Abstract:
The present invention provides an intervertebral implant for replacing intervertebral disc material and controlling relative motion between adjacent vertebral bodies. The intervertebral implant may have at least one end plate and an intermediate component which slides into engagement with the end plate. The intermediate component may be engageable with the end plate from an anterior approach, or a right or left lateral approach. The intermediate component can be a set of bearing surfaces which articulate to provide relative motion between two vertebral bodies, an elastic insert which deforms to provide motion between two vertebral bodies, or a rigid insert which prevents relative motion between two vertebral bodies. The intermediate component is replaceable with a different intermediate component which provides a different function. The present invention also provides one or more retaining members which snap into engagement with the end plate and secure the intermediate component to the end plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the following:  
         [0002]     U.S. Provisional Application No. 60,720,513, filed Sep. 26, 2005, which carries Applicants&#39; Docket No. MLI-45 PROV and is entitled MODULAR ARTICULATING AND FUSION SPINAL DISC IMPLANT SYSTEM;  
         [0003]     U.S. Provisional Application No. 60/720,514, filed Sep. 26, 2005, which carries Applicants&#39; Docket No. MLI-46 PROV and is entitled UNIVERSAL SPINAL DISC IMPLANT SYSTEM FOR PROVIDING INTERVERTEBRAL ARTICULATION AND FUSION; and  
         [0004]     U.S. Provisional Application No. 60/741,513, filed Nov. 30, 2005, which carries Applicants&#39; Docket No. MLI-50 PROV and is entitled SYSTEM AND METHOD FOR INTERVERTEBRAL IMPLANT DELIVERY AND REMOVAL.  
         [0005]     All of the foregoing are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0006]     1. The Field of the Invention  
         [0007]     The present invention relates generally to spinal orthopedics, and more precisely, to intervertebral implants.  
         [0008]     2. The Relevant Technology  
         [0009]     Severe back pain can be caused by a number of different ailments, including spinal stenosis, degenerative disc disease, spondylolisthesis, and the like. Many such ailments can be corrected by controlling or limiting relative motion between the affected vertebrae. Accordingly, a variety of devices including artificial discs and fusion devices have been proposed.  
         [0010]     Such devices are limited in that they typically provide only one mode of correction. Many such devices cannot be replaced or corrected. This is particularly true with intervertebral implants, in which bone-growth is often stimulated to integrate the implants with the surrounding bone tissue. Thus, if the device fails to solve the problem, there may be no other recourse for the patient.  
         [0011]     Further, many known devices are expensive or difficult to manufacture, or are difficult to implant. Some known intervertebral devices require the adjacent vertebrae to be distracted excessively, thereby endangering the surrounding ligaments and other connective tissues. Accordingly, there is a need in the art for a device that remedies these problems. Such a device would considerably enhance outcomes for patients with spinal disorders.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope.  
         [0013]      FIG. 1  is a perspective view illustrating a portion of a spine.  
         [0014]      FIG. 2  is a perspective lateral view of one embodiment of a total disc implant implanted in the spine, comprising inferior and superior end plates, inferior and superior bearings, and a plurality of clips.  
         [0015]      FIG. 3  is a perspective anterior view of the total disc implant of  FIG. 2  in an disassembled state.  
         [0016]      FIG. 4  is a bottom elevation view of the clip shown in  FIG. 3 .  
         [0017]      FIG. 5  is a top elevation view of the clip shown in  FIG. 3 .  
         [0018]      FIG. 6  is a perspective side view of the clip shown in  FIG. 3 .  
         [0019]      FIG. 7  is a perspective posterior view of the bone engaging side of the inferior end plate shown in  FIG. 3 .  
         [0020]      FIG. 8  is a lateral end view of the inferior end plate shown in  FIG. 3 .  
         [0021]      FIG. 9  is a perspective anterior view of the bearing engaging side of the inferior end plate shown in  FIG. 3 , and one clip.  
         [0022]      FIG. 10  is a perspective posterior view of the bearing engaging side of the superior end plate shown in  FIG. 3 , and one clip.  
         [0023]      FIG. 11  is a perspective posterior view of the caudal side of the inferior bearing and one clip shown in  FIG. 3 .  
         [0024]      FIG. 12  is an anterior side view of the inferior bearing shown in  FIG. 3 .  
         [0025]      FIG. 13  is a perspective anterior view of the cephalad side of the inferior bearing shown in  FIG. 3 .  
         [0026]      FIG. 14  is a perspective anterior view of the cephalad side of the superior bearing and one clip shown in  FIG. 3 .  
         [0027]      FIG. 15  is an anterior side view of the superior bearing shown in  FIG. 3 .  
         [0028]      FIG. 16  is a perspective posterior view of the caudal side of the superior bearing shown in  FIG. 3 .  
         [0029]      FIG. 17  is an anterior side view of the total disc implant of  FIG. 3 , in a partially assembled state.  
         [0030]      FIG. 18  is a perspective lateral view of one embodiment of a fusion block implant, shown in a portion of the spine.  
         [0031]      FIG. 19  is a perspective anterior view of the cephalad side of the fusion block shown in  FIG. 16 .  
         [0032]      FIG. 20  is an anterior side view of the fusion block shown in  FIG. 16 .  
         [0033]      FIG. 21  is a perspective anterior-cephalad view of the fusion block shown in  FIG. 16 , and two clips.  
         [0034]      FIG. 22  is a perspective lateral side view of the fusion block implant of  FIG. 18 , in a partially assembled state.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     The present invention relates to human spinal disc replacement systems. Those of skill in the art will recognize that the systems and methods described herein may be readily adapted for other modular implant systems for anatomic replication of orthopedic joints by man made implant systems.  
         [0036]     Referring to  FIG. 1 , a perspective view illustrates a portion of a spine  10 .  FIG. 1  illustrates only the bony structures; accordingly, ligaments, cartilage, and other soft tissues are omitted for clarity. The spine  10  has a cephalad direction  12 , a caudal direction  14 , an anterior direction  16 , a posterior direction  18 , and a medial/lateral axis  20 , all of which are oriented as shown by the arrows bearing the same reference numerals. In this application, “left” and “right” are used with reference to a posterior view, i.e., a view from behind the spine  10 . “Medial” refers to a position or orientation toward a sagittal plane (i.e., plane of symmetry that separates left and right sides from each other) of the spine  10 , and “lateral” refers to a position or orientation relatively further from the sagittal plane.  
         [0037]     As shown, the portion of the spine  10  illustrated in  FIG. 1  includes a first vertebra  24 , which may be the L5 (Fifth Lumbar) vertebra of a patient, and a second vertebra  26 , which may be the L4 (Fourth Lumbar) vertebra of the patient. The systems and methods may be applicable to any vertebra or vertebrae of the spine  10  and/or the sacrum (not shown). In this application, the term “vertebra” may be broadly interpreted to include the sacrum.  
         [0038]     As shown, the first vertebra  24  has a body  28  with a generally disc-like shape and two pedicles  30  that extend posteriorly from the body  28 . A posterior arch, or lamina  32 , extends between the posterior ends of the pedicles  30  to couple the pedicles  30  together. The first vertebra  24  also has a pair of transverse processes  34  that extend laterally from the pedicles  30  generally along the medial/lateral axis  20 , and a spinous process  36  that extends from the lamina  32  along the posterior direction  18 .  
         [0039]     Similarly, the second vertebra  26  has a body  48  from which two pedicles  50  extend posteriorly. A posterior arch, or lamina  52 , extends between the posterior ends of the pedicles  50  to couple the pedicles  50  together. The second vertebra  26  also has a pair of transverse processes  54 , each of which extends from the corresponding pedicle  50  generally along the medial/lateral axis  20 , and a spinous process  56  that extends from the lamina  52  along the posterior direction  18 . The vertebrae  24 ,  26  are separated from each other by an intervertebral disc  66 .  
         [0040]     Referring to  FIG. 2 , a perspective view illustrates one embodiment of an implant  70 , which may be referred to as a total disk implant. The implant  70  is designed for placement between spinal vertebrae to replace degenerated intervertebral disc material. More specifically, the implant  70  is designed to be inserted between the vertebral bodies  28 ,  48  of the first and second vertebrae  24 ,  26 , respectively, after removal of the intervertebral disc  66 . The implant  70  has end plates which secure the implant to the vertebral bodies, and an intermediate component which engages with the end plates, to control or prevent relative motion between the vertebral bodies. The intermediate component may be a first bearing surface configured to articulate with a second bearing surface, to provide relative motion between the vertebral bodies. The intermediate component may also be a deformable elastic insert which provides motion between the vertebral bodies, or a rigid insert to promote fusion, thus preventing relative motion between the vertebral bodies. Retention members, which in this embodiment of the invention take the form of clips, secure the intermediate component to the end plates.  
         [0041]     In the embodiment depicted in  FIG. 2 , the assembled implant  70  is of a generally rectangular box-like shape with rounded edges, with top and bottom surfaces which form a slight wedge. In alternative embodiments, it is appreciated that implant  70  need not have a rectangular box shaped configuration but can be square, circular, or have any other polygonal or irregular configuration. Furthermore, it is appreciated that the implant  70  can be configured at any desired wedge angle or can have substantially parallel top and bottom surfaces. The implant  70  comprises an inferior end plate  100 , an inferior bearing  300 , a superior bearing  400 , and a superior end plate  200 . When fully assembled, the inferior bearing  300  is releasably attached to the inferior end plate  100  by a plurality of clips  500 . The superior bearing  400  is releasably attached to the superior end plate  200  by a plurality of clips  500 .  
         [0042]      FIG. 3  depicts the implant  70  in a disassembled state, to illustrate the individual components of the implant. The inferior bearing  300  has a rounded dome, surrounded by a trough, which fits into a cup, surrounded by a ridge, on the superior bearing  400 . The inclusion of the ridge and trough in the bearing design allows the patient less impeded flexion/extension and lateral bending.  
         [0043]     During the implantation procedure, initially the inferior  100  and superior  200  end plates are placed in the intervertebral space, adjacent to the vertebral bodies  28 ,  48 . In the embodiment depicted the end plates  100 ,  200  are identical, but are inserted in an opposite orientation from one another. Thus, as depicted in  FIGS. 2 and 3 , the inferior end plate  100  is inserted adjacent to vertebral body  28  with a bone engaging side  102  in a caudal direction  14  and a bearing engaging side  104  in a cephalad direction  12 . Conversely, the superior end plate  200  is inserted adjacent to vertebral body  48  with a bone engaging side  202  in a cephalad direction  12  and a bearing engaging side  204  in a caudal direction  14 . Because the features of the end plates  100 ,  200  in this embodiment are identical except in orientation, only the inferior end plate  100  will be described in detail below. All description of the structure of the inferior end plate  100  also applies to the superior end plate  200 . However, it is appreciated that in alternative embodiments of the invention, end plates of varying configurations may be used in combination.  
         [0044]      FIG. 4  depicts a bottom side view of the clip  500 . In this embodiment of the invention the clip  500  is roughly quadrilateral in shape, with substantially parallel top and bottom sides, and is bilaterally symmetrical. However, it is appreciated that in alternative embodiments, the clip  500  may vary in configuration and/or use. It has a top side  510 , a bottom side  512 , an interior edge  514  and an exterior edge  516 . The interior edge  514  is chamfered on the bottom side  512 . The exterior edge  516  has a tab  502  which extends perpendicularly from the edge. The tab  502  is configured to fit a gap in each end plate  100  or  200 , and moving the tab into the gap snaps the clip  500  to the end plate  100  or  200 . A body  506  extends from the tab  502  and the exterior edge  516  toward the interior edge  514 . One arm  504  extends perpendicularly from each lateral side of the body  506 , on either side of the tab  502 . A T-shape is formed by the arms  504  and the body  506 . At the interior end of the body  506 , two prongs  508  extend initially perpendicularly from the body  506 , then, turning right angles, extend back toward the arms  504 .  
         [0045]     Referring to  FIG. 5 , a top side view of the clip  500  is shown. The outside edges of the prongs  508  are chamfered on the top side  510 , forming chamfered edges  518 . The chamfered edges  518  of the prongs  508  are slightly angled in their lengthwise orientation; that is, the width of the clip  500  at the interior edge  514  is slightly narrower than its width at the arms  508  and exterior edge  516 . A protrusion  520  projects from the body  506  on the top side  510 . The protrusion  520  appears as a square projecting upward from the top side  510 , and is centered between the arms  508 . The protrusion  520  is wedge-shaped in profile, the higher end adjacent to the tab  502 , and the opposite end slanting down until it is flush with the body  506  at the midpoint of the body  506 .  
         [0046]      FIG. 6  depicts a side profile view of the clip  500 . The clip  500  has a spring bias  522 , such that when the clip  500  is not secured to another component, the body  506 , the arms  504  and the tab  502  are held at a slight downward angle relative to the prongs  508 ; that is, the body  506 , the arms  504  and the tab  502  angle in the direction of the bottom side  512 .  
         [0047]     Referring to  FIG. 7 , a bone-engaging side view of the inferior end plate  100  is shown. In this embodiment the inferior end plate  100  is of a quadrilateral shape with rounded corners, and is bilaterally symmetrical. It has an anterior end  120 , a posterior end  122 , a right end  124  and a left end  126 . The inferior end plate  100  has a bone engaging face  106  and a bearing engaging face  108  which are connected by a support member  1   10 . Projecting from the bone engaging face  106  is a plurality of bone engaging spikes  112 . Each bone engaging spike  112  is columnar in form and projects perpendicularly in the caudal direction  14  from the bone engaging face  106 . The protruding end of each bone engaging spike  112  tapers and terminates in an acute angle. This angled tapering creates a point which facilitates seating the inferior end plate  100  in the adjacent vertebral body  28  during the implantation process; the point will more easily penetrate the vertebral body  28  than would a blunt end.  
         [0048]     A hollow grafting channel  114  runs through the center of each bone engaging spike  112 . Each grafting channel  114  originates on the bearing engaging face  108 , runs through the support member  110 , and ends at the pointed termination of the bone engaging spike  1   12 . This hollowed point configuration may be compared to the hollow point of a hypodermic needle, and further facilitates the penetration of the vertebral body  28  by the bone engaging spikes  112 . The grafting channels  114  also allow for the growth of bony columns from the vertebral body  28  through the channels, thereby fusing the inferior end plate  100  to the vertebral body  28 .  
         [0049]     A plurality of grafting ports  116  is present in the inferior end plate  100 . Each grafting port is an opening from the bearing engaging face  108  through the support member  110  to the bone engaging face  106 . The grafting ports  116  allow for the growth of bony material from the vertebral body  28  through the ports, thereby fusing the inferior end plate  100  to the vertebral body  28 .  
         [0050]     A groove  118  is present on each outer corner of the inferior end plate  100 . Each groove  118  is an indentation into the support member  1   10 . Each groove  118  is designed to fit closely around the end of an insertion tool such that the insertion tool (not shown) may securely grip the inferior end plate  100  during insertion or removal of the end plate.  
         [0051]     Referring to  FIG. 8 , a lateral end view of the inferior end plate  100  is shown. It is slightly wedge shaped when viewed from either lateral end. That is, the height of the inferior end plate  100  at the posterior end  122  is shorter than its height at the anterior end  120 . This is because this embodiment of the invention is designed for the lumbar portion of the spine, which is curved such that the intervertebral space is wider at the anterior end than at the posterior end. However, it is appreciated that the inferior end plate  100  can be configured at any desired wedge angle or can have substantially parallel top and bottom surfaces.  
         [0052]      FIG. 9  illustrates the bearing engaging side  104  of the inferior end plate  100 , with one clip  500  snapped on the left side. The end plate  100  has symmetrically placed lateral gaps  128  in the right end  124  and the left end  126 . The anterior end  120  has an anterior gap  130  which is identically shaped as the lateral gaps  128  but is located perpendicular to them. The lateral  128  and anterior  130  gaps are shaped to accommodate the insertion of clips  500 , which releasably hold the inferior bearing  300  to the inferior end plate  100 . The caudal or lower portions of the gaps  128 ,  130 , which are cut out of the bone engaging side  102  of the inferior end plate  100 , are shaped to fit the tab  502  of the clip  500 . The cephalad or upper portions of the gaps  128 ,  130  are wider to accommodate passage of arms  504  of the clip  500 . The walls  132  of the gaps  128 ,  130  are formed from the support member  110 , and are perpendicular to the bearing engaging side  104  of the inferior end plate  100 .  
         [0053]     Moving toward the center of the inferior end plate  100 , each lateral  128  and anterior  130  gap is adjacent to a pocket  134 , which is recessed into the bearing engaging face  108 . The sides of the pocket  134  are part of the support member  1   10 . The pocket  134  is T-shaped to fit the arms  504  and the body  506  of the clip  500  when it is snapped to the inferior end plate  100 . The floor of the pocket  134  is sloped such that the pocket arms  136  are deeper than the pocket body  138 . The floor of the pocket body  138  slopes upward until it is flush with a slot  140 .  
         [0054]     Continuing toward the center of the inferior end plate  100 , the slot  140  extends from the pocket  134  to the central grafting port  116 . The slot  140  is also recessed into the bearing engaging face  108  but to a lesser extent than the pocket  134 . Each slot  140  has two side walls which are chamfered, forming two opposing chamfered edges  142  into which prongs  508  of the clip  500  fit closely. The opposing chamfered edges  142  of each slot  140  are slightly angled toward one another as the edges approach the center of the inferior end plate  100 . A chamfered wall  144  is formed by the continuation of the posterior chamfered edges  142  of the two lateral slots  140 ; the two posterior chamfered edges  142  continue past their respective slots  140  and meet, forming the chamfered wall  144 . The gaps  128 ,  130 , the pocket  134  and the slot  140  are shaped to hold the clip  500  in place once it has been inserted. Once the clip  500  has been inserted, the chamfered edges  142  retain the prongs  508  of the clip  500 , while the tab  502  of the clip fits into the lateral  128  or anterior  130  gap. The spring bias  522  of the clip  500  causes the body  506 , arms  504  and tab  502  to be held in the pocket  134  against the bearing engaging side  104  of the inferior end plate  100  once the clip  500  has been inserted and the tab  502  snapped into the gap  128  or  130 .  
         [0055]     In the embodiment depicted, the inferior end plate  100  is intended to be implanted using one of three approaches into the intervertebral area with two clips  500  snapped to it. Implantation may be from an anterior approach, a right lateral approach, or a left lateral approach. If implantation is from the anterior approach, the anterior gap  130  is left empty with no clip secured, and with clips  500  snapped in the right and left lateral gaps  128 . When the inferior bearing  300  is later inserted, an anterior clip  500  will inserted with it, and once inserted, the anterior clip  500  is snapped to the inferior end plate  100 , into the empty anterior gap  130 .  
         [0056]     If implantation is from the right lateral approach, the right lateral gap  128  is left empty with no clip secured, and clips  500  are snapped in the anterior gap  130  and left gap  128  prior to implantation. When the inferior bearing  300  is later inserted, a right lateral clip  500  will be inserted with it, and once inserted, the right lateral clip  500  is snapped to the inferior end plate  100 , into the empty right lateral gap  128 .  
         [0057]     If implantation is from the left lateral approach, the left lateral gap  128  is left empty with no clip secured and clips  500  are snapped in the anterior gap  130  and right gap  128  prior to insertion. When the inferior bearing  300  is later inserted, a left lateral clip  500  will be inserted with it, and once inserted, the left lateral clip  500  is snapped to the inferior end plate  100 , into the empty left lateral gap  128 . It is appreciated that in alternative embodiments of the invention, the number and location of gaps and associated clips may vary.  
         [0058]     Referring to  FIG. 10 , a bearing facing side of the superior end plate  200  is shown, with one clip  500  snapped on the right side. The superior end plate  200  has a bone facing side  206  and a bearing facing side  208 . It has a plurality of grafting channels  214  and grafting ports  216 . An anterior end  220  has an anterior gap  230 , shaped to receive a clip  500 . The end plate  200  has a right lateral end  224  with a gap  228 , and a left lateral end with a gap  228 . Each gap  228 ,  230  leads into a pocket  234 . Each pocket  234  extends into a slot  240 , with chamfered edges  242 . The posterior chamfered edges  242  of the two lateral slots  240  meet, forming a chamfered wall  244 .  
         [0059]     Returning to  FIGS. 2 and 3 , following the surgical placement of the inferior end plate  100  with two clips  500  snapped to it, the superior end plate  200  is similarly placed, but in a superior orientation on the superior vertebral body  48 . The superior end plate  200  will also have two clips  500  snapped to it, in the same positions of the clips  500  snapped to the inferior end plate  100 .  
         [0060]     Referring to  FIG. 11 , a caudal side of the inferior bearing  300  is shown, with one clip  500 . The bearing  300  is quadrilateral in form with rounded edges, and is bilaterally symmetrical. It has a caudal side  302 , a cephalad side  304 , a posterior end  306 , an anterior end  308 , a right end  310  and a left end  312 . The caudal side  302  has an end plate engaging surface  314 . Adjacent to the anterior end  308  is an anterior pocket  316 , which is recessed into the end plate engaging surface  314 . Similarly, adjacent to the right end  310  is a right pocket  318  and adjacent to the left end  312  is a left pocket  320 . Each pocket  316 ,  318 ,  320  is recessed into the end plate engaging surface  314 , and is shaped to fit around the protrusion  520  of the clip  500 .  
         [0061]     Referring to  FIG. 12 , a side profile view of the inferior bearing  300  is shown. A square detent  322  is located on the end plate engaging surface  314 , laterally centered but slightly displaced toward posterior end  306 . The detent  322  is elevated from the end plate engaging surface  314  and has chamfered edges  324 . The cephalad side  304  has an inferior articulation surface  330  from which arises a round dome  332 .  
         [0062]      FIG. 13  illustrates the cephalad side  304  of the inferior bearing  300 . The dome  332  is encircled by a trough  334 , which is a recessed ring surrounding the dome  332 . The dome  332  and its encircling trough  334  are centered laterally on the cephalad side  304  of the inferior bearing  300 , but are slightly displaced toward the posterior end  306 . Recessed into the interior articulation surface  330  on the anterior end  308  is a notch  336 . The notch is recessed partway into the interior articulation surface  330  and extends perpendicularly from the edge of the anterior end  308  to the trough  334 . Similar notches  336  are present on the right  310  and left  312  ends of the inferior bearing  300 . The notches  336  are designed to fit closely around the end of an insertion tool such that the insertion tool may securely grip the inferior bearing  300  during insertion or removal of the bearing. It is appreciated that the design and placement of notches may vary in other embodiments of the invention.  
         [0063]     Referring to  FIG. 14 , the superior bearing  400  with one clip  500  in the right lateral position is shown. The superior bearing  400  is quadrilateral in form with rounded edges. It has a cephalad side  402 , a caudal side  404 , a posterior end  406 , an anterior end  408 , a right end  410  and a left end  412 . The cephalad side  402  has an end plate engaging surface  414 . Adjacent to the anterior end is an anterior pocket  416 , which is recessed into the end plate engaging surface  414 . Similarly, adjacent to the right end  410  is a right pocket  418  and adjacent to the left end  412  is a left pocket  420 . Each pocket  416 ,  418 ,  420  recessed into the end plate engaging surface  414 , and is configured to fit around the protrusion  520  on the clip  500 .  
         [0064]     Referring to  FIG. 15 a  side profile view of the superior bearing  400  is shown. A square detent  422  is located on the end plate engaging surface  414 , laterally centered but slightly displaced toward posterior end  406 . The detent  422  is elevated from the end plate engaging surface  414  and has chamfered edges  424 . The caudal side  404  has a superior articulation surface  430  into which is depressed a circular cup  432 . The cup  432  is encircled by a ridge  434 , which appears as a raised ring or donut surrounding the cup  432 .  
         [0065]     Referring to  FIG. 16 , a caudal side view of the superior bearing  400  is shown. The cup  432  and its encircling ridge  434  are centered laterally on the caudal side  404  of the superior bearing  400 , but are slightly displaced toward the posterior end  406 . Recessed into the superior articulation surface  430  on the anterior end  408  is a notch  436 . The notch is recessed partway into the superior articulation surface  430  and extends perpendicularly from the edge of the anterior end  408  to the ridge  434 . Similar notches  436  are present on the right  410  and left  412  ends of the superior bearing  400 . The notches  436  are designed to fit closely around the end of an insertion tool such that the insertion tool may securely grip the superior bearing  400  during insertion or removal of the bearing. It is appreciated that the design and placement of notches may vary in other embodiments of the invention.  
         [0066]     The inferior bearing  300  and the superior bearing  400  are inserted together into the space between the end plates  100 ,  200 . Inserting the bearings  300 ,  400  together requires less distraction of the vertebral bodies  28 ,  48  than if they were inserted separately. If inserted separately, additional distraction would be required to allow the dome  332  on the inferior bearing  300  to pass by the ridge  434  on the superior bearing  400 . When inserted together, the dome  332  is fit into the cup  432 , allowing the two bearings  300 ,  400  to fit into the smallest space possible. The bearings  300 ,  400  can be inserted from an anterior approach, a right lateral approach, or a left lateral approach; they will be inserted using whichever approach was chosen for the placement of the end plates  100 ,  200  during the same surgical procedure. However, it is appreciated that should there be any subsequent procedure for replacement or adjustment of the bearings  300 ,  400  such procedure may be carried out from any one of the three approaches.  
         [0067]     Returning to  FIG. 11 , one clip  500  is held in place next to the inferior bearing  300  as it is being inserted. The clip may be placed in an anterior position, a right lateral position, or a left lateral position, depending upon which surgical approach is implemented. If an anterior approach is implemented, the chamfered interior edge  514  of the clip is placed under the anterior chamfered edge  324  of the detent  322  of the inferior bearing  300 . Thus, the top side  510  of the clip  500  is held against the caudal side  302  of the inferior bearing  300 . The protrusion  520  of the clip  500  fits into the anterior pocket  316  of the inferior bearing  300 , when the inferior bearing  300  and the clip  500  are held together.  
         [0068]     If a right lateral approach is implemented, the chamfered interior edge  514  of the clip is placed under the right chamfered edge  324  of the detent  322  of the inferior bearing  300 . The protrusion  520  of the clip  500  fits into right pocket  318  of the inferior bearing  300 , when the inferior bearing  300  and the clip  500  are held together. If a left lateral approach is implemented, the chamfered interior edge  514  of the clip is placed under the left chamfered edge  324  of the detent  322  of the inferior bearing  300 . The protrusion  520  of the clip  500  fits into left pocket  320  of the inferior bearing  300 , when the inferior bearing  300  and the clip  500  are held together.  
         [0069]     Returning to  FIG. 14 , one clip  500  is held in place next to the superior bearing  400  as it is being inserted. The clip may be placed in an anterior position, a right lateral position, or a left lateral position, depending upon which surgical approach is implemented. If an anterior approach is implemented, the chamfered interior edge  514  of the clip is placed under the anterior chamfered edge  424  of the detent  422  of the superior bearing  400 . Thus, the top side  510  of the clip  500  is held against the cephalad side  402  of the superior bearing  400 . The protrusion  520  of the clip  500  fits into the anterior pocket  416  of the superior bearing  400 , when the superior bearing  400  and the clip  500  are held together.  
         [0070]     If a right lateral approach is implemented, the chamfered interior edge  514  of the clip is placed under the right chamfered edge  424  of the detent  422  of the superior bearing  400 . The protrusion  520  of the clip  500  fits into right pocket  418  of the superior bearing  400 , when the superior bearing  400  and the clip  500  are held together. If a left lateral approach is implemented, the chamfered interior edge  514  of the clip is placed under the left chamfered edge  424  of the detent  422  of the superior bearing  400 . The protrusion  520  of the clip  500  fits into left pocket  420  of the superior bearing  400 , when the superior bearing  400  and the clip  500  are held together.  
         [0071]     Referring to  FIG. 17 , the bearings  300 ,  400  are shown immediately prior to being inserted in between the end plates  100 ,  200  from a left lateral direction. As the inferior bearing  300  and the superior bearing  400  and their associated clips  500  are inserted, they are held together and slid along the medial-lateral axis  20  into the space between the end plates  100 ,  200 . The bearings  300 ,  400  and clips  500  are slid between the end plates  100 ,  200  and into the open left lateral gaps  128 ,  228  until the leading chamfered edge  324  of the detent  322  engages under the interior edge  514  of the opposite lateral clip  500  already in place. Simultaneously, on the superior bearing  400 , the leading chamfered edge  424  of the detent  422  engages under the interior edge  514  of the opposite lateral clip  500  already in place. Thus engaged, the bearings cannot slide in any further. As the clips  500  are slid in with the bearings  300 ,  400  the chamfered edges  518  on the prongs  508  also slide under the chamfered edges  142 ,  242  of the pockets.  
         [0072]     If an anterior approach is used, the bearings  300 ,  400  and clips  500  are slid in a posterior direction parallel to the end plates  100 ,  200  so that the detents  322 ,  422  slide into the empty anterior gaps  130 ,  230 . They are slid until the leading chamfered edge  324  of the detent  322  engages under the chamfered wall  144  at the end of the slot  140 . Simultaneously, on the superior bearing  400 , the leading chamfered edge  424  of the detent  422  engages under the chamfered wall  244  at the end of the slot  240 .  
         [0073]     After the bearings  300 ,  400  and clips  500  are fully slid in with all chamfered edges engaged, the tabs  502  are pinched downward or caudally following the spring bias  522  so that the tabs  502  snap into the anterior gaps  130  or lateral gap  128  of the inferior end plate  100 , and the arms  504  are seated in the pockets  134 . Similarly, the tabs  502  on the clips  500  adjacent to the superior end plate  200  are pinched upward in the cephalic direction, so that the tabs  502  snap into the anterior gaps  230  or lateral gaps  228  of the superior end plate  200 , and the arms  504  are seated in the pockets  234 . Once the tabs  502  are thus pinched, the spring bias  522  holds the clips  500  against end plates  100 ,  200 , and bearings  300 ,  400  are prevented from slipping in a lateral, anterior, posterior, caudal or cephalad direction.  
         [0074]     Should revision of the initial implantation be necessary, it can be accomplished by any of the three approaches: anterior, right lateral, or left lateral. For example, if the bearings  300 ,  400  need to be replaced by those of a different size or configuration (or by an elastic insert or the fusion block that will be described below), the surgery may be approached from a different direction than the initial implantation, thus avoiding disturbance of scar tissue. To remove the bearings  300 ,  400 , the tabs  502  of the two clips  500  on one approach (anterior, right or left) are pinched together. For example, if the anterior approach is used, the tab  502  of the anterior clip  500  on the inferior end plate  100  is pinched toward the tab  502  of the anterior clip  500  on the superior end plate  200 . The pinching action will free the tab arms  504  from the pockets  134 ,  234  of the inferior  100  and superior  200  end plates. Simultaneously, the protrusions  520  on the clips  500  will fit into the pockets  316 ,  416  in the inferior  300  and superior  400  bearings. The clips are then pulled perpendicularly away from the end plates  100 ,  200 , with the bearings  300 ,  400  held between them. The replacement bearings  300 ,  400  can then be inserted with clips  500  in place, as described in the original insertion procedure.  
         [0075]      FIG. 18  illustrates a fusion block  600 , shown assembled with the inferior end plate  100  and the superior end plate  200 , within a portion of the spine. A plurality of clips  500  are used to position the fusion block  600  and attach it to the end plates  100 ,  200 . The fusion block  600  may be inserted in place of the inferior bearing  300  and the superior bearing  400 , if fusion of the involved vertebrae is desired. Insertion of the fusion block  600  may occur during the initial procedure, following the placement of the inferior and superior end plates  100 ,  200 . Alternatively, it can be used to replace the inferior and superior bearings  300 ,  400  after they have been implanted in the patient for some period of time. As with the end plates  100 ,  200  and the bearings  300 ,  400 , the fusion block  600  may be inserted from the anterior, right lateral, or left lateral approach.  
         [0076]     As viewed in  FIG. 19 , fusion block  600  is of a quadrilateral shape with rounded corners. It has a cephalad side  602  and a caudal side  604 , which are substantially parallel. It has an anterior end  606 , a posterior end  608 , a right end  610  and a left end  612 . The cephalad side has a first end plate engaging surface  640 , and the caudal side has a second end plate engaging surface  642 . In this embodiment of the invention the cephalad and caudal sides  602 ,  604  are identical, and the fusion block  600  is bilaterally symmetrical, with identical right and left ends  610 ,  612 . However, it is appreciated that in alternative embodiments, sides and ends may vary from one another and symmetry may or may not occur.  
         [0077]     A plurality of grafting ports  614 , passing through a support member  620 , is present on the fusion block  600 . In this embodiment of the invention, these grafting ports  614  are configured to line up with the grafting ports  116 ,  216  on the inferior and superior end plates  100 ,  200 . These adjacent openings extend through the entire implant to allow growth of bone material through the fusion block  600  and the end plates  100 ,  200 , thus fusing the fusion block  600 , end plates  100 ,  200 , and vertebral bodies  28 ,  48  together. Before, during or after positioning of the end plates  100 ,  200  between the vertebral bodies, the fusion block  600  is at least partially packed with an osteogenic substance. In this application, “osteogenic substance” is broadly intended to include natural bone, such as autogenous bone graft or bone allograft, synthetic bone, growth factors and cytokines (including bone morphogenic proteins), and/or combinations thereof.  
         [0078]     Referring to  FIG. 20 , an anterior side view of the fusion block  600  is shown. A plurality of grafting holes  616  is present on each side of the block, passing through the support member  620 . As with the grafting ports  614  previously described, the grafting holes  616  allow growth of bone material throughout the fusion block  600 . Grooves  618  are recessed into the support member  620  on each outer corner of the block. Each groove  618  is designed to fit closely around the end of an insertion tool such that the insertion tool (not shown) may securely grip the fusion block  600  during insertion of the block.  
         [0079]     A square first detent  622  is located on the cephalad side  602  while a similar, square second detent  624  is on the caudal side  604 . The detents  622 ,  624  are laterally centered but slightly displaced toward the posterior end  608 . The detents  622 ,  624  project outward from the end plate engaging surfaces  640 ,  642  of each side  602 ,  604 . The first detent  622  has chamfered edges  626 , and the second detent  624  has chamfered edges  628 .  
         [0080]     As depicted in  FIG. 21 , adjacent to the anterior end  608  is a pocket  630 , which is recessed into the first end plate engaging surface  640 . Similar pockets  630  are adjacent to the right end  610  and the left end  612 . Each pocket  630  is recessed into the first end plate engaging surface  640 , and is configured to fit around the protrusion  520  on the clip  500 . Three pockets  630  are similarly located on the second end plate engaging surface  642 .  
         [0081]     Returning to  FIG. 18 , before insertion of the fusion block  600 , the end plates  100 ,  200  are placed against the vertebral bodies  28 ,  48  as previously described, each with two clips  500  snapped in place. The clips  500  are used to position the fusion block  600  during the insertion process, and then hold the fusion block in place once inserted. Prior to insertion of the block, the interior edge  514  of a clip  500  is slid under a chamfered edge  626  of the first detent  622  on the cephalad side  602 . Once the clip is in place, its protrusion  520  fits into the facing pocket  630  on the first end plate engaging surface  640 . Similarly, the interior edge  514  of a clip  500  is slid under the matching chamfered edge  628  of the second detent  624  on the caudal side  604 , with that clip&#39;s protrusion  520  in the facing pocket  630  of the second end plate engaging surface  642 . As a result, the fusion block  600  is sandwiched between two clips  500 , the clips being in matching anterior, right lateral or left lateral positions.  
         [0082]     Referring to  FIG. 22 , the fusion block  600  is shown as is it being inserted from the anterior approach into the gap between the end plates  100 ,  200 . The fusion block  600  and its associated clips  500  are held together and slid into the empty anterior gaps  130 ,  230  on the end plates  100 ,  200 . The fusion block  600  and clips  500  are held parallel to the end plates and slid posteriorly into place such that the lateral chamfered edges  626  of the first detent  622  engage under the chamfered edges  242  of the open pocket  234  on the superior end plate  200 . Simultaneously, the lateral chamfered edges  628  of the second detent  624  engage under the chamfered edges  142  of the open pocket  134  on the inferior end plate  100 .  
         [0083]     If an anterior approach is used, the fusion block  600  and clips  500  are slid until the leading chamfered edges  626 ,  628  of the detents  622 ,  624  engage under the chamfered walls  244 ,  144  at the end of the slots  240 ,  140 . If a lateral approach is used, the fusion block  600  and clips  500  are slid until the leading chamfered edges  626 ,  628  of the detents  622 ,  624  engage under the interior edges  514  of the opposite lateral clips  500  already in place. Thus engaged, the fusion block cannot slide in any further. As the clips  500  are slid in with the fusion block  600 , the chamfered edges  518  on the prongs  508  also slide under the chamfered edges  142 ,  242  of the pockets  134 ,  234 .  
         [0084]     After the fusion block  600  and clips  500  are fully slid in with all chamfered edges engaged, the tabs  502  are pinched individually toward the end plates following the spring bias  522  so that the tabs  502  snap into the anterior gaps  130 ,  230  or lateral gaps  128 ,  228  of the end plates  100 ,  200  and the arms  504  are seated in the pockets  134 ,  234 . Once the tabs  502  are thus snapped into place, the spring bias  522  holds the clips  500  against the end plates  100 ,  200  and the fusion block  600  is prevented from slipping in a lateral, anterior, posterior, caudal or cephalad direction.  
         [0085]     As with the bearings, if revision of the initial implantation of the fusion block is necessary, it can be accomplished by any of the three approaches: anterior, right lateral, or left lateral. For example, if the fusion block  600  needs to be replaced by one of a different size or configuration, the surgery may be approached from a different direction than the initial implantation, thus avoiding disturbance of scar tissue. To remove the fusion block  600 , the tabs  502  of the two clips  500  on one approach (anterior, right or left) are pinched together. For example, if the anterior approach is used, the tab  502  of the anterior clip  500  on the cephalad side  602  is pinched toward the tab  502  of the anterior clip  500  on the caudal side  604 . The pinching action will free the tab arms  504  from the pockets  134 ,  234  of the inferior  100  and superior  200  end plates. Simultaneously, the protrusions  520  on the clips  500  will fit into the pockets  630  in the first and second end plate engaging surfaces  640 ,  642 . The clips are then pulled perpendicularly away from the end plates  100 ,  200 , with the fusion block  600  held between them. A replacement fusion block  600 , elastic insert, or bearings  300 ,  400  can then be inserted with clips  500  snapped in place, as described in the original insertion procedure.  
         [0086]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. It is appreciated that various features of the above-described examples can be mixed and matched to form a variety of other alternatives. As such, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.