Abstract:
A revisable intervertebral implant may include two end plates designed to detachably receive a variety of intermediate components including articulating bearing inserts, elastic inserts, and fusion blocks. Each intermediate component may be secured to a snap insert that snaps into engagement with the corresponding end plate in response to pressure urging the intermediate component toward the end plate along a cephalad-caudal direction. The end plates may first be secured to the corresponding vertebral bodies, and then the intermediate component(s) may be snapped into locking engagement with the implanted end plates to complete in-situ assembly of the intervertebral implant. The implant may easily be revised by snapping the intermediate component(s) out of engagement with the end plates, removing the intermediate component(s), inserting the new intermediate component(s) into the space between the end plates, and snapping the new intermediate component(s) into engagement with the end plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the following: 
   U.S. Provisional Application No. 60,720,513, filed Sep. 26, 2005, which is entitled MODULAR ARTICULATING AND FUSION SPINAL DISC IMPLANT SYSTEM; 
   U.S. Provisional Application No. 60/720,514, filed Sep. 26, 2005, which is entitled UNIVERSAL SPINAL DISC IMPLANT SYSTEM FOR PROVIDING INTERVERTEBRAL ARTICULATION AND FUSION; and 
   U.S. Provisional Application No. 60/741,513, filed Nov. 30, 2005, which is entitled SYSTEM AND METHOD FOR INTERVERTEBRAL IMPLANT DELIVERY AND REMOVAL. 
   All of the foregoing are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates generally to spinal orthopedics, and more precisely, to intervertebral implants. 
   2. The Relevant Technology 
   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. 
   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. 
   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 
     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. 
       FIG. 1  is a perspective view of the total disc implant in a portion of the spine, according to one embodiment of the invention. 
       FIG. 2  is a perspective view of the total disc implant shown in  FIG. 1  in a disassembled state. 
       FIG. 3  is a perspective view of the bone-facing side of the inferior end plate shown in  FIG. 2 . 
       FIG. 4  is a perspective lateral side view of the inferior end plate shown in  FIG. 2 . 
       FIG. 5  is a cephalad view of the bearing-facing side of the inferior end plate shown in  FIG. 2 . 
       FIG. 6  is a perspective view of the superior end plate shown in  FIG. 2 . 
       FIG. 7  is a perspective view of the caudal side of the inferior bearing shown in  FIG. 2 . 
       FIG. 8  is a perspective view of the cephalad side of the inferior bearing shown in  FIG. 2 . 
       FIG. 9  is a perspective view of the cephalad side of the superior bearing shown in  FIG. 2 . 
       FIG. 10  is a perspective view of the caudal side of the superior bearing shown in  FIG. 2 . 
       FIG. 11  is a perspective view of the bearing-facing side of the snap shown in  FIG. 2 . 
       FIG. 12  is a perspective view of the end plate-facing side of the snap shown in  FIG. 2 . 
       FIG. 13  is a lateral view of the snap shown in  FIG. 2 . 
       FIG. 14  is a perspective view of an alternative embodiment of a total disc implant, in a disassembled state. 
       FIG. 15  is a perspective view of an interbody disc fusion implant, in a disassembled state. 
       FIG. 16  is a perspective view of the fusion cage shown in  FIG. 15 . 
       FIG. 17  is a perspective view of another alternative embodiment of a total disc implant, in a disassembled state. 
       FIG. 18  is a perspective view of a bone-facing side of the inferior endplate shown in  FIG. 17 . 
       FIG. 19  is a perspective view of a bearing-facing side of the inferior endplate shown in  FIG. 17 . 
       FIG. 20  is a perspective view of a caudal side of the inferior bearing shown in  FIG. 17 . 
       FIG. 21  is a perspective view of a cephalad side of the inferior bearing shown in  FIG. 17 . 
       FIG. 22  is a perspective view of a cephalad side of the superior bearing shown in  FIG. 17 . 
       FIG. 23  is a perspective view of a caudal side of the superior bearing shown in  FIG. 17 . 
       FIG. 24  is a perspective view of a bone-facing side of the snap fastener shown in  FIG. 17 . 
       FIG. 25  is an enlarged perspective side view of the snap fastener shown in  FIG. 17 . 
       FIG. 26  is a perspective view of a bearing-facing side of the snap fastener shown in  FIG. 17 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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  FIG. 1 , a perspective view illustrates one embodiment of an implant  50 , which may be referred to as a total disc implant, implanted in a portion of the spine. In this embodiment of the invention, the total disc implant includes two end plates  100 ,  200 , two bearings  300 ,  400 , and two snap fasteners  500  (not visible in  FIG. 1 ) which releasably hold the bearings to the end plates. The implant  50  is designed for placement between spinal vertebrae to replace degenerated intervertebral disk material. More specifically, the implant  50  of  FIG. 1  is designed to be inserted between the vertebral bodies  22 ,  42  of the first and second vertebrae  20 ,  40 , respectively, after removal of the intervertebral disc (not shown). The vertebral bodies  22 ,  42  are rasped and flat surfaces on them are prepared to fit the end plates  100 ,  200 . 
   The procedure to implant the total disc implant may be conducted from any of three approaches: anterior, right lateral, or left lateral. In addition, should there be any subsequent procedure for adjustment of the implant  50  or replacement of any component thereof, such procedure may be carried out from any one of the three approaches. 
     FIG. 2  illustrates the implant  50  in a disassembled state, so that all components are visible. During the implantation procedure, the end plates  100 ,  200  are pressed into place onto the vertebral bodies, with the inferior end plate  100  in a caudal position on vertebral body  22 , and superior end plate  200  in a cephalic position on vertebral body  42 . The end plates  100 ,  200  may be implanted in either order (inferior first or superior first). Once implanted, the two end plates  100 ,  200  appear as mirror images of one another with their bearing facing sides facing one another. Next, the inferior  300  and superior bearings  400  are attached to the end plates, using the snap fasteners  500  as releasable connectors. A set force delivered by the implantation instrumentation (not shown) presses each snap fastener  500  into place. The inferior bearing  300  is attached to the inferior end plate  100  with one snap fastener  500  between them, and the superior bearing  400  is attached to the superior end plate  200  with another snap fastener  500  between them. Like the end plates, the bearings  300 ,  400  may also be attached in either order. 
     FIG. 3  illustrates a bone-facing side of one end plate. In the illustration, the end plate depicted is the inferior end plate  100 , and so the bone-facing side  102  is in the caudal direction. In this embodiment of the invention the superior end plate  200  is identical to the inferior end plate  100  in every way except in orientation once implanted in the body. Thus, when the superior end plate  200  is implanted, its bone-facing side will be in the cephalic direction. With this exception due to orientation noted,  FIGS. 3 and 4  and the description of the end plate below also apply to the superior end plate  200 . However, it is appreciated that in alternative embodiments of the invention, the end plates may or may not be identical in size, shape, or configuration. 
   As viewed in  FIGS. 3 and 4 , the inferior end plate  100  is quadrilateral in form, with rounded corners, and is bilaterally symmetrical. It has a bone-facing side  102 , a bearing-facing side  104 , an anterior end  106 , a posterior end  108 , a right end  110  and a left end  112 . The end plate is slightly wedge-shaped, with the height of the anterior end  106  slightly greater than the posterior end  108 . This is to match the natural lordotic angle of the lumbar vertebrae as closely as possible. In alternative embodiments, it is appreciated that the end plates  100 ,  200  need not have a quadrilateral configuration but can be square, circular, or have any other polygonal or irregular configuration. Furthermore, it is appreciated that the end plates  100 ,  200  can be configured at any desired wedge angle or can have substantially parallel top and bottom surfaces. 
   The inferior end plate  100  has a bone engaging face  114  and a bearing engaging face  116  which are connected by a support member  118 . Projecting from the bone engaging face  114  is a plurality of anchoring members in the form of bone engaging spikes  120 . Each bone engaging spike  120  is columnar in form and projects perpendicularly in the caudal direction from the bone engaging face  114 . The caudal end of each bone engaging spike  120  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  22  during the implantation process; the point will more easily penetrate the vertebral body  22  than would a blunt end. 
   A hollow grafting channel  122  runs through the center of each bone engaging spike  120 . Each grafting channel  122  originates on the bearing engaging face  114 , runs through the support member  118 , and ends at the pointed termination of the bone engaging spike  120 . This hollowed point configuration may be compared to the point of a hypodermic needle, and further facilitates the penetration of the vertebral body  22  by the bone engaging spikes  120 . The grafting channels  122  also allow for the growth of bony columns from the vertebral body  22  through the channels, thereby fusing the inferior end plate  100  to the vertebral body  22 . 
     FIG. 5  illustrates the bearing-facing side  104  of the inferior end plate  100 . Near the corner formed by the posterior end  108  and the left end  112  is a peg port  124 . The peg port  124  is a circular opening originating on the bearing-engaging face  116  and recessed into the support member  118 . Partway through the support member  118 , the width of the peg port  124  constricts and the port continues as a grafting channel  122 , exiting through a bone engaging spike  120  on the bone-facing side  102 . A similar peg port  124  is located near the right posterior corner. 
   Centered on the anterior end  106  of the bearing-facing side  104  is a pocket  126 . Similar pockets are centered on the right end  110  and the left end  112 . Each pocket  126  is a rectangular segment cut from the edge of the bearing-engaging face  116  and extending caudally into the support member  118 . Once the cutaway area is below the bearing-engaging face  116 , the slot widens on either lateral side, and deepens perpendicularly into the support member  118 , toward the center of the end plate. The pockets  126  are places where implantation instruments (not shown) may grip or otherwise connect with the end plates during the implantation procedure. The number, size, configuration and placement of pockets may vary in other embodiments of the invention. 
   As seen in  FIGS. 3 ,  4  and  5 , a snap port  130  is located on the end plate  100 , laterally centered but slightly displaced toward posterior end  108 . The snap port  130  is an opening from the bearing-facing side  104  to the bone-facing side  102 , circumscribed by a tapered wall  132 . The tapered wall  132  angles outward toward the bone-facing side  102 , such that the cross-sectional area of the snap port  130  on the bearing-facing side  104  is smaller than the cross-sectional area of the same snap port  130  on the bone-facing side  102 . 
     FIG. 6  is a perspective view of the superior end plate  200 . Note that as discussed earlier, the superior end plate  200  is identical to the inferior end plate  100  in every way except in orientation once implanted. However, as illustrated, this does mean that the right end  210  and left end  212  of the superior end plate  200  are reversed from the right end  110  and left end  112  of the inferior end plate  100 . 
   Once the end plates  100 ,  200  are implanted, the bearings  300 ,  400  are inserted and attached to the end plates.  FIG. 7  illustrates the caudal side of the inferior bearing  300 . The inferior bearing  300  is of the same approximate quadrilateral shape and dimension as the inferior end plate  100 . It has a caudal side  302 , a cephalad side  304 , an anterior end  306 , a posterior end  308 , a right end  310  and a left end  312 . On the caudal side  302  is an end plate-engaging face  314 . Centered along the anterior end  306  is an instrument port  316 , which is an opening originating on the end plate engaging face  314 , passing through a support member  318 , and terminating on an inferior articulation surface  330 . Additional instrument ports  316  are centered on the right end  310  and the left end  312 . Protruding from the end plate-engaging face  314  near the posterior right and left corners are two pegs  320 . The pegs  320  fit into the peg ports  124  shown in  FIG. 5 , when the inferior bearing  300  is attached to the inferior end plate  100 . The fitting of the pegs  320  into the peg ports  124  assist in reducing shear stress on the implant. 
   Occupying the central area of the inferior bearing  300  is a cap  322 , surrounded by a trough  324 . The cap is a quadrilateral protrusion from the end plate engaging face  314 , and the surface of the cap  322 , while parallel to the end plate engaging face  314 , is slightly elevated from it. The trough  324  which surrounds the cap is recessed from the end plate engaging face  314  into the support member  318 . The outer boundary of the trough is a tapered wall  326 . The tapered wall  326  angles inward from the bottom of the trough  324  to the top, such that the cross sectional area of the trough  324  at its deepest point is larger than its cross sectional area where it meets the surface of the end plate engaging face  314 . 
     FIG. 8  displays the cephalad side  304  of the inferior bearing  300 . The cephalad side has an inferior articulation surface  330  from which arises a rounded dome  332 . The dome  332  is centered laterally on the cephalad side  304  of the inferior bearing  300 , but is slightly displaced toward the posterior end  308 . 
     FIG. 9  illustrates the cephalad side  402  of the superior bearing  400 . It has a cephalad side  402 , a caudal side  404 , an anterior end  406 , a posterior end  408 , a right end  410  and a left end  412 . On the cephalad side  404  is an end plate-engaging face  414 . Centered along the anterior end  406  is an instrument port  416 , which is an opening originating on the end plate engaging face  414 , passing through a support member  418 , and terminating on a superior articulation surface  430 . Additional instrument ports  416  are centered on the right end  410  and the left end  412 . Protruding from the end plate-engaging face  414  near the posterior right and left corners are two pegs  420 . The pegs  420  fit into the peg ports  224  shown in  FIG. 6 , when the inferior bearing  400  is attached to the superior end plate  200 . The fitting of the pegs  420  into the peg ports  224  assist in reducing shear stress on the implant. 
   Occupying the central area of the superior bearing  400  is a cap  422 , surrounded by a trough  424 . The cap  422  is a flat-topped protrusion from the end plate engaging face  414 , and the surface of the cap  422 , while parallel to the end plate engaging face  414 , is slightly elevated from it. The trough  424  which surrounds the cap is recessed from the end plate engaging face  414  into the support member  418 . The outer boundary of the trough is a tapered wall  426 . The tapered wall  426  angles inward from the bottom of the trough  424  to the top, such that the cross sectional area of the trough  424  at its deepest point is larger than its cross sectional area where it meets the surface of the end plate engaging face  414 . 
   The caudal side  404  of the superior bearing  400  is illustrated in  FIG. 10 . A rounded cup  432  is recessed into the support member  418  of the caudal side  404 . The cup  432  is centered laterally on the caudal side  404 , but is slightly displaced toward the posterior end  408 . A ridge  434  encircles the cup  432 . The ridge is raised substantially from the support member  418 . A smooth superior articulation surface  430  overlays the ridge  434  and the cup  432  such that where they meet, there is no discernable transition between the two features. 
   As seen in  FIG. 2 , the snap  500  serves as the connector between the inferior end plate  100  and the inferior bearing  300 , and between the superior end plate  200  and the superior bearing  400 .  FIGS. 11 ,  12  and  13  illustrate the snap  500  alone. In this embodiment of the invention, the snap  500  is quadrilateral and generally dish-like in form, with a bone-facing side  502  which is a substantially flat plane, and a bearing facing side  504  which is a flat plane circumscribed by a raised rim  506 . It is appreciated that in alternative embodiments of the invention, the snap feature may be quadrilateral, circular or any other shape or configuration. The outer edge of the rim  506  is formed by a dual-tapered wall  508 . As seen best in  FIG. 13 , the dual-tapered wall  508  is equally wide at the bone-facing side  502  and at the bearing-facing side  504 , but constricts at the midpoint between the two sides  502 ,  504 . 
     FIG. 2  best illustrates how all the components of the implant  50  fit together. During or after manufacture, but before the implantation procedure, one snap  500  is fitted over the cap  322  of the inferior bearing  300 , and a second snap  500  is fitted over the cap  422  of the superior bearing  400 . As the rim  506  of the snap  500  is pressed into the trough  324  of the inferior bearing  300 , the dual-tapered wall  508  compresses to pass into the trough  324 , then expands out into place such that the dual-tapered wall  508  fits against the tapered wall  326  of the trough. Because the widest part of the dual-tapered wall  508  is wider than the opening of the trough  324 , the snap  500  is locked into place, and can only be removed from the inferior bearing  300  with significant force. The second snap  500  is attached to the superior bearing  400  in the same manner. 
   The inferior end plate  100  is implanted in the vertebral body  22 , and the superior end plate  200  is implanted in the vertebral body  42 . The inferior bearing  300  is pressed into place in the inferior end plate  100 . The bone-facing side  502  of the snap  500 , now protruding from the caudal side  302  of the inferior bearing  300 , is pressed into the snap port  130  of the inferior end plate  100 . As the bone-facing side  502  of the snap  500  is pressed into the snap port  130 , the dual-tapered wall  526  compresses to pass into the snap port  130 , then expands out into place such that the dual-tapered wall  526  fits against the tapered wall  132  of the inferior end plate  132 . Because the widest part of the dual-tapered wall  526  is wider than the opening of the snap port  130 , the snap  500  is locked into place, and can only be removed from the inferior end plate  100  with significant force. 
   The superior bearing  400  and its snap  500  are attached to the superior end plate  200 , in the same manner as described above for the inferior end plate  100  and bearing  300 . Then the inferior articulation surface  330  is allowed to contact the superior articulation surface  430 . Although in this description, the inferior bearing and its snap were attached first, followed by the superior bearing and its snap, it is appreciated that the bearings may be attached in either order. It is also appreciated that should there be any subsequent procedure for replacement or adjustment of any of the end plates, bearings or snaps, such procedure may be carried out from any one of the three approaches; anterior, left lateral or right lateral. 
   Other embodiments of the invention can provide the same function while employing alternate snap connections.  FIG. 14  depicts a disassembled total disc implant  60 , which employs an alternate snap feature to lock the bearings to the end plates. In this embodiment, the inferior bearing  300  is connected to the inferior end plate  100  via a ring-shaped snap  500 . Similarly, the superior bearing  400  is connected to the superior end plate  200  by the same ring-shaped snap  500 . The mechanism by which the snap locks the bearings to the end plates is equivalent to the snap feature described in the first embodiment; in both embodiments the snap feature compresses to pass through a constrictive feature, and then expands out to lock the components in place. 
   If fusion of the vertebrae is required, an embodiment of the invention including a fusion block may be implemented.  FIG. 15  depicts an interbody disc fusion implant  70 , in a disassembled state. In this embodiment, the implant consists of an inferior end plate  100 , a superior end plate  200 , two ring-shaped snaps  500  and a fusion cage  600 . The interbody disc fusion implant  70  may be implanted from an anterior approach, a right lateral approach, or a left lateral approach. It may be implanted as part of the initial implantation procedure, or it may replace inferior and superior bearings, upon their removal. 
     FIG. 16  illustrates the fusion cage  600 . In this embodiment of the invention, the fusion cage  600  is quadrilateral and box-like in shape. It has a caudal side  602 , a cephalad side  604 , an anterior end  606 , a posterior end  608 , a right end  610  and a left end  612 . It is symmetrical such that the right and left ends  610 ,  612  are mirror images of one another and the caudal and cephalad sides  602 ,  604  are also mirror images. A plurality of notches  630 , designed for gripping by implantation instruments (not shown) are at the edges of the caudal and cephalad sides  602 ,  604 . 
   A plurality of grafting holes  614  perforates each end of the fusion cage. Before, during or after positioning of the end plates between the vertebral bodies, the fusion cage  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. After implantation, growth of bone material through the grafting holes will assist in the fusion of the fusion cage and end plates to the vertebrae. 
   A larger grafting port  616  is centered on the fusion block, with its openings on the caudal and cephalad sides. Recessed into the surface of the fusion block  600  and circumscribing the grafting port  616 , is a trough  618 . Around each opening of the grafting port, but to the inside of the trough  618 , is a raised rim  620 . The raised rim  620  protrudes from surface of the fusion block  600 . The inner wall  622  of the raised rim  620  is smooth and is a continuous part of the grafting port  616 . The outer wall  624  of the raised rim  620  constricts between the top of the rim and where it joins the trough  618 . This constriction is designed to hold the snap ring  500 , seen in  FIG. 15 . 
   Referring to  FIG. 17 , an alternative embodiment of a total disk implant is shown. The implant  1050  comprises an inferior end plate  1100 , a superior end plate  1200 , an inferior bearing  1300 , a superior bearing  1400 , and two snap fasteners  1500 . As with the implant  50 , the implant  1050  is designed for placement between spinal vertebrae to replace degenerated intervertebral disk material. Methods for placement, assembly and implantation of the implant  1050  are the same as those described for the implant  50 . 
   Referring to  FIG. 18 , an enlarged view of a bone-facing side of the end plate  1100  is shown. The end plates  1100 ,  1200  are identical to one another, differing only in their orientation as they are placed between the vertebral bodies. End plate  1100  will be described in detail, but it is appreciated that the same description applies to the end plate  1200 . The end plate  1100  has a bone-facing side  1102 , and a bearing-facing side  1104 . An irregularly shaped snap port  1130  occupies the center of the end plate  1100 , creating an opening from the bone-facing side  1102  to the bearing-facing side  1104 . A plurality of bone-engaging spikes  1120  are located on the bone-facing side  1102 , each adjacent to a grafting channel  1122 . Each bone-engaging spike  1120  is of a crescent shape, protruding from the bone-facing side  1102  and terminating with an acute edge. Several small diameter bone-engaging spikes  1121 , with small grafting channels  1123  are interspersed with the bone-engaging spikes  1120  and grafting channels  1122 . 
   The large size of the grafting channels  1122  creates favorable conditions for bone ingrowth once the implant  1150  is in place. Also, the crescent shapes of the bone-engaging spikes  1120  allow for good engagement with the vertebral body, but without requiring an excessive amount of force to press into place. The spikes  1122 ,  1121  also provide shear resistance once the end plate  1100  is implanted in the vertebral body. 
   The snap port  1130  occupies much of the surface area of the end plate  1100 . The large opening size of the snap port  1130  maximizes space available for bone ingrowth. The irregular shape of the snap port  1130  allows more contact area for the snap connection, and offers more torsional resistance than a regularly shaped, round port. The snap port  1130  is encircled by a wall  1132 . At several points on the wall  1132 , a recess  1134  is indented into the wall  1134 . 
   Referring to  FIG. 19 , an enlarged view of the bearing-facing side  1104  of the end plate  1100  is shown. The end plate  1100  has an anterior end  1106  and a posterior end  1108 . The grafting channels  1122 ,  1123  open out on the bearing facing side  1104 , as does the snap port  1130 . Three pockets  1126  are indented into sides of the end plate  1100 , on the anterior end  1106  and the two lateral sides. The pockets  1126  are shaped to engage with the instruments used to insert the end plate  11100 . 
   Referring to  FIG. 20 , a caudal side of the inferior bearing  1300  is shown. The inferior bearing  1300  has a caudal side  1302 , a cephalad side  1304 , an anterior end  1306  and a posterior end  1308 . Three instrument ports  1316  perforate the inferior bearing  1300 , one on the anterior end  1306  and one on each lateral side. A rounded cap  1322  protrudes from the center of the caudal side  1302 , and is surrounded by a trough  1324 . The trough  1324  is surrounded by a wall  1326 . Indented into each lateral side of the wall  1326  is a long recess  1328 . 
   Referring to  FIG. 21 , the cephalad side  1304  of the inferior bearing  1300  is shown. The three instrument ports  1316  open out on the cephalad side  1304 . A round dome  1332  rises from the surface of the cephalad side  1304 . 
   Referring to  FIG. 22 , a cephalad side of the superior bearing  1400  is shown. The superior bearing  1400  has a cephalad side  1402 , a caudal side  1404 , an anterior end  1406 , and a posterior end  1408 . Three instrument ports  1416  perforate the inferior bearing  1400 , one on the anterior end  1406  and one on each lateral side. A rounded cap  1422  protrudes from the center of the caudal side  1402 , and is surrounded by a trough  1424 . The trough  1424  is surrounded by a wall  1426 . Indented into each lateral side of the wall  1426  is a long recess  1428 . 
   Referring to  FIG. 23 , the caudal side  1404  of the superior bearing  1400  is shown. The three instrument ports  1416  open out on the caudal side  1404 . A circular ridge  1434  rises from the caudal side  1404  of the superior bearing  1400 . In the center of the circle formed by the ridge  1434 , a cup  1432  is depressed into the superior bearing  1400 . The cup  1432  on the superior bearing  1400  and the dome  1432  on the inferior bearing  1300  form the bearing surfaces when the implant  1050  is implanted. 
   Referring to  FIG. 24 , a bone-facing side  1502  of one snap fastener  1500  is shown. The bone-facing side  1502  is flat and has a generally square shape, with a central body  1506  and an irregular outer edge  1508 . The snap fastener has an anterior end  1510 , a posterior end  1512 , and two lateral sides  1514 . Two connection slots  1516  perforate the snap fastener, each generally parallel to a lateral side  1512  of the body  1506 . Four connection ports  1518  are located just inside the outer edge  1508 , one each on the anterior and posterior ends  1510 ,  1512 , and one on each lateral side  1514 . There is a gap  1520  in the outer edge  1508  adjacent to each connection port  1518 , such that the outer edge  1508  is not continuous but each connection port  1518  has an opening to the outside of the fastener  1500 . Formed onto the outer edge  1508  immediately adjacent to each gap  1520  is a tab  1522 , each tab  1522  being a protrusion from the outer edge  1508 , extending in the same plane as the body  1506 . 
   Referring to  FIG. 25 , an enlarged side view of a snap fastener  1500  is shown, in order to depict the tabs  1522  in greater detail. Each tab  1522  has a sloped bone-facing side  1532  and a sloped bearing-facing side  1534 . The slope of the bearing-facing side  1534  is steeper than the slope of the bone-facing side  1532 . This is so that when the tabs  1522  are snapped into the recesses  1134  in the walls  1132  of the end plate  1100 , more force is required to remove the snap fastener  1500  from the end plate  1100  than it takes to snap the snap fastener  1500  to the end plate  1100  or  1200 . 
   Referring to  FIG. 26 , a bearing-facing side  1504  of the snap fastener  1500  is shown. In the center of the body  1506 , a raised rim  1536  surrounds a rectangular dish  1538 . Protruding on each lateral side of the rim  1536  is a long tab  1540 . The long tabs  1540  are configured to fit into the long recesses  1328 ,  1428  on the bearings  1300 ,  1400  when the snap fastener  1500  is snapped to the bearing. Returning to  FIG. 25 , each long tab  1540  has a bone-facing side  1542  and a bearing-facing side  1544 . The slope of the bone-facing side  1542  is 90 degrees, and the slope of the bearing-facing side  1544  is less steep, approximating 45 degrees. This is so that when the snap fastener  1500  is snapped on to the inferior or superior bearing  1300 ,  1400 , it will require considerably less force to snap the fastener  1500  on the bearing than to remove it. 
   When the snap fastener  1500  is snapped on to the end plate  1100 , the bone-facing side  1532  of the tab  1522  pushes against the bearing-facing side  1104  of the end plate  1100 , and the outer edge  1508  flexes slightly until the tab  1522  is forced into the recess  1134 . Since the slope on the bearing-facing side  1534  of the tab  1522  is steeper, it would take much more force to remove the tab  1522  from the recess  11134 . 
   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, each of which may have a different bearing set, fusion block, or snap connection system according to the invention. 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.