Patent Publication Number: US-7909878-B2

Title: Artificial spinal disc, insertion tool, and method of insertion

Description:
RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 10/973,795, filed Oct. 26, 2004, incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention broadly concerns an artificial disc, insertion tool, and method of insertion for treating conditions of the human spine. More particularly, it is concerned with providing a system and method which advantageously permits insertion of the artificial disc from the posterior region of the patient and provides a unilateral approach with posterior stabilization through fixation through the pedicles of the vertebrae while still allowing the patient to enjoy a range of motion in the area of the artificial disc. 
     2. Description of the Prior Art 
     A number of conditions can result in damage or deterioration of one or more intervertebral discs of the human spine (hereinafter “discs”). A disc may become damaged by external injury or degenerative disc disease due to advanced age in combination with other factors, just to mention two examples. One condition which results in significant pain and potential nerve damage, and may require surgery, is a herniated disc. 
     One type of surgery which has benefitted patients with disc problems is fusion of the vertebrae adjacent the disc. In this surgery, the natural disc is replaced with a spacer and the vertebrae are fixed relative to one another. While such surgery provides a degree of relief to the patient, it necessarily limits flexibility of the spine, which presents a disadvantage for patients with an active lifestyle. Another disadvantage is the placement of greater loads on the adjacent vertebrae, resulting in what is commonly known as “adjacent segment disease.” Later efforts in the treatment of human disc problems is the use of artificial human discs. Conventionally, this treatment involves the removal of the natural human disc and the insertion of two plates through the abdomen, i.e. the anterior side of the patient. The reason that insertion through the anterior of the patient is thought to be the conventional method is that it avoids the necessity of passing the artificial disc past one or more of the spinal facets or articular processes, or the spinal cord. These body parts present significant obstacles to attempts to insert an artificial disc posteriorly. One problem with such conventional disc replacement systems and methods is that the anterior insertion of the disc does necessitate surgical involvement of the two major blood vessels, the vena cava and the aorta. Anterior insertion requires spreading the abdominal muscles and the aorta and vena cava in order to gain access to the vertebrae and the disc. Because rupture of either of the vena cava or aorta is life-threatening, such surgery requires the attendance of both a vascular surgeon as well as a spinal surgeon. Another problem is the difficulty presented in providing artificial discs in two pieces which provide satisfactory support and balance along a single load bearing point as close to the center load line of the spine as possible. 
     As a result, there has developed a need for an improved artificial disc which may be inserted unilaterally from the posterior side of the patient. 
     Moreover, there has developed a need for an improved artificial disc which provides support for the spinal column while providing the desired flexibility of movement. 
     Furthermore, there is a need for an improved artificial disc which minimizes the risk of injury to the patient during the surgical process. 
     In addition, there is a need for an improved method of insertion of an artificial disc which reduces the potential risk to the patient during surgery, minimizes or eliminates the necessity of spreading the abdominal muscles, and yet provides satisfactory insertion and stabilization of the disc. 
     SUMMARY OF THE INVENTION 
     These and other objects will be largely met by the artificial spinal disc, insertion tool and method of insertion of the present invention. That is to say, the present invention provides a device which is particularly designed for posterior insertion, and further provides insertion in a unilateral direction, while being minimally invasive to the patient. In this regard, only a portion of the patient&#39;s natural disc needs to be removed to receive the new artificial disc hereof. 
     Broadly speaking, the present invention includes an artificial disc which includes two opposing plate members each having an outrigger which includes a facet to replace the natural facet joint between the inferior articular process of one vertebra and the superior articular process of the adjacent vertebra. Portions of these inferior and superior articular processes are removed during the surgery to permit insertion of the plates from one side of the posterior position of the vertebra. The plate members are complementally configured to permit limited relative motion therebetween. They are also particularly configured with the ribs substantially centered longitudinally and provided with a recess in the ribs to enable mounting of the outriggers. Further, the plate members include a rounded nose configured complementally with the cross-sectional curvature of the natural disc and a relieved edge configured to avoid involvement with the spinal cord. One of the plate members is thus preferably provided with a convex portion which faces a convex portion of the opposing plate member. Ribs extend along the opposite surfaces of the plate members for limiting lateral motion of the plate members once inserted, and permit a unilateral insertion of the plate members between the adjacent vertebrae. The plate members may beneficially be provided with a bio-ingrowth surface to promote bone growth and thus more secure attachment to the vertebra. The outriggers include rods extending through the pedicles of the vertebrae which are attached to the plate members. In addition, the outriggers include respective superior and inferior posts connected to the respective rods, and facet heads on each post which are complementally configured to permit limited relative movement therebetween. The outriggers serve as artificial facets to replace the facet joint between the articular processes removed during surgery, so that support for the patient&#39;s spine is not compromised by removal of one of the facets of the vertebra. The other facet of the vertebra can also be replaced if desired using pedicle screws or a second attachment to the plate members of the artificial disc. The artificial facets may be enclosed in a flexible skirt, or alternatively a rigid collar can be provided in surrounding relationship to the artificial facets to help maintain the facets in proximity and further limit the amount of relative movement between the facets. 
     The present invention also beneficially provides for the cut for receiving and locating the plate members to be made along a single line of insertion—i.e. unilaterally—from generally the posterior of the patient. The provision for a unilateral cut through the natural disc and along each of the adjacent vertebrae provides a good support against lateral movement of the plate members, a favorable interface with the bone, leaves a portion of the natural disc intact for support and cushioning, and makes it easier for the surgeon to make the right cut and avoid errors. 
     The invention hereof also includes a novel tool for use during attachment of the outriggers. The tool functions as a guide for use in drilling the channel through the pedicles for receiving the outrigger rods. The tool, which includes a carrier and a drill guide, is advantageously temporarily mounted on one of the plate members and then the other, or two such tools can be employed simultaneously, so that the channel which is drilled is related to the position of the plate members after insertion into the spine, and more particularly the opening into the recess of the plate members. Moreover, the tool is adjustable whereby the surgeon can accommodate variations in individual physiology so that the channel is most advantageously located for the patient. The drill guide is preferably tubular in configuration, having a central axis, and beneficially the central axis intersects at a common point—the opening to the recess of the plate members—when the guide is shifted on the carrier so that the channel created by the drill remains aligned with the opening into which a part of the outrigger is inserted into the plate member. 
     The present invention also includes an improved method of insertion of an artificial disc into a patient. Broadly speaking, the method includes cutting a passage through the natural disc and respective slots in adjacent vertebrae from a generally posterior position of the patient, removing at least a part of one of the spinal superior articular processes and at least a part of one of inferior articular processes on the adjacent vertebra. The surgeon then inserts the plate members along the slots in a unilateral approach, first using a specialized spacer for creating additional space between the adjacent vertebrae, if necessary. This spacer includes a handle and a block fixed thereto, so that the block turns with the handle to which it is affixed. The block is wider in one direction and narrower in a direction perpendicular to the one direction, and preferably is rectangular with rounded corners. The block is inserted into a tunnel cut in the natural disc between adjacent vertebra, with the narrow width of the block oriented on the spinal axis, and then turned so that the wider width is oriented substantially on the spinal axis to increase the spacing between the adjacent vertebrae. The tool used for drilling the channels through the pedicles is then attached in sequence to each of the plate members (or two such tools may be used simultaneously) and the channels are drilled through the pedicles to permit attachment of the outriggers. The rods of the outriggers are then inserted through the channels and connected to their respective plate members, and the posts are affixed and secured to the rods. 
     It is believed that the present invention will attain the goal of reducing the pain experienced by the patient and restoring most of the original range of motion of the spine. Moreover, it should reduce the risks attendant to anterior insertion of artificial discs whereby the presence of a vascular surgeon will no longer be necessary. These and other benefits will be readily appreciated by those skilled in the art with reference to the drawings and the detailed description set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary rear elevational view of the patient&#39;s spine showing the plate members of the artificial disc inserted and the left facet joint removed, but before outriggers of the artificial disc have been attached; 
         FIG. 2  is a fragmentary side elevational view of the patient&#39;s spine showing the plate members of the artificial disc inserted and the tool attached thereto for acting as a drill guide for the channels to receive the outrigger rods; 
         FIG. 3  is a horizontal cross-sectional view taken along line  3 - 3  of  FIG. 1  with portions of adjacent vertebrae removed for clarity, and with an arcuate weight-bearing member of one of the plate members shown in phantom lines, and showing the tool attached to the plate members; 
         FIG. 4  is an enlarged, fragmentary vertical sectional view taken along line  4 - 4  of  FIG. 3 , showing the spacer in position between adjacent vertebrae before insertion of the plate members; 
         FIG. 5  is a end elevational view showing the tool attached to the plate members; 
         FIG. 6  is an enlarged, fragmentary vertical cross-sectional view taken along line  6 - 6  of  FIG. 3 , showing the artificial disc plate members having complemental weight-bearing surfaces and recesses for receiving the rods of the outriggers; 
         FIG. 7  is a fragmentary vertical cross-sectional view through a portion of the spine showing the channels drilled in the pedicles of the vertebrae in alignment with the openings in the plate members; 
         FIG. 8  is an enlarged, fragmentary vertical cross-sectional view similar to  FIG. 7  but showing the rods of the outriggers inserted in the channels, with the upper rod shown prior to expansion in the recess and the lower rod after expansion, and with multiaxial gripping heads mounted on the rods; 
         FIG. 9  is an enlarged, fragmentary vertical cross-sectional view similar to  FIG. 8 , showing the posts with the artificial facet joint thereon coupled to the multiaxial gripping heads on the rods; 
         FIG. 10  is an exploded view of the tool for guiding the drill, with the artificial disc plate members shown in partial section; 
         FIG. 11  is a perspective view of the tool as shown in  FIG. 10 , and showing a drill inserted in the guide of the tool for drilling the channel into one of the pedicles for receiving the rod; 
         FIG. 12  is an exploded view of the artificial disc, showing the plate members, the rods and posts of the outriggers, and the artificial facet with its casement bisected for clarity; 
         FIG. 13  is an enlarged vertical sectional view of the artificial disc hereof, showing an alternative configuration of the artificial facets and rigid collar instead of a flexible skirt surrounding the artificial facets, the collar being provided with an arcuate-shaped inner wall and permitting one of the rods to pass through an opening in the collar and move relative to the collar; 
         FIG. 14  is a fragmentary vertical sectional view of the artificial disc as shown in  FIG. 13 , wherein the upper facet is turned and shifted downwardly relative to the lower facet; and 
         FIG. 15  is a fragmentary horizontal sectional view taken approximately on line  15 - 15  of  FIG. 14  to illustrate the camming effect into the inner wall of the collar caused when the convex face of the upper facet turns or slides upwardly a sufficient distance relative to the concave face of the lower facet. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, an artificial spinal disc  20  in accordance with the present invention is shown in  FIGS. 9 and 12  and broadly includes first plate member  22  and second plate member  24  with a connecting outrigger  26 . The invention further includes a tool  28  used in drilling channels for receiving the outrigger  26 , and a spacer tool  30 . The artificial spinal disc  20  is designed for use between adjacent first vertebra  32  and second vertebra  34 . As used herein, the terms “first vertebra” and “second vertebra” are used merely as a references to distinguish between two adjacent, superior and inferior vertebra, and not in the medical sense as in “sixth cervical vertebra.” As seen in  FIGS. 1 ,  2  and  3  for example, each of the vertebrae  32  and  34  present a vertebral foramen  36  for receiving therein the spinal cord  38 , a body  40  which provides a weight bearing capacity for the spinal column  42 , left and right pedicles  44  and  46 , left and right transverse processes  48  and  50 , left and right superior articular processes  52  and  54 , left and right inferior articular processes  56  and  58 , and a spinous process  60 . The inferior articular processes  56  and  58  of first vertebra  32  and the superior articular processes  52  and  54  of second vertebra therebelow work together to provide natural facet joints  62  which provide support and a limited range of movement between the vertebrae. In addition, a natural human disc  64  is located between the first and second vertebra. 
     In greater detail, the first plate member  22  includes an elongated panel  66  having a first surface  68  for abutment with the first vertebra  32  and a second surface  69  generally facing the second plate member  24  and the second vertebra  34 , a longitudinally extending, laterally centered rib  70  extending toward the first vertebra  32  from the first surface  68 , and an arcuate, generally convex bearing  72  extending toward the second vertebra  34  from the second surface  34 . The panel  66  has a surrounding edge  74  which includes a rounded nose  76 , generally straight and relatively parallel side edges  78  and  80 , a generally straight back edge  82  extending substantially perpendicular from side edge  78 , and relieved edge  84  which is inwardly arcuate and extends from the side edge  80  to the back edge  82 . The rounded nose  76  is generally tapered in the longitudinal direction so that when a line is extended perpendicular to the junction of the side edge  78  and the nose  76 , the opposite side edge  80  extends past this perpendicular line, as seen in  FIG. 3 . The rounded nose thus mirrors the rounded anterior margin of the body  40  and the natural disc  64 . The relieved edge  84  is recessed to avoid or at least minimize any intrusion of the plate member  22  with the spinal cord. The back edge  82  further includes a longitudinally extending hole  86  which is oriented along the longitudinal axis of the plate member  22  and parallel to the longitudinal extension of the rib  70 . The hole  86  is preferably internally threaded for mounting tool  28  thereto, as shown in  FIGS. 10 and 11 . Bearing  72  is located on second surface  69  in a position not necessarily laterally or longitudinally centered on the panel  66 , but rather positioned as shown in  FIG. 3  to be centered on what is substantially the weight bearing axis of the spinal column. The plate member  22  may be cast or machined of any suitable metal such as cobalt-chrome stainless steel or titanium, and except for bearing  72 , provided with a bio-ingrowth coating or texture, such as, for example, hydroxyappetite or porous beads. Bearing  72  may be cast or machined along with the remainder of the plate member  22  so that it is integrally formed of metal, such as cobalt-chrome stainless steel or titanium, or may be attached to the first surface and provided of a durable and friction-resistant synthetic material such as nylon. The bearing  72  is shown as being domed and semi-hemispherical, but could also be ellipsoid or of other arcuate shapes. 
     Rib  70  includes a remote surface  87  and a pair of generally angled, divergent (in a direction from the remote surface  87  toward the surface  68 ) side surfaces  88  and  90 . A recess  92  extends in longitudinal alignment into the rib  70  and panel  66 . The recess  92  may be cylindrical and/or threaded, but most preferably is generally frustoconical in configuration and has an axis which is angled relative to the remote surface  87  to provide a desired angle of attachment and approach for the outriggers, as seen in  FIGS. 6 ,  7 ,  8 ,  9  and  12 . The frustoconical shape of the recess  92  is such that an inner wall  94  generally diverges as the depth into the plate member  22  increases. 
     Plate member  24  shares similar features with plate member  22 . In that regard, second plate member  24  includes an elongated panel  96  having a first surface  98  for abutment with the second vertebra  34  and a second surface  99  generally facing the disc  64  and the first vertebra  32 , a longitudinally extending, laterally centered rib  100  extending toward the second vertebra  34  from the first surface  98 , and an arcuate, generally concave bearing-receiving depression  102  extending inwardly into the panel  96 . The panel  96  has a surrounding edge  104  which includes a rounded nose  106 , generally straight and relatively parallel side edges  108  and  110 , a generally straight back edge  112  extending substantially perpendicular from side edge  108 , and relieved edge  114  which is inwardly arcuate and extends from the side edge  110  to the back edge  112 . The rounded nose  106  is generally tapered in the longitudinal direction so that when a line is extended perpendicular to the junction of the side edge  108  and the nose  106 , the opposite side edge  10  extends past this perpendicular line. Thus, as with panel  66 , the rounded nose  106  of the panel  96  thus mirrors the rounded anterior margin of the body  40  and the natural disc  64 . The relieved edge  114  is recessed to avoid or at least minimize any intrusion of the plate member  24  with the spinal cord. The back edge  112  further includes a longitudinally extending hole  116  which is oriented along the longitudinal axis of the plate member  24  and parallel to the longitudinal extension of the rib  110 . Plate members  22  and  24  are thus designed so that their respective panel edges are substantially congruent when placed in longitudinal alignment in superior-inferior relationship, with the holes  86  and  116  being oriented in parallel relationship. The hole  116 , as with hole  86 , is preferably internally threaded for mounting tool  28  thereto, as shown in  FIGS. 10 and 11 . Bearing-receiving depression  102  is located on second surface  99  in a position not necessarily laterally or longitudinally centered on the panel  66 , but rather positioned as shown in  FIG. 3  to be centered on what is substantially the weight bearing axis of the spinal column. The plate member  22  may be cast or machined of any suitable metal such as cobalt-chrome stainless steel or titanium, and except for bearing-receiving depression  102 , provided with a bio-ingrowth coating or texture, such as, for example, hydroxyappetite or porous beads. Bearing-receiving depression  102  has a curvature complemental to the curvature of the bearing  72 , but in order to permit rocking of the bearing  72 , the depth of the bearing-receiving depression  102  is less than the height of the bearing  72 . Thus, the respective surfaces  69  and  99  of panels  66  and  96  have a space S therebetween when the panels are in an initial, parallel orientation. The bearing receiving depression  102  is machined or otherwise formed in the plate member  24  to be smooth, and is preferably integrally formed of metal, such as cobalt-chrome stainless steel or titanium. If desired, a friction-resistant coating of lining of a durable and friction-resistant synthetic material such as nylon may be provided. 
     Rib  100  includes a remote surface  118  and a pair of generally angled, divergent (in a direction from the remote surface  118  toward the surface  98 ) side surfaces  120  and  122 . A recess  124  extends in longitudinal alignment into the rib  110  and panel  96 . The recess  124  may be cylindrical and/or threaded, but most preferably is generally frustoconical in configuration and has an axis which is angled relative to the remote surface  118  to provide a desired angle of attachment and approach for the outriggers, as seen in  FIGS. 6 ,  7 ,  8 ,  9  and  12 . The frustoconical shape of the recess  124  is such that an inner wall  126  generally diverges as the depth into the plate member  24  increases. 
     Outrigger  26  provides an artificial facet joint  127 . The outrigger  26  includes first and second rods  128  and  130  respectively connected to first and second plate members  22  and  24 . The first and second rods  128  and  130  each mount respective first and second posts  132  and  134  having respective, mating, complementally configured first and second facets  136  and  138  on the posts. A retaining skirt  140  encloses the first and second facets  136  and  138 . 
     The first and second rods  128  and  130  each include a connector  142  for attachment to the plate members at one end and a coupler  144  for attachment to the posts at the other end. The connectors  142  permit removable fastening of the rods to respective ones of the plate members  22  and  24 . In the preferred embodiment illustrated in the drawings, the connectors  142  include a threaded pin  146  received in a tubular cannula  148  of each rod  128  and  130 . The tubular cannula  148  each have a slotted tip  150  distal to the coupler  144  for insertion into the recesses  92  and  124 . The slotted tip  150  includes a plurality of finger-like extensions  152  each having an enlarged, bulb-like end  154 . The cannula  148  have an enlarged, rounded knuckle  156  at the proximate end  158  adjacent to and received by the coupler  144 , and an externally threaded barrel  160  intermediate the tip  150  and the rounded knuckle  156 . The tubular cannula  148  also have internal walls  162  defining a passage  164  for receiving the threaded pin  146  therein. As may be seen in  FIG. 8 , for example, the initial configuration of the passage  164  interior of the slotted tip  150  is convergent in a direction from the coupler toward the tip  150 , such that the diameter of the bulb-like end  154  is initially approximately the same as the unthreaded portion  166  of the tubular cannula remote from the threaded barrel  160 . At least a portion  168  of the internal walls  162  also have threads  170  thereon. The rounded knuckle  156  is rounded along its distal surface, and includes at its proximate surface slots  172  or other tool-receiving slot or socket such as for receiving a hex or other wrench to permit driving and removal of the tubular cannula  148 . The threaded pin  146  is complementally sized for axial insertion into the passage  164  and has threads  174  thereon for permitting advancement and removal of the pin from the tubular cannula  148 . The pin  146  preferably includes a shank  176  and a head  178 , one of which is provided with the threads  174 . As illustrated, the head  178  is externally threaded for engagement with the threads  170  of the tubular cannula  148 . The head  178  also includes a hex socket  180  or similar structure whereby a tool, such as a screwdriver or an Allen wrench, may be inserted for advancing the pin  146  into the tubular cannula  148 . The coupler  144  preferably is provided as a multi-axial head  182  and includes a collar  184  having arcuate inner walls  186  which are sized and configured to receive the rounded knuckle  156  in snap-on engagement, whereby once a tubular cannula  148  is inserted into the multi-axial head  182  with the rounded knuckle  156  installed in the collar  184 , the rounded knuckle  156  may swivel and pivot within collar  184 . The multi-axial head  182  further includes opposed ears  188  and  190  having internally facing opposed teeth  192  thereon. The opposed teeth  192  serve both to grip a post inserted between the ears  188  and  190  and to threadably receive a locking cap  194 . 
     The first and second posts  132  and  134  are inserted between the ears  188  and  190  whereby their respective first and second facets  136  and  138  are in opposed engagement. The facets  136  and  138  are complementally configured, one facet having a generally concave mating surface  198  and the other having a convex mating surface  196  and both being inclined in a superior to inferior direction. It is to be understood that either the superior facet  136  or the inferior facet  138  may have either a convex or a concave face, but as illustrated the convex mating surface  196  is provided on the superior facet while the concave mating surface  198  is provided on the inferior facet  138 . Retaining skirt  140  is provided over the facets  136  and  138  to aid in their initial retention. The skirt  140  is preferably tubular (shown bisected in  FIG. 12 ) and provided of a flexible, resilient and biologically inert synthetic resin material. The skirt may be held in position by clamps  200 , drawstrings, resilient bands at each end, or by its own resiliency, for example. For ease of use, the posts including the facets may be provided already connected by the skirt  140  so that the surgeon need not assemble the skirt around the facets during surgery. 
     In order to facilitate insertion of the artificial disc  20  between the vertebrae, and because of possible compression of the natural disc  64 , it may be necessary to provide additional spacing therebetween. Additional space between adjacent vertebrae may be provided by spacer  30  shown in  FIG. 4 . The spacer  30  is configured for insertion in a tunnel initially cut in the natural disc  64 . The spacer  30  includes a block  202  connected to an elongated handle  204 . The block  202  is generally rectangular in transverse shape, having a primary dimension A which is smaller than a secondary dimension B transverse thereto, parallel long sides  206  and parallel short sides  208  perpendicular to the long sides  206 , and rounded corners  210  which facilitate turning of the spacer  30  between the vertebrae. 
     Further, the tool  28  of the present invention is provided to aid the surgeon installing the artificial disc with regard to aligning the drill for drilling channels in the pedicles for the rods of the outrigger  26 . As shown in  FIG. 7 , the surgeon may employ one tool  28  sequentially or use two tools  28  on respective ones of the plate members  32  and  34 . The tool  28  broadly includes a carrier  212 , and a drill guide  214 , and is preferably made of a corrosion-resistant metal such as stainless steel. 
     Carrier  212  is preferably provided to include a slide  216  and an elongated rod  218  having a threaded stem  220  at one end thereof, the threaded stem being sized and configured for threading into the longitudinally extending holes  86  and  116  in the back edges of each of the plate members  32 ,  34 . However, it may be appreciated that the stem  220  and its receiving longitudinally extending holed  86  and  116  could be smooth, or polygonal, may be provided with a snap connection, or other mounting configurations for holding the tool  28  temporarily on the respective plate member. The opposite end of the elongated rod  218  is provided with a mounting section  222  for receiving the slide  216 . The mounting section  222  may be threaded or include a snap connection. In the embodiment illustrated, the mounting section  222  is provided as a flat-sided flange  224  for receiving the protractor thereon. The slide  216  includes a coupler  226  complementally configured with the mounting section  222  for facilitating coupling of the arcuate-shaped slide to the rod  218 , and a beam  228 . The beam  228  is preferably arcuate in configuration, with the coupler  226  preferably laterally offsetting the beam  228  relative to the rod  218 . A set screw  230  may be provided to act as a securement to hold the mounting section  222  to the rod  218 . The drill guide  216  includes an elongated tube  232  and a slider  234 . The slider  234  mounts the tube  232  thereon and includes structure defining a slot  244  which is complementally configured to the beam  228  so that the drill guide  216  fits snugly to the beam but the position of the slider  234  may be adjusted along the beam. The tube  232  has a central axis A, and advantageously, the central axis A will always orient toward the recess in the rib of the respective plate member  22 ,  24  to which the tool  28  is mounted, such that the central axis A always intersects at the same point as the drill guide is shifted along the beam  228  of the carrier  212 . The slider  234  offsets the tube  232  relative to the beam  228  so that there is no interference between the beam and the tube  232 , and also whereby the tube  232  is substantially co-planar with the carrier  212 . A thumbscrew  236  or other fastener may threaded through a companion opening in the slider  234  if additional securement to hold the drill guide stationary in use is necessary. The tube  232  is sized to receive a drill bit  238  attached to a drill  240 , the diameter of the tube  232  and the drill bit  238  preferably being sized so that a channel  242  created by the drill bit  238  is substantially of the same diameter as the outside diameter of the unthreaded portion  166  of the rods as seen, for example, in  FIG. 9 . 
     A desired method of insertion of the artificial spinal disc  20  of the present invention is also provided. After initial incisions and spreading of the skin and muscles to gain access to the spinal region from a posterior direction (preferably with the patient in a supine position, the surgeon cuts away a portion of the inferior articular process  56  of the first vertebra  32  and a portion of the superior articular process  52  of the second vertebra  34  immediately therebeneath in order to provide a route for the introduction of the plate members  22  and  24  therethrough. The surgeon then cuts a tunnel  244  into the natural disc of the patient of a width substantially corresponding to the width of the plate members between the respective side edges of one panel  78  and  80  and  108  and  110  of the other panel. If the spacing between the first vertebra  32  and the second vertebra  34  have been reduced to too great an extent, the block  206  of the spacer  30  is inserted into the tunnel  244  in an initial position as shown in dashed lines in  FIG. 4 . The block  206  is then turned 90° to the position indicated in solid lines in  FIG. 4  to cause additional space to be created between the first vertebra  32  and the second vertebra  34 . The spacer may then be removed. Thereafter, a chisel or similar tool is used to cut troughs  246  and  248  into the body  40  of each of the vertebra  32  and  34 . The troughs  246  and  248  oppose one another and are oriented in a parallel direction to the tunnel and preferably substantially centered laterally. Further, the troughs  246  and  248  are sized to receive respective ribs  70  and  110 . 
     The surgeon then places the plate members  22  and  24  in mating juxtaposition with one another whereby the bearing  72  is received in the bearing receiving depression  102 . The surgeon then slides the assembled plate members  22  and  24  into the tunnel  244  with the ribs  70  and  110  received in and sliding along respective trough  246  and  248  until the center of the bearing is located substantially at the load bearing axis of the spinal column in the region between the first vertebra  32  and the second vertebra  34  as illustrated in  FIGS. 1 ,  3  and  6 . It may thus be appreciated that the surgeon is employing a substantially unilateral approach from the posterior direction. In addition, the ribs  70  and  110  act both as a guide during insertion of the plate members  22  and  24  but also provide additional stabilization to minimize movement of the artificial disc off of the weight bearing axis of the spinal column. Further, a substantial portion of the natural disc of the patient is left intact for additional cushioning and support and the bone ingrowth coating or surface on the plate members  22  and  24  should help promote natural fixation of the artificial disc  20 . 
     Because the natural facet joint between the adjacent inferior articular process and the superior articular process on one side of the first vertebra  32  and the second vertebra is lost as a result of the insertion of the plate members  22  and  24  from one side in a posterior position, the present invention provides the outrigger  26  to provide an artificial facet joint. After the plate members  22  and  24  are in place, the surgeon then attaches the tool  28  to the plate members  22  and  24  by affixing the carriers to the holes  86  and  116  as shown, for example, in  FIGS. 2 ,  3 ,  10  and  11 . The surgeon moves the drill guide  216  along the beam of the slide  216  until the channel  242  to be drilled is aligned to pass through substantial bone mass through the pedicle  44  on the desired side of each of the first vertebra  32  and the second vertebra  34 . Beneficially, the drill guide  216  remains aligned with the entry of recesses  92  and  124  along the top of the ribs so that the surgeon will need to be concerned only with finding the appropriate bone mass in the pedicle knowing that the drill guide will align the drill bit with the respective recess  92  or  124  without it being seen. The surgeon then inserts the drill bit  238  into the tube  232  and drills the channel  242  through the pedicles  44  on one side of each of the first vertebra  32  and second vertebra  34  for receiving the rods  128  and  130 . With the channels thus drilled as shown in  FIG. 7 , the tool  28  may then be removed from the plate members  22  and  24 . 
     The rods  128  and  130  are then mounted on their respective ribs  70  and  110  of the plate members  22  and  24  as illustrated by  FIG. 8 . The surgeon initially inserts the rod as shown with respect to rod  128  in  FIG. 8 . While the threads may be omitted from the externally threaded barrel  158  on the exterior of the rods, the threads thereon aid in holding the rod in position. Additional attachment support is provided by advancing the pin  144  along the passage. As the pin  146  advances, the extensions  152  of the slotted tip  150  expand so that the bulb-like ends  154  spread and engage the inner surfaces  94  and  126  of the frustoconically shaped recesses  92  and  124 , respectively. Because of the recesses  92  and  124  have a slightly greater transverse dimension at their end wall as opposed to their entry opening on the surface of the rib, the bulb-like ends  152  expand and help lock the rods in place once the pins are fully seated as shown in  FIG. 9 . 
     With the rods  128  and  130  fully inserted as desired, the surgeon then installs the posts  132  and  134  onto the multi-axial head  182  by positioning the posts between the gripping surfaces of the ears  190  and  192  of each multi-axial head. As may be seen in  FIG. 9 , the multi-axial heads  182  permit pivoting and rotation of the ears so that the posts  132  and  134  may be in superposed alignment, with the facets  136  and  138  in mating concave-convex relationship. The posts may be secured in this position further by threading thereon the locking cap  194  as shown in  FIG. 9 . Any excess length of the posts  132  and  134  extending beyond the multi-axial heads  182  may be cut away by the surgeon. The primary weight to be carried along the spinal column will pass between the plate members, with the facets  136  serving as a limiter against excessive twisting or vertical movement rather than for bearing weight. The skirt  140  being preplaced around the facets  136  and  138 , the incision may be closed in accordance with ordinary surgical procedures. 
     Among the many benefits provided by the present invention are the minimization of risk of damage to the spinal cord by adopting a posterior, trans-pedicular approach from the side, the fact that one side of the vertebrae involved are left alone because of the unilateral approach which is minimally invasive, minimizing the possibility that the artificial disc will spin or shift one inserted in position, retaining substantial flexibility in the joint to avoid transferral of stress to other vertebrae, and the ability to leave the anterior ligament intact. 
     One further benefit of the present invention is that should the need arise, the artificial disc  20  hereof may be readily removed from the patient. By using threaded connections and a unilateral approach, if fusion of the adjacent vertebrae later becomes necessary, the entire artificial disc hereof may be removed. First, to remove the outrigger  26 , the surgeon removes the locking caps  194  from the multi-axial heads and prises the posts  132  out from between the ears  190  and  192 . The surgeon may then gain access to the pin within the tubular cannula, and using a wrench or similar tool, back out the threaded pin from the tubular cannula in which it is received. Because the rounded knuckle  156  includes slots  172  or other tool receiving structure, once the pin  146  is backed out, the extensions  152  of the slotted tip  150  are free to collapse as the tubular cannula is threaded out of the bone and the tip  150  is withdrawn from the respective recess. With the rods removed, the surgeon may insert threaded bolts or other tools into the holes  86  and  118  to use to gain a purchase on the plate members and remove them along the tunnel initially cut through the natural disc. Alternatively, if it was necessary to fix the first vertebra  32  against movement relative to the second vertebra, the plate members  22  and  24  could be left in position, but a solid bridge-type structure inserted either between the multi-axial heads or be attached between the plate members  22  and  24  and connected by threaded fasteners inserted into the holes  86  and  116 . 
     As an alternative to the flexible retaining skirt  140  illustrated in  FIGS. 9 and 12 , a collar  250  may be utilized in connection with the artificial facets  136   a  and  138   a . The collar  250  is initially manufactured having an upper cap  252  and a lower housing  254  which receive the posts  132  and  134  respectively. The post  132  passes through an access  256  in the upper cap  252  and the post  134  passes through an opening  258  in the lower housing  254 . The access  256  generally presents a greater clearance between the collar  250  and the post  132  than the clearance between the collar  250  and the lower post  134  provided by opening  258  as illustrated in  FIGS. 13 and 14 . The upper cap  252  and lower housing  254  are then assembled and secured by welding, mechanical fasteners or the like along their respective mating edges so that the facets  136   a  and  138   a  are retained within the assembled collar  250 , which is thereby provided as a unit along with the posts  132  and  134 , so that the assembly including the collar  250  enclosing the artificial facets  136   a  and  138   a , along with the posts  132  and  134 , is provided to the surgeon ready for installation on the respective rods. Preferably, the lower post  134  is secured to the housing  254  at the opening  258  by welding, adhesive or a mechanical fastener so that no relative motion is permitted between the lower post  134  and the housing  254 . 
     The collar  250  is configured with an inner wall  260 . The inner wall  260  has a side surface  262  which is preferably generally arcuate in horizontal cross section, and most preferably, the side surface  262  which is generally ovoid in horizontal cross-section, being substantially circular in cross section along an anterior portion  264  and more prolate or eccentrically shaped along a posterior portion  266  as illustrated by  FIG. 15 . The inner wall  260  also closely conforms to the respective outer surfaces  268  and  270  of the facets  136   a  and  138   a  but permits limited relative movement therebetween. Most preferably, the inner wall  260  includes an upper wall surface  272  which is positioned closely adjacent an upper margin  274  of the superior facet  136   a  so that the mating, convex and concave faces  196  and  198  of the facets  136   a  and  138   a  are maintained in constant engagement and the facets do not separate when in the full distraction position as shown in  FIG. 13 . Moreover, the side surface  262  is most preferably substantially upright and perpendicular to the upper wall surface  272  along the anterior portion  264 , but the posterior portion  266  is slightly inclined so as to present a more conical configuration when viewed in vertical cross-section as seen in  FIGS. 13 and 14 . The outer surface  270  of the inferior facet  138   a  is preferably provided with a lip  278  which extends posteriorly so that in horizontal cross-section, it has an ovoid shape complemental to and closely adjacent the inner wall  260 , as seen in  FIG. 15 . The outer surface  268  of the superior facet  136   a  is preferably generally ovoid in cross-section, such that the anterior side  280  of the superior facet  136   a  is generally circular while the posterior side  282  is prolate and more closely conforms to the configuration of the inner wall  260 , having a generally tapered posterior side  282  such that the distance of the posterior side  282  from the anterior portion  264  of the inner wall  260  increases in a downward direction away from post  132 . The configuration of the upper facet  136   a  relative to the collar  250  is such as to limit separation between the facets  136   a  and  138   a , but still providing sufficient clearance between the posterior side  282  of the outer surface  268  of the superior facet  136   a  and the posterior portion  266  of the inner wall  260  to permit limited sliding of the superior facet  136   a  downwardly along the inferior facet  138   a  and limited relative rotational movement of the superior facet  136   a  relative to the inferior facet  138   a.    
     The relative configurations of the collar  250  and the facets  136   a  and  138   a  provide improved stability while permitting limited relative movement between the posts  132  and  134  and their respective facets  136   a  and  138   a  in the artificial spinal disc  20  hereof.  FIG. 13  shows the position of the posts and facets  136   a  and  138   a  when the facets are in their fully distracted position. As patient moves (as illustrated here, bends so that the upper vertebra  32  moves clockwise relative to the lower vertebra  34 ), the superior facet  136   a  is permitted to slide downwardly along the inferior facet  138   a  until the outer surface  268  engages the side surface  262  of the inner wall  260 , thereby limiting the amount of relative movement between the facets  136   a  and  138   a , and also providing additional stability for the patient. As the upper facet  136   a  moves beyond the position shown in  FIG. 14 , the outer surface  268  will engage the inclined posterior portion of the side surface  262  of the inner wall. The extended lip on the inferior facet  138   a  enables the concave and convex mating surfaces to substantially remain in contact and provide continuous support in the region as well as continuous resistance so that the patient does not experience rapid slippage as the frictional engagement changes. 
     In addition, a limited amount of relative rotational movement is permitted between the facets  136   a  and  138   a .  FIGS. 14 and 15  illustrate that as the superior facet  136   a  begins to rotate relative to the inferior facet  138   a , the generally convex mating surface or face  196  begins to ride up along the opposing, and fixed, concave mating face  198  of the inferior facet  138   a . When the upper facet  136   a  shifts down along the lower facet  138   a , an axis A of the inferior post  132  remains stationary, while the axis B of the superior post  134  moves off of its initial alignment with axis A in a generally anterior to posterior direction. As shown in  FIG. 15 , when the upper facet  136   a  also has limited rotational movement relative to the lower facet  138   a , the axis B also moves to one side of an anterior to posterior plane bisecting the lower facet  138 . The upper facet  138   a  is permitted to continue this relative rotational movement as the patient twists until the outer surface  268  of the superior facet  136   a  moves into engagement with the side surface  262  of the inner wall  260  of the collar  250 . The combination effect of the convex and concave face engagement with the camming effect when the upper facet engages the inner wall provides increased resistance as the patient&#39;s torso twists up to the point of engagement, as well as a beneficial centering function. 
     Advantageously, the configuration of the facets  136   a  and  138   a  and the collar  250  is complemental and synergistic in regard to limiting the amount of patient movement. That is to say, the more movement the patient undertakes to cause the upper facet  136   a  to slide downwardly along the lower facet  138   a , the less relative rotational movement is permitted. This is because when the upper facet  136   a  fully slides downwardly, it comes into full engagement with the side surface  262  so that no relative rotational movement is permitted. This inhibits excessive movement which might lead to injury to the patient. Thus, the configuration of the inner wall  260  benefits the patient in which the artificial spinal disc has been implanted by permitting some movement, but also by limiting the amount of movement based on a combination of physical factors related to how much stress the patient is placing on the spinal region. 
     Although preferred forms of the invention have been described above, it is to be recognized that such disclosure is by way of illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. By way of example only, the rods of the outrigger  26  could be attached to the plate members by threading or other means of fastening, a separate synthetic resin or metal member could be used as a weight bearing member between the plate members each having a generally convex bearing receiving surface, and a bio-ingrowth surface coating could be applied to the exterior of the rods to promote bone growth therearound. In addition, the collar  250  could have a tapered inner wall to allow sliding movement while maintaining contact with the facets, could be configured with a generally cylindrical inner wall  260 , or both facets could be free to move relative to the collar. It is also to be understood that the plate members could be configured to receive additional outriggers whereby both natural facet joints could be removed and artificial facets provided in lieu thereof. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of their invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set out in the following claims.