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 . referring to fig1 , a perspective view illustrates a portion of a spine 10 . fig1 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 . as shown , the portion of the spine 10 illustrated in fig1 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 . 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 . 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 . referring to fig2 , 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 . in the embodiment depicted in fig2 , 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 . fig3 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 . 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 fig2 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 . fig4 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 . referring to fig5 , 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 . fig6 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 . referring to fig7 , 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 . 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 . 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 . 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 . referring to fig8 , 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 . fig9 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 . 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 . 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 . 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 . 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 . 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 . referring to fig1 , 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 . returning to fig2 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 . referring to fig1 , 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 . referring to fig1 , 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 . fig1 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 . referring to fig1 , 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 . referring to fig1 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 . referring to fig1 , 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 . 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 . returning to fig1 , 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 . 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 . returning to fig1 , 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 . 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 . referring to fig1 , 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 . 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 . 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 . 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 . fig1 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 . as viewed in fig1 , 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 . 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 . referring to fig2 , 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 . 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 . as depicted in fig2 , 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 . returning to fig1 , 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 . referring to fig2 , 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 . 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 . 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 . 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 . 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 .
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 .