Abstract:
Devices and surgical methods treat various types of adult spinal pathologies, such as degenerative spondylolisthesis, spinal stenosis, degenerative lumbar scoliosis, and kypho-scoliosis. Various types of spinal joint replacement prostheses, surgical procedures for performing spinal joint replacements, and surgical instruments are used to perform the surgical procedures.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/658,089, filed Sep. 9, 2003, which is a divisional of co-pending U.S. patent application Ser. No. 10/615,727, filed Jul. 9, 2003, which is a divisional of co-pending application Ser. No. 09/693,272, filed Oct. 20, 2000, entitled “Facet Arthroplasty Devices and Methods” (now U.S. Pat. No. 6,610,091) which claims priority to U.S. Provisional Patent Application Ser. No. 60/160,891, filed Oct. 22, 1999, entitled “Facet Arthroplasty Devices and Methods,” which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to devices and surgical methods for the treatment of various types of spinal pathologies. More specifically, the present invention is directed to several different types of spinal joint replacement prostheses, surgical procedures for performing spinal joint replacements, and surgical instruments which may be used to perform the surgical procedures.  
       BACKGROUND OF THE INVENTION  
       [0003]     Back pain is a common human ailment. In fact, approximately 50% of persons who are over 60 years old suffer from lower back pain. Although many incidences of back pain are due to sprains or muscle strains which tend to be self-limited, some back pain is the result of more chronic fibromuscular, osteoarthritic, or ankylosing spondolytic processes of the lumbosacral area. Particularly in the population of over 50 year olds, and most commonly in women, degenerative spine diseases such as degenerative spondylolisthesis and spinal stenosis occurs in a high percentage of the population. Iida, et al, 1989.  
         [0004]     Degenerative changes of the adult spine have traditionally been determined to be the result of the interrelationship of the three joint complex; the disk and the two facet joints. Degenerative changes in the disc lead to arthritic changes in the facet joint and vice versa. See Farfan and Sullivan, 1967; see also Farfan, 1969; see also Farfan, 1980.  
         [0005]     One cadaver study of 19 cadavers with degenerative spondylolisthesis showed that facet degeneration was more advanced than disc degeneration in all but two cases. Farfan. In mild spondylolisthetic cases, the slip appeared to be primarily the result of predominantly unilateral facet subluxation. Other studies into degenerative changes of the spine have revealed extensive contribution of facet joint degeneration to degenerative spinal pathologies such as degenerative spondylolisthesis, central and lateral stenosis, degenerative scoliosis, and kypho-scoliosis, at all levels of the lumbar spine. See Kirkaldy-Willis et al, 1978; see also Rosenberg, 1975.  
         [0006]     It has been determined that facet joint degeneration particularly contributes to degenerative spinal pathologies in levels of the lumbar spine with sagittally oriented facet joints, i.e. the L4-L5 level.  
         [0007]     When intractable pain or other neurologic involvement results from adult degenerative spine diseases, such as the ones described above, surgical procedures may become necessary. Traditionally, the surgical management of disease such as spinal stenosis consisted of decompressive laminectomy alone. Herkowitz, et al, The Diagnosis and Management of Degenerative Lumber Spondylolisthesis, 1998. Wide decompressive laminectomies remove the entire lamina, and the marginal osteophytes around the facet joint. Because a lot of degenerative spine disease has been demonstrated to be caused by facet joint degeneration or disease, this procedure removes unnecessary bone from the lamina and insufficient bone from the facet joint.  
         [0008]     Furthermore, although patients with one or two levels of spinal stenosis tend to do reasonably well with just a one to two level wide decompressive laminectomy, patients whose spinal stenosis is associated with degenerative spondylolisthesis have not seen good results. Lombardi, 1985. Some studies reported a 65% increase in degree of spondylolisthesis in patients treated with wide decompressive laminectomy. See Johnson et al; see also White and Wiltse. The increase in spinal slippage especially increased in patients treated with three or more levels of decompression, particularly in patients with radical laminectomies where all of the facet joints were removed.  
         [0009]     To reduce the occurrence of increased spondylolisthesis resulting from decompressive laminectomy, surgeons have been combining laminectomies, particularly in patients with three or more levels of decompression, with multi-level arthrodesis. Although patients who undergo concomitant arthrodesis do demonstrate a significantly better outcome with less chance of further vertebral slippage after laminectomy, arthrodesis poses problems of its own. Aside from the occurrence of further spondylolisthesis in some patients, additional effects include non-unions, slow rate of fusion even with autografts, and significant morbidity at the graft donor site. Furthermore, even if the fusion is successful, joint motion is totally eliminated at the fusion site, creating additional stress on healthy segments of the spine which can lead to disc degeneration, herniation, instability spondylolysis, and facet joint arthritis in the healthy segments.  
         [0010]     An alternative to spinal fusion has been the use of an invertebral disc prosthesis. There are at least 56 artificial disc designs which have been patented or identified as being investigated. McMillin C. R. and Steffee A. D., 20th Annual Meeting of the Society for Biomaterials (abstract) (1994). Although different designs achieve different levels of success with patients, disc replacement mainly helps patients with injured or diseased discs; disc replacement does not address spine pathologies such as spondylolisthesis and spinal stenosis caused by facet joint degeneration or disease.  
       SUMMARY OF THE INVENTION  
       [0011]     There is a need in the field for prostheses and prosthetic systems to replace injured and/or diseased facet joints, which cause, or are a result of, various spinal diseases. There is also a need for surgical methods to install such prostheses. There is also a need for prostheses and prosthetic systems to replace spinal fusion procedures.  
         [0012]     The present invention overcomes the problems and disadvantages associated with current strategies and designs in various treatments for adult spine diseases. The present inventive spinal arthroplastic systems avoid the problems of spine stiffness, increased loads on unfused levels, and predictable failure rates associated with spinal arthrodesis.  
         [0013]     The present invention pertains to spinal prostheses designed to replace facet joints and/or part of the lamina at virtually all spinal levels including L1-L2, L2-L3, L3-L4, L4-L5, L5-S-1, T11-T12, and T12-L1. Various types of joint replacement prostheses are described for treating different types of spinal problems.  
         [0014]     One aspect of the invention provides a facet prosthesis, which suitable for use in virtually all levels of the spine, including all lumbar levels, lower thoracic levels, and the first sacral level. The facet prosthesis may comprise, e.g., a body which attaches to a pedicle and includes a surface defining a facet.  
         [0015]     Another aspect of the invention provides a bilateral facet arthroplasty system. The bilateral facet arthroplasty system may comprise, e.g., an inferior lamina/facet prosthesis that spans the distance from one inferior facet joint to another and replaces both inferior facet segments and any inferior section of a lamina which has been cut. The bilateral facet arthroplasty system may also comprise, e.g., facet prostheses which have replaced the superior facets to form a complete prosthetic facet joint with the inferior facet prosthesis.  
         [0016]     Another aspect of the invention provides a hemi-lamina/facet prosthesis, which may replace parts of a lamina and inferior facet which have been removed in a hemiarthroplasty with or without wide decompressive laminectomy.  
         [0017]     Another aspect of the invention provides surgical procedures for performing replacements of various facets and lamina in the spine, as well as surgical instruments for facilitating performance of the disclosed surgical procedures, including spinal fusion.  
         [0018]     Another aspect of the invention allows sequential replacements of all facet joints from S1 to T11, allowing for motion on all levels.  
         [0019]     Features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a lateral view of a spine with degenerative spondylolisthesis at L4-L5;  
         [0021]      FIG. 2  is a front view of a universal facet replacement prosthesis;  
         [0022]      FIGS. 2A, 2B , and  2 C are view of an alternative embodiment of a universal facet replacement prosthesis;  
         [0023]      FIG. 3  is a lateral view of a spine with a superior universal facet prosthesis installed in a L5 vertebra;  
         [0024]      FIG. 4  is a superior view of a L5 vertebra with an installed superior universal facet prosthesis;  
         [0025]      FIG. 5  is a superior view of a L5 vertebra depicting removal of the prominent bone of the superior articular process;  
         [0026]      FIG. 6  is a diagram illustrating the trimming of the superior facet to decompress a nerve root prior to reaming;  
         [0027]      FIG. 7  is a superior view of a L5 vertebra depicting the reaming of the facet into the pedicle;  
         [0028]      FIG. 8  is a front view of a facet reamer;  
         [0029]      FIG. 9  is a superior view of a vertebral body depicting broaching an opening into a vertebral body;  
         [0030]      FIG. 10  is a superior view of a vertebral body depicting two universal facet prostheses which have been installed in a vertebral body to form two superior facets;  
         [0031]      FIG. 11  is a posterior view of a spine depicting an installed inferior lamina/facet prosthesis;  
         [0032]      FIG. 12  is a superior view of a vertebral body depicting complete prosthetic facet joints comprising an inferior lamina/facet prosthesis and two superior universal facet prostheses;  
         [0033]      FIG. 13  is a lateral view of an installed complete prosthetic facet joint;  
         [0034]      FIG. 14  is a superior view of a vertebral body depicting sagittally oriented arthritic facets with lateral stenosis;  
         [0035]      FIG. 15  is a superior view of a vertebral body depicting removal of the inferior one eighth of the spinous process;  
         [0036]      FIG. 16  is a superior view of a vertebral body after an inferior lamina/facet resection;  
         [0037]      FIG. 17  is a posterior view of a spine at an L4-L5 showing a spinous process resection line and inferior facet resection line;  
         [0038]      FIG. 18  is a posterior view of an L4-L5 after part of the lamina and inferior facets have been removed, showing an installed universal facet prosthesis;  
         [0039]      FIG. 19  is a posterior view of an L4-L5 after part of the lamina and inferior facets have been removed with an alternative V-type laminal cut, showing an installed universal facet prosthesis;  
         [0040]      FIG. 20  is a posterior view of a L4 vertebra with an alternative shaped inferior lamina/facet prosthesis installed over a V-type laminal cut;  
         [0041]      FIG. 21  is a posterior view of one embodiment of an installed hemi-lamina/facet prosthesis of the present invention;  
         [0042]      FIG. 22  is a front view of one embodiment of a hemi-lamina/facet prosthesis of the present invention;  
         [0043]      FIG. 23  is a posterior view of a spine, at an L4-L5 joint which has undergone hemiarthroplasty with wide decompressive laminectomy, with two base members of a hemi-lamina/facet prosthesis in the process of being installed onto the L4-L5;  
         [0044]      FIG. 24  is a posterior view of one embodiment of an installed hemi-lamina/facet prosthesis of the present invention;  
         [0045]      FIG. 25  is a posterior view of one embodiment of an installed hemi-lamina/facet prosthesis of the present invention;  
         [0046]      FIG. 26  is a posterior view of the L4-L5 depicting various cuts which may be made into the lamina a facets for a hemiarthroplasty with or without wide decompressive laminectomy;  
         [0047]      FIG. 27  is a lateral view of the L4 and L5 vertebrae;  
         [0048]      FIG. 28  is a superior view of the L4 and L5 vertebrae in a separated condition;  
         [0049]      FIG. 29  is a front elevation view of a single-side prosthesis that embodies the feature of the invention;  
         [0050]      FIG. 30  is a side elevation view of the prosthesis shown in  FIG. 29 ;  
         [0051]      FIG. 31  is a lateral view of the L3, L4, and L5 vertebrae, with the prosthesis shown in  FIG. 29  secured to the L4 vertebral body;  
         [0052]      FIG. 32  is a lateral view of the L3 and L4 vertebrae, with a link secured to the L4 vertebral body;  
         [0053]      FIG. 33  is a lateral view of the L3 and L4 vertebrae, with a link secured to the L4 vertebral body;  
         [0054]      FIG. 34  is a front elevation view of another single-side facet prosthesis that embodies the feature of the invention;  
         [0055]      FIG. 35  is a lateral view of the L3 and L4 vertebrae, with the prosthesis shown in  FIG. 34  secured to the L4 vertebral body;  
         [0056]      FIG. 36  is a front elevation view of a double-side facet joint link assembly that embodies the feature of the invention, being formed of two criss-crossing, mating link bodies;  
         [0057]      FIGS. 37 and 38  are front elevation views of the link bodies forming the joint link assembly shown in  FIG. 36 , being shown in a mutually separated condition;  
         [0058]      FIG. 39  is a front elevation view of an alternative embodiment of a link body that, when assembled with a mating link body, forms a joint link assembly like that shown in  FIG. 36 ;  
         [0059]      FIG. 40  is a front elevation view of the double-side facet joint link assembly shown in  FIG. 36  in relation to its location on a vertebral body;  
         [0060]      FIG. 41  is a side view of a prosthesis, like that shown in  FIG. 29, 34 , or  36 , secured for use on the pedicle of a vertebral body (shown in lateral view); and  
         [0061]      FIG. 42  is a side view of the lower end of the prosthesis shown in  FIG. 41 , forming the inferior half of a facet joint, the superior half of the facet joint being formed by a superior universal facet prosthesis shown in  FIG. 2 .  
     
    
       [0062]     The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0000]     I. Anatomy of Lumbar Vertebrae  
         [0063]      FIGS. 27 and 28  show the fourth and fifth lumbar vertebrae L4 and L5, respectively, in a lateral view (while in anatomic association) and in a superior view (separately). The lumbar vertebrae (of which there are a total of five) are in the lower back, also called the “small of the back.” 
         [0064]     As is typical with vertebrae, the vertebrae L4 and L5 are separated by an intervertebral disk  25 . The configuration of the vertebrae L4 and L5 differ somewhat, but each (like vertebrae in general) includes a vertebral body  10 , which is the anterior, massive part of bone that gives strength to the vertebral column and supports body weight. The vertebral arch  12  is posterior to the vertebral body  10  and is formed by the right and left pedicles  14  and lamina  16 . The pedicles  14  are short, stout processes that join the vertebral arch  12  to the vertebral body  10 . The pedicles  14  project posteriorly to meet two broad flat plates of bone, called the lamina  16 .  
         [0065]     Seven other processes arise from the vertebral arch. Three processes—the spinous process  18  and two transverse  20  processes—project from the vertebral arch  12  and afford attachments for back muscles, forming levers that help the muscles move the vertebrae. The remaining four processes, called articular processes, project superiorly from the vertebral arch (and are thus called the superior articular processes  22 ) and inferiorly from the vertebral arch (and are thus called the inferior articular processes  24 ). The superior and inferior articular processes  22  and  24  are in opposition with corresponding opposite processes of vertebrae superior and inferior adjacent to them, forming joints, called zygapophysial joints or, in short hand, the facet joints or facets. The facet joints permit gliding movement between the vertebrae L4 and L5. Facet joints are found between adjacent superior and inferior articular processes along the spinal column.  
         [0066]     The facet joints can deteriorate or otherwise become injured or diseased, causing lack of support for the spinal column, pain, and/or difficulty in movement.  
         [0067]     As described in this Specification, a facet joint has a superior half and an inferior half. The superior half of the joint is formed by the vertebral level below the joint, and the inferior half of the joint is formed by the vertebral level above the joint. For example, in the L4-L5 facet joint, the superior half of the joint is formed by structure on the L-5 vertebra, and the inferior half of the joint is formed by structure on the L-4 vertebra.  
         [0000]     II. Superior Universal Facet Prosthesis  
         [0000]     A. Structure  
         [0068]     A superior universal facet prosthesis  330  is shown in  FIG. 1  that embodies features of the invention. The prosthesis  330  is designated “superior” because it creates an artificial facet surface for the superior half of the facet joint. The artificial surface articulates with the inferior half of the facet joint. The prosthesis  330  allows for the replacement of injured, diseased and/or deteriorating components along the superior half of facet joints, to provide improved support for the spinal column.  
         [0069]     The universal facet prosthesis  330  may be constructed and configured in various ways. The universal facet prosthesis  330  may, e.g., comprise a cup member  315 . The cup member  315  itself may be made of various materials commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, titanium alloys, chrome cobalt, surgical steel, or any other total joint replacement metal and/or ceramic, bony in-growth surface, sintered glass, artificial bone, any uncemented metal or ceramic surface, or a combination thereof. The cup member  315  may also be any appropriate shape including, but not limited to, rectangular, disc shaped, trough shaped, or cup shaped. The cup member may be fixed or anchored directly to a vertebra with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-away anchors and/or wires to facilitate any future removal of the prosthesis, or a combination thereof, or any other means known in the art.  
         [0070]     As shown in  FIG. 2 , the cup member  315  is made of any joint materials commonly used in the prosthetic arts, including, but not limited to, metals, ceramics, titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bony in-growth surfaces, artificial bone, uncemented surface metals or ceramics, or any combination thereof, preferably covered with a bony in-growth surface.  
         [0071]     In the illustrated embodiment, the cup member  315  is fixed to a stem  310 , e.g., pre-welded, or glued with a biocompatible adhesive, or removably secured using a frictional Morse taper. If desired, the stem  310  can incorporate one or more fins or ribs (not shown), extending outward from the stem  310 , which desirably reduce and/or eliminate rotation of the stem  310  once positioned within the targeted bone. In addition, the stem  310  can be cannulated, if desired, to allow the use of guide pins during insertion of the stem, as is well known in the art.  
         [0072]     The stem  310  may itself be made of any joint materials commonly used in the prosthetic arts, including, but not limited to, metals, ceramics, titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bony in-growth surfaces, artificial bone, uncemented surface metals or ceramics, or a combination thereof. In a preferred embodiment, the stem  310  is covered with a bony in-growth surface.  
         [0073]     In the illustrated embodiment, the cup member  315  carries a surface member, which is made of a material, e.g. polyethylene, ceramic, or metal, which provides glide and cushioning ability for any potential contacting components, such as the articular head members described below. In one embodiment (see  FIG. 2   b ), the surface member  325  can be formed in a gently upwardly curving shape, similar in shape to a catcher&#39;s mitt. In another embodiment (see  FIG. 2   c ), the surface member  325  is rectangular in shape with rounded corners. The cup member  315  is sized to be larger than the articulating superior half of the facet joint, to allow for motion of the joint.  
         [0074]     The surface member  325  may be a separate component that is fixed to the cup member  315 , e.g., with a biocompatible adhesive, screws, nails, or comprise a formed part of the cup member  315 . The surface member  325  may also be held into the cup member  315  with compressive forces or friction (e.g., using a Morse taper).  
         [0075]     As shown in  FIGS. 2   a  and  2   b , the stem  310   a  could alternately comprise a threaded portion, such as in a pedicle screw, with the head or pedestal  315   a  incorporating a depression  316   a  sized to accommodate a hexagonal driver or other surgical driving tool well know in the art. In addition, the prosthesis  320   a  could incorporate a lower insert  321   a  sized to fit into the depression  316   a  in the head  315   a . If desired, the insert  321   a  could comprise a Morse taper. In this embodiment, the stem  310   a  can be screwed into the bone, with the insert  321   a  positioned or otherwise secure within the depression  316   a . The stem  310   a  could be placed by tapping without screwing. If revision surgery is required, or some other condition required removal of the prosthesis, the insert  321   a  can be removed from the stem  310   a , and the stem  310   a  can subsequently be removed from the bone.  
         [0076]     As  FIG. 2   a  shows, the stem  310   a  can also include an enlarged projection or collar  311   a  abutting the cup member  315   a . The collar  311   a  serves to prevent unintended ingress of the stem  310   a  further into the pedicle, beyond a desired distance.  
         [0077]      FIG. 1  depicts a spondylolisthetic spine with slippage at the L4-L5 joint between the L4 and L5 vertebrae.  FIG. 3  and  FIG. 4  depict a universal facet prosthesis  330  which has been installed into an L5 vertebra  105  to replace the inferior half  305  of a facet joint. In one embodiment, the stem  310  of universal facet prosthesis  330  is fixed into the L5 vertebra  105  with poly (methylmethacrylate) bone cement, hydroxyapatite, a ground bone composition, or a combination thereof. In another embodiment, both the stem  310  and the cup member  315  are fixed to a vertebra with stainless steel wire to provide addition stability.  
         [0078]     The new support provided by a universal facet prosthesis  330  helps correct degenerative spine diseases such as spondylolisthesis, spinal stenosis, or any spine disease. As demonstrated by comparing  FIG. 1  showing a spondylolisthetic spine with slippage between the L4 vertebra  100  and the L5 vertebra  105  with  FIG. 3  where the diseased superior half  305  of the facet joint has been replaced with a superior universal facet prosthesis  330  of the present invention, correcting spondylolisthesis at the L4-L5 joint and preventing further spondylolisthesis. Similarly, where correction of scoliosis and/or kypho-scoliosis is desired, the size and/or shape of the prosthesis may be chosen to re-orient the affected level(s) of the spine.  
         [0079]     The superior universal facet prosthesis  330  described above may be used as a replacement for the superior half of one or more of facet joints at any facet joint at any level of the spine. In the preferred embodiment, the universal facet prosthesis  330  is used to replace the superior half of one or more facet joints in one or more facet joints. The superior facet prosthesis  330  is designed such that it has the appropriate cephalad and caudad directions as well as the appropriate medial/lateral angulation for the given level of the spine where the implant occurs.  
         [0080]     In further embodiments, one or more surfaces of a universal facet prosthesis  330  may be covered with various coatings such as antimicrobial, antithrombotic, and osteoinductive agents, or a combination thereof. See, e.g., U.S. Pat. No. 5,866,113, which is incorporated herein by reference. These agents may further be carried in a biodegradable carrier material with which the pores of the stem and/or cup member of certain embodiments may be impregnated. See, e.g., U.S. Pat. No. 5,947,893, which is also incorporated herein by reference.  
         [0081]     In still further embodiments of the present invention, a universal facet prosthesis may be attached to strengthened or fortified bone. Vertebrae may be strengthened prior to or during fixation of the prostheses using the methods, e.g., described in U.S. Pat. No. 5,827,289, which is incorporated herein by reference. This type of bone strengthening is particularly suggested for osteoporotic patients who wish to have facet replacement.  
         [0000]     B. Surgical Method for Facet Replacement Using the Superior Universal Facet Prosthesis  
         [0082]     A surgical procedure that embodies features of the invention replaces the superior half of a facet joint with the superior universal facet prosthesis  330  described above. The surgical procedure comprises exposing the spinous process, lamina, and facet joints at a desired level of the spine using any method common to those of skill in the medical arts. The prominent bone  306   b  (see  FIG. 5 ) may then be rongeured using any means common in the field. The superior facet  305  may also be trimmed, as depicted in  FIG. 6 , to decompress the nerve root  203 . A reamer  400 , or any other instrument that is useful for grinding or scraping bone, may be used to ream the facet  305   b  into the pedicle  304   b  as depicted in  FIG. 7  and  FIG. 8 .  
         [0083]     In a preferred embodiment (see  FIG. 9 ), an opening  407  is made into the vertebral body  107  with a broach  405 . The universal facet prosthesis  330   b  is installed into the opening  407  made by the broach  405 , as shown in  FIG. 10 . The opening  407  may be partly filled with bone cement, hydroxyapatite, or any bone adhesive before installation of the universal facet prosthesis  330   b.    
         [0084]     In an alternative embodiment, the stem  310  of the superior universal facet prosthesis  330  may be constructed in such a way that the superior universal facet prosthesis  330  can be directly screwed or tapped into the vertebral body  107 .  
         [0085]     In another arrangement, the cup member  315  of the universal facet member  330  may additionally be fixed to the vertebral body  107  with bone cement, hydroxyapatite, or any other biocompatible adhesive. In yet another arrangement, a universal facet prosthesis without a stem  310  may be attached to the vertebral body with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-away anchors to facilitate later removal of the prosthesis, or a combination thereof, or any other means known in the art.  
         [0086]     In a further embodiment of the present invention, the universal facet prosthesis  330  may be fixed into strengthened or fortified bone. Vertebrae may be strengthened prior to or during fixation of the prosthesis using the methods described in U.S. Pat. No. 5,827,289, which is incorporated herein by reference. This type of bone strengthening procedure is particularly suggested for osteoporotic patients who wish to have facet replacement surgery.  
         [0000]     III. Inferior Lamina/Facet Prosthesis  
         [0000]     A. Structure  
         [0087]     An inferior lamina/facet prosthesis  500  that embodies features of the invention is shown in  FIG. 11 . The prosthesis  500  is designated “inferior” because it creates an artificial facet surface for the inferior half of a facet joint. The artificial surface articulates with the superior half of the facet joint. The prosthesis  330  allows for the replacement of injured, diseased and/or deteriorating components along the inferior halves of facet joints to provide improved support for the spinal column.  
         [0088]     The prosthesis  330  may span the distance from a region on one side of a vertebra to a region of the other side of the vertebra. It can thus replace both inferior halves of a facet joint.  
         [0089]      FIG. 14  depicts a superior view of a vertebral body depicting sagitally oriented arthritic facets with lateral stenosis, showing how the spinal process  631  presses forward onto the nerve roots  205  and  200 . The prosthesis  500  allows for replacement of diseased and deteriorating inferior regions of the vertebra and partial replacement of lamina (see  FIG. 12 ), which may be pressing on the spinal nerves, to relieve pain. The prosthesis  500  creates artificial facet surfaces for the inferior half of facet joints in the spine, which provide improved support for the spinal column.  
         [0090]     As  FIG. 12  shows, a superior universal facet prosthesis  330 , as described above, may also be installed to replace the superior halves of the facet joints and, with the inferior lamina/facet prosthesis  500  replacing the inferior halves of the facet joints, forming a total facet replacement system that can result in entire artificial facet joints along a length of the spinal column. Alternatively, just the inferior half one or more facet joints, or just the superior half of one or more facet joints, may be replaced. The inferior and/or superior halves of facet joints may be replaced on one side of a given vertebra (unilateral), on the both sides of a given vertebra (bilateral), or a combination of each along a length of the spinal column.  
         [0091]     The inferior lamina/facet prosthesis  500  may be constructed in various ways. As shown in  FIG. 1 , the prosthesis  500  can comprise a base member  505 . The base member  505  may be made of any joint materials commonly used in the prosthetic arts, including, but not limited to, metals, ceramics, titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bony in-growth surfaces, artificial bone, uncemented surface metals or ceramics, or a combination thereof. The base member  505  may also be any appropriate shape to give appropriate support to the spine and to appropriately and sturdily attach to the inferior portions of a vertebral body. The base member  505  may be fixed or anchored directly to the inferior portion of a vertebral body with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-away screws to facilitate any future removal of the prosthesis, or a combination thereof, or any other means known in the art.  
         [0092]     In a preferred arrangement, as depicted in  FIG. 11 ,  FIG. 12 , and  FIG. 13 , the base member  505  of the inferior lamina/facet prosthesis  500  is attached to each pedicle  102   a  and  102   b  with bilateral pedicle screws  520   a  and  520   b . The base member  505  of the inferior lamina/facet prosthesis  500  may further be attached to the spinous process  630  with a trans-spinous-process screw  515  to provide additional stability.  
         [0093]     In another embodiment, the inferior lamina/facet prosthesis  500  may have a head member  510  for articulation with the cup member  315  of a superior universal facet prosthesis  330  or with a superior articular process of the adjoining vertebral body. The head member  510  may be made of various materials commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, tantalum, titanium, chrome cobalt, surgical steel, bony in-growth surfaces, ceramics, artificial bone, or a combination thereof. The head member  510  may further be any shape which facilitates attachment to the rest of the inferior lamina/facet prosthesis  500  and to smooth connection to, and movement in orientation to, a universal facet prosthesis  330  or a superior articular process of an adjoining vertebral body. In one embodiment, a head member  510  is attached to the base member  505  of the inferior facet/lamina prosthesis  500  with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, or any other means known in the art. The head member  510  may also be removably attached by frictional engagement (e.g., using a Morse taper).  
         [0094]     In a preferred embodiment (see  FIGS. 11 and 12 ), the inferior facet/lamina prosthesis  500  comprises two head members  510   a  and  510   b  formed in the shape of an articular head. The head members  510   a  and  510   b  preferably each have a Morse taper  512  at their upper surface to allow them to lock into the base member  505  of the inferior facet/lamina prosthesis  500 . Of course, either or both head members  510   a  and  510   b  could be formed integrally with the prosthesis  500 . In the preferred arrangement, a complete prosthetic facet joint  560  is provided (see  FIG. 11 ), in which the head members  510   a  and  510   b  articulate with the cup member  315  of the superior universal facet prosthesis  330 .  
         [0095]     In further embodiments, one or more surfaces of the inferior lamina/facet prosthesis  500  may be covered with various coatings such as antimicrobial, antithrombotic, and osteoinductive agents, or a combination thereof. See, e.g., U.S. Pat. No. 5,866,113, which is incorporated herein by reference. These agents may further be carried in a biodegradable carrier material with which the pores of the base member and/or any screws, bolts, or nails of certain embodiments may be impregnated. See, e.g., U.S. Pat. No. 5,947,893, which is incorporated herein by reference.  
         [0096]     In other arrangements, an inferior lamina/facet prosthesis  500  may be attached to strengthened or fortified bone. Vertebrae may be strengthened prior to or during fixation of the prosthesis using the methods described, e.g., in U.S. Pat. No. 5,827,289, which is incorporated herein by reference. This type of bone strengthening is particularly suggested for osteoporotic patients who wish to have facet replacement.  
         [0000]     B. Surgical Method for Partial Inferior Lamina/Facet Replacement Using the Inferior Lamina/Facet Prosthesis  
         [0097]     A surgical procedure that embodies features of the invention replaces inferior lamina and articulated processes with the inferior lamina/facet prosthesis  500  as described above. The surgical procedure exposes the spinous process, lamina, and facet joints at a desired level of the spine using any method common to those of skill in the medical arts. As  FIG. 15  shows, an inferior one eighth to one half of the spinous process  302  may be cut along the spinous process resection line  610  and removed, if the spinous process appears diseased or damaged. The cutting and removal of the spinous process may be performed using any means common in the field.  
         [0098]     As shown in  FIGS. 16 and 17 , the inferior half of the facet joint may also be cut at or near the inferior facet resection line  600 . In a preferred embodiment (see  FIGS. 16 and 17 ), most of the lamina  615  is preserved, as is the facet joint capsule  625 , which may be opened and folded back. In a preferred embodiment, the facet joint capsule  625  may be cut perpendicular to its direction. The inferior half  621  of the facet joint  620  may then be retracted from the superior half  622 . Once the facet joint  620  is separated, the cut inferior bone  615  of the upper joint (i.e. the cut inferior portion of the L4 vertebra in the L4-L5 joint) may be removed. Alternatively, it may be possible to remove the cut inferior bone  615  while simultaneously separating the facet joint  620 .  
         [0099]     In a preferred embodiment (see  FIGS. 18 and 19 ), a superior universal facet prosthesis  330  is then installed as previously described. Alternatively, the superior universal facet prosthesis  330  may be installed before the inferior bone is removed or even cut.  
         [0100]     An inferior lamina/facet prosthesis  500  as described above may be placed onto the facet joints and over the spinous process. The inferior lamina/facet prosthesis  500  may be fixed or anchored to the vertebral body with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-away screws, or a combination thereof to facilitate any future removal of the prosthesis, or any other means known in the art. In the preferred embodiment (see  FIG. 11 ,  FIG. 12 , and  FIG. 13 ), the inferior lamina/facet prosthesis  500  is attached to each pedicle  102   a  and  102   b  of the inferior facets with bilateral pedicle screws  520   a  and  520   b  and is further attached to the spinous process  630  with a trans-spinous-process screw  515  to provide additional stability.  
         [0101]     A head member  510  of an inferior lamina/facet prosthesis  500  may articulated into the cup member  315  of the superior universal facet prosthesis  330 , or into a inferior half of a facet joint if the inferior half has not been replaced, to create a complete prosthetic facet joint.  
         [0102]     In an alternative embodiment, as depicted by  FIG. 19 , the inferior facet resection line  610  may be a V-type cut. If a V-type cut is used, an appropriately shaped inferior lamina/facet prosthesis  550  should be used, such as depicted in  FIG. 20 . The inferior facet resection line may alternatively be cut in other ways, which are apparent to one of skill in the art of orthopedic surgery and will require inferior lamina/facet prostheses of varying shapes to appropriately fit the cut vertebra.  
         [0103]     In a further embodiment of the present invention, a universal facet prosthesis and/or an inferior lamina/facet prosthesis may be fixed into strengthened or fortified bone. Vertebrae may be strengthened prior to or during fixation of the prosthesis using the methods described, e.g., in U.S. Pat. No. 5,827,289, which is incorporated herein by reference. This type of bone strengthening procedure is particularly suggested for osteoporotic patients who wish to have facet replacement surgery.  
         [0000]     IV. Hemi-Lamina/Facet Prosthesis  
         [0000]     A. Structure  
         [0104]     A hemi-lamina/facet prosthesis  700  that embodies features of the invention (see  FIG. 21 ) may be used to replace parts of a lamina and inferior processes, some or all which may have been removed in a primary procedural bone resection, (i.e. with or without wide decompressive laminectomy). The hemi-lamina/facet prosthesis  700  may be designed similarly, or even identically, to the inferior lamina/facet prosthesis  500  described above, depending on how much of the bone is removed.  
         [0105]     The hemi-lamina/facet prosthesis  700  may be constructed in various ways. In one embodiment, hemi-lamina/facet prosthesis  700  may, e.g., comprise a base member  705 . The base member  705  may be made of any joint materials commonly used in the prosthetic arts, including, but not limited to, metals, ceramics, titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bony in-growth surfaces, artificial bone, uncemented surface metals or ceramics, or a combination thereof. The base member  705  may be any shape which gives appropriate support to the spine and can be appropriately attached to the bone of the remaining lamina. The base member  705  may be fixed or anchored directly to the inferior portion of a vertebral body with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-away screws to facilitate any future removal of the prosthesis, a combination thereof, or any other means known in the art.  
         [0106]     In a preferred embodiment (see  FIG. 21 ) of a prosthesis for hemiarthroplasty (depicted as cut line  800  and further described below) without decompressive laminectomy, the base member  705  of the hemi-lamina/facet prosthesis  700  is attached to superior pedicle  102   b  with a pedicle screw  720 . In another preferred embodiment, the base member  705  of the hemi-lamina/facet prosthesis  700  may further be attached to the spinous process  630  with a trans-spinous-process screw  715  to provide additional stability.  
         [0107]     In a preferred embodiment (see  FIGS. 24 and 25 ) of a prosthesis for hemiarthroplasty with wide decompressive laminectomy, the hemi-lamina/facet prosthesis  700  comprises at least one base member  705 . The base member  705  may further comprise a pedicle attachment hole  725  through which a pedicle screw  720 , or a nail, anchor, break-away anchor, bolt, or any other fastening means, may be installed to help secure the hemi-lamina/facet prosthesis  700  to the inferior pedicle. In the preferred embodiment, the base member  705  may also have at least one lamina attachment hole, with two lamina attachment holes  741  and  742  pictured in  FIG. 22 , to further secure the hemi-lamina/facet prosthesis  700  to the remaining laminal bone with screws, nails, anchors, break-away anchors, bolts, or any other fastening means. Parts of the hemi-lamina/facet prosthesis  700  which overlap bone may be additionally fixed with bone cement, or any biocompatible adhesive.  
         [0108]     A hemi-lamina/facet prosthesis  700  may further comprise a connection plate, similar to the connection plate  750  depicted in  FIG. 24 , to connect two base members, i.e.  705   a  and  705   b , together. The connection plate  750  may be fixed to each base member  705   a  and  705   b  with a biocompatible adhesive, screws, nails, bolts, compressive force, a combination thereof, or any other means common to those of skill in the art. Alternatively, a hemi-lamina/facet prosthesis  700  may further comprise at least one stabilization bar, similar to the stabilization bars  761  and  762  depicted in  FIG. 25 . A stabilization bar or bars may be fixed to each base member  705   a  and  705   b  with a biocompatible adhesive, screws, nails, bolts, compressive force, a combination thereof, or any other means common to those of skill in the art. A hemi-lamina/facet prosthesis  700  may have any type of bridging or stabilizing members, or no bridging members at all, and may be comprised of any number of base members to provide appropriate stability to the spine. The bridging members may be made of any joint materials commonly used in the prosthetic arts, including, but not limited to, metals, ceramics, titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bony in-growth surfaces, artificial bone, uncemented surface metals or ceramics, or a combination thereof.  
         [0109]     In another embodiment, a hemi-lamina/facet prosthesis  700  may have a head member  710  for articulation with the cup member  315  of a superior universal facet prosthesis  330  or with the superior articular process of an adjoining superior pedicle. The head member  710  may be made of various materials commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, or a combination thereof. The head member  710  may further be any shape which allows it to attach to the rest of the hemi-lamina/facet prosthesis  700  and to smoothly connect to, and move in orientation to, the universal facet prosthesis  330  or superior articular facet of the adjoining superior pedicle. In one embodiment, the head member  710  is attached to the rest of the hemi-lamina/facet prosthesis with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, a combination thereof, or any other means known in the art. The head member  710  may be removably attached, using, e.g., a Morse taper.  
         [0110]     In a preferred embodiment, hemi-lamina/facet prosthesis  700  comprises a head member  710  made in the shape of an articular head. The head member  710  preferably has a Morse Taper at its upper surface to allow it to lock into hemi-lamina/facet prosthesis  700 .  
         [0111]     In further embodiments, one or more surfaces of a hemi-lamina/facet prosthesis  700  may be covered with various coatings such as antimicrobial, antithrombotic, and osteoinductive agents, or a combination thereof. See, e.g., U.S. Pat. No. 5,866,113, which is incorporated herein by reference. These agents may further be carried in a biodegradable carrier material with which the pores of the base member and/or any screws, bolts, or nails of certain embodiments may be impregnated. See, e.g., U.S. Pat. No. 5,947,893, which is incorporated herein by reference.  
         [0112]     In still further embodiments of the present invention, a hemi-lamina/facet prosthesis  700  may be attached to strengthened or fortified bone. Vertebrae may be strengthened prior to or during fixation of the prosthesis using the methods described, e.g., in U.S. Pat. No. 5,827,289, which is incorporated herein by reference. This type of bone strengthening is particularly suggested for osteoporotic patients who wish to have facet replacement.  
         [0000]     B. Hemiarthroplasty with or without Wide Decompressive Laminectomy Using the Hemi-Lamina/Facet Prosthesis  
         [0113]     A surgical procedure that embodies features of the invention removes at least part of a lamina and at least one superior portion of a facet joint and replacing it with a hemi-lamina/facet prosthesis  700  as described above. The general surgical procedure is generally similar to the inferior lamina/facet replacement previously described, with the main difference being the types of cuts made into the laminal bone, and that two separate prostheses are used to replace the superior portions of two facet joints (left and right) of a given vertebra.  
         [0114]     One embodiment of the surgical procedure comprises exposing the spinous process, lamina, and facet joints at a desired level of the spine using any method common to those of skill in the medical arts. The inferior facet joint and part of the lamina may be cut with a hemiarthroplasty resection line  800  as depicted in  FIG. 26  for a hemiarthroplasty. The lamina may additionally be cut for a wide decompressive laminectomy along the decompression resection line  810  as depicted in  FIG. 26 . The inferior facet joint may be cut on one side or both sides of the lamina. Likewise, the lamina may be cut along a decompression resection line on one side or both sides.  
         [0115]     In a preferred embodiment of a hemiarthroplasty without a wide decompressive laminectomy, leaving the cut inferior facet bone  300  in place, the facet joint capsule  625  may be opened and folded back. In the preferred embodiment, the facet joint capsule  625  may be cut perpendicular to its direction. The inferior half  621  of the facet joint  620  may then be retracted from the superior half  622 . Once the facet joint  620  is separated, the cut inferior facet bone  825  may be removed. Alternatively, it may be possible to remove the cut inferior facet bone  825  while simultaneously separating the facet joint  620 .  
         [0116]     In a preferred embodiment of a hemiarthroplasty with a wide decompressive laminectomy, a superior universal facet prosthesis  330  is then installed as previously described, and depicted in  FIG. 18 .  
         [0117]     A base member  705  of hemi-lamina/facet prosthesis  700  as described in any of the embodiments above may be placed onto at least one facet joint and at least one pedicle as depicted in  FIG. 23 , and over the spinous process if it has not been removed for hemiarthroplasty without decompressive laminectomy as depicted in  FIG. 21 . The hemi-lamina/facet prosthesis  700  may be fixed or anchored to the vertebral body with poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-away screws to facilitate any possible future removal of the prosthesis, a combination thereof, or any other means known in the art. In the preferred embodiment, as depicted in  FIG. 21 ,  FIG. 24 , and  FIG. 25 , the hemi-lamina/facet prosthesis  500  is attached to each pedicle with bilateral pedicle screws  720 .  
         [0118]     A hemi-lamina/facet prosthesis  700  that may be used in hemiathroplasty without wide decompressive laminectomy, depicted in  FIG. 21 , may further be attached to the spinous process  630  with a trans-spinous-process screw  715  to provide additional stability. A hemi-lamina prosthesis  700  that may be used in hemiathroplasty with wide decompressive laminectomy, as depicted in  FIGS. 23, 24 , and  25 , may further be attached to remaining laminal bone with screws, bolts, nails, anchors, or breakaway anchors through at least one lamina attachment hole  741  to provide additional stability.  
         [0119]     In embodiments where a hemi-lamina/facet prosthesis  700  with more than one base member  705  is installed, a connection plate, depicted as connection plate  750  in  FIG. 24 , at least one stabilization bar, depicted as stabilization bars  761  and  762  in  FIG. 25 , or any other connecting or stabilizing means known in the art, may be installed with the base members to provide additional stability to the spine.  
         [0120]     At least one head member, depicted as head member  710  in  FIGS. 21, 23 ,  24 , and  25 , of a hemi-lamina/facet prosthesis  700  may be articulated into a cup member of a superior universal facet prosthesis  330  to create a prosthetic facet joint capsule.  
         [0121]     The embodiments may be used to replace one or more facet joints for the entire length of the spine from S1 to T11, on one side of a given vertebra, or both sides of a given vertebra, or a combination thereof along a length of the spine. If only one facet joint at a given level is to be replaced, the unilateral arthroplasty prosthesis for the inferior half of the joint may be fixed to the superior ipso-lateral pedicle and include a box fitted over the spinous process, combined with screw fixation. The spinous process box is similar to the spinous process box in the bilateral total facet arthroplasty embodiment previously discussed.  
         [0122]     In a further embodiment of the present invention, a universal facet prosthesis  330  and/or a hemi-lamina/facet prosthesis  700  may be fixed into strengthened or fortified bone. The vertebrae may be strengthened prior to or during fixation of the prosthesis using the methods described, e.g., in U.S. Pat. No. 5,827,289, which is incorporated herein by reference. This type of bone strengthening procedure is particularly suggested for osteoporotic patients who wish to have facet replacement surgery.  
         [0000]     V. Other Facet Prostheses  
         [0000]     A. Single Side  
         [0123]      FIGS. 29 and 30  show an inferior prosthesis  26  that embodies features of the invention. The prosthesis  26  is designated “inferior” because it creates an artificial facet surface in the inferior half of a facet joint. The artificial surface articulates with the superior half of the facet joint. The prosthesis  26  is particularly well suited to single-sided procedures and/or for procedures involving vertebral bodies which are not symmetrical.  
         [0124]     When the processes on one side of a vertebral body are differently spaced from those on the other side of the same body, the prostheses on each side would desirably be of differing sizes as well. Moreover, it is often difficult and/or impossible for a surgeon to determine the precise size and/or shape necessary for a prosthesis until the surgical site has actually been prepared for receiving the prosthesis. In such a case, the surgeon typically needs a family of prostheses possessing differing sizes and/or shapes immediately available during the surgery. The surgeon cannot wait for a custom-fitted device to be created during the surgery, so a number of prostheses of varying sizes and/or shapes must be available for each procedure.  
         [0125]     The prosthesis  26  can be conveniently formed in different sizes and shapes, to offer an array of prostheses  26  from which the surgeon can pick and choose as surgery proceeds. This allows a surgeon to create a “custom” implant during the surgical procedure.  
         [0126]     In the illustrated embodiment (see  FIGS. 29 and 30 ), the prosthesis  26  comprises a body  28  sized and shaped to span the distance between a pedicle  14  and an inferior articular process  24  on the same side of a vertebral body (see  FIG. 31 ). The body  28  may be formed of a material commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, or a combination thereof.  
         [0127]     The upper section of the body  28  desirably includes an opening  32 . The opening  32  accommodates a pedicle screw  34  (see  FIG. 41 ), which secures the upper end of the body  28  into the pedicle  14  of the vertebral body. The opening  32  could be elongated, to allow for varying orientations and/or sizes of the pedicle screw  34 . The remainder of the link body  28  can be secured to the exterior of the vertebra using, e.g., biocompatible adhesive.  
         [0128]     The lower section of the body  28  is oriented to serve as the superior half of a facet joint. The lower section of the body  28  desirably incorporates a head  30 . The head  30  can be permanently affixed to the body  28 , using, e.g., adhesive. Alternatively, the head can be frictionally secured, e.g., using a Morse taper, for removal and replacement (as  FIG. 41  shows). Like the body  28 , the head  30  can be formed of a material commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, or a combination thereof. The head  30  possesses a curvilinear shape that desirably curves along a gradual arc (as  FIG. 42  shows), or can present a “button” shape.  
         [0129]     If desired, the lower section of the joint link body  28  could be angled, to more naturally mimic the orientation of a non-diseased facet joint. In one alternative embodiment, the lower section of the joint link body  28  could rotate relative to the upper section, and could be rotationally secured in a desired position by a surgeon using a locking screw or other locking means known in the art. Such an embodiment would allow the surgeon to alter the orientation of the lower section to fit the particular needs of a patient during the actual surgical procedure.  
         [0130]     In use (see  FIG. 31 ), the head  30  articulates with the superior half of the facet joint. The superior facet  22  can comprise the natural superior articular process itself (as  FIG. 31  shows), or it can comprise a superior prosthetic facet created, e.g., by the previously described universal facet prosthesis  330  (as  FIG. 42  shows). The surface member  320  of the universal facet prosthesis  330  can comprise a metal material made of, e.g., titanium, cobalt, chrome, etc., or a plastic material such as, e.g., polyethylene, or a ceramic material. Thus the surgeon can select the same or different materials to form the joint interface between the head  30  and facet prosthesis  330 .  
         [0131]      FIGS. 34 and 35  show another embodiment of an inferior universal prosthesis  36  that embodies features of the invention. The prosthesis  36 , like the prosthesis  26 , is designated “inferior” because it creates an artificial facet surface in the inferior half of the facet joint. The artificial surface articulates with the superior half of the facet joint. Like the prosthesis  26 , the prosthesis  36  is particularly well suited to single-sided procedures and/or for procedures involving vertebral bodies which are not symmetrical.  
         [0132]     The prosthesis  36  comprises a body  38  sized and shaped to span the distance between a pedicle  14  and an inferior articular process  24  (see  FIG. 35 ). The body  38  may be formed of the same types of material as the link body  28 . Like the link body  28 , the upper section of the joint link body  38  desirably includes an opening  42 , to accommodate a pedicle screw  34  (see  FIG. 35 ), which secures the upper end of the body  38  into the pedicle  14  of the vertebral body, in similar fashion as generally shown in  FIG. 41 . As before described with reference to the link  26 , the opening  42  in the link body  38  could be elongated, to allow for varying orientations and/or sizes of the pedicle screw  34 . The remainder of the link body  28  can be secured to the exterior of the vertebra using, e.g., biocompatible adhesive.  
         [0133]     Unlike the link body  28 , the link body  38  includes an intermediate opening  44 . In use (see  FIG. 35 ), the spinous process  18  (if present) can extend through the opening  44 , to stabilize the link body  38  on the vertebral body. Desirably, a trans-spinous-process screw  45  can be used to provide additional stability.  
         [0134]     The lower section of the joint link body  38  is oriented to serve as the inferior half of a facet joint. The lower section of the joint link body  38  desirably incorporates a head  40 , which can be constructed in the same fashion as the head  30  of the link  26 . Like the head  30 , the facet head  40  can be permanently affixed to the body  38  or can be secured in with a frictional fit (e.g., using a Morse taper) for removal and replacement. Like the head  30 , the head  40  can be formed of a material commonly used in the prosthetic arts.  
         [0135]     In use (see  FIG. 35 ), the head  40  articulates with the superior half of the facet joint with the next adjacent vertebra level. As before explained for the link  26 , the superior facet  22  can comprise the natural superior articular facet  22  itself, or it can comprise a prosthetic facet created, e.g., by the previously described universal facet prosthesis  330 .  
         [0136]      FIG. 32  shows a superior prosthetic link  26 ′ that also embodies features of the invention. The prosthetic link  26 ′ is designated “superior” because it creates an artificial facet surface in the superior half of a facet joint. The artificial surface articulates with the inferior half of the facet joint. The superior prosthesis link  26 ′, like the prosthesis  26 , is particularly well suited to single-sided procedures and/or for procedures involving vertebral bodies which are not symmetrical.  
         [0137]     A stem  37  extends out from the upper end of the link  26 ′. The stem  37  is inserted (by screwing or tapping) into the pedicle, to thereby secure the link  26 ′ to the vertebral body.  
         [0138]     As  FIG. 32  shows, the upper end of the link  26 ′ is shaped to form a cup  36 , which articulates with the inferior half of the facet joint.  
         [0139]     The inferior half of the facet joint can comprise the natural inferior articular process  24  itself (as  FIG. 32  shows), or it can comprise the head  30  of an inferior prosthesis  26  or link  26 ′ attached to the next adjacent upper vertebra level (as  FIG. 33  shows).  
         [0140]     The lower end of the link  26 ′ can also carry a head  30  for articulation with the superior half of a facet joint with the next adjacent lower vertebra. The superior half of the facet joint can comprise the natural superior articular process  22  itself, or it can comprise the cup of a link  26 ′ attached to the next adjacent lower vertebra level.  
         [0141]     It can thus be appreciated that the link  26 ′ is well suited for use in procedures requiring replacement of multiple levels of facet joints, and can be interlinked in superior and inferior pairs, like a structure formed out of interlinking tinker-toy pieces. The link  26 ′ also allow subsequent surgeries to build upon already replaced levels, rather than requiring the removal and replacement of an existing implant to accommodate replacement of failing facet joints in an adjacent level. It should be appreciated that the upper end of the prosthesis  36  can also be shaped to form a cup to articulate with the superior half of the facet joint with the next adjacent upper vertebra level.  
         [0142]     The prosthesis  26 ,  36 , or link  26 ′ are well suited for use in a single side of the vertebral body, such as where the facet joints need only be replaced on a single side of the vertebral body. The prosthesis  26 ,  36 , or link  26 ′ are also well suited for use in a dual-sided procedure where the vertebral body is either symmetrical or non-symmetrical. In this arrangement, other prostheses  26 ,  36 , or links  26 ′ can be secured on the opposite side of the vertebral body, allowing both sides of the vertebral body to be treated. Because the surgeon can pick prostheses  26 ,  36 , and links  26 ′ of varying sizes, depending upon the size of the vertebral site, and can individually position each prosthesis  26  or link  26 ′ relative to the vertebral body, the surgeon can tailor the linked implant system to the individual&#39;s needs.  
         [0000]     B. Multiple Level, Sequential Link Assemblies  
         [0143]      FIG. 36  shows a universal prosthetic joint link assembly  56  that embodies features of the invention. The joint link assembly  56  is particularly well suited to double-sided procedures and for sequential, multiple level procedures.  
         [0144]     In the illustrated embodiment (see  FIG. 36 ), the joint link assembly  56  comprises two criss-crossing link bodies  58  and  60 . Each body  58  and  60  (shown mutually separated in  FIGS. 37 and 38 , respectively) may be formed of a material commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, or a combination thereof.  
         [0145]     As  FIG. 36  shows, the link bodies  58  and  60  are desirably locked together for use at an intermediate key-way  62 , to form the x-shaped, criss-crossing assembly  56 . The key-way  62  is formed by a shaped opening  68  in one body  60  (see  FIG. 37 ) and a mating shaped key  70  in the other body  58  (see  FIG. 38 ). The key  70  nests within the opening  60  (as  FIG. 36  shows), to frictionally hold the bodies  58  and  60  together and resist relative rotation between the bodies  58  and  60 .  
         [0146]     Of course, the shape of the opening  68  and key  70  can vary. In  FIGS. 36, 37 , and  38 , the opening  68  and key  70  are generally square or rectilinear in shape. In  FIG. 39 , an alternative link body  58  is shown, which possesses a key  70 ′ that is generally octagonal in shape, sized to nest within a corresponding octagonal opening in the other link (not shown). In this arrangement, the two link bodies  58  and  60  can be mutually assembled in different arcuately spaced orientations, allowing for variations in facet joint size and positioning. If desired, the key-way  62  could alternately be formed in a tooth and gear arrangement, which would desirably allow a multiplicity of potential arcuately spaced orientations for the two link bodies  58  and  60  forming the assembly  56 .  
         [0147]     The key  70  desirable peripherally defines an opening  72  (see  FIG. 38 ), through which the spinous process  18  can (if present) project during use. This is generally shown in phantom lines by  FIG. 41 .  
         [0148]     Alternatively, the link bodies  58  and  60  could be formed in a criss-crossing shape as a single, unitary body.  
         [0149]     The upper section of each link body  58  and  60  desirably includes a cup  64 . The cups  64  form the left and right superior halves of a facet joint and, in use, articulate with the left and right inferior halves of the facet joint.  
         [0150]     A stem  65  extends out from the upper end of each link body  58  and  60 . The stem  67  is inserted (by screwing or tapping) into the pedicle, to thereby secure the link bodies  58  and  60  to the vertebral body. In use, the stems  67  secure the upper end of the bodies  58  and  60  into an opposite pedicle  14  of a vertebral body.  
         [0151]     As  FIG. 40  best shows, the bodies  58  and  60  are each sized, shaped and mutually oriented to span the distance between a pedicle  14  on one side of the vertebral body and the region of the inferior articular process on the opposite side of the vertebral body. The remainder of the link bodies  58  and  60  can be secured to the exterior of the vertebra using, e.g., biocompatible adhesive. A trans-spinous-process screw  63  can also be used to provide additional stability.  
         [0152]     The lower section of each link body  58  and  60  is oriented to serve as the inferior half of a facet joint. As  FIG. 40  shows, the link body  58 , secured to the right pedicle, is positioned to serve as the inferior half of the facet joint on the left side of the vertebra. The link body  60 , secured to the left pedicle, is positioned to serve as the inferior half of the facet joint on the right side of the vertebra. For this purpose, the lower section of each link body  58  and  60  desirably incorporates a head  66 . As before explained, the head  66  can be permanently affixed to each body  58  and  60  or it can be secured in a frictional way using, e.g., a Morse taper for removal and replacement. Like the bodies  58  and  60 , the head  66  can be formed of a material commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, or a combination thereof.  
         [0153]     In use, the heads  66  articulate with the superior halves of the left and right facet joints with the next adjacent vertebra level. As earlier described with reference to the single link structures, the superior halves of the facet joints can comprise the natural superior articular process  22  itself, or it can comprise a prosthetic facet created, e.g., by the cups  64  of another link assembly  56  secured to the next adjacent lower vertebra.  
         [0154]     The interlocking of the criss-crossing link bodies  58  and  56  increases the strength of the overall link assembly  56 . The link assembly  56  distributes forces to both of the pedicles (and the spinous process, if desired), rather than relying upon fixation to a single pedicle.  
         [0155]     Like the link  26 ′, the link assembly  56  is well suited for implantation in procedures requiring replacement of multiple levels of facet joints, and can be interlinked in superior and inferior pairs, like a structure formed out of interlinking tinker-toy pieces. Like the link  26 ′, the link assembly  56  also allows subsequent surgeries to build upon already replaced levels, rather than requiring the removal and replacement of an existing implant to accommodate replacement of failing facet joints in an adjacent level.  
         [0156]     The size and shape of any prosthesis disclosed herein are desirably selected by the physician, taking into account the morphology and geometry of the site to be treated. The shape of the joint, the bones and soft tissues involved, and the local structures that could be harmed if move inappropriately, are generally understood by medical professionals using textbooks of human anatomy along with their knowledge of the site and its disease and/or injury. The physician is also desirably able to select the desired shape and size of the prosthesis and its placement in and/or around the joint based upon prior analysis of the morphology of the targeted joint using, for example, plain film x-ray, fluoroscopic x-ray, or MRI or CT scanning. The shape, size and placement are desirably selected to optimize the strength and ultimate bonding of the prosthesis to the surrounding bone and/or tissue of the joint.  
         [0157]     Other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. All documents referenced herein are specifically and entirely incorporated by reference. The specification and examples should be considered exemplary only with the true scope and spirit of the invention indicated by the following claims. As will be easily understood by those of ordinary skill in the art, variations and modifications of each of the disclosed embodiments can be easily made within the scope of this invention as defined by the following claims.