WO 2004/016205 PCT/US2003/025535 INTERVERTE%(RAL DISC IMPLANT 5 FIELD OFD.tWE INVENTION IL [001] Thelinvention is related to d6yices and methods for the treatment of trauma and diseases of the spine, More particularly, the invention relates to intervertebral disc replacement. 10 BACKGROUND OF THE INVENTION [002] A variety of conditions suctas spondylolysis, disc herniation, compression of spi al cord nerve roots, degenerative disc disease, and trauma are known to causesevere discomfort, requiring medical attention. Among the procedures currently used to alleviate such conditions are spinal fusion, such as intervertebral and posterolateral fusion or 15 arthrodesis. In these procedures, two adjacent vertebral bodies are fused together. The afL Tcted intervertebral disc is first excised, and an implant is inserted which accommodates boc growth between the two vertebral bodies to effectively bridge the gap left by the disc removal. A number of different implant m terials and implant designs have been used for fion with varying success. Although intervertebral and posterolateral fusion are widely 20 usd drawbacks to their use include a reduced physiologic range of motion and other fusion rel ed complications such as degeneration of adjacent discs and destabilization of the fuxIctional spinal unit. As a result, alternative treatments with fewer complications, but sirgilar efficacy to fusion, are desirable. Ode such alternative to spinal fusion is artbroplasty an& the use of a prosthetic or artificial disc. 25 [M3] In general, arthroplasty is used in the replacement of diseased joints. Arhroplasty involves a set of procedures directed to maintaining motion of the joint, thereby pr serving its integrity and keeping the adjacent motion segments from deteriorating, as they tend to do after fusion. Depending on the location and the condition of the affected joint, spc ific artlroplasty procedures may be used. For example, interpositional reconstruction 30 swmgery, which reshapes the joint and adds ii:prosthetic disk between the two bones forming the joint is commonly used on elbow, shoulder, ankle, and fingerjoints. Totaljoint replacement, or total joint arthroplasty, replaces the entire diseased joint with an artificial WO 2004/016205 PCT/US2003/025535 5 prosthesis and, in recent years, has become the operation of choice for most kne and hip problems. [004] Hip and knee replacements are particularly widespread with nearly 300,000 hip replacements and about as many' knee replacements performed in the United States in -2001. With respect to the knee and hip joint replacement surgeries, there are several implants. 10 or prosthetics available. For the hip prosthetic, in an exemplary design, there are two components, one is a metal ball attached to a metal stem which is fitted into the femur, and the second is a matching plastic socket whikh is implanted into the pelvis. The metal pieces are; generally formed from stainless steel, alloys of cobalt and chrome, titanium; and alloys of titanium; the plastic pieces are generally formed from high-density polyethylene. For the 15 knee prosthetics, in an exemplary embodiment, metal and plastic components are again used to replace the damaged bone ends and cartilage. The metal pieces are generally formed from stainless steel, alloys of cobalt and chrome, titanium, and alloys of itanium; the plastic pieces are generally formed from high-density polyethylene, [005] Although the evolution of spinal arthroplasty and the use of prosthetics in the 20 spine has been similar to that of other joints. in the body, evolving from fusing the joint to replacing the functional joint, the advent of spinal athroplasty, however, has been slower than arthroplasty in other majorjoints in the body. A few of the possible reasons why spinal artbroplasty has been delayed are that spinal problems related to disc degeneration are difficult to diagnose, spinal procedures are typically crisis-driven and thus conservative 25' soliations such as fusion are acceptable, and spinal anatomy is complex. (006] Over the past 40 years spinal: arthroplasty technologies have been under deelopment and in the last 10 years spinal arthroplasty has won the attention of leading surgeons and implant manufacturers. The evolution of spinal arthroplasty essentially began in the 1950's and one of several emerging concepts was the spherical concept of the disc 30 prostheses, The spherical concept is simply the placement of a ball, essentially cirmferential, in the cavity of the nucleus-pulposus after a discectomy procedure has been performed. The annulus is kept in place and the ball serves as a nucleus replacement device. Various materials have been experimented with for the spherical concept. For example, in the early 1960's, implants using silicone ball bearings were implanted into the cervical 35 regions of patients, but the outcomes were uncertain. In the mid 1960's, stainless-steel (ball beaig) prostheses were implanted'into patients. The results of the procedure were initially 2 WO 2004/016205 PCT/US2003/025535 5 promising but over time the disc spaces lost height due to subsidence of the steel blls into the-vertebral bodies. Presently, the concept-of a spherical prosthesis continues to le examined using different materials, the latest of which is a modified carbon fiber. [007] Another emerging concept is the necbanical concept design. The mechanical concept design is essentially a total disc replacement product which is intended to restore the 10 range of motion of the vertebral motion segment unit. These devices are often comprised of metallic endplates fixed to the adjacent vertebral bodies via a stabilization mechanism and a core fonned from polyethylene or other polymeric materials. Alternatively, instead of a core, bearing surfaces can be used, the bearing surface materials being ceramic-on-ceranic, metal on metal; or metal-on-polyethylene. The mechanical design concept is based on the same 15 principles as joint reconstruction products, such as knee and hip replacements, and a variety of mechanical design prostheses concepts have been proposed and continue to be proposed. [008] Another concept is the physiological concept. The physiological concept uses a hydrogel, elastomer, or polyurethane-based core which is intended to restore the aisc function by absorbing and emitting fluid between the patient's vertebral endplateswhile also 20 maintaining the natural shock absorbing or cushioning function of the disc. The physiological concept devices are generally considered only a partial solution as they are designed to replace only the nucleus or a portion of the disc. [009] All-of the approaches to disc-replacement are aimed at some or all of the following: alleviating discogenic pain, restoring range of motion, maintaining the natural 25 shock absorbing function of the disc, restoring normal form or disc height, and resIoring physiological kinematics. Generally, four exemplary types of artificial intervertebral discs have been developed for replacing a portion -or all of an excised disc: elastomer/fuid filled discs, ball and socket type discs, mechanical spring discs and hybrid discs. [0010] Elastomer/fluid filled discs typically include an elastomer cushion or a fluid 30 . filled chamber positioned between lower and upper rigid endplates. The cushions and chambers of these implants advantageously function, in mechanical behavior, similar to the removed intervertebral disc tissue. [0011] Ball and socket type discs typically incorporate two plate members having cooperaing inner ball and socket portions which permit articulating motion of the Members 35 during movement of the spine. 3 WO 2004/016205 PCT/US2003/025535 5 [0(12] Mechanical spring discs typically incorporate one or more coiled springs disposed between metal endplates. The coiled springs define a cumulative spring constant. that is designed to be sufficient to maintain the spaced arrangement of the adjacent vertebrae while allowing normal movement of the vertebrae during flexion and extension- of the spine in.any direction. 10 [Q13) The fourth type of artificial intervertebral disc, the hybrid disc incorporates tw6 or more of the aforementioned design principles. For example, one known hybrid disc. arr ngement includes a ball and socket joint surrounded by an elastomer ring. [0414] While each of the foregoing prostheses addresses some of the problems. relting to intervertebral disc replacement each of the implants presents signifidait 15 drawbacks. Thus, there is a need for an intervertebral implant that accommodates the anAtomy and geometry of the intervertebral space sought to be filled as well as the anatomy and geometry of the ends of adjacent vertebral bodies, while providing reliability and sim city in design. More particularly, there is a need for a spinal disc implant which provides stability for supporting the high loads applied to the vertebrae, permits sufficient 20 mobility to. allow the patient an approximate normal range of motion, provides for'axial coxrpression between adjacent vertebrae, and has shock absorption abilities, SUMMARY OF THE INVENTION [o 15] The invention relates to an intervertebral disc that is preferably designed to restore disc height and lordosis, allow for a natural range of motion, absorb shook and 25 prqevide resistance to motion and axial compression. Furthermore, the intervertebral disc may be sed in the cervical, the thoracic, or the lumbar regions of the spine. [0016] The intervertebral disc includes a body having a footprint that is preferably co orming in size and shape with at least a portion of the ends of adjacent vertebrae. The shaes of the intervertebral disc include, but are not limited to, circular, oval, ellipsoid, 30 . ddpey-bean, anmular, C-shaped, D-shaped, etc. 00j7} In one embodiment, the body of the intervertebral disc includes an upper endlate, a lower endplate, and an elastic membrane disposed between the upper and lower .en tes. Alternatively, the elastic membrane may surround and encapsulate the endplates, elastic membrane defines an interior that is at least partially filled with a fluid 35 erably,.the fluid is selected from the group consisting of a gas, a liquid, a gel or any 4 WO 2004/016205 PCT/US2003/025535 .5 combination thereof. In addition, the fluid inay be compressible, and may be selected from thd group consisting of, for example, gas, liqnid or hydrogen, or may be incompressible,'and may be selected from the group consisting f: for example, saline. . [0(18] The disc also preferably includes a valve for permitting insertion of fluid to th interior-of the intervertebral disc. The VIalv6 may be disposed on the elastic membrane, 10 altrnatively, however the valve can. be located in the upper and lower endplates ofthe disc. [0019] The upper and lower endplates are preferably formed of metal, such as titanium, stainless steel, titanium alloys, cobalt-chromium alloys, or amorphous alleys. Alternatively, however, the tipper and lower endplates may be formed of ceramics,. cohIposites, polymers, such as poly-ether-ether-ketone (i.e., PEEK) or an ultra high molecular 15 weight polyethylene (i.e., UHIWPE), bone, including cortical, cancellous, allograft, auograft, xenogratt, demineralized. or partially demineralized bone, or any other materials able to serve as load bearing supports. The materials chosen for the endplates, in co bination with the desired fluid, are preferably selected to reduce the amount of wear, and thus increase the life of the joint. 20 [0920] The outer surface of the upp§r and lower endplates may be substantially flat, wedge-shaped, ete. The outer surfaces of the upper and lower endplates also may be dome h uped with their radii defined in the sagittal and coronal planes to generally match those of th ends of the adjacent vertebra. The dome shape allows the upper and lower endplates to beer conform with the ends of the adjacent vertebrae for a better fit in situ. 25 [0U21] The intervertebral disc also preferably includes migration-resistant structures pr vided on the outer surface of at least one or both of the endplates to impede movement, distodging, or expulsion of the endplates within and from the ends of the adjacent vertebrae. The migration-resistant structures include, but are not limited to, flaps, spikes, teeth fins, d loyablespikes, deployable teeth, flexible spikes, flexible teeth, alternatively shaped teeth, 30 in rtble or expandable fins, screws, hooks, serrations, ribs, and textured surfaces. [0 22] Furthermore, the upper and lower endplates also preferably coated with a bone growth inducing or conducting substance to promote bony ingrowth to permanently-secure the disc to the adjacent vertebrae. Alternatively, the upper and lower endplates may have a ro 9hened- surface; a porous surface; laser treated endplate layers; integrate an 35 osteaconductive/osteoinductive scaffold; or may be provided with or made from an integral ostoconductive and/or osteoinductive material to promote bony ingrowth. The endplates 5 WO 2004/016205 PCT/US2003/025535 5 may further include a membrane and/or a btrier to limit the amount and/or deptlj f boAy ingrowth. [0023] The upper and lower ehdplati s may also have implant instrument tion attachment, guiding, and retainment structure. For example, the endplates may have holes, slots, threads, or a dovetail for implanting tie. implant and/or distracting the adjacent 10 vertebrae. For example, the disc may include.a slot formed in the upper and/or lower endplates, the slot being configured to recei e an implant insertion instrument, a.distractor or both. [0024] The upper and lower endplates may also preferably include articulating surfaces, thus providing the intervertebral disc with greater mobility. The articulating . 15 surfaces preferably including a surface polish or similar wear reducing finish such as diamond finish, TiNi finish, etc. in order to minimize wear, decrease particle generation, and increase'disc life. (0025] In some embodiments, in addition to the fluid or in place of the fluid, additional structures may be included to proyie additional stiffness. The structures include, 20: but are not limited to, springs, elastomers, bellow, balloons, closed reservoirs. hollow bodies, biocompatible fibers, and cables. [0026] In some embodiments, the inevertebral disc also preferably has an articulating mechanism to allow the endpla* ;to pivot with respect to one another such that associated portions of the endplates may come closer together under compression while . 25 different associated portions of the endplates may separate under tension. The aiticulation mechanism may be in the form of a center pivot axis or fulcrum. Preferably, the . intervertebral disc also allows and provides a mechanism, or is configured to allow the location of the pivot axis within the disc to change in response to the loading conditions, thus providing a moving instantaneous axis of rotion. The intervertebral disc also preferably 30 coniprises a mechanism, such as providing a fluid, an elastomer, a spring, a cable, etc. to absorb axial compression forces and to provide a shock absorbing effect. [0027] In some embodiments the intervertebral disc includes an upper end, a lower ehd, and an outer sidewall disposed therebetween. The disc may have an interior volume defined betireen the upper and lower ends and the outer sidewall, with the interior volume 35 preferably including a center pivot and at lenst one chamber, the chamber being peripheral to and surrounding the center pivot. Preferably, the center pivot includes a central wall defining 6 WO 2004/016205 PCT/US2003/025535 5 a central chamber, and the at least one periph ral chamber is disposed between-the :uter sidewall and the central wall. A first fluid may be disposed in the at least one peripheral chamber, A second fluid may be disposed inthe central chamber. The first and second fluids may or may not be the same. The intervertebral disc may include additional periphral chambers which may or may not be in fluid communication with the central chamb r and 10 eac other. Furtherthe sidewall may be fanned of a first material while the centrawall may be.formed of a second material, with the first material having a different stiffness than'the second material. Preferably, the center pivot and/or central chamber may permit th 9 upper and lowers ends to pivot with respect to each other, and may include a resilient element such as a spring. 15 [0028} In another embodiment, the intervertebral disc includes a body havi an upper surface spaced from and opposing a lower surface. The spacing between the pper surface and the lower surface may be selectable. The body further includes an oute sidewall Corning an outer wall and a thra-hole formig-an inner wall, with the inner wall de ing an opening. Trther, the body may be substantially C-shaped. A chamber may also b disposed 20 within the.body. In addition, there may be at least one portion extending from the body for contacting a"vertebrae, with the portion defining a hole for receiving a fastener. [0029] The intervertebral disc may b implanted in a modular fashion, if paosible, or it may be:implanted preassembled. An anterior, anteriolateral, or lateral surgical ap roach' may be used to implant the intervertebral disc. Furthermore, depending on the inte ertebral 25 disc to be implanted, a minimally invasive surgical method or a simultaneous distrattion and injlantation surgical method may be used. Also depending on the intervertebral disc to be implanted, the Anterior Longitudinal Ligament may be attached directly to the disc to the adjacent-vertebral bodies. The Anterior Lorigitudinal Ligament may be formed from partially demineralized or demineralized autograft, alograft, or xenograft. Alternatively, thdAntcior 30 Longitudinal Ligament may be formed from biocompatible materials such as elastofoers, or braided polymers. To assist with the implantation of the intervertebral disc, the inteivertebral disc may include alignment markers. BRIEF DESCRiPTION -OF THE DRAwiNGS {0030] To facilitate an understanding f and for the purpose of illustrating We present 35 invention, exemplary and preferred features and embodiments arc disclosed in the 7 WO 2004/016205 PCT/US2003/025535 5 acctornpanying drawings, it being understood however, thut the invention is not limited to the precise arrangements and instrumentalities hown, and wherein similar reference characters deriote similar elements throughout the several views, and wherein. [003] Figure 1 is a perspective vie*.of a first embodiment of an artificial. intervertebral disc according to the present invention; 10 (032] Figure 2 is a cross-sectional iew of the artificial intervertebral -disc.of Figure 1 taken dng line A-A; [0433] Figure 2a is an alternate-cros -sctional view of the artificial intervertebral dis of Figure 1 taken along line A-A; [00434] Figure 3 a is a side view of a deployable spike according to the present 15 inv tion. [0q35] Figure 3b is a side view of at other deployable spike according to the present *mtntion. [0X 36] Figure 3c is side view of a i ble spike according to the present invention. 20 [0 .1 Figure 3d is a aide view of alternatively shaped teeth according to the present [0138] Figure 3e is a side view of aichiors according to the present invention. (0439] Figure 4 is a perspective view of a second embodiment of an intervertebral dish according to -the present invention; [0440] Figure 5 is a cross-sectional ew of the intervertebral disc of Figure 4 taken 25 alob line B-B; [041] Figure 6 is a perspective view o'f an alternative embodiment of the intervertebral disc of Figure 4, 042] Figure 7 is a perspective.vie of a third embodiment of an interveriebraldisc according-to-the present invention; 30 [0C043] - Figure 8 is a cross-sectional View of the intervertebral disc of Figure.7 taken alo flini-C-C; [0C44] Figure 9 is a cross-sectional iipw of an alternative embodiment of the inti rvetebal disc of Figure 7 taken along Ii e D-D; 0045] Figure 10 is a perspective viewof a fourth embodiment of an intervertebral 35 disc according to the present invention. 8 WO 2004/016205 PCT/US2003/025535 5 [0046] Figure 11 is a side view of t11. fourth embodiment of the intervertbral disc of Figure 12; (0047] Figure 12 is schematic view tf a fifth embodiment of an intervertebral disc according to the present invention. DETAILED DESCRIPTION OF TiE PREFERREtI EMBODIMENTS 10 (0048] Any of a wide variety of different implant structures can be prepared according to the teachings shown by the illustrative examples of the intervertebral discs disclosed herein. The intervertebral discs of the present invention are preferably designed to restore spinal lardosis, disc height, to allowfor a natural range of motion, absorb shock and to provide resistance to motion and axial cqnpression. 15 [0049] The intervertebral discs preferably are sized and adapted, for use in the cervical, thoracic, and lumbar regions of the spine. Also, the intervertebral discs can be tailored for each individual patient allowing for disc characteristics appropriate for the individual-patient. For example, the core of dhe disc can include different assemblies, different components, and/or various types of materials to create the desired characteristics 20.. for each individual patient. (0050] Furthermore, the intervertebtal discs may allow flexion, extension lateral bending, rotation, and translation. Flexion movementt that brings two parts of joint or body into a bent position; in the spine, this is a movement in which the spine starts straight and moves.into forward bending. Extension is a movement that daws two parts away from 25 eadh other; in the spine, this is a movementqn.which the spine starts straight and noves into backward bending. Lateral bending is a bending movement toward a lateral side; in the spine, this movement generally involves bending (lateral) and coupled rotation. Rotation is a movement that results in a portion of the spiie twisting, rotating or turning with respect to the axis of the spinal column. Translation is a limited movement that is generally transverse to 30 the axis of the spinal column. [0051] Additionally, similar to a natural intervertebral disc, the artificial intervertebral discs preferably.allow for a moving instantaneous axis of rotation. At every instant for g body in plane motion there is a line in the body or a hypothetical extension of this line that does not move.. The instantaneous axis of rotation is this line. A moving instantaneous axis. 35 of rotation refers to the ability of the instantaneous axis of rotation to move (i.e., translate) as 9 WO 2004/016205 PCT/US2003/025535 5 a result of different loading conditions; in otter words, the location of the instantaneous axis of rotation moves with respect to the disc. The preferred mean location of the moving instantaneous axis of rotation for the lumbar region of the spine is preferably in the posterior half of the disc space or proximal to an adjacent (superior or inferior) endplate, preferably proximal to the inferior/caudal endplate, the preferred mean location of the moving 10 instantaneous axis ofrotation for the thoracic-region of the spine is preferably in the inferior; portion of the disc space and proximal to the caudal vertebral body extending posteriorly into the.spinal canal, and the preferred mean location of the moving instantaneous axis.ofrotation for the cervical region of the spine is preferably in the posterior half of the caudal vertebral body. 15 [0652] Also similar to a natural intervertebral disc, the response characteristics of the artificial intervertebral disc are preferably non-linear. For example, in response to continued axial compression, the artificial intervertebral disc preferably undergoes a large initial amount of compression followed by non-linearly decreasing amounts of compression. [003. Referring to the accompanying drawings, preferred embodiments and features 20 of the artificial intervertebral disc will be described in detail. It is to be noted however that these descriptions of specific embodiments and features are merely illustrative. It is contemplated that one or more features or eleinents of the various embodiments may be combined or used singularly, and that modifications of the various embodiments, as well as other embodiments are contemplated and will be apparent to those persons skilled hr the art. 25 [0054J Referring initially to Figures 1.and 2, a perspective view of an exemplary first. embodiment of an artificial intervertebral disc 10 is shown. As shown, the disc 10 has a generally iddney-bean shaped footprint which includes an anterior side 11, a posterior side 13,and Erst and second lateral sides 15, 17,respectively. The anterior side 11 and lateral sides 15, 17 are all substantially convex in shape while the posterior side 13 is substantially 30 concave in.shape. However, the disc 10 may take on other shapes that preferably confonn geometically and anatomically with the adjacent vertebral bodies including, but not limited to circular, oval, ellipsoid, annular, D-shaped, C-shaped, etc. [0055] As shown, the intervertebral disc 10 includes an upper endplate 12,.a lower endplate 14 and an elastic membrane 16, the-elastic membrane 16 generally extending from 35 theupper endplate 12 to the lower endplate 14 and is located, preferably, proximate'to the outer periphery of the disc 10. Alternatively, the elastic membrane 16 may surround and/or 10 WO 2004/016205 PCT/US2003/025535 5 endapsulatc the upper and lower endplates 12 14. The elastic membrane 16 in combination with the upper and lower endplates 12, 14 may define an interior volume that may be at least partially filled with a fluid 22. The elastic membrane 16 preferably is formed from an elastomer such as polyurethane, silicone, a braided polymer, or any other appropriate elastic material known in the art. The elastic membrane may b6 non-permeable. Alternatively the 10 elastic membrane 16 may be permeable or semi-permeable to allow fluid to flow into and out of the interior of the disc (as described in more detail below). Preferably, the elastic menbrane 16 may resist translational motion between the upper and lower endplates 12,14, and may also prevent soft tissue ingrowth between the endplates 12, 14 as well as contain any wear particles generated within the interior volume. The elastic membrane 16 may be 15 attached to-the upper and lower endplates 1i 14 by any fixation method known in. the art including, but not limited to, bonding agents, ultrasonic welding, screws, nails, mechanical we going, andpins. [0456] Alternatively, the elastic meriibrane 16 may be in the form of a bellow, the bellow assuming an "accordion" shape, enabling it to expand and contract under the various 20 loading conditions. The bellow may be rigidy attached to the upper and lower endplates 12, 14 by any method known in the art including, but not limited to a circular groove formed in each endplate 12, 14, bonding agents; ultrasonic welding, screws, nails, mechanical wedging, and pins. Preferably, the bellow is made from a metal, although other material such as elastomers or polymers may be used. 25 (0Q57 The disc 10 also may include a valve 20, the valve 20 providing access to the interior 19 of disc 10 so that fluid maybe injected into, or removed from, the interior19 of the disc 10. The valve 20 preferably is a one-way valve, as known to those-skilled in the art, so the fluid, once injected, can not escape from the interior 19 of the disc 10. As shown in Figures 1 and 2, the valve 20 preferably is:disposed within the elastic membrane 16, 30 alternatively however, the valve 20 may be disposed within the upper and/or lower endplates 12.]14. as shown in Figure 2a. When the valve is disposed on the upper and/or lower en lates 12, 14, a passageway 30 preferabl' is included to interconnect the valve 20 with the interior 19-of the disc 10. [O0S8J The fluid 22 provided in the interior volume may be a gas, a liquid, a. gel, or 35 an4 combination thereof. Where a gas is provided as the fill media for the interior volume, or whire a combination of gas and liquid or ge is provided, the ultimate gas pressure within the 11 WO 2004/016205 PCT/US2003/025535 5 interior 'olume should be selected to provide adequate shock absorption during axial compression of the disc 10. The fluid may.also permit limited articulation or movement of theiupper epdplate 12 and lower endplate 14 with respect to one another. Preferably, the fluid is an incompressible liquid, for example, a saline solution. In use, the fluid 22 maybe injected into the interior 19 of the disc 10 before insertion of the disc 10. between adjacent 10' verebrae. 'Alternatively, the fluid 22 may be injected in situ to facilitate insertion of discO10 and:subsequent distraction between adjacent vertebrae. The rigidity and distraction capabilities.of the disc 10 may be a function of the amount of fluid 22 injected into the interior 19'6f the disc 10 and/or the elastic nature of the membrane 16. Generally,. the more fluid 22 provided in the interior 19 of the digc 10, the more rigid the disc 10, and the greater 15 thedistraction capability. Furthermore, pliability and increased articulation may b realized by 1lingonly a portion of the interior voliie 19 of the disc 10. Finally, variably filling the interior 19 of the disc 10 with fluid 22 permits the overall height H of the disc 10 to be varied as necessary depending on the needs of thelindividual patient. [0059] As shown in Figure 2a, the upper endplate 12 may have an inner surface 20 provided with an arcuate socket 32, while the lower endplate 14 may have an inner surface provided with an arcuate protrusion 34, or vice versa. The socket 32 and protusfon 34 are configured'and dimensioned to mate, or to correspond generally with each other. The type and :amount of articulation desired may dictate the curvature of the socket 32 and protmsion 34 provided. For example, if the protrusion 34 has the same radius as the socket 32, then the 25 disc 10 may provide greater support but moire constrained movement, Alternatively, if the socket 32 has a larger radius than the protrusioa 34, the disc will provide iner6ased ardculation. Furthermore, the protrusion 34.and/or socket 32 may also incorporate a flattened paition which may allow translational movement of the upper endplate 12 with respect to the lower endplate 14. By allowing translation the disc 10 may provide a moving instantaneous 30 axi of rotation as previously explained. [0060] It is'also possible for the socket 32 and protrusion 34 to take on contours other thaai those described above in order to achieve a desired articulation. Moreover, while the sodket 32 and protrusion 34 are shown with-contours that generally permit mating of their surfaces, it is possible to provide non-mating contours for the socket 32. and p-trusion 34 to 35 achieve a: desired articulation. 12 WO 2004/016205 PCT/US2003/025535 5 [0061] The use of a fluid filled interior volume 19 in combination with an attiulatiig surface may permit the socket 32 and protruion 34 to translate more easily with respect to each other by reducing friction between the sliding surfaces. [062] Altdrnatively, where the fluid is a compressed gas, the articulation surfaces may net be constantly engaged, but may onlybecome engaged when sufficient compressive . 10 fore is placed in the disc by the adjacent vertebrae. Thus, the disc of this embodiment would ha e a dual performance aspect, under one loading scenario performing like a fluid-iled diso, and under a second scenario performing like a mechanical protrusion/socket articulating disk [oq63] Depending on the location in the spine where the disc 10 is implanted: the disc 15 10 preferably may restore height in the rang from about 4 millimeters (mm) to about 26 mnm. In addition the disc 10 preferably may restoire-lordosis in the range between about-O" to about 204. The disc 10 preferably may also restore stiffness in the range from about 1 Newton meker per degree (Nm/deg) to about 11 Nm/deg in axial rotation, about 0 Nm/deg to about 7 Nrdeg in flexion/extension, and about 0 Nm/deg to about 5 Nm/deg in lateral bending. hi 20 addition, the disc 10 preferably provides a dOmpression stiffness from about 100 N/mm to abput 5000 N/mm and tension stiffness fromabout 50 N/mm to about 1000 N/m; Furthermore, depending on the location of the spine where the disc 10 is implanted, the intervertebral disc 10 preferably allows for a range of motion of from about 5* to about 45* in fl4ion/extension, from about 30 to about 3S, in lateral bending, and from about ".. to. about 25 60' in, axial rotation. The intervertebral disc 10 preferably also allows for axial compression in Ie range from about 0.2 mm to about 2 rmm. [0 64 ' Preferably, the upper and lower endplates 12, 14 are formed of metal, such as tita ium, stainless steel, titanium alloys, cobalt-chromium alloys, or amorphous alloys. All arnatively, however, the upper and lower endplates 12, 14 may be formed of ceramics, 30 co posites, polymers, such as PEEK or UIWPE, bone, including cortical, cancellous, all graft, autograf xenograft, demineralized or partially demineralized bone, or any other miateialS-appropriate to serve as load bearing suprs oepeferaly the materials chosen for the endplates, in combination with the fluid, may be chosen so as to minimize 35 [5] ' Furthermore, preferably, any articulating surfaces in the intervertebral discs of theipresent invention includes a surface polih or similar wear reducing finish such.as 13 WO 2004/016205 PCT/US2003/025535 5 diamond imish, TiNi finish, etc. in order to minimize wear, decrease particle generaion, and increase disc life. [0066] The outer surface of the upper and lower endplates may be substantially flat, wedge-shaped, etc. The outer surfaces of the upper and lower endplates 12, 14 also-may be dome shaped with their radii defined in the sagittal and coronal p lanes to generally match the 10 shap of the ends of the adjacent vertebral, thereby providing a better fit in situ. [0067] In -addition, as shown in Figdres 1 through 2a, the disc 10 may include niration resistant features, such as, for example, spike-like structures 18 on the outer sutraces ofthe upper and lower endplates 12 14. The migration resistant features may facilitate engagement of the disc 10 with the ends of the adjacent vertebra by providing a 15 mechanical interlock as a result of penetration-and/or deformation of the ends of the adjacent verItebrae. The initial mechanical stability afforded by spikes I$, for example, minimizes the rist of post-operative instability, movement dislodging or expulsion of the disc 10.: Other mijration resistant features may include, without limitation, flaps, teeth, deployable teeth, deployable spikes, flexible spikes, flexible teeth, fis, insertable or expandable fins, anchors, 20 screws, ridges, serrations, or other similar texturing on the outer surfaces of the upper and loner endplates 12, 14. As shown in Figure 3a, deployable spikes 21 may be provided, and a carm mechanism 23 maybe used to deploy the spikes 21. Alternatively, as shown. itiFigure 3bi an instrument may be used to deploy thespikes 21. As shown in Figures 3c through 3e, respectively, examples of flexible spikes 24 shaped teeth 25, and anchors 26 are shown. 25 Alfematively or in addition, bonding agents such as calcium phosphate cements, etc. may also be used to secure the disc 10 to adjacent vertebra. [068 Furthermore, the upper and lower endplates 12, 14 may also be coated with a bon e growth inducing substance, such as hydroxyapatite, to promote bony ingrowth to pemanently secure the disc 10 to the adjacent vertebrae. Alternatively, the upper and lower 30 endplates 12, 14 may have a roughened or porous surface to facilitate bony ingrowth. Altematively, the upper and lower endplates 12, 14 may have laser treated endplate .layers to create a porous stmoture, or may integrate an osteoconductive/osteoinductive scaffold. The endplates 12, 14 may also be made from an bsteoconductive and/or osteoinductive material to rP pronte bony ingrowth. The endplates 12,14 may further include a membrane and/or barrier 35 to l1mit the depth of bony ingrowth permitted. 14 WO 2004/016205 PCT/US2003/025535 5 [W0|69] The upper and lower endplatis 12, 14 may also have implant instrumentation attchment;:guiding, and retaining structures. For example, the endplates 12, 14 may have holes, slots, threads, or a dovetail for accepting a tool used to implant the disc 10. and/or to distract the vertebrae. For example, the diso;may include a slot formed in the upper and/or lower endplates 12, 14, the slot configured to receive an implant insertion instrument, a 10 distractor er both. £0970] As a result ofthe material and structural components used, the disc 10 can allcw flexion/extension, lateral bending, axial rotation, and translation, depending on the loading imparted on the intervertebral disc. In addition, under the various spinal loading conditions resulting frorm spinal movements, the fluid 22 may move within the interior 15 vohume, either compressing (in the case of a gas), or moving radially outward as the .membrane expands, allowing the end plates to move with respect to each other. This varying .movement or displacement of fluid 22 provides a moving instantaneous axis of rotation. [0071] As shown in Figures 4 and 5, a second exemplary embodiment of an. artificial .dis is provided. Disc 100 generally has an annular shape and includes an upper surfacS 102, 20 a.1 wer surface 104, an outer sidewall 106 arming an outer wall, and an inner sidewall 107 delining an.opening 108 (i.e., a thra-hole). However, the disc 100 may take on other shapes that preferably conform geometrically and anatomically with adjacent vertebrae bodies, including, but not limited to, kidney-bean shape, circular, oval, ellipsoid, C-shape, D-shape etc The disc 100. is preferably made from an elastomeric material that forms a closed 25 resorvoirhaving an interior volume 103. The disc 100 may further include a valve!118 for int'oducing or withdrawing fluid 120 from the interior volume 103 of disc 100 as.previously described Preferably, the valve 11 comprises a one-way valve and is located onithe outer sid'wall 106, as shown in Figure 5, however, the valve 118 may also be located onthe upper surface 102, the lower surface 104, or on the inner wall 107. 30 :[0072]. As best shown in Figure 5, the -disc 100 may further include a metal mesh 105 molded onto or otherwise secured to the upper surface 102 and/or lower surface 104. The metal mesh 105 may impart additional strength and rigidity to the disc 100. The metal mesh 105 may also be flexible so as to adapt to the concavity of the ends of the adjacent Vertebral bodies to thereby facilitate a high degree ofisurface contact with the disc. The metal mesh 35. 105may also be textured, its surface may be porous, and it may be used in conjunction with 15 WO 2004/016205 PCT/US2003/025535 5 borje-growth inducing or conducting substances to further enhance engagement and4 fsion with the adjacent vertebral elements. [0t3] Preferably, the through-hole108 may be filled with an elastomeric material (not shown). The elastomeric material mayhave a stiffness different from that of the disc 109. Preferably, the elastomeric material has a higher stiffness than the stiffness of disc 100 10 the by allowing the through hole 108 to be pore rigid and thus to act as a center pivot or center strut.about which the upper and lower surfaces 102, 104 may articulate. The center pivpt may allow one portion or side of the disc 100 to compress while at the same time pemitting another portion of the disc 100 tg expand, In an alternative embodiment, the elaomeric material may have a lower stiffness than the stiffness of disc 100. Alternatively, .15 th r through-hole 108 may be filled with a hyrdogel. {0974] h# addition, the upper and lower surfaces 102, 104 of disc 100 may include miton resistant features, permanent fixation means and/or implant instrumentation attbchment, guiding, and retaining structures as previously described in regards to the disc 10 of Figures 1 through 3. Preferably, disc 100 may be provided with at least one securing 20 fu t ure (i.e.. flap) 110 to facilitate engagement of the disc 100 with the vertebral bodies ofthe adacent vertebra. As shown in Figure 4, preferably two flaps 110 are provided, one flap 110 fo1 the upper surface 102 and one flap 110 for the lower surface 104. Flaps 110 maybe provided as one piece which extends beyond the upper and lower surfaces 102, 104, or flaps I .may be provided as two or more pieces. Flaps 110 preferably extend above and below 25 sufaces-102, 104, respectively, from lateraf side 106, and are sized to abut a portion ofthe ex erior surface of the vertebral bodies of the adjacent vertebrae, Flaps 110 may include through-holes 114 for receiving fasteners such as, for example, fixation screws (not shown), The fixation screws can be used to secure disc 100 to the vertebral bodies of the adjacent ve tebrae. 30 [0975] Alternatively, as shown in Figure 6, disc 100 may further include a-gap 126 in its circumference, producing opposed end faces 122, 124 which give the disc 100-a general "C" shaped appearance. Preferably, end fades 122, 124 are configured to be resiliently biased apk however, end faces 122, 124 may be naturally disposed apart from each other, without ree ilient biaking. The gap 126 formed between -end faces 122, 124 provide the disq 100 with 35 increased flexibility thus facilitating insertin and placement of the disc 100 between vertebrae. The gap 126 pennits the diameter of disc 100 to be decreased by press ends 16 WO 2004/016205 PCT/US2003/025535 5 .122, 124 together. The gap 126 also may alto4 the disc to be unfolded by pulling ends 122, 124 apart, 'hus, the gap 126, allows-the di'4 100 to be configured to have at least one smaller outer dimension as compared to its rest state, which in turn may allow the disc 100 to be inserted nto an anatomical region through a cavity or other opening that is smaller than the uncompressed (i.e. at rest) size of disc 1#0, thus making posterior insertion possible. 10 [0076] Depending on the location o the spine where the disc 100 is implanted, the disc 100 preferably restores height, lordosis stiffness, offers compression stiffness, and allows a range of motion similar to that described in relation to previous embodiments. [00.77) As a result of the materials, geometry, and components used, disc 100 can allow flexion/extension, lateral bending, axial rotation, and translation, depending on the 15 loading imparted on the intervertebral disc. Similar to the embodiment of Figures I through 2a under various spinal loading conditions resulting from spinal movements, the fliAd 22 may movevithin the interior volume, either.eompressing (in the case of a gas), or moving radially outward as the membrane expands, alowing the end plates to move with respect to each other This varying movement or displacement of fluid 22 provides a moving 20 Intantaneous.axis of rotation. [OQf78] With reference to Figures 7 through 9, a third exemplary embodiment of an artificial disc will be described. Preferably, disc 150 has a generally cylindrical shape with a circular footprint and has an upper end 152alower end 154, and an outer sidewall 156 disposed therebetween. The disc further includes an interior volume as defined between the 25 upper and lower ends 152, 154 and-the outer idewall 156. Although illustrated as a cylinder, the disc: 150 may take on any other shape that preferably conform geometrically and. ankonically with adjacent vertebral bodies, including, but not limited to, kidney-bean shapdL anular, oval, ellipsoid, D-shaped, Cshaped. etc. [0791 The disc 150 may be made fron any material known in the art capable of 30 serving as a load bearing support including; lut not limited to, elastomers, polymers, ce'amics,.composites, etc. The disc 150 may further include a valve (not shown).for intoducing fluid 158 into the interior of disc as previously described in relation to other embodiments. [0080] The disc 150 may further include upper and lower end plates (not shown) as 35 previously described with regards to other ntbodiments. Alternatively, the disc 150 may in lude a metal mesh molded onto or other sei secured to the upper surface 152 and/or lower 17 WO 2004/016205 PCT/US2003/025535 5 surfale&154 as previously described in relation to other embodiments. In additionJthe disc 150 may further include migration resistant features, permanent fixation means anor implant instrumentation attachment, guiding, and retaining structures as previously described in relation to other embodiments. [0081] Depending on the location of the spine where the disc 150 is implanted, the 10 disc 150-preferably restores height, lordosisa:stiffhess, offers compression stiffhes, and allows a range of motion similar to that described in relation to previous embodiments. [0082] With reference to Figure 8, the interior of disc 150 is shown. Preferably the interior of disc 150 includes a plurality of interconnected peripheral chambers 160; and a separate central chamber 162. The multi-chambered interior of disc 150 permits controlled 15 fluid flow within the intervertebral disc 150 so that under loading, controlled articulation or motion is permitted. The peripheral chamber 160 may be in fluid communication"with the : central chamber. 162 by way of an open passageway, a porous central wall 165,:an osmotic membrane, etc. Preferably, however, the peripheral chambers 160 are in fluid communication with the central chamber 162by way of a baffle and/or valve. More 20 preferably, the baffle and/or valve is configured to provide for selective exchange of fluid such that the fluid 158 from the peripheral chambers 160 may flow more easily or quickly into the. central chamber- 162 than the fluid 1$8 would flow out of the central charter 162. alternatively the central chamber 162 may bp sealed with respect to the peripher4- chambers 16& in this case, the peripheral chamber 160 and central chamber 162 may be filled with the. 25 same or different fluids. [0083] The peripheral chambers 160 are defined by walls 163, while the central . chaber.142 is separated from the peripheial chambers 160 by a central wall 165, In addition to defining the geometry of chambers 160, 162, walls 163, 165 also serve as supports between surfaces 152, 154 by resisting loads acting upon the disc 150 when in use. 30 [0084] Preferably the central chamber 162 and outer periphery chambers 14 are arranged so that the central chamber 162 is more rigid than the outer peripheral ch mbers 160 (such as by dmpletely filling with incompressible fluid), thus enabling the central chamber 162 to act as a center pivot or center strut about which the upper and lower surfaces 152, 154 may articulate. The center pivot allows one portion or side of the disc 150 to com press while 35 at the same time permitting another portion of the disc 150 to expand. The walls 163 of the peripheral. chambers 160 may be formed ofta material having a lower stiffness than the 18 WO 2004/016205 PCT/US2003/025535 5 erial.used to produce the central wall 16$, thereby allowing the central chamber'162 to be more rigid and act as a center pivot. Alter4ively, the walls 163 of the peripheral chambers 160 maybe formed of the same material as te central wall 165, but with a geometry that provides a lower stiffness than the geometr .of the central wall 165 of central chamber 162 thereby allowing'the central chamber 162 tq act as a center pivot for disc 150. Furthermore, a 10 combination of material and geometric char eteristics of the chamber walls 163, 165 may be selected to make the central chamber 162 mnre rigid than the peripheral chambers 160 so.that the central chamber 162 may act as a centejivot about which the disc 1-50 pivot. [00853 The geometry of chambers 1l0, 162, the geonietry and material of the walls 163, 165, along with the fluid(s) disposed therein can be selected to obtain the desired 15 characteristics of the disc, including the desied stiffness, height, pliability, and preferably the relative.stiffness of the central chamber 1621with respect to the peripheral chambers 160 to provide the desired articulation between the upper and lower ends 152, 154. Thus, the disc 150 will move, deform or extend in flexion tension, lateral bending, axial rotation, and translation depending on the loadings impacted on the intervertebral disc since under various 20 spial loading conditions, the fluid can translate between the peripheral chambers 160 and/or .the central chamber 162. This movement oithe chambers with respect to each other, as well as the movement of the fluid within and bet een the chambers allows for a moving instantanedus axis of rotation of the disc 15b.. It should be noted that the central chamber 162 needn't be located in the center of the disc, ut rather may be positioned in any other location 25 within the disc appropriate to produce the d sired movement of the endplates relative to. each other. [0086] Alternatively, as shown in Figure 9, the central chamber 162 may house a spring 167 The spring 167 serves as additi nal support for disc 150 further enabling the ceitral chamber 162 to act as a center pivo and/or strat. When a spring 167 is provided in 30 the central chamber 162, fluid may or may not also be provided. The spring 167 nay be formed from any material known in the art for example, cobalt-chronium alloys, titanium alloys. stainless steel, amorphous alloys, p lymers, or composites. [0087] Alternatively, the central cl br 162 may house a bladder (not shown). The bWdder may be integrally formed with, or connected to, ends 152, 154. Alternatively, the 35 bl der may be separate from the ends 1524 154. This bladder may articulate, compress, a1/or translate within the central chamber 62, providing the disc with a moving: 19 WO 2004/016205 PCT/US2003/025535 5 instantaneous axis of rotation, which under various loading conditions, may allow fbra greater degree of articulation or movement of disc 150. In addition, the central bladder may serve as additional support for disc 150 so that the central chamber 162 may act a center pivot and also permit the. desired motion. [0 881 With reference to Figures 10 and 11, a fourth embodiment of an artificial 10 intervertebral disc will be described. Disc 250, has a generally kidney-bean shapedo totprint with an upper endplate 252, a lower endplate 254, and at least one cable element 256, 258. Although disc 250- is shown as having a kidney-bean shaped footprint, the disc 250 may take oniany other shape that generally conforms geometrically and anatomically with adjacent -verebral bodies, including, but not limited to, circular, annular, oval, ellipsoidal, f-shaped, 15 C-shaped, etc. In addition, the endplates 252, 254 preferably include migration resistant fe tures, pemanent fixation means and/or implant instrumentation attachment guiding, and retaining structures as previously described in relation to previous embodiments. [0489] Preferably, the upper and lower endplates 252, 254 are formed of nietal, such as titanium, stainless steel, titanium alloys, cobalt-chromium alloys, or amorphou alloys, 20 4lfnatively, the upper 'and lower endplat6s 252, 254 may be formed of ceramics composites, polymers, such as PEEK or UtvNWPE, bone, including cortical, cancellous, al$graft, autogi-aft, xenograft, denineralized or partially domineralized bone, or any other mterials appropriate to serve as load bearing supports. [0090] -. The outer surface of the upp and lower endplates may be substantially.fat, 25 wqdge-shaped, etc. Alternatively, the outer surfaces of the upper and lower endpites 252, 2$ may be dome shaped with their radii defined in the sagittal and coronal planes to generally match the shape of the ends of the adjacent vertebrae, thereby providing-a better fit in-\situ. [0991] The disc'250 may also. include an elastic membrane, the elastic mcibrane 30 generally extending from the upper endplate 252 to the lower endplate 254 as preiously described in relations to previous embodiments. The disc 250 may also include a valve, the valve providing access to the interior of the disc 250 so that a fluid may be at least partially injected into the interior of the disc as descibed in relation to previous embodiments. [O 92] Depending on the location 46f the spine where thc disc 250 is implanted, the 35 di c 250 preferably restores height, lordos1 $stiffness, offers compression stiffnes, and allows a range of-motion similar to that described in relation to previous embodiments. 20 WO 2004/016205 PCT/US2003/025535 5 [0093] As shown, disc 250 includes a plurality of peripheral cable elements 256 and a central cable element 258. The peripheral cable elements 256 may be located near the perimeter cif disc 250, while the center cable: element 258 is preferably located near the center of the disc. The peripheral cable elements ?S and the center cable element 258 are attached to the upper and lower endplates 252, 254 by any fixation means know; in the art including, 10 but not limited to, bonding agents, ultrasonic welding, screws, nails, mechanical wedging and pins. Preferably, however, the cable elements 256, 258 engage the upper and lower endplates 252, 254 via boreholes 260 formed on the upper and lower endplates 252, 254. The ends of cable elements 256, 258 are crimped where they penetrate the outer surface of the upper and lower endplates 252, 254. This permits surgeons to appropriately size the disc 250 just prior 15 to implantation by means of crimping/attachin' appropriately sized cables to the endplates. The peripheral cable elements 256 and central cable element 258 may be made- from metals, polymers, composites, or any other appropriate material known in the art. [0094] In one embodiment, the center cable element 258 is shorter than the peripheral cable elenients 256. This causes the periphei'l elements 256 to assume a curved or bowed 20 shape between the endplates 252, 254, As a result, the length of the central cable -element" 258 determines the maximum distance between the upper and lower endplates 252i254 under tension. Furthermore, as a result of the peripheral cable elements 256 being longer than the central cable element 258, the shorter central-table element 258 causes the longer peripheral cable elements 256 to be held in compression The resilience of the bowed peripheral cable 25 elements 256 provides shock absorption, axial compression and articulation characteristics to the disc 450. [0095]. As a result of the materials, geometry, and components used, disc 250 can allow flexion/extension, lateral bending, axial rotation, and translation, depending on the loading conditions. In addition, under various spinal loading conditions resulting from spinal 30 movements, the peripheral cable elements 256 can bend or compress varying amounts, Such variable bending/compression provides the desired moving instantaneous axis ofrotation. [0096] With reference to Figure 12:, an exemplary installation procedure will be described. Generally speaking the disc 300 includes an upper endplate 302, a lower endplate 304 and a core mechanism 306, the core mechanism being any cable, elastomer, fiber, or 35 flid filled disc previously described. The intervertebral discs 300 may be implanted in a: modular fashion, for example, the endplates 302, 304 of disc 300 are inserted into the 21 WO 2004/016205 PCT/US2003/025535 5 intervertebral cavity using instruments such as a distractor and/or holder instrument. The intervertebral disc space may be distracted using a standard spinal distractor which engages the endplates 302,.304. Trial spacers are then preferably used to determine the appropriate size of the core mechanism 306 to be inserted in the resulting disc space. In an exemplary embodiment, the core mechanism, 306 is inserted and attached to endplates 302,304 through 10 the use of a dovetail, slot, or similar connection. This modulor insertion technique avoids over-distracting the intervertebral space, which may damage surrounding tissue and/or blood vessels. [0097] Alternatively, the intervertebral disc 300 may be inserted preassembled with the use -of particular insertion tools. For example, an endplate holding clip may bused that 15 allows the endplates 302, 304 to be held and locked in a parallel and spaced relationship as they are inserted into the intervertebral space. Once implanted, the clip may be unlocked and removed from the endplates 302, 304. The slip may then be removed from the intervertebral space- In addition, the disc 300 may be implanted in a compressed state to prevent over distraction. The introduction of the disc 300 in a compressed state may be accomplished via 20- a surgical insertion instrument or by an internal mechanism located in the disc 300. [0098]- An anterior, lateral, or anterolateral surgical approach may be used for the intervertebral disc 300. Furthermore, depending on the intervertebral disc 300 to-be implanted, a minimally invasive surgical method or a simultaneous distraction and implantation surgical method may be used. Simultaneous distraction and implantation.rnay 25 be accomplished, for example, by using slots formed on the outer surface of the ehdplates 302, 304 to guide the implant down the distractor during implantation. Also, depending on the intervertebral disc to be implanted, an artificial Anterior Longitudinal Ligament or the natural Anterior Longitudinal Ligament may be attached directly to the disc or to: the adjacent vertebral bodies. Attachment of the Anterior Longitudinal Ligament may assist in preventing 30 movement, dislodging or expulsion of the implant. To assist with the implantation of the intervertebral discs, the intervertebral discs may include alignment markers. [0099] While various descriptions okthe present invention are described above,. it should be understood that the various features can be used singly or in combination thereof, Therefore, this invention is not to be limited to the specific preferred embodiments depicted 35 herein. 22 WO 2004/016205 PCT/US2003/025535 5 [00100] Further, it should be understood that variations and modifcations within the. spirit and scope of the invention may occur to those skilled in the art to which the inventio .pertains. For example, some portions of the implants disclosed herein may be formed of bone, such as allografts, autografts, and xenografts, which may be partially or fully demineralized. In addition, some implants may include bone material or other bone. growth 10 inducing material in their interiors or on/intheir.endplates. Such substances in theiinteriors may be permitted to interact with the surrounding anatomy, as with channels or other holes formed in the implant walls, Also, intra and post-operative alignment markers maybe used to assist with implantation of the intervertebti discs, Furthermore, the intervertebral discs can be made rigid in situations where fusion is necessary. The intervertebral discs may be 15 made rigid by, for-example, allowing fusion between the endplates, inserting spacers between the endplates, or by injecting a solidifying liquid between the endplates. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included aS flrther: embodiments of the present inventions. The scope of the present invention is 20 eciprln defined as set forth in the appendd claims. 23