable retaining member through an articular process such that the end with the preformed retainer can engage the articular process. 
         [0090]    In one embodiment, the wire or cable retaining member is secured to the articular processes with sufficient laxity or length between the secured ends or between the prosthesis and one secured end so that the two articular processes are not fixed in position relative to each other and remain capable of performing movements such as flexion, extension, lateral flexion and/or rotation. In one embodiment, the retaining member comprises a cable of braided polymer, including but not limited to a braided polymer such as PEEK or PEKK, or a braided metal, such as braided cobalt chromium or titanium. The cable can be selected with different degrees of flexibility to provide different degrees of movement at that facet joint. The cable has a first segment capable of engaging the prosthesis at its retaining interface to limit the movement 
         [0091]    2. Screw/Bolt Retaining Member 
         [0092]    In one embodiment of the invention, shown in  FIG. 36A , the retaining member comprises a screw or bolt  126  with a proximal end  128 , body  130  and distal end  132 . The distal end  132  of the screw or bolt is capable of forming a mechanical interfit with a complementary retaining interface  134  on the prosthesis or spacer  136 . The distal end  132  typically comprises threads, but one skilled in the art will understand that other configurations may be used to form a mechanical interfit. The complementary retaining interface  134  on the prosthesis  136  could be a threaded through hole or preferably, a close-ended hole. The proximal end  128  of the screw or bolt  126  has a hex or other type of interface known in the art, capable of engaging a rotating tool to manipulate the screw or bolt  126 . The body of the screw or bolt  126  has a length sufficient to at least span the length of the hole or conduit created through the articular process for securing the prosthesis. In  FIG. 36B , the retaining member further comprises a pivotable washer  127  with a pivot surface  129  that articulates with the proximal end  128  of the screw  126 . In one embodiment, the pivotable washer  127  is capable of a range of positions relative to the screw  126  and provides the screw  126  with a better surface area contact with the bone. 
         [0093]      FIG. 37  is a cross-sectional view of a facet joint  28  with a spacer  136  bolted to one articular process  20  of a facet joint  28 . The spacer  136  position is fixed relative to one facet  24  of the joint  28 , but provides for spacing and movement of the other facet  26  with respect to the spacer  136 . In embodiments of the invention comprising a two-part prosthesis, shown in  FIGS. 38 and 39 , each disc may have its own screw or bolt retaining member.  FIG. 38  depicts a flat two-part prosthesis  138  and  FIG. 39  depicts a curved two-part prosthesis  140 . 
         [0094]    3. Projection Retaining Member 
         [0095]    In some embodiments of the invention, shown in  FIGS. 40A through 41B , the retaining member is integral with or attached to the prosthesis and comprises a projection  142  from the prosthesis  144  that is adapted to engage the adjacent articular process or surrounding tissue. In one embodiment, the projection comprises at least one spike  142  or hook projecting from one face of the prosthesis  144 . In one embodiment, the spike  142  or hook can be ribbed, barbed or threaded to resist separation after insertion into bone or tissue.  FIG. 42  depicts the prosthesis  144  of  FIG. 40A  engaged to a facet  24  of the facet joint  28 . In one embodiment comprising a two-part prosthesis  146 , shown in  FIG. 43 , each disc  148  may have its own projection-retaining member  142 . In some embodiments of the invention, as depicted in  FIG. 44 , more than one projection  150  is provided on the prosthesis  152 .  FIG. 45  illustrates the prosthesis of  FIG. 44  placed in a facet joint  28 . The projections  150  may be angled with respect to the prosthesis  152  to resist dislodgement by the movement at the joint. 
         [0096]      FIGS. 46A to 47B  illustrate embodiments of the invention where the retaining member comprises a projection  154  extending laterally such as from the side of the prosthesis  156 , and adapted to engage the soft tissue surrounding the facet joint, rather than a bony or cartilaginous articular process. In one example, the prosthesis of  FIG. 46  could be inserted into a facet joint through an incision made in the joint capsule, but the integrity of the joint capsule opposite the incision site is maintained and used as an anchoring site for the prosthesis. The orientation of the projection can be fixed as in  FIG. 44 , or flexible.  FIG. 47  depicts a flexible tether such as a wire  158  with its proximal end  160  embedded in or otherwise attached to the prosthesis and one or more barbs which may be attached to its distal end  162 . A flexible projection may provide greater selection of soft tissue anchoring sites for the prosthesis. 
         [0097]    In one embodiment of the invention, the joint capsule is closed after placement of the prosthesis. Closure may be performed using adhesives, suturing, stapling or any of a variety of closure mechanisms known in the art. 
       E. Accessing the Facet Joints 
       [0098]    1. Surgical Approach to the Cervical Spine 
         [0099]    In one embodiment of the invention, general anesthesia is achieved and the patient is positioned prone on a turning frame or three-point head rest attached to the table. Skeletal traction is performed using tongs. The patient is prepped and draped in the usual sterile fashion. Pre-operative radiographic films are reviewed and any vertebral anomalies or variations are noted. In one embodiment, the spinous processes are palpated to identify the location of the cervical vertebrae and a skin incision is made over the desired vertebrae, as shown in  FIG. 48 . In another embodiment, a paraspinous skin incision is made over the desired facet joint. The exposed skin edges and subcutaneous tissue are injected with epinephrine 1:500,000 solution to facilitate hemostasis. Dissection to the spinous processor facet joint is performed using an electrocautery knife. In one embodiment, shown in  FIG. 49 , dissection is performed along the nuchal ligament  164  to avoid cutting into vascular muscle tissue. Soft tissue retractors are used to maintain tissue tension and aid the dissection process. The ligamentous attachments to the spinous process  16  are detached and the facet joints are exposed. In another embodiment, dissection is performed through the muscle tissue to directly access the facet joint. The joint capsule of the facet joint is opened by incision or piercing. The facets of the facet joint are distracted as required to provide access to the joint space. In one embodiment, the affected facet joint is sized and a joint prosthesis is selected. In one embodiment, the articular process or processes are prepared for receiving the joint prosthesis, including but not limited to roughening the articular surface of the articular process and/or drilling a hole for the prosthesis anchor or retaining member. The prosthesis is inserted into the facet joint space and the anchor or retaining member, if any is attached to the articular process. The steps are repeated until all the joint prostheses have been inserted. The surgical site is closed in layers with a suction tube or drainage tube in place. The surgical site is cleaned and dressed. 
         [0100]    2. Surgical Approach to the Thoracic Spine 
         [0101]    In one embodiment of the invention, general anesthesia is achieved and the patient is positioned prone on a padded spinal operating frame. The patient is prepped and draped in the usual sterile fashion. Pre-operative radiographic films are reviewed and any vertebral anomalies or variations are noted. In one embodiment, shown in  FIG. 50 , a midline skin incision is made over the desired vertebrae. In another embodiment, a paraspinous skin incision is made over the desired facet joint. The exposed skin edges, subcutaneous tissue and erector spinae muscles are injected with epinephrine 1:500,000 solution to facilitate hemostasis. Dissection is performed using an electrocautery knife or scalpel through the superficial and lumbodorsal fascia to the tips of the spinous processes. The erector spinae muscle is reflected laterally to the tips of the transverse processes, thereby exposing the posterior arch. After exposure of all the desired vertebrae is achieved, an intra-operative x-ray is obtained to confirm access to the desired vertebrae. The facets of the facet joint are distracted as required to provide access to the joint space. The joint capsule of the facet joint is opened by incision or piercing. In one embodiment, the affected facet joint is sized and a joint prosthesis is selected. In one embodiment, the articular process or processes are prepared for receiving the joint prosthesis, including but not limited to roughening the articular surface of the articular process and/or drilling a hole for the prosthesis anchor or retaining member. The prosthesis is inserted into the facet joint space and the anchor or retaining member, if any is attached to the articular process. The steps are repeated until all the joint prostheses have been inserted. The surgical site is closed in layers with a suction tube or drainage tube in place. The surgical site is cleaned and dressed. 
         [0102]    3. Surgical Approach to the Lumbar Spine 
         [0103]    In one embodiment of the invention, general anesthesia is achieved and the patient is positioned prone or kneeling on a padded spinal operating frame. In one embodiment, by allowing the abdomen to hang free, intravenous pressure is reduced and blood loss during the procedure is decreased. The patient is prepped and draped in the usual sterile fashion. Pre-operative radiographic films are reviewed and any vertebral anomalies or variations are noted.  FIG. 51A  illustrates a midline skin incision is made over the desired vertebrae. The exposed skin edges and subcutaneous tissue are injected with epinephrine 1:500,000 solution to facilitate hemostasis. In  FIGS. 51B and 51C , dissection is continued to the lumbodorsal fascia and the surgical site is exposed by retracting the skin and subcutaneous tissue laterally. In  FIGS. 51D and 51E , blunt finger dissection is used between the multifidus and longissimus muscles to access the facet joints. Self-retaining Gelpi retractors are inserted between the muscle groups. Electrocautery or elevators are used to separate the transverse fibers of the multifidus from their heavy fascial attachments. Exposure of the transverse processes and fascial planes is continued. Cautery may be used to provide hemostasis from the lumbar arteries and veins along the base of the transverse processes. The facets of the facet joint are distracted as required to provide access to the joint space. The joint capsule of the facet joint is opened by incision or piercing. In one embodiment, the affected facet joint is sized and a joint prosthesis is selected. In one embodiment, the articular process or processes are prepared for receiving the joint prosthesis, including but not limited to roughening the articular surface of the articular process and/or drilling a hole for the prosthesis anchor or retaining member. The prosthesis is inserted into the facet joint and the anchor or retaining member, if any is attached to the articular process. The steps are repeated until all the joint prostheses have been inserted. The surgical site is closed in layers over a suction tube and the skin flaps are sutured down to the fascia to eliminate any dead space in the tissue. The surgical site is cleaned and dressed. 
         [0104]    4. Minimally Invasive Approach to the Cervical Spine 
         [0105]    In one embodiment of the invention, general or local anesthesia is achieved and the patient is positioned prone on a turning frame or three-point head rest attached to the table. Skeletal traction is performed using tongs. The patient is prepped and draped in the usual sterile fashion. Pre-operative radiographic films are reviewed and any vertebral anomalies or variations are noted. The spinous processes are palpated to identify the location of the cervical vertebrae and a small 1 cm skin incision is made over the desired insertion site. Hemostasis is achieved with infiltration of epinephrine 1:500,000 solution around the incision site. Under fluoroscopy, a trocar or needle is inserted through the incision site and joint capsule to the desired facet joint. The needle or trocar is replaced with an introducer. In one embodiment, insertion is performed along the nuchal ligament to avoid cutting into vascular muscle tissue. In another embodiment, insertion is performed directly through the skin and muscle overlying the facet joint. The facets of the facet joint are distracted as required to provide access to the joint space. In one embodiment, the affected facet joint is sized by injecting a radio-contrast agent into the facet joint and a joint prosthesis is selected. In one embodiment, the articular process or processes are prepared for receiving the joint prosthesis, including but not limited to roughening the articular surface of the articular process and/or drilling a hole using endoscopic instruments known in the art. The prosthesis is inserted into the facet joint space through the introducer and an anchor or retaining member, if any is attached to the articular process. The steps are repeated until all the joint prostheses have been inserted. The surgical site is closed, cleaned and dressed. 
         [0106]    5. Minimally Invasive Approach to the Thoracic Spine 
         [0107]    In one embodiment of the invention, general or local anesthesia is achieved and the patient is positioned prone on a padded spinal operating frame. The patient is prepped and draped in the usual sterile fashion. Pre-operative radiographic films are reviewed and any vertebral anomalies or variations are noted. A small 1 cm skin incision is made over the desired insertion site. Hemostasis is achieved by injecting epinephrine 1:500,000 solution around the incision site. Under fluoroscopy, a trocar or needle is inserted through the superficial and lumbodorsal fascia, the erector spinae muscle and joint capsule to access the facet joint. The trocar or needle is replaced with an introducer. The facets of the facet joint are distracted as required to provide access to the joint space. An intra-operative x-ray or fluoroscopy is obtained to confirm access to the desired facet joint. In one embodiment, the affected facet joint is sized and a joint prosthesis is selected. In one embodiment, the articular process or processes are prepared for receiving the joint prosthesis, including but not limited to roughening the articular surface of the articular process and/or drilling a hole for the prosthesis anchor or retaining member, using endoscopic instruments known in the art. The prosthesis is inserted into the facet joint space and the anchor or retaining member, if any is attached to the articular process. The steps are repeated until all the joint prostheses have been inserted. The surgical site is closed, cleaned and dressed. 
         [0108]    6. Minimally Invasive Approach to the Lumbar Spine 
         [0109]    In one embodiment of the invention, general or local anesthesia is achieved and the patient is positioned prone or kneeling on a padded spinal operating frame. In one embodiment, by allowing the abdomen to hang free, intravenous pressure is reduced and blood loss during the procedure is decreased. The patient is prepped and draped in the usual sterile fashion. Pre-operative radiographic films are reviewed and any vertebral anomalies or variations are noted. A small 1 cm skin incision is made over the desired insertion site. Hemostasis is achieved by injecting epinephrine 1:500,000 solution around the incision site. Under fluoroscopy, a trocar or needle is inserted through the lumbodorsal fascia. The trocar or needle is replaced with an introducer. In one embodiment, radio-contrast agent is injected through the introducer to identify the junction between the lumbodorsal fascia and the multifidus and longissimus muscles. A blunt dissector is inserted through the introducer to dissect between the multifidus and longissimus muscles and pierce the joint capsule to access the facet joints. The facets of the facet joint are distracted as required to provide access to the joint space. In one embodiment, the affected facet joint is sized and a joint prosthesis is selected. In one embodiment, the articular process or processes are prepared for receiving the joint prosthesis, including but not limited to roughening the articular surface of the articular process and/or drilling a hole for the prosthesis anchor or retaining member. The prosthesis is inserted into the facet joint space and the anchor or retaining member, if any is attached to the articular process. The steps are repeated until all the joint prostheses have been inserted. The surgical site is closed, cleaned and dressed. 
         [0110]    While embodiments of this invention have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention. For all of the embodiments described above, the steps of the methods need not be performed sequentially.