Patent Publication Number: US-8118838-B2

Title: Inter-cervical facet implant with multiple direction articulation joint and method for implanting

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Provisional Application No. 60/687,765, entitled “Inter-Cervical Facet Implant With Multiple Direction Articulation Joint And Method For Implanting,” filed Jun. 6, 2005, which is incorporated herein by reference in its entirety. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. Provisional Patent Application incorporates by reference all of the following co-pending applications and issued patents: 
     U.S. Provisional Application No. 60/635,435, entitled “Inter-Cervical Facet Implant and Method,” filed Dec. 13, 2004; 
     U.S. application Ser. No. 11/053,399, entitled “Inter-Cervical Facet Implant and Method,” filed Feb. 8, 2005; 
     U.S. application Ser. No. 11/053,624, entitled “Inter-Cervical Facet Implant and Method,” filed Feb. 8, 2005; 
     U.S. application Ser. No. 11/053,735, entitled “Inter-Cervical Facet Implant and Method,” filed Feb. 8, 2005; 
     U.S. application Ser. No. 11/093,557, entitled “Inter-Cervical Facet Implant with Locking Screw and Method,” filed Mar. 30, 2005; 
     U.S. Provisional Application No. 60/679,363, entitled “Inter-Cervical Facet Implant with Implantation Tool,” filed May 10, 2005; 
     U.S. Provisional Application No. 60/679,361, entitled “Inter-Cervical Facet Implant with Implantation Tool,” filed May 10, 2005; 
     U.S. Provisional Application No. 60/679,377, entitled “Inter-Cervical Facet Implant with Implantation Tool,” filed May 10, 2005; 
     U.S. application Ser. No. 11/429,905, entitled “Inter-Cervical Facet Implant with Implantation Tool,” filed May 8, 2006; 
     U.S. application Ser. No. 11/429,726, entitled “Inter-Cervical Facet Implant with Implantation Tool,” filed May 8, 2006; 
     U.S. application Ser. No. 11/429,733, entitled “Inter-Cervical Facet Implant with Implantation Tool,” filed May 8, 2006; 
     International Patent Application No. PCT/US2005/044979, entitled “Inter-Facet Implant,” filed Dec. 13, 2005; 
     U.S. application Ser. No. 11/053,346, entitled “Inter-Cervical Facet Implant and Method,” filed Feb. 8, 2005; 
     U.S. application Ser. No. 11/093,689, entitled “Inter-Cervical Facet Implant and Method for Preserving the Tissues Surrounding the Facet Joint,” filed Mar. 30, 2005; 
     U.S. Provisional Application No. 60/668,053, entitled “Inter-Cervical Facet Implant Distraction Tool,” filed Apr. 4, 2005; and 
     U.S. application Ser. No. 11/397,220, entitled “Inter-Cervical Facet Implant Distraction Tool, filed Apr. 4, 2005. 
    
    
     TECHNICAL FIELD 
     This invention relates to interspinous process implants. 
     BACKGROUND OF THE INVENTION 
     The spinal column is a bio-mechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebral disks. The bio-mechanical functions of the spine include: (1) support of the body, which involves the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs, (2) complex physiological motion between these parts, and (3) protection of the spinal cord and the nerve roots. 
     As the present society ages, it is anticipated that there will be an increase in adverse spinal conditions which are characteristic of older people. By way of example only, with aging comes an increase in spinal stenosis (including, but not limited to, central canal and lateral stenosis), and facet arthropathy. Spinal stenosis results in a reduction foraminal area (i.e., the available space for the passage of nerves and blood vessels) which compresses the cervical nerve roots and causes radicular pain. Humpreys, S. C. et al.,  Flexion and traction effect on C 5- C 6 foraminal space , Arch. Phys. Med. Rehabil., vol. 79 at 1105 (September 1998). Another symptom of spinal stenosis is myelopathy, which results in neck pain and muscle weakness. Id. Extension and ipsilateral rotation of the neck further reduces the foraminal area and contributes to pain, nerve root compression, and neural injury. Id.; Yoo, J. U. et al.,  Effect of cervical spine motion on the neuroforaminal dimensions of human cervical spine , Spine, vol. 17 at 1131 (Nov. 10, 1992). In contrast, neck flexion increases the foraminal area. Humpreys, S. C. et al., supra, at 1105. 
     In particular, cervical radiculopathy secondary to disc herniation and cervical spondylotic foraminal stenosis typically affects patients in their fourth and fifth decade, and has an annual incidence rate of 83.2 per 100,000 people (based on 1994 information). Cervical radiculopathy is typically treated surgically with either an anterior cervical discectomy and fusion (“ACDF”) or posterior laminoforaminotomy (“PLD”), with or without facetectomy. ACDF is the most commonly performed surgical procedure for cervical radiculopathy, as it has been shown to increase significantly the foramina dimensions when compared to a PLF. 
     It is desirable to eliminate the need for major surgery for all individuals, and in particular, for the elderly. Accordingly, a need exists to develop spine implants that alleviate pain caused by spinal stenosis and other such conditions caused by damage to, or degeneration of, the cervical spine. 
     The present invention addresses this need with implants and methods for implanting an apparatus into at least one facet joint of the cervical spine to distract the cervical spine while preferably preserving mobility and normal lordotic curvature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a lateral view of two adjacent cervical vertebrae and spinous processes, highlighting the cervical facet joint. 
         FIG. 2  depicts a lateral view of the cervical spine with spinal stenosis. 
         FIG. 3A  depicts correction of cervical stenosis or other ailment with a wedge-shaped embodiment of the implant of the invention positioned in the cervical facet joint. 
         FIG. 3B  depicts correction of cervical kyphosis or loss of lordosis with a wedge-shaped embodiment of the invention with the wedge positioned in the opposite direction as that depicted in  FIG. 3A . 
         FIG. 4  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention including a screw fixation device for attaching to a single vertebra. 
         FIG. 5  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising screw fixation of two implants, one implant fixed to each of two adjacent vertebrae. 
         FIG. 6  shows cervical spine kyphosis, or loss of lordosis. 
         FIG. 7  shows correction of cervical kyphosis, or loss of lordosis, with a further embodiment of the implant of the invention comprising two facet implants with screw fixation. 
         FIG. 8  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet implant and a keel. 
         FIG. 9  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising facet implant, a keel, and screw fixation. 
         FIG. 10  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet implant with teeth. 
         FIG. 11  depicts correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet implant with teeth and screw fixation. 
         FIG. 12  depicts correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants having bony ingrowth surfaces. 
         FIG. 13  depicts correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants having bony ingrowth surfaces and posterior alignment guide. 
         FIG. 14  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants with increased facet joint contact surfaces. 
         FIG. 15  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants having bony ingrowth surfaces and screw fixation. 
         FIG. 16  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants with articular inner surfaces. 
         FIG. 17  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet joint implant with a roller. 
         FIG. 18  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet joint implant with a plurality of rollers. 
         FIG. 19  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet joint implants, screw fixation, and elastic restraint. 
         FIG. 20  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet joint implants, screw fixation, and spring restraint. 
         FIG. 21  shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet joint implants, screw fixation, and magnetic restraint. 
         FIG. 22A  shows a perspective view of a further embodiment of implant of the invention. 
         FIG. 22B  shows a perspective exploded view of the embodiment of the invention shown in  FIG. 22A . 
         FIG. 23A  depicts a posterior view of the embodiment of the implant of the invention shown in  FIG. 22A . 
         FIG. 23B  shows a posterior view of a locking plate of the embodiment of the implant of the invention shown in  FIG. 22A . 
         FIG. 24A  depicts a lateral side view of the embodiment of the implant of the invention shown in  FIG. 22A . 
         FIG. 24B  shows a lateral side view of the keel of the locking plate of the embodiment of the implant of the invention shown in  FIG. 22A . 
         FIG. 25A  shows a perspective view of a further embodiment of the implant of the invention. 
         FIG. 25B  shows a side view of the embodiment of the implant of the invention in  FIG. 25A , having a curved, uniformly-thick artificial facet joint spacer or inter-facet spacer including a tapered end 
         FIG. 26A  shows a perspective view of a further embodiment of the implant of the invention having a locking cam in a first position. 
         FIG. 26B  shows a perspective view of a further embodiment of the implant of the invention having a locking cam in a second position. 
         FIG. 27A  depicts a side view of the embodiment of the implant of the invention shown in  FIGS. 26A and 26B , implanted in a cervical spine. 
         FIG. 27B  shows a posterior perspective view of the embodiment of the implant of the invention depicted in  FIGS. 26A ,  26 B and  FIG. 27A . 
         FIG. 28A  depicts a posterior perspective view of a further embodiment of the implant of the invention. 
         FIG. 28B  depicts a side view of the embodiment of the implant of the invention shown in  FIG. 28A . 
         FIG. 29A  depicts a side view of an embodiment of a sizing tool of the invention. 
         FIG. 29B  depicts a top view of an embodiment of the sizing tool of the invention depicted in  FIG. 29A . 
         FIG. 29C  depicts a perspective view of an embodiment of the sizing tool of the invention depicted in  FIGS. 29A and 29B . 
         FIG. 29D  depicts a side view of the head of the sizing tool of the invention depicted in  FIG. 29A . 
         FIG. 29E  depicts a cross-sectional view of the head of the sizing tool of the invention depicted in  FIGS. 29A-29C . 
         FIG. 30  is a flow diagram of an embodiment of a method of the invention. 
         FIG. 31A  is posterior view of a further embodiment of the implant of the invention. 
         FIG. 31B  is a side view of an embodiment of a locking screw of the implant of the invention depicted in  FIG. 31A . 
         FIG. 32  is a posterior view of a further embodiment of the implant of the invention. 
         FIGS. 33A and 33B  depict initial and final insertion positions of the embodiment of the invention depicted in  FIG. 32 . 
         FIGS. 34A and 34B  illustrate a top and bottom plan view of an alternative embodiment of an inter-cervical facet implant in accordance with the present invention. 
         FIG. 35  is a partially exploded perspective view of the implant of  FIGS. 34A and 34B . 
         FIGS. 36A and 36B  illustrate side views of the implant of  FIGS. 34A and 34B  illustrating a general range of motion of the implant. 
         FIG. 37  is a side view of still another embodiment of an implant in accordance with the present invention. 
         FIG. 38  is a flow diagram of an alternative embodiment of a method in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide for a minimally invasive surgical implantation method and apparatus for cervical spine implants that preserves the physiology of the spine. In particular, embodiments provide for distracting the cervical spine to increase the foraminal dimension in extension and neutral positions. Such implants, when implanted in the cervical facet joints, distract, or increase the space between, the vertebrae to increase the foraminal area or dimension, and reduce pressure on the nerves and blood vessels of the cervical spine. 
     The facet joints in the spine are formed between two vertebrae as follows. Each vertebra has four posterior articulating surfaces: two superior facets and two inferior facets, with a superior facet from a lower vertebra and an inferior facet of an upper vertebra forming a facet joint on each lateral side of the spine. In the cervical spine, the upward inclination of the superior articular surfaces of the facet joints allows for considerable flexion and extension, as well as for lateral mobility. Each facet joint is covered by a dense, elastic articular capsule, which is attached just beyond the margins of the articular facets. The capsule is larger and looser in the cervical spine than in the thoracic and lumbar spine. The inside of the capsule is lined by a synovial membrane which secretes synovial fluid for lubricating the facet joint. The exterior of the joint capsule is surrounded by a capsular ligament. It is this ligament and the joint capsule that must be cut in the embodiments of the method described herein for inserting the artificial facet joint. 
     In a specific preferred embodiment, an implanted interfacet spacer of 1.5 mm to 2.5 mm in width can result in interfacet distraction that increases foraminal dimension in extension and neutral. Other interfacet spacer dimensions also are contemplated by the invention described herein below. The present embodiments also preserve mobility of the facet joints. 
     Further embodiments of the present invention accommodate the distinct anatomical structures of the spine, minimize further trauma to the spine, and obviate the need for invasive methods of surgical implantation. Embodiments of the present invention also address spinal conditions that are exacerbated by spinal extension. 
       FIG. 1  shows a simplified diagram of a portion of the cervical spine, focusing on a cervical facet joint  1  formed between two adjacent cervical vertebrae. The spinous processes  3  are located posteriorly and the vertebral bodies  5  are located anteriorly, and a nerve root canal  7  is visible. Each vertebra has four posterior articulating surfaces: two superior facets and two inferior facets, with a superior facet from a lower vertebra and an inferior facet of an upper vertebra forming a facet joint on each lateral side of the spine. In the cervical spine, the upward inclination of the superior articular surfaces of the facet joints allows for considerable flexion and extension, as well as for lateral mobility. Each facet joint is covered by a dense, elastic articular capsule, which is attached just beyond the margins of the articular facets. The capsule is large and looser in the cervical spine than in the thoracic and lumbar spine. The inside of the capsule is lined by a synovial membrane which secretes synovial fluid for lubricating the facet joint. The exterior of the joint capsule is surrounded by a capsular ligament. It is this ligament that may be pushed out of the way in the embodiments of the method for inserting the facet joint spacer or inter-facet spacer, described herein. 
       FIG. 2  depicts cervical foraminal stenosis. From the drawing, the nerve root canal  7  is narrowed relative to the nerve root canal  7  depicted in  FIG. 1 . The spinal canal and/or intervertebral foramina also can be narrowed by stenosis. The narrowing can cause compression of the spinal cord and nerve roots. 
       FIG. 3A  shows a first embodiment  100  of the present invention, which is meant to distract at least one facet joint, in order to increase the dimension of the neural foramen while retaining facet joint mobility. The wedge-shaped embodiment  100  is a wedge-shaped implant that can be positioned in the cervical facet joint  101  to distract the joint and reverse narrowing of the nerve root canal  107 . In this embodiment  100 , the implant is positioned with the narrow portion of the wedge facing anteriorly. However, it is also within the scope of the present invention to position embodiment  100  ( FIG. 3B ) with the wide portion of the wedge facing anteriorly, to correct for cervical kyphosis or loss of cervical lordosis. 
     Referring to  FIG. 4 , the embodiment  200  of the implant has a joint insert or inter-facet spacer  210 , also herein referred to as a facet joint spacer or inter-facet spacer, that is positioned in the cervical facet joint  101 . The joint insert or inter-facet spacer  210  can be wedge-shaped with the narrow part of the wedge facing anteriorly. Alternatively, the joint insert or inter-facet spacer  210  need not be wedge-shaped but can be of substantially uniform thickness, the thickness determined by an individual patient&#39;s need for distraction of the cervical facet joint  201 . As with embodiment  100 , one objective of this embodiment is facet joint distraction, and joint mobility after implantation. The joint insert or inter-facet spacer  210  is continuous with a posterior sheath  220  bent at an angle from the joint insert or inter-facet spacer  210  to align substantially parallel with the bone. The posterior sheath can lie against the lamina, preferably against the lateral mass. The posterior sheath  220  can have a bore  230  which can accept a bone screw  240 . Alternatively, the bore  230  can accept any other appropriate and/or equivalent fixation device capable of fixing the embodiment  200  to the spine. The device is thereby affixed to the vertebra, preferably by fixing to the lateral mass. 
       FIG. 5  shows embodiment  300 , which is the use of two embodiments  200 , each fixed to one of two adjacent cervical vertebrae. As with embodiment  200 , the implanted facet joint is distracted and joint mobility is retained. A joint insert or inter-facet spacer  310  from each of the two implants is inserted and positioned in the cervical facet joint  301 . In this embodiment, the joint inserts or inter-facet spacers  310  are substantially flat and parallel to each other and are not wedge-shaped. Attentively, the joint inserts or inter-facet spacers  310  can together define a wedge-shaped insert that is appropriate for the patient. The two joint inserts or inter-facet spacers  310  combined can have, by way of example, the shape of the joint insert or inter-facet spacer  210  in  FIG. 4 . Embodiment  300  then can be fixed to the spine with a screw  340  or any other appropriate fixation device, inserted through a bore  330  in the posterior sheath  320 . The posterior sheath  320  can be threaded to accept a screw. The screw can be embedded in the lamina, preferably in the lateral mass, where possible. 
     It is within the scope of the present invention to use and/or modify the implants of the invention to correct cervical spine kyphosis, or loss of lordosis.  FIG. 6  depicts a cervical spine lordosis.  FIG. 7  demonstrates an embodiment  400  which contemplates positioning two implants to correct for this spinal abnormality while retaining facet joint mobility. The joint insert or inter-facet spacer  410  of each implant is shaped so that it is thicker at its anterior portion. Alternatively, the implants can be shaped to be thicker at the posterior ends, for example as depicted in  FIG. 3A . The posterior sheath  420  of each implant is bent at an angle from the joint insert or inter-facet spacer  410  to be positioned adjacent to the lateral mass and/or lamina, and has a bore  430  to accept a screw  440  or other appropriate and/or equivalent fixation means to fix the embodiment  400  to the spine, preferably to the lateral mass. The placement of two joint inserts or inter-facet spacers  410  in the cervical facet joint  401  distracts the facet joint, which shifts and maintains the vertebrae into a more anatomical position to preserve the physiology of the spine. 
       FIG. 8  shows a further embodiment  500  of the implant of the invention, wherein the joint insert or inter-facet spacer  510  has a keel  550  on an underside of the joint insert or inter-facet spacer  510 . The keel  550  can be made of the same material or materials set forth above. The surfaces of the keel  550  can be roughened in order to promote bone ingrowth to stabilize and fix the implant  500 . In other embodiments, the keel  550  can be coated with materials that promote bone growth such as, for example, bone morphogenic protein (“BMP”), or structural materials such as hyaluronic acid “HA,” or other substances which promote growth of bone relative to and into the keel  550 . 
     The keel  550  can be embedded in the facet bone, to facilitate implant retention. The keel  550  can be placed into a channel in the facet bone. The channel can be pre-cut. Teeth (not shown), preferably positioned posteriorly, also may be formed on the keel  550  for facilitating retention of the implant  500  in the cervical facet joint  501 . As noted above, the joint insert or inter-facet spacer  510  can be substantially flat or wedge-shaped, depending upon the type of distraction needed, i.e., whether distraction is also necessary to correct abnormal curvature or lack of curvature in the cervical spine. Because the joint is not fused, mobility is retained, as with the embodiments described above and herein below. 
       FIG. 9  illustrates that a further embodiment  600  of the implant of the invention can have both screw fixation and a keel  650  for stability and retention of the implant  600 . On embodiment  600 , the joint insert or inter-facet spacer  610  is continuous with a posterior sheath  620  having a bore hole  630  to accept a screw  640  which passes through the bore  630  and into the bone of the vertebrae, preferably into the lateral mass, or the lamina. The bore  630  can be threaded or not threaded where it is to accept a threaded screw or equivalent device. Alternatively, the bore  630  need not be threaded to accept a non-threaded equivalent device. The keel  650  is connected with the joint insert or inter-facet spacer  610  and embeds in the bone of the cervical facet joint  601  to promote implant retention. 
     A further alternative embodiment  700  is illustrated in  FIG. 10 . In this embodiment  700 , the joint insert or inter-facet spacer  710  has on a lower side at least one tooth  760 . It should be clear to one of ordinary skill in the art that a plurality of teeth  760  is preferable. The teeth  760  are able to embed in the bone of the cervical facet joint  701  to facilitate retention of the implant  700  in the joint  701 . The teeth  760  can face in a direction substantially opposite the direction of insertion, for retention of the implant  700 . As above, the joint insert or inter-facet spacer  710  can be wedge-shaped or substantially even in thickness, depending upon the desired distraction. Because the implant distracts and is retained without fusion, facet joint mobility is retained. 
       FIG. 11  depicts a further embodiment  800  of the implant of the invention. In this embodiment  800 , the joint insert or inter-facet spacer  810  is continuous with a posterior sheath  820  having a bore  830  for accepting a fixation device  840 , as described above. The fixation device  840  can be a screw which fits into a threaded bore  830 ; alternatively, the fixation device  830  can be any other compatible and appropriate device. This embodiment  800  further combines at least one tooth  860  on an underside of the joint insert or inter-facet spacer  810  with the posterior sheath  820 , bore  830  and fixation device  840  to address fixation of the implant  800  in a cervical facet joint  801 . It will be recognized by one of ordinary skill in the art that the implant  800  can have a plurality of teeth  860  on the underside of the joint insert or inter-facet spacer  810 . 
       FIG. 12  shows yet another embodiment  900  of an implant of the present invention. In this embodiment  900 , the joint inserts or inter-facet spacers  910  of two implants  900  are positioned in a cervical facet joint  901 . As described above, the joint inserts or inter-facet spacers  910  can be wedge-shaped as needed to restore anatomical curvature of the cervical spine and to distract, or the joint inserts or inter-facet spacers  910  can be of substantially uniform thickness. The implants  900  each comprise a joint insert or inter-facet spacers  910  with an outer surface  970  that interacts with the bone of the cervical facet joint  901 . On the upper implant  900 , the surface  970  that interacts with the bone is the upper surface  970  and on the lower implant  900 , the surface  970  that interacts with the bone is the lower surface  970 . Each surface  970  can comprise a bone ingrowth surface  980  to create a porous surface and thereby promote bone ingrowth and fixation. One such treatment can be with plasma spray titanium, and another, with a coating of sintered beads. Alternatively, the implant  900  can have casted porous surfaces  970 , where the porous surface is integral to the implant  900 . As a further alternative, the surfaces  970  can be roughened in order to promote bone ingrowth into these defined surfaces of the implants  900 . In other embodiments, the surfaces  970  can be coated with materials that promote bone growth such as for example bone morphogenic protein (“BMP”), or structural materials such as hyaluronic acid (“HA”), or other substances which promote growth of bone on other external surfaces  970  of the implant  900 . These measures facilitate fixation of the implants  900  in the facet joint, but do not result in fusion of the joint, thereby retaining facet joint mobility, while also accomplishing distraction of the joint. 
       FIG. 13  depicts yet another embodiment  1000  of the implant of the present invention. In this embodiment  1000 , the joint inserts or inter-facet spacers  1010  of two implants  1000  are positioned in a cervical facet joint  1001 . As described above, the joint inserts or inter-facet spacers  1010  can be wedge-shaped as needed to restore anatomical curvature of the cervical spine and to distract, or the joint inserts or inter-facet spacers  1010  can be of substantially uniform thickness. The implants  1000  each comprise a joint insert or inter-facet spacer  1010  with an outer surface  1070  that interacts with the bone of the cervical facet joint  1001 . On the upper implant  1000 , the surface  1070  that interacts with the bone is the upper surface and on the lower implant  1000 , the surface  1070  that interacts with the bone is the lower surface. As set forth above, each outer surface  1070  can comprise a bone ingrowth surface  1080  to create a porous surface and thereby promote bone ingrowth and fixation, without facet joint fusion and loss of mobility. In one preferred embodiment, the bone ingrowth surface  1080  can be created with plasma spray titanium, and/or with a coating of sintered beads. In an alternative preferred embodiment, the implant  1000  can have casted porous surfaces  1070 , where the porous surface is integral to the implant  1000 . In a further alternative preferred embodiment, the surfaces  1070  can be roughened in order to promote bone ingrowth into these defined surfaces of the implants  1000 . In other preferred embodiments, the surfaces  1070  can be coated with materials that promote bone growth such as for example BMP, or structural materials such as HA, or other substances which promote growth of bone on other external surfaces  1070  of the implant  1000 . 
     The implant  1000  can have a posterior alignment guide  1090 . The posterior alignment guides  1090  of each implant  1000  can be continuous with the joint inserts or inter-facet spacers  1010 . The posterior alignment guides substantially conform to the bone of the vertebrae when the joint inserts or inter-facet spacers  1010  are inserted into the cervical facet joint  1001 . The posterior alignment guides  1090  are used to align the implants  1000  so that the joint inserts or inter-facet spacers  1010  contact each other and not the bones of the cervical facet joint  1001  when the joint inserts or inter-facet spacers  1010  are positioned in the cervical facet joint  1001 . 
       FIG. 14  depicts a further embodiment  1100  of the implant of the present invention. In this embodiment  1100 , the joint inserts or inter-facet spacers  1110  of two implants  1100  are inserted into the cervical facet joint  1101 . Each of the joint inserts or inter-facet spacers  1110  is continuous with a cervical facet joint extender or facet-extending surface  1192 . The bone contacting surfaces  1170  of the joint inserts or inter-facet spacers  1110  are continuous with, and at an angle to, the bone contacting surfaces  1193  of the cervical facet joint extenders  1192 , so that the cervical facet joint extenders  1192  conform to the bones of the vertebrae exterior to the cervical facet joint  1101 . The conformity of the cervical facet joint extenders  1192  is achieved for example by forming the cervical facet joint extenders  1192  so that when the join inserts or inter-facet spacers  1110  are positioned, the cervical facet joint extenders  1192  curve around the bone outsider the cervical facet joint  1101 . 
     The cervical facet joint extenders have a second surface  1184  that is continuous with the joint articular surfaces  1182  of the joint inserts or inter-facet spacers  1110 . The second surfaces  1184  extend the implant  1100  posteriorly to expand the joint articular surfaces  1182  and thereby to increase contact and stability of the spine at least in the region of the implants  1100 . It is to be understood that such facet joint extenders  1192  can be added to the other embodiments of the invention described and depicted herein. 
     The embodiment depicted in  FIG. 15  shows two implants  1200  positioned in a cervical facet joint  1201 , having bony ingrowth surfaces as one preferred method of fixation, and using screws as another preferred method of fixation. In this embodiment, each of two implants  1200  has a joint insert or inter-facet spacer  1210  positioned in a cervical facet joint  1201 . As described above, the joint inserts or inter-facet spacers  1210  can be wedge-shaped as needed to restore anatomical curvature of the cervical spine and to distract, or the joint inserts or inter-facet spacers  1210  can be of substantially uniform thickness. The implants  1200  each comprise a joint insert or inter-facet spacer  1210  with an outer surface  1270  that interacts with the bone of the cervical facet joint  1201 . On the upper implant  1200 , the surface  1270  that interacts with the bone is the upper surface and on the lower implant  1200 , the surface  1270  that interacts with the bone is the lower surface. As set forth above, each outer surface  1270  can comprise a bone ingrowth surface  1280  to create a porous surface and thereby promote bone ingrowth and fixation. In one preferred embodiment, the bone ingrowth surface  1280  can be created with plasma spray titanium, and/or with a coating of sintered beads. In an alternative preferred embodiment, the implant  1200  can have casted porous surfaces  1270 , where the porous surface is integral to the implant  1200 . In a further alternative embodiment, the surfaces  1270  can be roughened in order to promote bone ingrowth into these defined surfaces of the implants  1200 . In other preferred embodiments, the surfaces  1270  can be coated with materials that promote bone growth such as for example BMP, or structural materials such as HA, or other substances which promote growth of bone on other external surfaces  1270  of the implant  1200 . 
     Screw fixation or other appropriate fixation also can be used with implants  1200  for fixation in the cervical facet joint  1201 . The joint insert or inter-facet spacer  1210  is continuous with a posterior sheath  1220  bent at an angle from the joint insert or inter-facet spacer  1210  to align substantially parallel with the bone, preferably the lateral mass or lamina. The posterior sheath  1220  can have a bore  1230  which can accept a bone screw  1240 , preferably into the lateral mass or lamina. Alternatively, the bore  1230  can accept any other appropriate and/or equivalent fixation means for fixing the embodiment  1200  to the spine. 
       FIG. 16  depicts a further preferred embodiment of the present invention. In this embodiment  1300 , two joint inserts or inter-facet spacers  1310  are positioned in the cervical facet joint  1301 . The joint inserts or inter-facet spacers each have outer surfaces  1370  that interact with the bone of the vertebrae forming the cervical facet joint. These outer surfaces  1370  of the embodiment  1300  can be treated to become bone ingrowth surfaces  1380 , which bone ingrowth surfaces  1380  contribute to stabilizing the two joint inserts or inter-facet spacers  1310  of the implant  1300 . In one preferred embodiment, the bone ingrowth surface  1380  can be created with plasma spray titanium, and/or with a coating of sintered beads. In an alternative preferred embodiment, the implant  1300  can have casted porous surfaces  1370 , where the porous surface is integral to the implant  1300 . In a further alternative embodiment, the surfaces  1370  can be roughened in order to promote bone ingrowth into these defined surfaces of the implants  1300 . In other preferred embodiments, the surfaces  1370  can be coated with materials that promote bone growth such as for example BMP, or structural materials such as HA, or other substances which promote growth of bone on other external surfaces  1370  of the implant  1300 . This fixation stabilizes the implant  1300  in the facet joint without fusing the joint, and thus the implant preserves joint mobility, while accomplishing distraction and increasing foraminal dimension. 
     Also shown in  FIG. 16  are articular inner surfaces  1382  of the implants  1300 . These surfaces can be formed from a metal and polyethylene, the material allowing flexibility and providing for forward bending/flexion and backward extension of the cervical spine. The embodiment  1300  of  FIG. 16  can be made in at least two configurations. The first configuration includes a flexible spacer  1382  made, by way of example, using polyethylene or other suitable, flexible implant material. The flexible spacer  1382  can be permanently affixed to the upper and lower joint insert or inter-facet spacer  1310 . The spacer  1382  can be flat or wedge-shaped or have any other shape that would correct the curvature of the spine. In other configurations, the spacer  1382  can be affixed to only the upper insert or inter-facet spacer  1310  or to only the lower insert or inter-facet spacer  1310 . Alternatively, a spacer  1382  can be affixed to each of an upper insert or inter-facet spacer  1310  and a lower insert or inter-facet spacer  1310  with the upper insert or inter-facet spacer  1310  and the lower insert or inter-facet spacer  1310  being separate units. 
       FIG. 17  shows a further preferred embodiment of the implant of the present invention. In this embodiment  1400 , the implant has a roller  1496  mounted on a joint insert or inter-facet spacer  1410 , the roller being a further means of preserving joint mobility while accomplishing distraction. Both the roller  1496  and the joint insert or inter-facet spacer  1410  are positioned in the cervical facet joint  1401 . The joint insert or inter-facet spacer  1410  as in other embodiments has a bone-facing surface  1470  and joint articular surface  1482 . The bone-facing surface  1470  can interact with the lower bone of the cervical facet joint  1401 . Alternatively, the bone-facing surface can interact with the upper bone of the cervical facet joint  1401 . Between the bone-facing surface  1470  and the joint articular surface  1482  is an axis about which the roller  1496  can rotate. The roller  1496  rotates in a cavity in the joint insert or inter-facet spacer  1410 , and interacts with the top bone of the cervical facet joint  1401 . Alternatively, where the bone-facing surface  1470  of the joint insert or inter-facet spacer  1410  interacts with the top bone of the cervical facet joint  1401 , the roller  1496  rotates in a cavity in the joint insert or inter-facet spacer  1410  and interacts with the lower bone of the cervical facet joint  1401 . The rotation of the roller  1496  allows flexion and extension of the cervical spine. Alternatively, a roller such as roller  1496  can be secured to an upper and a lower insert such as inserts or spacers  410  in  FIG. 7 . As depicted in  FIG. 18 , a plurality of rollers  1496  also is possible. 
       FIG. 19  depicts a further embodiment of the implant of the present invention. In this embodiment, two implants  1500  are implanted in the cervical facet joint  1501 . Screw fixation or other appropriate fixation is used with implants  1500  for fixation in the cervical facet joint  1501 . The joint insert or inter-facet spacer  1510  is continuous with a posterior sheath  1520  bent at an angle from the joint insert or inter-facet spacer  1510  to align substantially parallel with the bone, preferably the lateral mass or lamina. The posterior sheath  1520  of each implant  1500  can have a bore  1530  which can accept a bone screw  1540 , preferably into the lateral mass or lamina. Alternatively, the bore  1530  can accept any other appropriate and/or equivalent fixation means for fixing the embodiment  1500  to the spine. The head of the screw  1540  in each posterior sheath  1520  of each implant  1500  has a groove  1598  or other mechanism for retaining an elastic band  1597 . The elastic band  1597  is looped around each of the two screws  1540  to restrain movement of the cervical spine without eliminating facet joint mobility. The band  1597  preferably can restrain flexion and lateral movement. The elastic band  1597  can be made of a biocompatible, flexible material. 
       FIG. 20  shows an alternative to use of an elastic band as in  FIG. 19 . In the embodiment in  FIG. 20 , the elastic band is replaced with a spring restraint  1699 , which extends between the heads of two screws  1640 , one screw fixing each of two implants  1600  in the cervical facet joint  1601 . 
       FIG. 21  shows another alternative to using an elastic band and/or a spring as in  FIG. 19  or  20 . In  FIG. 21 , magnets  1795  is used for restraint between the two screws  1740 . The magnet  1795  can either be comprised of two opposing magnetic fields or two of the same magnetic fields to operate to restrain movement. The head of one of the two screws  1740  is magnetized, and the head of the other screw  1740  is magnetized with either the same or opposite field. If the magnets  1795  have the same polarity, the magnets  1795  repel each other and thus limit extension. If the magnets  1795  have opposite polarities, the magnets  1795  attract each other and thus limit flexion and lateral movement. 
       FIGS. 22A-24B , depict a further embodiment  1800  of the implant of the present invention. In this embodiment, a facet joint spacer (or insert) or inter-facet spacer (or insert)  1810  is connected with a lateral mass plate (also referred to herein as an anchoring plate)  1820  with a hinge  1822 . The hinge  1822  allows the lateral mass plate  1820  to bend at a wide range of angles relative to the facet joint spacer or inter-facet spacer and preferably at an angle of more than 90 degrees, and this flexibility facilitates positioning and insertion of the facet joint spacer or inter-facet spacer  1810  into a patient&#39;s facet joint, the anatomy of which can be highly variable among individuals. This characteristic also applies to embodiments described below, which have a hinge or which are otherwise enabled to bend by some equivalent structure or material property. The hinge  1822  further facilitates customizing the anchoring of the implant, i.e., the positioning of a fixation device. The hinge enables positioning of the lateral mass plate  1820  to conform to a patient&#39;s cervical spinal anatomy, and the lateral mass plate  1820  accepts a fixation device to penetrate the bone. The facet joint spacer or inter-facet spacer  1810  can be curved or rounded at a distal end  1812  ( FIG. 23A ), and convex or dome-shaped on a superior surface  1813  to approximate the shape of the bone inside the facet joint. The inferior surface  1815  can be flat or planar. Alternatively, the inferior surface  1815  can be concave. As another alternative, the inferior surface  1815  can be convex. 
     The lateral mass plate  1820 , when implanted in the spine, is positioned outside the facet joint, preferably against the lateral mass or against the lamina. The lateral mass plate  1820  has a bore  1830  therethrough. The bore  1830  can accept a bone screw  1840 , also referred to as a lateral mass screw, to secure the lateral mass plate  1820  preferably to the lateral mass or alternatively to another part of the spine, and thus to anchor the implant. The lateral mass screw  1840  preferably has a hexagonal head to accept an appropriately-shaped wrench. As described below, the head accepts a compatible probe  1826  from a locking plate  1824 . 
     The locking plate  1824  includes a keel  1828  with a wedge shaped distal end to anchor the implant, preferably in the lateral mass or in the lamina, outside the facet joint and to prevent rotation of the lateral mass plate  1820  and the locking plate  1824 . The keel  1828  aligns with a groove  1823  through an edge of the lateral mass plate  1820  to guide and align the keel  1828  as the keel  1828  cuts into a vertebra. 
     As noted above, the locking plate  1824  includes a probe  1826  that fits against the head of the lateral mass screw  1840 . The locking plate further includes a bore  1831  that can accept a machine screw (not shown) which passes through to an aligned bore  1829  in the lateral mass plate  1820  to hold the locking plate  1824  and the lateral mass plate  1820  together without rotational displacement relative to each other. The locking plate  1824  thus serves at least two functions: (1) maintaining the position of the lateral mass screw  1840  with the probe  1826 , so that the screw  1840  does not back out; and (2) preventing rotation of the implant with the keel  1828  and machine screw relative to the cervical vertebra or other vertebrae. 
     It is to be understood that other mechanisms can be used to lock the locking plate  1824  to the lateral mass plate  1820 . For example, the locking plate can include a probe with barbs that can be inserted into a port in the lateral mass plate. The barbs can become engaged in ribs that define the side walls of the port in the lateral mass plate 
     In the preferred embodiment depicted in  FIGS. 25A ,  25 B, the lateral mass plate  1920  includes a recessed area  1922  for receiving the locking plate  1924  so that the locking plate  1924  is flush with the upper surface  1925  of the lateral mass plate  1920  when the probe  1926  is urged against the lateral mass screw  1940  and the keel  1928  is inserted into the lateral mass or the lamina of the vertebra. In the preferred embodiment depicted in  FIGS. 25A ,  25 B, the shape and contours of the facet joint spacer or inter-facet joint spacer  1910  can facilitate insertion of the facet joint spacer or inter-facet joint spacer  1910  into the cervical facet joint. In this embodiment, the facet joint spacer or inter-facet joint spacer  1910  has a rounded distal end  1912 . The distal end  1912  is tapered in thickness to facilitate insertion. The tapered distal end  1912  meets and is continuous with a proximal mid-section  1916  which, in this preferred embodiment, has a uniform thickness, and is connected flexibly, preferably with a hinge  1922 , to the lateral mass plate  1920 , as described above. The facet joint spacer (or insert) or inter-facet joint spacer (or insert)  1910 , with its proximal mid-section  1916  and tapered distal end  1912 , is curved downward, causing a superior surface  1913  of the facet joint spacer or inter-facet joint spacer  1910  to be curved. The curve can cause the superior surface  1913  to be convex, and the convexity can vary among different implants  1900  to suit the anatomical structure of the cervical facet joint(s) of a patient. An inferior surface  1915  accordingly can be preferably concave, flat, or convex. The curved shape of the implant can fit the shape of a cervical facet joint, which is comprised of an inferior facet of an upper vertebra and a superior facet of a lower adjacent vertebra. The convex shape of the superior surface  1913  of the facet joint spacer or inter-facet joint spacer  1910  fits with a concave shape of the inferior facet of the upper cervical vertebrae. The concave shape of the inferior surface  1915  of the facet joint spacer or inter-facet joint spacer  1910  fits with the convex shape of the superior facet of the cervical vertebrae. The degree of convexity and concavity of the facet joint spacer or inter-facet joint inferior and superior surfaces can be varied to fit a patient&#39;s anatomy and the particular pairing of adjacent cervical vertebrae to be treated. For example, a less-curved facet joint spacer or inter-facet joint spacer  1910  can be used where the patient&#39;s cervical spinal anatomy is sized (as described below) and found to have less convexity and concavity of the articular facets. Generally for the same level the input for the right and left facet joint will be similarly shaped. It is expected that the similarity of shape of the facet joint spacer or inter-facet joint spacer and the smooth, flush surfaces will allow distraction of the facet joint without loss of mobility or damage to the bones of the cervical spine. Further, and preferably, the width of the mid-section  1916  is from 1.5 mm to 2.5 mm. 
     Except as otherwise noted above, the embodiment shown in  FIGS. 22A-24B  is similar to the embodiment shown in  FIGS. 25A ,  25 B. Accordingly the remaining elements on the  1900  series of element numbers is preferably substantially similar to the described elements in the  1800  series of element numbers, as set forth above. Thus, by way of example, elements  1923 ,  1928 ,  1929  and  1930  are similar, respective elements  1823 ,  1828 ,  1829  and  1830 . 
       FIG. 30  is a flow chart of the method of insertion of an implant of the invention. The embodiment  1800  or  1900  of the present invention preferably is inserted in the following manner (only elements of the embodiment  1800  will be set forth herein, for purposes of the written description of a method of the invention). First the facet joint is accessed. A sizing tool  2200  (see  FIGS. 29A-C ) can be inserted to select the appropriate size of an implant of the invention for positioning in the cervical facet joint. This step may be repeated as necessary with, if desired, different sizes of the tool  2200  until the appropriate size is determined. This sizing step also distracts the facet joint and surrounding tissue in order to facilitate insertion of the implant. Then, the natural (made from animal bone) or artificial facet joint or inter-facet joint spacer  1810  is urged between the facets into the facet joint. The facet itself is somewhat shaped like a ball and socket joint. Accordingly, in order to accommodate this shape, the natural or artificial facet joint spacer or inter-facet joint spacer  1810  can have a rounded leading edge shaped like a wedge or tissue expander to cause distraction of the facet joint as the natural or artificial facet joint spacer or inter-facet joint spacer is urged into the facet joint of the spine. The natural or artificial facet joint spacer or inter-facet joint spacer  1810  also includes the convex surface  1813  in order to more fully accommodate the shape of the facet joint of the spine. However, as set forth above and as depicted in  FIG. 25B , it is possible in the alternative to have a curve-shaped natural or artificial facet joint spacer (or insert) or inter-facet joint spacer (or insert)  1910  with a convex superior surface  1913  and a concave inferior surface  1915 , the distal end  1912  tapering to facilitate insertion, while the remainder of the natural or artificial facet joint spacer or inter-facet joint spacer  1910 , (i.e., the proximal section  1916 ) has a uniform thickness. 
     Once the natural or artificial facet joint spacer or inter-facet joint spacer  1810  is positioned, the lateral mass plate  1820  is pivoted downward about the hinge  1822  adjacent to the vertebrae and preferably to the lateral mass or to the lamina. Thus, the lateral mass plate  1820  may be disposed at an angle relative to the natural or artificial facet joint spacer or inter-facet joint spacer  1810  for a representative spine configuration. It is to be understood that as this embodiment is hinged the final position of the lateral mass plate  1820  relative to the natural or artificial facet joint spacer or inter-facet joint spacer  1810  will depend on the actual spine configuration. It is to be understood that embodiments of the invention can be made without a hinge, as long as the connection between the natural or artificial facet joint spacer or inter-facet joint spacer and the lateral mass plate is flexible enough to allow the lateral mass plate to be bent relative to the natural or artificial facet joint spacer or inter-facet joint spacer in order to fit the anatomy of the patient. Once the lateral mass plate  1820  is positioned, or prior to the positioning of the lateral mass plate  1820 , a bore can be drilled in the bone to accommodate the bone screw  1824 . Alternatively the screw  1824  can be self-tapping. The screw is then placed through the bore  1830  and secured to the bone, preferably the lateral mass or the lamina, thereby holding the natural or artificial facet joint spacer or inter-facet joint spacer  1810  in place. In order to lock the bone screw  1824  in place and to lock the position of the natural or artificial facet joint spacer or inter-facet joint spacer  1810  and the lateral mass plate  1820  in place, the locking plate  1824  is positioned over the lateral mass plate  1820 . So positioned, the probe  1826  is positioned through the bore  1830  and against the head of the bone screw to keep the bone screw from moving. The keel  1828 , having a sharp chisel-shaped end, preferably can self-cut a groove in the bone so that the keel  1828  is locked into the bone as the keel  1828  is aligned by, and received in, a groove  1831  of the lateral mass plate  1820 . Alternatively, a groove can be pre-cut in the bone to receive the keel  1828 . As this occurs the bore  1829  of the locking plate  1824  aligns with the threaded bore  1831  of the lateral mass plate  1820  and a machine screw can be inserted to lock the locking plate relative to the lateral mass plate. This locking prevents the lateral mass plate  1820  and the natural or artificial facet joint spacer or inter-facet joint spacer  1810  from rotating and, as previously indicated, prevents the bone screw  1840  from backing out from the vertebra. Preferably the implant is between the C5 and C6 vertebrae level, or the C6 and C7 vertebrae level. It is noted that two implants preferably will be implanted at each level between vertebrae. That is, an implant  1800  will be placed in a right facet joint and also in a left facet joint when viewed from a posterior view point. This procedure can be used to increase or distract the foraminal area or dimension of the spine in an extension or in neutral position (without having a deleterious effect on cervical lordosis) and reduce the pressure on the nerves and blood vessels. At the same time this procedure preserves mobility of the facet joint. 
       FIGS. 26A-27B  show a further embodiment of the implant of the invention, with the embodiment  2000  implanted in the cervical spine as depicted in  FIGS. 27A and 27B . The implant  2000  comprises a first natural or artificial facet joint spacer (or insert) or inter-facet joint spacer (or insert)  2010  and a second natural or artificial facet joint spacer (or insert) or inter-facet joint spacer (or insert)  2010 . Each natural or artificial facet joint spacer or inter-facet joint spacer can have a distal end  2012  that is tapered or wedge-shaped in a way that facilitates insertion into the cervical facet joints on both sides of two adjacent cervical vertebrae at the same level. The natural artificial facet joint spacers or inter-facet joint spacers further can be dome-shaped, or convex on a superior surface  2013 , to approximate the shape of the cervical facets of the cervical facet joints. 
     The first and second natural or artificial facet joint spacers or inter-facet joint spacers  2010  are bridged together by a collar  2015 . The collar  2015  passes between the spinous processes of the adjacent cervical vertebrae. As can be seen in  FIG. 26B , the implant can preferably be “V” shaped or “boomerang” shaped. The entire implant  2000  or the collar  2015  of the implant can be made of a flexible material such as titanium, so that it is possible to bend the collar  2015  so that it conforms preferably to the shape of the lateral mass or the lamina of the cervical vertebrae of the patient and thereby holds the implant in place with the natural or artificial facet joint spacers or inter-facet joint spacers  2010  inserted in the cervical facet joints. Bores  2029  are preferably provided through implant  2000  adjacent to the natural or artificial facet joint spacer or inter-facet joint spacer  2010  respectively. These bores  2029  can receive bone screws to position the implant  2000  against the lateral mass or the lamina as shown in  FIGS. 27A ,  27 B. The description of the embodiment  2100 , in  FIGS. 28A ,  28 B provide further details concerning the method of affixing the implant  2000  to the vertebrae. The implant  2100  also can be made of PEEK or other materials as described herein. Embodiment  2000  (the “boomerang” shape depicted in  FIG. 27B ) further can have a locking plate as, for example, the locking plate  1824  in  FIG. 22A . The locking plate for embodiment  2000  (not shown) can have the same features as locking plate  1824 , that is: (1) a probe  1826  that interacts with the bone screws to prevent the bone screws from backing out of the bone, the likely consequence of which would be displacement of the implant  2000 ; and (2) a keel  1828  with a chisel end to embed in the bone and thus to prevent rotational displacement of the implant. However, given the collar  2015  configuration of embodiment  2000 , a chisel may not serve the same purpose as with the embodiments set forth above, which lack a collar stabilized by two bone screws. Therefore, a locking plate on embodiment  2000  can be provided without a keel. 
       FIGS. 28A and 28B  depict a further embodiment of the implant of the invention  2100 . In this embodiment  2100 , the collar  2115  can be made of a flexible material such as titanium, of a substantially inflexible material, or of other materials described herein. Substantial flexibility can also be derived from connecting a first natural or artificial facet joint spacer (or insert) or inter-facet joint spacer (or insert)  2110  with the collar  2115  using a first hinge  2117 , and connecting a second natural or artificial facet joint spacer or inter-facet joint spacer  2110  with the collar  2115  using a second hinge  2117 . Using the first hinge  2117  and the second hinge  2117 , the collar  2115  can be pivoted downward to conform to a particular patient&#39;s cervical spinal anatomy. In other words, the degree of pivoting will vary among different patients, and the first hinge  2117  and second hinge  2117  allow the implant  2100  to accommodate the variance. 
     In the hinged embodiment  2100 , and similar to the embodiment  2000 , the collar  2115  can have a first bore  2129  inferior to the first hinge  2117 , and a second bore  2129  inferior to the second hinge  2117 . A first bone screw penetrates the first bore  2129  and into the lateral mass or the lamina, and the second bone screw penetrates the second bore  2129  and into the lateral mass or the lamina, the first and second bone screws serving to anchor the implant. A bore, preferably in the lateral mass, can be drilled for the first bone screw and for the second bone screw. Alternatively, the bone screws can be self-tapping. A first locking plate similar to the plate  1924  ( FIG. 25A ) can be secured about the head of the first bone screw and a second locking plate can be secured about the head of the second bone screw to prevent displacement of the first and second bone screws  2140 . The first locking plate can block the first bone screw with a probe and the second locking plate can block to the second bone screw with a probe. 
     It should be noted that embodiments  2000  and  2100  also can be configured for accommodating treatment of cervical spinal stenosis and other cervical spine ailments where only a single cervical facet joint between adjacent vertebrae requires an implant, i.e., where treatment is limited to one lateral facet joint. In that case, the collar  2015 ,  2115  extends medially without extending further to join a second natural or artificial facet joint spacer or inter-facet joint spacer  2010 ,  2110 . For the hinged embodiment  2100 , the implant comprises a single hinge  2117 , and the collar  2115  has only one bore  2129  to accept one bone screw to secure the implant  2100 . 
       FIGS. 29A-E , depict a sizing and distracting tool  2200  of the invention. Sizing tool  2200  has a handle  2203  and a distal head  2210  that is shaped as a natural or artificial facet joint spacer or inter-facet joint spacer (e.g.,  1810 ) of an implant of the invention. That is, the head  2210  preferably will have essentially the same features as the natural or artificial facet joint spacer or inter-facet joint spacer  1810 , but the dimensions of the head  2210  will vary from one tool  2200  to the next, in order to be able to use different versions of the sizing tool  2200  to determine the dimensions of the cervical facet joint that is to be treated and then to select an appropriately-sized implant. The head  2210  preferably can be used to distract the facet joint prior to the step of implanting the implant in the facet joint. In this regard, the head  2210  is rounded at the most distal point  2212 , and can be a tapered to facilitate insertion into a cervical facet joint. The head  2210  also can have a slightly convex superior surface  2213 , the degree of convexity varying among different sizing tools  2200  in order to determine the desired degree of convexity of an implant to be implanted in the cervical facet joint. The head  2210  may have a uniform thickness along a proximal mid-section  2216 . Accordingly, the inferior surface  2215  preferably can be concave. Alternatively, the proximal mid-section  2212  may be convex on the superior surface  1813  without being uniform in thickness. Thus, the inferior surface  2215  can be flat or planar. The head also can be curved. 
     The head  2210  has a stop  2218  to prevent over-insertion of the head  2210  of the sizing tool  2200  into the facet joint. The stop  2218  can be a ridge that separates the head  2210  from the handle  2203 . Alternatively, the stop  2218  can be any structure that prevents insertion beyond the stop  2218 , including pegs, teeth, and the like. 
     Different sizing tools  2200  covering a range of dimensions of the head  2210  can be inserted successively into a cervical facet joint to select the appropriate size of an implant to position in the cervical spine, with the appropriate convexity and concavity of natural or artificial facet joint spacer or inter-facet joint spacer. Each preferably larger head also can be used to distract the facet joint. 
       FIG. 31A  depicts a posterior view of a further embodiment  2300  of the implant of the invention. Embodiment  2300 , as well as all of the embodiments herein, can benefit from some or all of the advantages described herein with regard to the other embodiments described herein. Further, in  FIG. 31A , embodiment  2300  has a natural or artificial facet joint spacer (or insert) or inter-facet joint spacer (or insert)  2310  that can have a tapered or thinned distal end  2312  so that the distal end  2312  facilitates insertion of the natural or artificial facet joint spacer or inter-facet joint spacer  2310  into a cervical facet joint. The distal end  2312  can be rounded, as seen in the plan view of  FIG. 31A , in order to conform to the roundness of the facet joint. The natural or artificial facet joint spacer or inter-facet joint spacer  2310  further can be curved so that a superior surface  2313  of the natural or artificial facet joint spacer or inter-facet joint spacer  2310  is convex, and an inferior surface  2315  is concave, to approximate the natural shape of the cervical facet joint that is to receive the implant  2300 . The curve can have a uniform thickness, or it can have a varied thickness. Further, the lateral edges of the natural or artificial facet joint spacer or inter-facet joint spacer  2310  are curved or rounded, for distribution of load-bearing stress. As with other embodiments described herein, the natural or artificial facet joint spacer or inter-facet joint spacer  2310  also can be made of a flexible, biocompatible material, such as PEEK, to maintain joint mobility and flexibility. 
     The natural or artificial facet joint spacer or inter-facet joint spacer  2310  is connected flexibly with a lateral mass plate  2320 , the flexible connection preferably being a hinge  2322 . As seen in the plan view of  FIG. 31A , the implant  2300  is substantially hour-glass shaped. This shape, as well as the shape of  FIG. 32 , will be discussed further below. The hinge  2322  is narrower than the natural or artificial facet joint spacer or inter-facet joint spacer  2310 , with the hinge  2322  sitting at substantially the isthmus  2317  between the natural or artificial facet joint spacer or inter-facet joint spacer  2310  and the lateral mass plate  2320 . The curved edges, or fillets, about the hinge  2322  serve to distribute more evenly the load-bearing stress on the implant  2300 , and thus prevent concentrating the stress about the edges. 
     The hinge  2322  allows the implant  2300  to bend at the hinge  2322 , bringing a lateral mass plate  2320  adjacent to the lateral mass and/or lamina of the patient&#39;s spine, and to conform to a particular patient&#39;s anatomy. The lateral mass plate  2320  is made of a biocompatible flexible material, preferably titanium or any other biocompatible flexible material as described herein, for example PEEK, that will support the use of bone screws and other hardware, as described below. The lateral mass plate  2320  bends downward at the hinge  2322  over a wide range of angles relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2310 , and preferably at an angle of more than 90 degrees, and this flexibility facilitates positioning and insertion of the natural or artificial facet joint spacer or inter-facet joint spacer. This flexibility of the lateral mass plate  2320  relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2310  further facilitates positioning of the lateral mass plate relative to the lateral mass and/or the lamina of the patient&#39;s spine. Once the lateral mass plate  2320  is positioned adjacent to the bone, preferably the lateral mass of a cervical vertebra, a first bone screw, such as bone screw  1840 , can be inserted through a first bore  2330  through the lateral mass plate  2320  and embedded into the bone of the lateral mass of the cervical vertebra. 
     The lateral mass plate  2320  further comprises a second bore  2329  which is preferably positioned medially, relative to the first bore  2330 . Thus, viewing the implant from a posterior perspective as in  FIG. 31A , the second bore  2329  in the lateral mass plate  2320  can be positioned either to the left or to the right of the first bore  2330 . The position of the second bore  2329  will depend upon whether the implant  2300  is intended to be inserted into a cervical facet joint on the left or right side of a patient. Specifically, an implant  2300  to be inserted into a right-side cervical facet joint (i.e., the patient&#39;s rights side) will have a second bore  2329  positioned to the left of the first bore  2330  as in  FIG. 31A , when implant  2300  is viewed from a posterior perspective, while an implant  2300  to be inserted into a left-side cervical facet joint will have a second bore  2329  positioned to the right of the first bore  2330 , when implant  2300  is viewed from a posterior perspective. 
     The second bore  2329  through the lateral mass plate  2320  is adapted to accept a second screw  2390  ( FIG. 31B ), which preferably is a locking screw with a chisel point  2391 . The locking screw  2390  is received by the second bore  2329  and the chisel point  2391  self-cuts a bore into the bone. The locking screw  2390  preferably is inserted through the second bore  2329  and embedded in the bone, after the bone screw is embedded in the bone through the first bore  2330 . The position of the second bore  2329 , i.e., medial to the first bore  2330 , positions the locking screw  2390  so that it embeds in stronger bone tissue than if the second bore  2329  were located more laterally. The locking screw, in combination with the bone screw, prevents rotational and/or backward displacement of the implant  2300 . As the locking screw  2390  is received by the second bore  2329 , the head  2392  of the locking screw  2390  aligns with the head of the first bone screw in the first bore  2330 , blocking the head of the first bone screw to prevent the first bone screw from backing out of the bone of the vertebra and the first bore  2330 . 
       FIG. 32  depicts a further embodiment  2400  of the implant of the invention, from a posterior view. Embodiment  2400  is adapted to be implanted in a manner that preserves the anatomy of the cervical facet joint, in particular, the soft tissues around the cervical facet joint, including the joint capsule. 
     Implant  2400 , like implant  2300  and other implants disclosed above, has a natural or artificial facet joint spacer or inter-facet joint spacer  2410 , flexibly connected, preferably by a hinge  2422 , to a lateral mass plate  2420 . As can be seen in  FIG. 32 , the implant  2400  including the natural or artificial facet joint spacer or inter-facet joint spacer  2410  and the hinge  2422  is substantially “P” shaped. As explained below, its “P” shape assists in the insertion of the implant  2400  into the facet joint with most of the facet capsule and facet capsule ligament and other soft tissue associated with the facet joint still left intact. The natural or artificial facet joint spacer or inter-facet joint spacer, as above for implant  2300  and the other implants disclosed above, can have a superior surface  2413  of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  that is convex, and an inferior surface  2415  that is concave, or any appropriate shaping to approximate the natural shape of the cervical facet joint that is to receive the implant  2400 . The thickness of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  can be uniform, or varied. The natural or artificial facet joint spacer or inter-facet joint spacer  2410  also can be made of a flexible, biocompatible material, such as PEEK, to maintain joint mobility and flexibility. The hinge  2422  can have smooth, rounded edges, for distribution of load stress, as disclosed above. Other features and advantages of the other embodiments can be, if desired, incorporated into the design of the embodiment of  FIG. 32 . For example, the natural or artificial facet joint spacer or inter-facet joint spacer  2410  further can have a tapered or thinned edge  2412  so that the edge  2412  facilitates insertion of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  into a cervical facet joint. The edge  2412  can be curved. In this embodiment  2400 , however, the thinned edge  2412  of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  preferably is not at the distal end of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  as is the thinned edge  2312  of the natural or artificial facet joint spacer or inter-facet joint spacer  2310 ; rather, the thinned edge  2412  preferably is positioned laterally, toward the hinge  2422  of the implant  2400 . The thinned edge  2412  coincides substantially with a lateral curvature  2440  of the natural or artificial facet joint spacer or inter-facet joint spacer  2410 , which is pronounced relative to the curvature on the medial side of the implant  2400 , i.e., a “P” shape. In other words, the curved part of the head of the “P”  2440  corresponds to the thinned edge  2412 , and serves as the leading edge of the implant  2400  to begin insertion of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  into a cervical facet joint, preferably through an incision in the soft tissue of the facet joint. The “P” shape narrows at isthmus  2417  where the natural or artificial facet joint spacer or inter-facet joint spacer  2410  that is joined by the hinge  2422  with the lateral mass plate  2420 . The smooth or rounded edges or fillets serve to distribute stresses on the implant  2400 . The above described “P” shape of implant  2400  allows the implant  2400  to be pivoted into place into a facet joint as described below. The thinned edge  2412  and leading lateral curvature  2440  of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  are adapted to facilitate urging implant  2400  into the cervical facet joint, through the incision in the joint capsule. The implant  2400  then is pivoted into position so that the lateral mass plate  2420  can be bent downward, relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2410 , to align with and lie adjacent to the lateral mass and/or the lamina. The lateral mass plate  2420  is then fastened to the bone. The lateral mass plate  2420  of implant  2400 , like the lateral mass plate for implant  2300 , is flexibly connected, preferably by the smooth-edged hinge  2422 , to the natural or artificial facet joint spacer or inter-facet joint spacer  2410  at the narrow lower part of the artificial facet joint. The lateral mass plate  2420  is made of a biocompatible flexible material, preferably titanium or any other biocompatible flexible material such as PEEK that will support the use of bone screws and other hardware, as described below. As with the facet joint spacer, the lateral mass plate of any of these embodiments can be made of a natural animal bone. 
     The lateral mass plate  2420  bends downward at a wide range of angles relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2410 , and preferably at an angle of more than 90 degrees. The flexibility of the lateral mass plate  2420  relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2410  further facilitates positioning of the lateral mass plate  2420  relative to the lateral mass and/or the lamina of the patient&#39;s spine. 
     Like embodiment  2300 , described above, the lateral mass plate  2420  has first bore  2430 , which is adapted to receive a bone screw  2440 , to help anchor implant  2400  in position. The lateral mass plate  2420  further includes a second bore  2429  adapted to be positioned medially, relative to the first bore  2430 , as disclosed above for implant  2300 . The position of the second bore  2429 , when viewing implant  2400  from a posterior perspective ( FIG. 32 ), will depend upon whether implant  2400  is intended to be implanted into a left-side or right-side cervical facet joint of a patient. Thus, implant  2400  with the second bore  2429  positioned to the left of the first bore  2430  is intended to be implanted in a right-side cervical facet joint of a patient, as depicted in  FIG. 32 , while an implant  2400  with a second bore  2429  positioned to the right of the first bore  2430  is intended to be implanted in a left-side cervical facet joint of a patient. 
     The second bore  2429  through the lateral mass plate  2420  is adapted to receive a second screw  2490  with head  2492 , which preferably is a locking screw with a chisel point, such as screw  2390 . The function and purpose of the bone screw disposed through bore  2430  and the locking screw disposed through bore  2429  are as described above with respect to the implant  2300 . 
     The present invention further includes a method of implanting the implant  2400  ( FIGS. 33A ,  33 B). To insert the natural or artificial facet joint spacer or inter-facet joint spacer  2410 , a facet joint is accessed and an incision or a pair of incisions is made in the capsular ligament, the joint capsule, and the synovial membrane so that the thinned edge  2412  of the implant  2400  can be urged into the cervical facet joint through these tissues. The capsular ligament and the joint capsule and other soft tissues around the cervical facet joint are allowed to remain substantially intact, except for the small incision, and will be sutured and allowed to heal around the implant  2400 . If desired, the cervical facet joint can be distracted prior to urging the curved section  2440  with the thinned edge  2412  of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  into the cervical facet joint. Once the curved section  2440  of the natural or artificial facet joint spacer or inter-facet joint spacer  2410  with the thinned edge  2412  is urged into the cervical facet joint, implant  2400  is pivoted, preferably about 90 degrees, so that the second bore  2429  is placed medially relative to the first bore  2430 . This allows the natural or artificial facet joint spacer or inter-facet joint spacer  2410  to be positioned in the facet joint. It is noted that the overall size, including the isthmus  2417 , of the natural or artificial fact joint spacer or inter-facet joint spacer  2410 , as that of  2310 , can be somewhat smaller than in prior embodiments to allow the natural or artificial facet joint spacer or inter-facet joint spacer to be positioned within the edges of the facet joint with the joint capsule substantially intact. The lateral mass plate  2420  then can be bent downward about the hinge  2422  into position adjacent the lateral mass or lamina of the spine of the patient, which position will depend upon the anatomy of an individual patient&#39;s cervical spine. 
     Once the lateral mass plate  2420  is positioned adjacent to the bone, preferably the lateral mass of a cervical vertebra, a first bone screw can be inserted through the first bore  2430  through the lateral mass plate  2420  and become embedded into the bone of the lateral mass of the cervical vertebra to anchor the implant  2400 . After the bone screw is embedded, a locking screw is inserted through the second bore  2429  of the lateral mass plate  2420 , the second bore  2429  medial to the first bore  2430 . The locking screw has a chisel end that allows the locking screw to dig into the bone without use of a tool to pre-cut a bore. Alternatively, a bore can be pre-cut and a locking screw without a chisel end can be used. As the locking screw is embedded in the bone, the locking head of the locking screw is brought into proximity with the head of the bone screw to block its backward movement so that the implant  2400  remains anchored with the bone screw, i.e., so that the bone screw cannot back out of the bone. The embedded locking screw also serves to prevent rotational displacement of implant  2400 , while blocking backward displacement of the first bone screw. 
     Referring to  FIGS. 34A through 36B , a still further embodiment of an implant  2500  in accordance with the present invention can include a natural or artificial facet joint spacer (or insert) or inter-facet joint spacer (or insert)  2510  connected with a lateral mass plate (also referred to herein as an anchoring plate)  2520  by a spheroidal joint arrangement  2538  or otherwise shaped multiple direction articulation joint arrangement. The natural or artificial facet joint spacer or inter-facet joint spacer  2510  has a load bearing structure sized and shaped to distribute, as desired, a load applied by opposing surfaces of superior and inferior facets to one another. As shown, the load bearing structure has a saucer shape, but as described in further detail below (and as described in previous embodiments above), in other embodiments the load bearing structure can have some other shape so long as a desired load distribution and separation between superior and inferior facets is achieved. The natural or artificial facet joint spacer or inter-facet joint spacer  2510  includes a handle-like structure connected with the load bearing surface, the handle-like structure necking at an isthmus  2517  and terminating at a pivot end  2526 . In an embodiment, the pivot end  2526  is substantially spherical, ovoidal, or similarly rounded in shape. As further described below, the natural or artificial facet joint spacer or inter-facet joint spacer  2510  can comprise a flexible material, for example a biocompatible polymer such as PEEK, or a more rigid material, for example a biocompatible metal such as titanium. As shown, the lateral mass plate  2520  has a generally square shape with rounded corners; however, in other embodiments the lateral mass plate  2520  can have any number of shapes so long as the lateral mass plate  2520  provides sufficient support for anchoring the implant  2500  in position and so long as the lateral mass plate  2520  allows a desired range of motion for the natural or artificial facet joint spacer or inter-facet joint spacer  2510 . The lateral mass plate  2520  includes a cavity  2527  within which the pivot end  2526  is held. The spheroidal joint arrangement  2538  comprises the pivot end  2526  and the cavity  2527  and as described below allows the natural or artificial facet joint spacer or inter-facet joint spacer  2510  to tilt and swivel relative to the lateral mass plate  2520 . 
       FIG. 34A  is a posterior view showing a posterior face  2532  of the lateral mass plate  2520 , while  FIG. 34B  is an anterior view showing an anterior face  2534  of the lateral mass plate  2520 . The lateral mass plate  2520  includes an anterior notch  2524  (see  FIG. 35 ) or other indentation formed along the edge of the anterior face  2534  and a posterior notch  2522  or other indentation formed along the posterior face  2532 . The posterior and anterior notches  2522 ,  2524  are generally aligned with one another along the edge of the lateral mass plate  2520  and are connected with the cavity  2527 . The notches  2522 ,  2524  confine movement of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  in the anterior and posterior directions relative to the lateral mass plate  2520 , allowing the natural or artificial facet joint spacer or inter-facet joint spacer  2510  to tilt at varying degrees of angle in an anterior and posterior direction. Referring to  FIG. 35 , the anterior notch  2524  can have a narrower width than the posterior notch  2522  which is sized to provide the pivot end  2526  of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  with access to the cavity  2527  so that the pivot end  2526  can be inserted into the cavity  2527 . Once the pivot end  2526  is positioned within the cavity  2527  a plug  2528  can be mated with the lateral mass plate  2520  to lock the pivot end  2526  in place within the cavity  2527  and to further limit freedom of movement of the natural or artificial facet joint spacer or inter-facet joint spacer  2510 , particularly limiting tilting of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  in a posterior direction. The plug  2528  can be press fit to the posterior notch  2522  and further welded or otherwise fixedly fastened with the lateral mass plate  2520 . A physician can select an appropriate and/or desired natural or artificial facet joint spacer or inter-facet joint spacer  2510 , lateral mass plate  2520 , and plug  2528  according to the motion segment targeted for implantation and/or the particular anatomy of the patient. Once an appropriate combination of components is identified, the natural or artificial facet joint spacer or inter-facet joint spacer  2510  and the lateral mass plate  2520  can be mated, and the natural or artificial facet joint spacer or inter-facet joint spacer  2510  can be locked in place by the plug  2528 . 
     As can further be seen in  FIGS. 34A through 35  the lateral mass plate  2520  has a first bore  2530  therethrough. The first bore  2530  can accept a bone screw  2540  (also referred to herein as a lateral mass screw) to secure the lateral mass plate  2520  preferably to the lateral mass, lamina, or alternatively to another part of the spine, and thus to anchor the implant  2500 . The lateral mass screw  2540  preferably has a head  2542  that can accept a tool chosen for the surgical procedure whether a wrench, screwdriver, or other tool. The lateral mass plate  2520  further has a second bore  2529  which is preferably positioned medially, relative to the first bore  2530 . Referring to  FIG. 34A , the second bore  2529  in the lateral mass plate  2520  can be positioned either to the left or to the right of the first bore  2530 . The position of the second bore  2529  will depend upon whether the implant  2500  is intended to be inserted into a cervical facet joint on the left or right side of a patient. Specifically, an implant  2500  to be inserted into a right-side cervical facet joint (i.e., the patient&#39;s rights side) will have a second bore  2529  positioned to the left of the first bore  2530  as in  FIG. 34A , when implant  2500  is viewed from a posterior perspective, while an implant  2500  to be inserted into a left-side cervical facet joint will have a second bore  2529  positioned to the right of the first bore  2530 , when implant  2500  is viewed from a posterior perspective. 
     The second bore  2529  through the lateral mass plate  2520  is adapted to accept a second screw  2590  which preferably is a locking screw having a chisel point  2591 . The locking screw  2590  is received by the second bore  2529  and the chisel point  2591  self-cuts a bore into the bone. The locking screw  2590  is preferably inserted through the second bore  2529  and embedded in the bone after the bone screw  2540  is embedded in the bone through the first bore  2530 . The medial position of the second bore  2529  relative to the first bore  2530  positions the locking screw  2590  so that it embeds in stronger bone tissue than if the second bore  2529  were located more laterally. The locking screw  2590 , in combination with the bone screw  2540 , prevents rotational and/or backward displacement of the lateral mass plate  2520 . As the locking screw  2590  is received by the second bore  2529 , the head  2592  of the locking screw  2590  aligns with the head  2542  of the first bone screw  2540  in the first bore  2530 , blocking the head  2542  of the first bone screw  2540  to prevent the first bone screw  2540  from backing out of the bone of the vertebra and the first bore  2530 . The posterior face  2532  can include a recessed portion  2539 , and/or the second bore  2529  can be countersunk, so that the locking screw  2590  does not protrude farther from the posterior face  2532  than desired. 
     In a preferred embodiment (as shown in  FIGS. 34A-37 ), the spheroidal joint arrangement  2538  includes a spherical pivot end  2526  and a cavity  2527  having a shape approximately conforming to the spherical pivot end  2526  so that the spheroidal joint arrangement  2538  is a ball-in-socket arrangement. The ball-in-socket arrangement  2538  allows the natural or artificial facet joint spacer or inter-facet joint spacer  2510  to move freely relative to the lateral mass plate  2520  where the natural or artificial facet joint spacer or inter-facet joint spacer  2510  is unobstructed by the lateral mass plate  2520 . For example, as shown in  FIG. 36A  the natural or artificial facet joint spacer or inter-facet joint spacer  2510  can tilt in an anterior direction (to position 1, for example) and can tilt in a posterior direction (to position 2, for example). As the natural or artificial facet joint spacer or inter-facet joint spacer  2510  tilts in an anterior direction, the isthmus  2517  moves within the anterior notch  2524  so that the natural or artificial facet joint spacer or inter-facet joint spacer  2510  can continue tilting without obstruction. Conversely, as the natural or artificial facet joint spacer or inter-facet joint spacer  2510  tilts in a posterior direction (to position 2, for example), the isthmus  2517  contacts the plug  2528 , limiting the amount of tilt of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  in a posterior direction. 
     Referring to  FIG. 36B , the ball-and-socket arrangement allows the natural or artificial facet joint spacer or inter-facet joint spacer  2510  to swivel (to position 3, for example) relative to the lateral mass plate  2520 , potentially providing a more conformal arrangement of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  with the surfaces of the superior and inferior facets. Further, the ability of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  to swivel can increase options for lateral mass plate  2520  anchor positions. A physician can anchor the lateral mass plate  2520  in a more conformal or advantageous orientation and/or position along the lateral mass, for example, by altering the arrangement of the lateral mass plate  2520  relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2510 . The amount of swiveling accommodated (and the degree of freedom of movement accommodated in general) depends on the geometries of the components. For example, where the isthmus  2517  is sufficiently narrow and long in length, a greater degree of swiveling in combination with tilt can be achieved, or, for example, where the plug  2528  extends over a portion of the natural or artificial facet joint spacer or inter-facet joint spacer  2510 , as shown in  FIGS. 36A and 36B , the amount of tilt possible in the posterior direction can be limited. One of ordinary skill in the art will appreciate that the freedom of movement of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  relative to the lateral mass plate  2520  is limited substantially or wholly by the geometries of the components, and therefore can be substantially altered to achieve a desired range of movement. The ball-and-socket arrangement need not include a ball that extends from the natural or artificial facet joint spacer or inter-facet joint spacer and a socket that is formed in the lateral mass plate. For example, the ball of such a joint can extend from a locking or anchoring plate and the socket can be included in the natural or artificial facet joint spacer or inter-facet joint spacer. Further, while the preferred embodiment has been described as a ball-and-socket arrangement, other arrangements can be employed with varied results. It should not be inferred that embodiments in accordance with the present invention need include a spheroidal shaped end mated with a rounded cavity. The scope of the present invention is not intended to be limited to ball-and-socket arrangements, but rather is intended to encompass all such arrangements that provide a plurality of degrees of freedom of movement and substitutability of components. 
     Referring again to  FIGS. 36A and 36B , the load bearing structure of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  includes a superior surface  2513  having a generally convex shape and an inferior surface  2514  having a slightly concave shape. The shape of the load bearing structure is intended to approximate a shape of opposing surfaces of the superior and inferior facets. The shape of the superior and inferior surfaces  2513 ,  2514  can vary between motion segments and between patients. For example, as shown in  FIG. 37 , where the cervical vertebra includes an inferior facet having a substantially convex natural surface, a physician may select a natural or artificial facet joint spacer or inter-facet joint spacer  2610  including a load bearing structure with an inferior surface  2614  having a more concave shape combined with a lateral mass plate  2620  having a bone screw  2640  more appropriately sized for the particular lateral mass to which it will be fixed. (As shown the bone screw  2640  has a shorter length and wider diameter.) A physician can be provided with natural or artificial facet joint spacers or inter-facet joint spacers having a multiplicity of load bearing structure shapes. As mentioned above, the ability to match different natural or artificial facet joint spacers or inter-facet joint spacers with different lateral mass plates can improve a physician&#39;s ability to provide appropriate treatment for a patient, and can further provide the physician flexibility to reconfigure an implant once a surgical site has been exposed and the physician makes a determination that a different combination of components is appropriate. 
     In yet another embodiment, the spheroidal joint arrangement  2538  of  FIGS. 34A-37  can be applied to collar structures, for example as shown in  FIGS. 26A-27B  so that the natural or artificial facet joint spacers or inter-facet joint spacers at each end of the collar structure include an increased range of motion to improve surface matching between the natural or artificial facet joint spacers or inter-facet joint spacers and the surfaces of the superior and inferior facets (i.e., increasing the amount of facet surface area contacting the natural or artificial facet joint spacers or inter-facet joint spacers). 
       FIG. 38  is a flow chart of an embodiment of a method in accordance with the present invention for implanting an implant as described in  FIGS. 34A through 37 . An incision must first be made to expose the surgical site and access the targeted facet joint (Step  2500 ). Once the facet joint is made accessible, the facet joint can be sized and distracted (Step  2502 ). A sizing tool  2200  (for example, see  FIGS. 29A-C ) can be inserted to select the appropriate size of an implant  2500  of the invention for positioning in the cervical facet joint. This step may be repeated as necessary with, if desired, different sizes of the tool  2200  until the appropriate size is determined. This sizing step also distracts the facet joint and surrounding tissue in order to facilitate insertion of the implant  2500 . Once the appropriate size is determine, the physician can select an appropriate natural or artificial facet joint spacer or inter-facet joint spacer  2510  with the lateral mass plate  2520  (Step  2504 ). The natural or artificial facet joint spacer or inter-facet joint spacer  2510  can then be urged between the facets into the facet joint (Step  2510 ). The facet itself is somewhat shaped like a ball and socket joint. Accordingly, in order to accommodate this shape, the natural or artificial joint spacer or inter-facet joint spacer  2510  can have a rounded leading edge shaped like a wedge or tissue expander to cause distraction of the facet joint as the natural or artificial facet joint spacer or inter-facet joint spacer is urged into the facet joint of the spine. The natural or artificial facet joint spacer or inter-facet joint spacer  2510  also includes the convex superior surface  2513  in order to more fully accommodate the shape of the facet joint of the spine. However, as set forth above and as depicted in  FIG. 37 , it is possible in the alternative to have a curve-shaped natural or artificial facet joint spacer or inter-facet joint spacer  2610  with a convex superior surface  2613  and a concave inferior surface  2614 , the distal end of the natural or artificial facet joint spacer or inter-facet joint spacer  2610  tapering to facilitate insertion, while the remainder of the natural or artificial facet joint spacer or inter-facet joint spacer  2610  has a uniform thickness. 
     Once the natural or artificial joint spacer or inter-facet joint spacer  2510  is positioned, the lateral mass plate  2520  is tilted and/or swiveled so that the lateral mass plate  2520  is adjacent to the vertebrae and preferably to the lateral mass or to the lamina (Step  2512 ). Thus the lateral mass plate  2520  may be disposed at an angle relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2510  for a representative spine configuration. It is to be understood that the final position of the lateral mass plate  2520  relative to the natural or artificial facet joint spacer or inter-facet joint spacer  2510  will depend on the actual spine configuration. Once the lateral mass plate  2520  is positioned, or prior to the positioning of the lateral mass plate  2520 , a bore can be drilled in the bone to accommodate the bone screw  2540 . Alternatively the screw  2540  can be self-tapping. The screw  2540  is then placed through the first bore  2530  and secured to the bone, preferably the lateral mass or the lamina, thereby holding the natural or artificial facet joint spacer or inter-facet joint spacer  2510  in place (Step  2514 ). In order to lock the bone screw  2540  in place and to lock the position of the natural or artificial facet joint spacer or inter-facet joint spacer  2510  and the lateral mass plate  2520  in place, a self-tapping locking screw  2590  is positioned within a second bore  2529  of the lateral mass plate  2520  and secured to the bone, thereby resisting undesirable movement of the lateral mass plate  2520  (Step  2516 ). A head  2592  of the locking screw  2590  can further block movement of the bone screw  2540  by trapping the bone screw head  2542  between the locking screw head  2592  and the first bore  2530 . The locking screw  2590  therefore prevents the lateral mass plate  2520  and the natural or artificial facet joint spacer or inter-facet joint spacer  2510  from rotating and, as previously indicated, prevents the bone screw  2540  from backing out from the vertebra. Preferably the implant is between the C5 and C6 vertebrae level, or the C6 and C7 vertebrae level. It is noted that two implants preferably will be implanted at each level between vertebrae. That is, an implant will be placed in a right facet joint and also in a left facet joint when viewed from a posterior view point. This procedure can be used to increase or distract the foraminal area or dimension of the spine in an extension or in neutral position (without having a deleterious effect on cervical lordosis) and reduce the pressure on the nerves and blood vessels. At the same time this procedure preserves mobility of the facet joint. 
     Materials for Use in Implants of the Present Invention 
     As alluded to above, and as described in further detail as follows, in some embodiments, the implant, and components of the implant (i.e., a lateral mass plate, a bone screw, a locking screw, etc.) can be fabricated from medical grade metals such as titanium, stainless steel, cobalt chrome, and alloys thereof, or other suitable implant material having similar high strength and biocompatible properties. Additionally, the implant can be at least partially fabricated from a shape memory metal, for example Nitinol, which is a combination of titanium and nickel. Such materials are typically radiopaque, and appear during x-ray imaging, and other types of imaging. Implants in accordance with the present invention, and/or portions thereof (in particular an artificial facet joint) can also be fabricated from somewhat flexible and/or deflectable material. In these embodiments, the implant and/or portions thereof can be fabricated in whole or in part from medical grade biocompatible polymers, copolymers, blends, and composites of polymers. A copolymer is a polymer derived from more than one species of monomer. A polymer composite is a heterogeneous combination of two or more materials, wherein the constituents are not miscible, and therefore exhibit an interface between one another. A polymer blend is a macroscopically homogeneous mixture of two or more different species of polymer. Many polymers, copolymers, blends, and composites of polymers are radiolucent and do not appear during x-ray or other types of imaging. Implants comprising such materials can provide a physician with a less obstructed view of the spine under imaging, than with an implant comprising radiopaque materials entirely. However, the implant need not comprise any radiolucent materials. 
     One group of biocompatible polymers is the polyaryletherketone group which has several members including polyetheretherketone (PEEK), and polyetherketoneketone (PEKK). PEEK is proven as a durable material for implants, and meets the criterion of biocompatibility. Medical grade PEEK is available from Victrex Corporation of Lancashire, Great Britain under the product name PEEK-OPTIMA. Medical grade PEKK is available from Oxford Performance Materials under the name OXPEKK, and also from CoorsTek under the name BioPEKK. These medical grade materials are also available as reinforced polymer resins, such reinforced resins displaying even greater material strength. In an embodiment, the implant can be fabricated from PEEK 450G, which is an unfilled PEEK approved for medical implantation available from Victrex. Other sources of this material include Gharda located in Panoli, India. PEEK 450G has the following approximate properties: 
                                             Property   Value                                                        Density   1.3   g/cc           Rockwell M   99           Rockwell R   126           Tensile Strength   97   MPa           Modulus of Elasticity   3.5   GPa           Flexural Modulus   4.1   GPa                        
PEEK 450G has appropriate physical and mechanical properties and is suitable for carrying and spreading a physical load between the adjacent spinous processes. The implant and/or portions thereof can be formed by extrusion, injection, compression molding and/or machining techniques.
 
     It should be noted that the material selected can also be filled. Fillers can be added to a polymer, copolymer, polymer blend, or polymer composite to reinforce a polymeric material. Fillers are added to modify properties such as mechanical, optical, and thermal properties. For example, carbon fibers can be added to reinforce polymers mechanically to enhance strength for certain uses, such as for load bearing devices. In some embodiments, other grades of PEEK are available and contemplated for use in implants in accordance with the present invention, such as 30% glass-filled or 30% carbon-filled grades, provided such materials are cleared for use in implantable devices by the FDA, or other regulatory body. Glass-filled PEEK reduces the expansion rate and increases the flexural modulus of PEEK relative to unfilled PEEK. The resulting product is known to be ideal for improved strength, stiffness, or stability. Carbon-filled PEEK is known to have enhanced compressive strength and stiffness, and a lower expansion rate relative to unfilled PEEK. Carbon-filled PEEK also offers wear resistance and load carrying capability. 
     As will be appreciated, other suitable similarly biocompatible thermoplastic or thermoplastic polycondensate materials that resist fatigue, have good memory, are flexible, and/or deflectable, have very low moisture absorption, and good wear and/or abrasion resistance, can be used without departing from the scope of the invention. As mentioned, the implant can be comprised of polyetherketoneketone (PEKK). Other material that can be used include polyetherketone (PEK), polyetherketoneetherketoneketone (PEKEKK), polyetheretherketoneketone (PEEKK), and generally a polyaryletheretherketone. Further, other polyketones can be used as well as other thermoplastics. Reference to appropriate polymers that can be used in the implant can be made to the following documents, all of which are incorporated herein by reference. These documents include: PCT Publication WO 02/02158 A1, dated Jan. 10, 2002, entitled “Bio-Compatible Polymeric Materials;” PCT Publication WO 02/00275 A1, dated Jan. 3, 2002, entitled “Bio-Compatible Polymeric Materials;” and, PCT Publication WO 02/00270 A1, dated Jan. 3, 2002, entitled “Bio-Compatible Polymeric Materials.” Other materials such as Bionate®, polycarbonate urethane, available from the Polymer Technology Group, Berkeley, Calif., may also be appropriate because of the good oxidative stability, biocompatibility, mechanical strength and abrasion resistance. Other thermoplastic materials and other high molecular weight polymers can be used. Further, the embodiments hereof can be made at least in part from a natural animal bone material. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.