WO2003028583A2

WO2003028583A2 - Artificial intervertebral disc having a grooved belleville washer force restoring element

Info

Publication number: WO2003028583A2
Application number: PCT/US2002/019654
Authority: WO
Inventors: James D. Ralph; Stephen Tatar
Original assignee: Third Millennium Engineering Llc
Priority date: 2001-10-01
Filing date: 2002-06-19
Publication date: 2003-04-10
Also published as: AU2002345747A1; AU2002345747A8; WO2003028583A3

Abstract

An artificial disc having a pair of opposing plates (100, 200) for seating against opposing vertebral bone surfaces, separated by at least one spring mechanism. The preferred spring mechanism is at least one belleville washer (130) having radially extending grooves,although belleville washers having radially spaced concentric grooves, and belleville washers having a radially varying thickness are also disclosed for use herein. Preferably, the belleville washer utilized has a narrow end that is modified to mount onto a semispherical protuberance (201) on a plate, and a wide end that seats for expansion against an opposing plate. The narrow end of the belleville washer is modified by having a curvate socket that can be rotatably and angulatably mounted to the semispherical protuberance.

Description

ARTIFICIAL INTERVERTEBRAL DISC HAVING A GROOVED BELLEVILLE WASHER FORCE RESTORING ELEMENT

FIELD OF THE INVENTION [0001] This invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to simultaneously provide stabilization and continued flexibility and proper anatomical motion, and more specifically to such a device that utilizes a grooved belleville washer as a restoring force providing element.

BACKGROUND OF THE INVENTION

[0002] The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri- joint complex that consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.

[0003] The spinal column is highly complex in that it includes these more than 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.

[0004] Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/ or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.

[0005] Referring now to Figures 8 and 9, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages 10 generally comprise tubular metal body 12 having an external surface threading 14. They are inserted transverse to the axis of the spine 16, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in Figure 9 the pair of cages 10 are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (SI). Two cages 10 are generally inserted side by side with the external threading 14 tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (SI) below. The cages 10 include holes 18 through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior 20 of the cage 10 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage 10.

[0006] These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which nearly completely mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution. [0007] It is, therefore, an object of the invention to provide an intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space.

[0008] It is further an object of the invention to provide an implant device that stabilizes the spine while still permitting normal motion.

[0009] It is further an object of the invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient's spine.

[0010] It is further an object of the invention to provide an artificial disc that has an plate attachment device (for attaching the plates of the artificial disc to the vertebral bones between which the disc is implanted) with superior gripping and holding strength upon initial implantation and thereafter.

[0011] It is further an object of the invention to provide an artificial disc plate attachment device that deflects during insertion of the artificial disc between vertebral bodies.

[0012] It is further an object of the invention to provide an artificial disc plate attachment device that conforms to the concave surface of a vertebral body.

[0013] It is further an object of the invention to provide an artificial disc plate attachment device that does not restrict the angle at which the artificial disc can be implanted.

[0014] It is further an object of the invention to provide an artificial disc that supports tension loads.

[0015] It is further an object of the invention to provide an artificial disc that provides a centroid of motion centrally located within the intervertebral space. [0016] Other objects of the invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION [0017] The preceding objects are achieved by the invention, which is an artificial intervertebral disc or intervertebral spacer device comprising a pair of support members (e.g., spaced apart plates), each with an exterior surface. Because the artificial disc is to be positioned between the facing surfaces of adjacent vertebral bodies, the plates are arranged in a substantially parallel planar alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) with the exterior surfaces facing away from one another. The plates are to mate with the vertebral bodies so as to not rotate relative thereto, but rather to permit the spinal segments to axially compress and bend relative to one another in manners that mimic the natural motion of the spinal segment. This natural motion is permitted by the performance of a spring disposed between the secured plates, and the securing of the plates to the vertebral bone is preferably achieved through the use of a vertebral body contact element including, for example, a convex mesh attached to the exterior surface of each plate. Each convex mesh is secured at its perimeter, by laser welds, to the exterior surface of the respective plate. While domed in its initial undeflected conformation, the mesh deflects as necessary during insertion of the artificial disc between vertebral bodies, and, once the artificial disc is seated between the vertebral bodies, the mesh deforms as necessary under anatomical loads to reshape itself to the concave surface of the vertebral endplate. Thus, the wire mesh is deformably reshapeable under anatomical loads such that it conformably deflects against the concave surface to securably engage the vertebral body endplate. Stated alternatively, because the wire mesh is convexly shaped and is secured at its perimeter to the plate, the wire mesh is biased away from the plate but moveable toward the plate (under a load overcoming the bias; such a load is present, for example, as an anatomical load in the intervertebral space) so that it will securably engage the vertebral body endplate when disposed in the intervertebral space. This affords the plate having the mesh substantially superior gripping and holding strength upon initial implantation, as compared with other artificial disc products. The convex mesh further provides an osteoconductive surface through which the bone may ultimately grow. The mesh preferably is comprised of titanium, but can also be formed from other metals and/ or non-metals. Inasmuch as the mesh is domed, it does not restrict the angle at which the artificial disc can be implanted. It should be understood that while the flexible dome is described herein preferably as a wire mesh, other meshed or solid flexible elements can also be used, including flexible elements comprises of non-metals and/ or other metals. Further, the flexibility, deflectability and/ or deformability need not be provided by a flexible material, but can additionally or alternatively be provided mechanically or by other means.

[0018] To enhance the securing of the plates to the vertebral bones, plates in some embodiments further comprise at least a lateral porous ring (which may be, for example, a sprayed deposition layer, or an adhesive applied beaded metal layer, or another suitable porous coating known in the art). This porous ring permits the long- term ingrowth of vertebral bone into the plate, thus permanently securing the prosthesis within the intervertebral space. The porous layer may extend beneath the domed mesh as well, but is more importantly applied to the lateral rim of the exterior surface of the plate that seats directly against the vertebral body.

[0019] Between the plates, on the exterior of the device, there may also be included a circumferential wall that is resilient and that prevents vessels and tissues from entering the interior of the device. This resilient wall may comprise a porous fabric or a semi-impermeable elastomeric material. Suitable tissue compatible materials meeting the simple mechanical requirements of flexibility and durability are prevalent in a number of medical fields including cardiovascular medicine, wherein such materials are utilized for venous and arterial wall repair, or for use with artificial valve replacements. Alternatively, suitable plastic materials are utilized in the surgical repair of gross damage to muscles and organs. Still further materials, which could be utilized herein, may be found in the field of orthopedic in conjunction with ligament and tendon repair. It is anticipated that future developments in this area will produce materials, which are compatible for use with this invention, the breadth of which shall not be limited by the choice of such a material. For the purposes of this description, however, it shall be understood that such a circumferential wall is unnecessary, and in some instances may be a hindrance, and thusly is not included in the specific embodiments set forth hereinbelow.

[0020] The spring disposed between the plates provides a strong restoring force directed outward against the opposing plates when a compressive load is applied to the plates, and also permits rotation and angulation of the two plates relative to one another. While there is a wide variety of artificial disc embodiments contemplated, each embodiment family described herein includes at least one belleville washer, as representative of preferred types.

[0021] Belleville washers are washers that are generally bowed in the radial direction. Specifically, they have a radial convexity (i.e., the height of the washer is not linearly related to the radial distance, but may, for example, be parabolic in shape). The restoring force of a belleville washer is proportional to the elastic properties of the material. In addition, the magnitude of the compressive load support and the restoring force provided by the belleville washer may be modified by providing grooves in the washer, or radially varying its thickness. In the first embodiment family, the belleville washer utilized as the force restoring member has at least one radially extending groove. Preferred embodiments in this family have radially extending grooves that decrease in width and depth from the outside edge of the washer toward the center of the washer. In the second embodiment family, the belleville washer utilized as the force restoring member has at least one radially spaced concentric groove. Preferred embodiments in this family have concentric grooves of uniform width and/ or depth, while other embodiments have concentric grooves of varying width and/ or depth. In the third embodiment family, the belleville washer utilized as the force restoring member has a radially varying thickness. These three embodiments families are illustrated using the same upper plate and lower plate for each embodiment. In the fourth embodiment family, an alternate set of plates is described for use with any of the belleville washers described herein. [0022] As a compressive load is applied to a belleville washer, the forces are directed into a hoop stress that tends to radially expand the washer. This hoop stress is counterbalanced by the material strength of the washer, and the strain of the material causes a deflection in the height of the washer. Stated equivalently, a belleville washer responds to a compressive load by deflecting compressively, but provides a restoring force which is proportional to the elastic modulus of the material in a hoop stressed condition. With grooves formed in the washer, it expands and restores itself far more elastically than a solid washer. Radially extending grooves allow the washer to expand radially as the grooves widen under the load, only to spring back into its undeflected shape upon the unloading of the spring. Concentric grooves allow the washer to deflect in height more easily, or to have a different or varied load-to-deflection ratio compared to a washer that is not concentrically grooved. In general, a belleville washer is one of the strongest configurations for a spring, and is highly suitable for use as a restoring force providing subassembly for use in an intervertebral spacer element that must endure considerable cyclical loading in an active human adult.

[0023] Preferably, any of the belleville washers is utilized in conjunction with a semispherical protuberance (e.g., a ball-shaped headed post) on which it is free to rotate and angulate through a range of angles (thus permitting the plates to rotate and angulate relative to one another through a corresponding range of angles). More particularly, embodiments of the first three embodiment families preferably comprise a pair of spaced apart plates, one of which is simply a disc shaped member (preferably shaped to match the endplate of an intervertebral disc) having an outer surface, or external face, that has the porous coating discussed above, and an internal face, or inner surface, that has an annular retaining wall having a purpose discussed below. The other of the plates is similarly shaped, having an exterior f ce with a porous coating, but further includes on its internal face a semispherical protuberance provide by a central post portion that rises out of the internal face at a nearly perpendicular angle, in that the top of this post portion includes a ball-shaped knob. The knob includes a central threaded axial bore that receives a deflection preventing element (e.g., a small set screw). Prior to the insertion of the set screw, the ball- shaped head of the post can deflect radially inward (so that the ball-shaped knob contracts). The insertion of the set screw eliminates the capacity for this deflection. [0024] As introduced above, a belleville washer is mounted to this ball- shaped knob in such a way that it may rotate and angulate freely through a range of angles equivalent to the fraction of normal human spine rotation (to mimic normal disc rotation). Each belleville washer of the invention is modified by including a curvate socket (e.g., an enlarged inner circumferential portion at the center of the washer) that accommodates the ball-shaped portion of the post. More particularly, the enlarged portion of the modified belleville washer includes a curvate volume, or curvate socket, having a substantially constant radius of curvature that is also substantially equivalent to the radius of the ball-shaped head of the post. The deflectability of the ball-shaped head of the post, prior to the insertion of the set screw, permits the head to be inserted into the interior volume at the center of the belleville washer. Subsequent introduction of the set screw into the axial bore of the post prevents the ball-shaped head from deflecting. Thereby, the washer can be secured to the ball-shaped head so that it can rotate and angulate thereon through a range of proper lordotic angles (in some embodiments, a tightening of the set screw locks the washer on the ball-shaped head at one of the lordotic angles). [0025] Embodiments of the fourth embodiment family preferably comprise alternate plates, preferably in conjunction with a shield member, to achieve the same functionality as the plates of the first, second and third embodiment families, and are for use with any of the belleville washers described herein. More particularly, a lower plate of the fourth embodiment family has a circular recess in the inner face (inwardly facing surface) of the plate, which circular recess has a circumferential wall having the purpose and functionality of the annular retaining wall described above. The lower plate also utilizes a shield member placed over the belleville washer (when the belleville washer is disposed in the circular recess) and secured to the plate at the perimeter of the shield, which shield member has the purpose and functionality of the annular retaining ring described above. Preferably, the shield member is frusto- conical in shape so that is has a central hole to permit passage therethrough of the ball-shaped head and post of the opposing plate during assembly. An opposing plate, having a semispherical protuberance with radial slots and an axial bore (for receiving a deflection preventing element such as, for example, a rivet), provides functionality similar to the ball-shaped headed post described above, but with a lower profile. Both of the plates have the convex mesh described above for securing to adjacent vertebral bones.

[0026] With the several types of plates, the several types of belleville washers, and the several manners in which they may be coupled together, it is possible to assemble a variety of artificial disc embodiments. Many examples are described herein as noted above, although many permutations that are contemplated and encompassed by the invention are not specifically identified herein, but are readily identifiable with an understanding of the invention as described. [0027] Each assembly enjoys spring-like performance with respect to axial compressive loads, as well as long cycle life to mimic the axial biomechanical performance of the normal human intervertebral disc. The radially extending grooves allow the washers to expand radially as the grooves widen under the load, only to spring back into an undeflected shape upon the unloading of the spring. The concentric grooves allow the washers to deflect in height more easily. As each washer compresses and decompresses, the annual retaining wall (or circular recess wall) maintains the wide end of the washer within a prescribed boundary on the internal face of the plate that it contacts. Further, the assemblies withstand tension loads on the outwardly facing surfaces, because the annular retaining ring (or retaining shield) maintains the wide end of the washer against the internal face, and the set screw (or rivet) in the axial bore prevents the semispherical protuberance (either variation) from deflecting, thus preventing it from exiting the curvate socket. Accordingly, once the plates are secured to the vertebral bones, the assembly will not come apart when a normally experienced tension load is applied to the spine, similar to the tension- bearing integrity of a healthy natural intervertebral disc.

[0028] Assemblies having the ball-and-socket joint also provide a centroid of motion centrally located within the intervertebral space, because the plates are made rotatable and angulatable relative to one another by the semispherical protuberance being rotatably and angulatably coupled in the curvate socket. The centroid of motion remains in the semispherical protuberance, and thus remains centrally located between the vertebral bodies, similar to the centroid of motion in a healthy natural intervertebral disc.

[0029] Finally, inasmuch as the human body has a tendency to produce fibrous tissues in perceived voids, such as may be found within the interior of the invention, and such fibrous tissues may interfere with the stable and/ or predicted functioning of the device, some embodiments of the invention (although not in preferred embodiments) will be filled with a highly resilient elastomeric material. The material itself should be highly biologically inert, and should not substantially interfere with the restoring forces provided by the spring-like mechanisms therein. Suitable materials may include hydrophilic monomers such as are used in contact lenses. Alternative materials include silicone jellies and collagens such as have been used in cosmetic applications. As with the exterior circumferential wall, which was described above as having a variety of suitable alternative materials, it is anticipated that future research will produce alternatives to the materials described herein, and that the future existence of such materials which may be used in conjunction with the invention shall not limit the breadth thereof.

BRIEF DESCRIPTION OF THE DRAWINGS [0030] Figure 1 is a side perspective view of an interbody fusion device of the prior art.

[0031] Figure 2 is a front view of the anterior portion of the lumbo-sacral region of a human spine, into which a pair of interbody fusion devices of the type shown in Figure 1 have been implanted.

[0032] Figures 3a and 3b are side cross-section views of the upper and lower opposing plates of a first embodiment family of the invention.

[0033] Figures 4a and 4b are top and side cross-section views of a belleville washer having radially extending grooves, for use in the first embodiment family. [0034] Figure 5a is a top view of the upper plate of Figure 3a, with the belleville washer of Figures 4a and 4b fitted within a retaining wall and a retaining ring of the upper plate.

[0035] Figure 5b is a top view of the lower plate of Figure 3b.

[0036] Figure 6 is a side cross-section view of an embodiment in the first embodiment family, which utilizes a belleville washer of the type shown in Figures 4a and 4b, showing the plates of Figures 5a and 5b assembled together.

[0037] Figures 7a-b are side cross-section views of upper and lower opposing plates of a second embodiment family of the invention.

[0038] Figures 8a-b are cross-section views of belleville washers, having radially varying thicknesses, of the type used in the second embodiment family, the belleville washer of Figure 8a having a washer shape with a thicker inner portion than outer, the belleville washer of Figure 8b having a washer shape with a thinner inner portion than outer.

[0039] Figures 9a-b are top views of the opposing plates of Figures 7a-b, and more particularly, Figure 9a is a top view of the plate having the circumferential skirt and retaining ring, in which a belleville washer of the type of either Figures 8a or 8b is disposed within the skirt, and Figure 9b is a top view of the plate having a post element that seats within the central opening of the belleville washer.

[0040] Figures lOa-b are side cross-section views of fully assembled embodiments in the second embodiment family, which utilize the corresponding belleville washers illustrated in Figures 8a-b mounted between the plates illustrated in Figures 9a-b.

[0041] Figures lla-b are top and side cross-section views of a belleville washer, having radially spaced concentric grooves of uniform width and depth, of a type used in a third embodiment family of the invention.

[0042] Figures 12a-c are top and side cross-section views of a belleville washer, having radially spaced concentric grooves of varying width and depth, of a type used in the third embodiment family.

[0043] Figure 13 is a top view of the upper plate of Figure 7a, with the belleville washer of Figures lla-b fitted within a retaining wall and a retaining ring of the upper plate.

[0044] Figure 14 is a side cross-section view of a fully assembled embodiment in the third embodiment family, which utilizes a belleville washer of the type shown in Figures lla-b, showing the plates of Figures 13 and 9b assembled together. [0045] Figure 15 is a top view of the upper plate of Figure 7a, with the belleville washer of Figures 12a-c fitted within a retaining wall and a retaining ring of the upper plate.

[0046] Figure 16 is a side cross-section view of a fully assembled embodiment in the third embodiment family, which utilizes a belleville washer of the type shown in Figures 12a-c, showing the plates of Figures 15 and 9b assembled together.

[0047] Figures 17a-c are bottom plan, side cross-section, and top plan views of a lower plate of a fourth embodiment family of the invention, having a circular recess and rivet holes. [0048] Figures 18a-c are bottom plan, side cross-section, and top plan views of an upper plate of the fourth embodiment family, having a semispherical protuberance.

[0049] Figure 19 is a side cross-section view of a fully assembled embodiment in the fourth embodiment family, which shows a radially grooved belleville washer having a curvate socket mounted between the plate of Figures 17a-c and the plate of Figures 18a-c.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0050] While the invention will be described more fully hereinafter with reference to the accompanying drawings, in which particular embodiments and methods of implantation are shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while achieving the functions and results of this invention. Accordingly, the descriptions that follow are to be understood as illustrative and exemplary of specific structures, aspects and features within the broad scope of the invention and not as limiting of such broad scope. Like numbers refer to similar features of like elements throughout.

[005i] Referring now to Figures 3a and 3b, side cross-section views of upper and lower plate members 100,200 of a first embodiment family of the invention are shown. (As with all embodiments described herein, "upper" and "lower" are merely visual designations to describe the positions of the plates in accordance with the illustrations; it should be understood that the invention encompasses embodiments where the "upper" plates serve as lower plates and "lower" plates serve as upper plates.) As the device is designed to be positioned between the facing surfaces of adjacent vertebral bodies, the plates include substantially flat external face portions 102,202 that seat against the opposing bone surfaces. In addition, the plates are to mate with the bone surfaces in such a way as to not rotate relative thereto. Therefore, the external faces (outer surfaces) of the plates include a porous coating 104,204 into which the bone of the vertebral body can grow. (Note that this limited fusion of the bone to the plate does not extend across the intervertebral space.) A hole (not shown) can be provided in the lower plate such that the interior of the device may be readily accessed if a need should arise.

[0052] The upper plate 100 includes an internal face (inner surface, or inwardly facing surface) 103 that includes an annular retaining wall 108 and an annular retaining ring 109. The upper plate 200 includes an internal face (inner surface, or inwardly facing surface) 203 that includes a semispherical protuberance (e.g. here, provided by a central ball-shaped headed post member 201) that rises out of the internal face 203 at a nearly perpendicular angle. The top of this post member 201 includes a ball-shaped head 207. The head 207 includes a series of slots that render it compressible and expandable in correspondence with a radial pressure (or a radial component of a pressure applied thereto). The head 207 includes a central axial bore 209 that extends down the post 201. This bore 209 is designed to receive a deflection preventing element (e.g., a rivet, plug, dowel, or set screw; a set screw 205 is used herein as an example, and thus the axial bore is threaded in this embodiment). Prior to the insertion of the set screw 205, the ball-shaped head 207 of the post 201 can deflect radially inward because of the slots (so that the ball-shaped head contracts). The insertion of the set screw 205 eliminates the capacity for this deflection.

[0053] Referring now to Figures 4a and 4b, a belleville washer 130 of the first embodiment family, having radially extending grooves, is provided in top and side cross-section views, respectively. The belleville washer 130 is a restoring force providing device that comprises a circular shape, having a central opening 132, and which is radially arched in shape (it should be understood that belleville washers having a straight radial extent, e.g., such that they are frusto-conical, can also be used). The belleville washer 130 has a radial convexity 134 (i.e., the height of the washer 130 is not linearly related to the radial distance, but may, for example, be parabolic in shape). The restoring force of the belleville washer 130 is proportional to the elastic properties of the material.

[0054] The belleville washer 130 comprises a series of grooves 133 formed therein. The grooves 133 extend radially from the outer edge of the belleville washer toward the center of the element. Preferably, the width 135 and depth 137 of each groove 133 decreases along the length of the groove 133 from the outer edge of the washer toward the center of the washer, such that the center of the washer is flat, while the outer edge of the washer has grooves of a maximum groove depth. It should be understood that in other embodiments, one or both of the depth and the width of each groove can be (1) increasing along the length of the groove from the outer edge of the washer toward the center of the washer, (2) uniform along the length of the groove from the outer edge of the washer toward the center of the washer, or (3) varied along the length of each groove from the outer edge of the washer toward the center of the washer, either randomly or according to a pattern. Moreover, in other embodiments, it can be the case that each groove is not formed similarly to one or more other grooves, but rather one or more grooves are formed in any of the above-mentioned fashions, while one or more other grooves are formed in another of the above-mentioned fashions or other fashions. It should be clear that any groove pattern can be implemented without departing from the scope of the invention.

[0055] As a compressive load is applied to the belleville washer 130, the forces are directed into a hoop stress that tends to radially expand the washer. This hoop stress is counterbalanced by the material strength of the washer, and the force necessary to widen the radial grooves 133 along with the strain of the material causes a deflection in the height of the washer. Stated equivalently, the belleville washer 130 responds to a compressive load by deflecting compressively; the radial grooves cause the washer to further respond to the load by spreading as the grooves in the washer expand under the load. The spring, therefore, provides a restoring force that is proportional to the elastic modulus of the material in a hoop stressed condition.

[0056] More particularly, the belleville washer has a curvate socket for receiving the semispherical protuberance, and in this respect for example, the central opening 132 of the belleville washer is enlarged. This central opening 132 includes a curvate socket (e.g., a curvate volume) 233, for receiving therein the ball-shaped head

207 of the post 201 of the lower plate 200 described above. More particularly, the curvate volume 233 has a substantially constant radius of curvature that is also substantially equivalent to the radius of the ball-shaped head 207 of the post 201. In this embodiment, the depth 137 of each groove 133 decreases along the length of the groove 133 from the outer edge of the washer toward the center of the washer, such that the center of the washer is flat, while the outer edge of the washer has grooves of a maximum groove depth. Therefore, the central opening 132 can be formed from flat edges. It should be understood that this is not required, but rather is preferred for this embodiment.

[0057] Referring now to Figure 5a, a top view of the upper plate 100 of Figure 3a, with the radially grooved belleville washer 130 of Figures 4a and 4b fitted within a retaining wall 108 and a retaining ring 109 of the upper plate 100, is shown. The diameter of the retaining wall 108 is preferably slightly wider than the diameter of the undeflected belleville washer 130 such that the loading thereof can result in an unrestrained radial deflection of the washer 130. Figure 5b shows a top view of the lower plate 200 of Figure 3b.

[0058] Referring also to Figure 6, a fully assembled embodiment in the first embodiment family of the invention is shown. The radially grooved belleville washer 130 is placed with its wide end against the top plate 100 within the annular retaining wall 108 as shown in Figure 5b. The annular retaining ring 109 is provided to hold the belleville washer 130 against the internal face 103 of the upper plate 100 within the retaining wall 108. The post 201 of the lower plate 200 is fitted into the central opening 132 of the belleville washer 130 (the deflectability of the ball-shaped head 207 of the post 201, prior to the insertion of the set screw 205, permits the head 207 to be inserted into the interior volume 233 at the center of the belleville washer 130. Subsequent introduction of the set screw 205 into the axial bore 209 of the post 201 eliminates the deflectability of the head 207 so that the washer 130 cannot be readily removed therefrom, but can still rotate and angulate thereon. (In some embodiments, but not in this embodiment, the post head 207 can be dimensioned so that it can be locked tightly within the central volume 233 of the belleville washer 130 by a further or alternate tightening of the set screw 205, to prevent any rotation of the plates 100,200.) Compressive loading of the assembly causes the washer 130 to deflect (with the radially extending grooves enhancing the deflection) so that the wide end radially expands while being maintained centrally against the upper plate 100 by the retaining wall 108 and the retaining ring 109. When the load is removed, the washer 130 springs back to its original shape.

[0059] With regard to the second embodiment family, and referring now to Figures 7a-b, side cross-section views of the top and bottom plate members 300,400 of a second embodiment family of the invention are shown. Similar to the plates 100,200 of the first embodiment family, the plates 300,400 have substantially flat surface portions 302,402 that seat against the opposing bone surfaces and a porous coating 304,404 into which the bone of the vertebral body can grow. As shown in Figures 7c-d, the most desirable upper and lower plate surface porous feature is a deflectable mesh 408 (preferably made of metal such as titanium) into which the bone can readily grow, and which mesh will deform to seat into the concave upper and lower bone faces. (Note that this limited fusion of the bone to the plate does not extend across the intervertebral space.) These features, while being preferred are not required, and further can be used with any of the embodiments described herein, as well as other embodiments and for other intervertebral spacer devices and the like.

[0060] Referring now also to Figure 9a, plate 300 further includes a circumferential skirt 306 that serves as a retaining wall, into which the wide end of a belleville washer may be seated. The diameter of the retaining wall 306 is preferably wider than the diameter of the undeflected belleville washer such that the loading thereof can result in an unrestrained radial deflection of the washer. The inner surface of the retaining wall 306 includes an annular recess into which a retaining ring 310 may be provided for holding the belleville washer in place (see, e.g., the assembled embodiments of Figures lOa-b). [0061] Referring now also to Figure 9b, plate 400, similar to the plate 200 of the first embodiment family, further includes a semispherical protuberance. More particularly, this is provided by a central post 406 having at its top end a deflectable ball-shaped head 410 with radial slots 412 and an axial bore 414 for receiving a deflection preventing element, (e.g., a rivet, plug, dowel, or set screw; a set screw 416 is used herein as an example) that rises out of the interior face 408 of the plate 400 at a nearly perpendicular angle.

[0062] Referring now to Figures 8a-b, side cross-section views of two belleville washers 330a,330b are provided. Each of these belleville washers 330a,330b is similar in form and function to the belleville washer of the first embodiment family, but with a significant difference in that the washers do not have radially extending grooves, but the thickness (the distance from the concave surface to the convex surface) of the material that comprises the washer varies from the central opening 332a,332b region to the outer circumference 334a,334b of the washer. (It should be understood, however, that the invention encompasses belleville washers have both a radially varying thickness in addition to radially extending grooves. Also, as concentrically grooved belleville washers will be discussed below, it should be understood that the invention encompasses belleville washers having various permutations and combinations of radial groove features, concentric groove features, and thickness variance features, such that one, two or all of such features, including each feature's various forms, can be applied to a belleville washer to provide additional belleville washers of the invention that have unique expansion and restoration characteristics.

[0063] More particularly with respect to the washer in Figure 8a (and shown within the circumferential ring of plate 300 in Figure 9a), the belleville washer 330a has a greater thickness at the outer edge 334a than it does at the inner edge 332a. As the restoring force of a belleville washer is proportional to the elastic properties of the material as well as the quantity of material being loaded, the reduction of the material at the edge of the inner opening 332a permits a load/ deflection profile in which the load which deflects the inner portion of the washer is less than the outer portion. This permits the washer to compress to initially compress easily under a light loading, but to rapidly (faster than a straight linear loading profile) become stiff and resist deflection. This loading profile is more anatomically relevant with respect to mimicking the performance of the cartilage present in a healthy intervertebral space. [0064] More particularly with respect to the washer in Figure 8b (and shown within the circumferential ring of plate 300 in Figure 9a), the belleville washer 330b has a smaller thickness at the outer edge 334b than it does at the inner edge 332b. As the restoring force of a belleville washer is proportional to the elastic properties of the material as well as the quantity of material being loaded, the reduction of the material at the outer edge 334b permits a load profile in which the load that deflects the outer portion of the washer is less than the inner portion. This permits the washer to compress to initially compress easily under a light loading (as a result of outer edge deflection), but to rapidly (faster than a straight linear loading profile) become stiff and resist deflection. This loading profile is more anatomically relevant with respect to mimicking the performance of the cartilage present in a healthy intervertebral space.

[0065] In addition, each of the belleville washers includes a curvate socket, provided for example in that the central openings of each of the belleville washers 330a,330b further include a curvate volume 336a,336b for receiving therein the ball- shaped head 410 of the post 406 of the lower plate 400, the curvate volume being similar in form and function to that of the belleville washer of the first embodiment family.

[0066] Referring now to Figures lOa-b, side cross-section views of fully assembled embodiments of the second embodiment family are provided. Each structure includes a belleville washer (selected from those illustrated in Figures 8a-b) having its wide end held against the plate 300 of Figure 7a by the retaining ring 310 and retaining wall 306, and its central opening 332a,332b rotatably and angulatably secured to the ball-shaped head 410 of the plate 400 of Figure 7b and 9b by a set screw 416 received in the threaded bore 414 of the head 410 (after the head 410 is placed in the central opening 332a,332b), similar in this respect to the assembly of the embodiments in the first embodiment family.

[0067] With regard to the third embodiment family, the opposing plates of Figures 7a-d and 9b are useful in these embodiments as well and will be referred to herein accordingly. Referring now to Figures lla-b, a belleville washer 730 of a third embodiment family of the invention, having radially space concentric grooves 733a-c, is provided in top and side cross-section views. The grooves 733a-c are concentric and radially spaced from the outer edge of the belleville washer toward the center of the element. Preferably, as shown, the width 735 of each groove is uniform along the length of the groove. Further preferably, the depth 737 of each groove is uniform along the length of the groove. Further preferably, each groove has a different width configuration and a different depth configuration than each other groove. More specifically, preferably, the width dimension and the depth dimension both vary from groove to groove, each increasing incrementally from groove to adjacent groove with increasing distance from the center of the washer. Stated alternatively, grooves that are relatively more narrow and more shallow than the other grooves are closer to the center of the washer, whereas grooves that are relatively wider and deeper than the other grooves are closer to the outer edge of the washer. This is illustrated by example in Figures lla-b, which shows three concentric grooves 733a-c, with the outermost groove 733c being deeper and wider than groove 733b, which is in turn deeper and wider than groove 733a. Further preferably, the radial spacing of the grooves is uniform.

[0068] It should be understood that in other embodiments, one or both of the depth and the width of each groove can be (1) increasing along the length of the groove, (2) decreasing along the length of the groove, or (3) varied along the length of each groove, either randomly or according to a pattern. Moreover, in other embodiments, it can be the case that each groove is not formed similarly to one or more other grooves, with or without respect to width and depth dimensions, but rather one or more grooves are formed in any of the above-mentioned fashions, while one or more other grooves are formed in another of the above-mentioned fashions or other fashions. Also, in other embodiments, it can be the case that the radial distance between the grooves is not the same, but rather the spacing increases the closer the space is to the outer edge of the washer, decreases the closer the space is to the outer edge of the washer, or varies either randomly or according to a pattern. Also, while the grooves preferably have lengths that form closed loops as illustrated, it should be noted that in other embodiments, the concentric grooves can have lengths that form open loops or arcs; for example, a two concentric grooves forming open loops or arcs can be used in place of a single concentric groove forming a closed loop. It should be clear that any concentric groove pattern can be implemented without departing from the scope of the invention.

[0069] To illustrate alternate embodiments showing an alternate radially spaced concentric groove pattern, Figures 12a-c show a belleville washer 930 having radially spaced concentric grooves 933a-c in top and side cross-section views, with each groove having a width and a depth each varying along the length of the groove, with each groove being formed differently than at least one other groove, with the radial spacing of the grooves being varied, and with both closed loops and open loops or arcs being used. In these alternate embodiments, the difference between the grooves is characterized in that the wider and deeper portion of any particular groove is on a different side of the washer than the wider and deeper portion of at least one other groove.

[0070] Each belleville washer has a curvate socket for coupling to a semispherical protuberance of a plate, provided in that the central opening 732,932 of each belleville washer 730,930 includes a curvate socket (e.g., a curvate volume 833,1033) for receiving therein the ball-shaped head 410 of the post 406 of the lower plate 400, the curvate volume 833,1033 being similar in form and function to that of the belleville washers of the first and second embodiment families. The central opening 732,932 is preferably formed from flat edges, in that the grooves 733a-c,933a- c preferably do not encroach on the center of the washer. It should be understood that this is not required, but rather is preferred.

[0071] Figures 13 and 15 are top views of the upper plate 300 of Figure 7a, each showing a concentrically grooved belleville washer 730,930 fitted within a retaining wall 306 and a retaining ring 310 of the upper plate 300, similar to the assembly of the embodiments in the first and second embodiment families. Figures 14 and 16 show fully assembled embodiments in the third embodiment family, each including a concentrically grooved belleville washer 730,930 with its wide end held against the plate 300 of Figure 7a by the retaining ring 310 and retaining wall 306, and its central opening 732,932 rotatably and angulatably secured to the ball-shaped head 410 of ύιe plate 400 of Figure 7b and 9b by a set screw 416 received in the threaded bore 414 of the head 410 (after the head 410 is placed in the central opening 732,932).

[0072] Each belleville washer 730,930 deflects under a compressive load similar to the belleville washer of the first embodiment family, with a difference inasmuch as the concentric grooves further enhance the deflection in height under the load, and or alter the load-to-deflection ratio of the washer, as compared with washers that are not concentrically grooved. When the load is removed, the washer 730,930 springs back to its original shape.

[0073] With regard to the fourth embodiment family, and referring now to Figures 17a-c and 18a-c, two alternate plates of the invention are shown in bottom plan views (Figures 17a and 18a), side cutaway views (where cross-sectional areas and surfaces viewable behind them are shown) (Figures 17b and 18b), and top plan views (Figures 17c and 18c). More specifically, Figures 17a-b show a bottom plan view and a side cutaway view, respectively, of an alternate lower plate 500a. Figures 18a-b show a bottom plan view and a side cutaway view, respectively, of an alternate upper plate 500b.

[0074] Each plate 500a-b has an exterior surface 508a-b. Because the artificial disc of the invention is to be positioned between the facing surfaces of adjacent vertebral bodies, the two plates used in the artificial disc are disposed such that the exterior surfaces face away from one another (as best seen in Figure 19, discussed below). The two plates are to mate with the vertebral bodies so as to not rotate relative thereto, but rather to permit the spinal segments to axially compress and bend relative to one another in manners that mimic the natural motion of the spinal segment. This motion is permitted by the performance of a belleville washer (any described herein or others) disposed between the secured plates. The mating of the plates to the vertebral bodies and the application of the belleville washer to the plates are described below.

[0075] More particularly, each plate 500a-b is a flat plate (preferably made of a metal such as, for example, titanium) having an overall shape that conforms to the overall shape of the respective endplate of the vertebral body with which it is to mate. Further, each plate 500a-b comprises a vertebral body contact element (e.g., a convex mesh 506a-b (preferably oval in shape) that is attached to the exterior surface (outer surface, or external face) 508a-b of the plate 500a-b to provide a vertebral body contact surface. The mesh 506a-b is secured at its perimeter, by laser welds, to the exterior surface 508a-b of the plate 500a-b. The mesh is domed in its initial undeflected conformation, but deflects as necessary during insertion of the artificial disc between vertebral bodies, and, once the artificial disc is seated between the vertebral bodies, deforms as necessary under anatomical loads to reshape itself to the concave surface of the vertebral endplate. This affords the plate having the mesh substantially superior gripping and holding strength upon initial implantation as compared with other artificial disc products. The mesh further provides an osteoconductive surface through which the bone may ultimately grow. The mesh is preferably comprised of titanium, but can also be formed from other metals and/ or non-metals without departing from the scope of the invention. [0076] Each plate 500a-b further comprises at least a lateral ring 510a-b that is osteoconductive, which may be, for example, a sprayed deposition layer, or an adhesive applied beaded metal layer, or another suitable porous coating. This porous ring permits the long-term ingrowth of vertebral bone into the plate, thus permanently securing the prosthesis within the intervertebral space. It shall be understood that this porous layer 510a-b may extend beneath the domed mesh 506a- b as well, but is more importantly applied to the lateral rim of the exterior surface 508a-b of the plate 500a-b that seats directly against the vertebral body.

[0077] It should be understood that the convex mesh attachment devices and methods described herein can be used not only with the artificial discs and artificial disc plates described or referred to herein, but also with other artificial discs and artificial disc plates, including, but not limited to, those currently known in the art. Therefore, the description of the mesh attachment devices and methods being used with the artificial discs and artificial disc plates described or referred to herein should not be construed as limiting the application and/ or usefulness of the mesh attachment device.

[0078] With regard to the disposition of a belleville washer between these two plates, each of the plates 500a-b comprises features for applying the belleville washer thereto, and the various application methods are described below. More specifically, the lower plate 500a includes an inwardly facing surface (inner surface, or internal face) 504a that includes a circular recess 502a for rotationally housing a wide end of a belleville washer and allowing the wide end to expand in unrestricted fashion when the belleville washer is compressed, and the inwardly facing surface 504a also accepts fasteners (e.g., rivets, plugs, dowels, or set screw; rivets 516a are used herein as examples) (shown in Figure 19) for securing a retaining element (e.g., a shield 518a) (the purpose and application of the shield are described below and shown on Figure 19).

[0079] The upper plate 500b includes an inwardly facing surface 504b that includes an inwardly directed semispherical (e.g., ball-shaped) protuberance 502b. The ball-shaped protuberance 502b includes a series of slots 520b that render the ball- shaped protuberance 502b radially compressible and expandable in correspondence with a radial pressure (or a radial component of a pressure applied thereto). The ball- shaped protuberance 502b further includes an axial bore 522b that accepts a deflection preventing element (e.g., a set screw, dowel, plug, or rivet; a rivet 524b is used herein as an example) (shown in Figure 19). (If a set screw is used, the axial bore can be threaded to accept it.) Prior to the insertion of the rivet 524b, the ball-shaped protuberance 502b can deflect radially inward because the slots 520b will narrow under a radial pressure. The insertion of the rivet 524b eliminates the capacity for this deflection. Therefore, the ball-shaped protuberance 502b, before receiving the rivet 524b, can be compressed to seat in a curvate socket portion of a belleville washer and, once the ball-shaped protuberance 502b has been seated in the curvate socket, the rivet 524b can be inserted into the axial bore 522b to ensure that the ball-shaped protuberance 502b remains held in the curvate socket. A hole can be provided in the opposing plate so that the interior of the device may be readily accessed if a need should arise. It should be understood that the specific dimensions of the ball-shaped protuberance, the mechanism for radial compressibility of the ball-shaped protuberance, and the mechanism for preventing radial compression of the ball- shaped protuberance are not limited to those shown, but rather can be varied and changed without departing from the scope of the invention. [0080] Referring now to Figure 19, a side cross-section view of a fully assembled embodiment in the fourth embodiment family is provided. The structure includes a radially grooved belleville washer 600 of a type described herein (although other belleville washers, including the others described herein, can be used in similar fashion) having a curvate socket of a type described herein, with its wide end held against the plate 500a of Figures 17a-c by a shield 518a encompassing the extent of the belleville washer. More specifically, the wide end of the belleville washer fits within the circular recess 502a with room to expand when the belleville washer is under compression. Because the diameter of the circular recess is greater than the diameter of the wide end of the belleville washer, unrestrained rotation of the belleville washer relative to the plate is enabled, and compressive loading of the artificial disc (and therefore the belleville washer) results in an unrestrained radial deflection of the belleville washer, both as necessary for proper anatomical response. To prevent removal of the wide end of the belleville washer from the circular recess when the artificial disc is loaded in tension, a shield 518a is placed over the belleville washer and secured by fasteners (e.g., rivets 516a). The shield 518a is preferably frusto-conical such that it has a central hole 520a through which the ball-shaped protuberance of the opposing plate can pass to accommodate efficient assembly of the artificial disc. The shield 518a can alternatively or additionally be formed from multiple shield parts. With regard to the narrow end of the belleville washer (the end having the curvate socket), this end is rotatably and angulatably coupled to the ball- shaped protuberance on the opposing plate, as described above.

[0081] In embodiments having a ball-and-socket joint as described herein, because the ball-shaped protuberance is held within the curvate socket by a rivet or set screw in the axial bore preventing radial compression of the ball-shaped protuberance, the artificial disc can withstand tension loading of the plates, as necessary for proper anatomical response. More particularly, when a tension load is applied to the plates, the ball-shaped protuberance in the curvate socket seeks to radially compress to fit through the opening of the curvate socket. However, the rivet or set screw in the axial bore of the ball-shaped protuberance prevents the radial compression, thereby preventing the ball-shaped protuberance from exiting the curvate socket. Further, as the wide end of the belleville washer seeks to separate from the plate when there is a tension load, the retaining ring (or the shield) prevents the separation when the belleville washer presses against the inner surface of the ring or shield. Therefore, the assembly does not come apart under normally experienced tension loads. This ensures that no individual parts of the assembly will pop out or slip out from between the vertebral bodies when the patient stretches or hangs while exercising or performing other activities. Thus, in combination with the securing of the plates to the adjacent vertebral bones via the mesh domes, the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc.

[0082] Further, because the plates in some embodiments are made angulatable relative to one another by the ball-shaped protuberance being rotatably and angulatably coupled in a curvate socket, the disc assembly provides a centroid of motion within the ball-shaped protuberance. Accordingly, in those embodiments, the centroid of motion of the disc assembly remains centrally located between the vertebral bodies, similar to the centroid of motion in a healthy natural intervertebral disc. [0083] Inasmuch as the human body has a tendency to produce fibrous tissues in perceived voids, such as may be found within the interior of the invention, and such fibrous tissues may interfere with the stable and/ or predicted functioning of the device, preferred embodiments of the invention will be filled with a highly resilient and biologically inert elastomeric material. Suitable materials may include hydrophilic monomers such as are used in contact lenses. Alternative materials include silicone jellies and coHagens such as have been used in cosmetic applications.

[0084] While there has been described and illustrated specific embodiments of an artificial disc, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the invention. The invention, therefore, shall not be limited to the specific embodiments discussed herein.

Claims

CLAIMSWhat is claimed is:

1. An intervertebral spacer device, comprising: first and second plates, said plates being disposed in a spaced apart relationship such that a plate surface of said first plate faces a plate surface of said second plate, the facing surfaces being inner surfaces, and alternative faces of each plate being outer surfaces; and at least one restoring force providing element disposed between the inner surfaces of said first and second plates, and disposed such that a compressive load applied to the outer surfaces of said first and second plates is counteracted by said at least one restoring force providing element, said at least one restoring force providing element including at least one belleville washer that is selected from the group consisting of a belleville washer having a radially varying thickness and a grooved belleville washer.

2. The intervertebral spacer device of claim 1, wherein the at least one belleville washer has a narrow end and a wide end and is oriented such that the wide end is in contact with the inner surface of one of said first and second plates.

3. The intervertebral spacer device of claim 1, wherein the at least one belleville washer has a central opening forming a curvate socket.

4. The intervertebral spacer device of claim 3, wherein at least one of said first and second plates has on its inner surface a semispherical protuberance that is couplable to the curvate socket.

5. The intervertebral spacer device of claim 4, wherein the semispherical protuberance comprises at least one radial slot such that the semispherical protuberance is radially deflectable upon the application of a radially inwardly directed force.

6. The intervertebral spacer device of claim 5, wherein the semispherical protuberance further comprises an axial bore into which a deflection preventing element is disposable to prevent the radial deflection of the semispherical protuberance.

7. The intervertebral spacer device of clai 6, wherein the deflection preventing element comprises a rivet.

8. The intervertebral spacer device of claim 4, wherein the semispherical protuberance is rotatably and angulatably coupleable to the curvate socket.

9. The intervertebral spacer device of claim 8, wherein the semispherical protrusion comprises a radially deflectable semispherical portion and the curvate socket has an interior volume and an opening leading to the interior volume, the curvate socket accommodating the semispherical portion for free rotation and angulation therein, the semispherical portion fitting through the opening only when radially deflected, ϋxe semispherical portion being adapted to receive a deflection preventing element that when applied to the semispherical portion prevents the semispherical portion from fitting through the opening.

10. The intervertebral spacer device of claim 9, wherein the deflection preventing element comprises a rivet.

11. The intervertebral spacer device of claim 4, wherein the semispherical protuberance is provided by a post structure extending outwardly from the inner surface of one of said first and second plates, which post structure includes a ball- shaped head.

12. The intervertebral spacer device of claim 11, wherein the post structure further includes a bore that extends axially from the ball-shaped head toward the inner surface of the one of said first and second plates, and which bore receives therein a deflection preventing element such that prior to an insertion of the deflection preventing element therein, the bore permits the ball-shaped head to compress radially inwardly, and such that after the insertion of the deflection preventing element the ball-shaped head is not readily radially compressible.

13. The intervertebral spacer device of claim 12, wherein the deflection preventing element comprises a set screw and the bore is threaded to accept the set screw.

14. The intervertebral spacer device of claim 12, wherein the curvate socket accommodates the ball-shaped head for free rotation and angulation within the curvate socket after receiving the deflection preventing element when disposed in the curvate socket.

15. The intervertebral spacer device of claim 1, wherein the belleville washer having, a radially varying thickness is thicker at the inner portion of the washer as compared with the outer portion.

16. The intervertebral spacer device of claim 1, wherein the belleville washer having a radially varying thickness is thicker at the outer portion of the washer as compared with the inner portion.

17. The intervertebral spacer device of claim 1, wherein the radially varying thickness varies continuously.

18. The intervertebral spacer device of claim 1, wherein the grooved belleville washer has at least one concentric groove.

19. The intervertebral spacer device of claim 18, wherein the at least one concentric groove in the grooved belleville washer comprises a plurality of radially spaced concentric grooves, and wherein at least one of the plurality of radially spaced concentric grooves in the grooved belleville washer has a length.

20. The intervertebral spacer device of claim 18, wherein the at least one of the plurality of radially spaced concentric grooves in the grooved belleville washer has a depth and a width, and at least one of the width and the depth is uniform along the length.

21. The intervertebral spacer device of claim 18, wherein the at least one of the plurality of radially spaced concentric grooves in the grooved belleville washer has a depth and a width, and at least one of the width and the depth varies along the length.

22. The intervertebral spacer device of claim 18, wherein the length forms a closed loop.

23. The intervertebral spacer device of claim 18, wherein the length forms an open loop.

24. The intervertebral spacer device of claim 18, wherein the radial spacing of the plurality of radially spaced concentric grooves is uniform.

25. The intervertebral spacer device of claim 18, wherein the radial spacing of the plurality of radially spaced concentric grooves is non-uniform.

26. The intervertebral spacer device of claim 18, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer has a respective length and a respective depth along the respective length, and wherein at least one of the depths is different than at least one other of the depths.

27. The intervertebral spacer device of claim 26, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance from an outer edge of the grooved belleville washer, wherein the , depths increase incrementally with decreasing the distances.

28. The intervertebral spacer device of claim 18, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer has a respective length, a respective width along the respective length, and wherein at least one of the widths is different than at least one other of the widths.

29. The intervertebral spacer device of claim 28, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance fro an outer edge of the grooved belleville washer, wherein the widths increase incrementally with decreasing the distances.

30. The intervertebral spacer device of claim 18, wherein the at least one concentric groove in the grooved belleville washer comprises a plurality of radially spaced concentric grooves, and wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer has a respective length, a respective depth along the respective length, and a respective width along the respective length, at least one of the respective depth and the respective width being uniform along the respective length.

31. The intervertebral spacer device of claim 30, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance from an outer edge of the grooved belleville washer, wherein the depths differ with respect to one another depending on the distances.

32. The intervertebral spacer device of claim 30, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance from an outer edge of the grooved belleville washer, wherein the widths differ with respect to one another depending on the distances.

33. The intervertebral spacer device of claim 1, wherein the grooved belleville washer has at least one radially extending groove.

34. The intervertebral spacer device of claim 33, wherein the at least one radially extending groove in the grooved belleville washer comprises a linear groove having a length extending from a locus on a peripheral edge of the grooved belleville washer toward a locus that is radially in from the peripheral edge, a depth along the length, and a width along the length.

35. The intervertebral spacer device of claim 34, wherein at least one of the width and the depth tapers along the length.

36. The intervertebral spacer device of claim 34, wherein each of the width and the depth decreases along the length.

37. The intervertebral spacer device of claim 33, wherein the at least one radially extending groove in the grooved belleville washer comprises a plurality of spaced apart radially extending grooves, each of which extends from a locus on a peripheral edge of the grooved belleville washer to a locus that is radially in from the peripheral edge.

38. The intervertebral spacer device of claim 37, wherein each of the plurality of radially extending grooves in the grooved belleville washer comprises a linear groove having a length extending from the locus on the peripheral edge toward the locus that is radially in from the peripheral edge, a depth along the length, and a width along the length.

39. The intervertebral spacer device of claim 38, wherein at least one of the width and the depth tapers along the length.

40. The intervertebral spacer device of claim 38, wherein each of the width and the depth decreases along the length.

41. The intervertebral spacer device of claim 1, wherein at least one of said first and second plates comprises a retaining element that maintains a wide end of the at least one belleville washer adjacent the inner surface of the at least one of said first and second plates.

42. The intervertebral spacer device of claim 41, wherein the retaining element comprises a retaining wall behind which the wide end is disposed.

43. The intervertebral spacer device of claim 42, further comprising a retaining ring between which retaining ring and the inner surface of the at least one of said first and second plates the wide end is disposed.

44. The intervertebral spacer device of claim 41, wherein the retaining element comprises a circular recess on the inner surface of the one of said first and second plates, within which circular recess the wide end of the at least one belleville washer is disposed.

45. The intervertebral spacer device of claim 44, wherein the retaining element further comprises a retaining shield between which retaining shield and the inner surface of the at least one of said first and second plates the wide end is disposed.

46. The intervertebral spacer device of claim 1, wherein at least one of the outer surfaces comprises an element that is deformably reshapeable under anatomical loads to securably engage a vertebral body endplate.

47. The intervertebral spacer device of claim 46, wherein the element comprises a mesh.

48. The intervertebral spacer device of claim 47, wherein the mesh has a resting shape in the shape of a dome convexly extending from the at least one of the outer surfaces.

49. The intervertebral spacer device of claim 47, wherein the mesh is laser- welded to the at least one of the outer surfaces.

50. The intervertebral spacer device of claim 47, wherein the mesh comprises titanium.

51. The intervertebral spacer device of claim 47, further comprising an osteoconductive feature adjacent the mesh.

52. The intervertebral spacer device of claim 51, wherein the osteoconductive feature adjacent the mesh comprises a porous area on the at least one of the outer surfaces.

53. The intervertebral spacer device of claim 1, wherein at least one of the outer surfaces comprises an osteoconductive feature.

54. The intervertebral spacer device of claim 53, wherein the osteoconductive feature comprises a porous coating.

55. The intervertebral spacer device of claim 53, wherein the osteoconductive feature comprises a wire mesh mounted to the at least one of the outer surfaces.

56. The intervertebral spacer device of claim 55, wherein the wire mesh is deflectable.

57. The intervertebral spacer device of claim 55, wherein the wire mesh is convex.

58. An artificial intervertebral disc, comprising: first and second plates disposed to provide opposed respective inwardly facing support surfaces of said plates, and to provide respective outwardly facing surfaces of said plates; and at least one belleville washer selected from the group consisting of a belleville washer having a radially varying thickness and a grooved belleville washer, said at least one belleville washer being disposed between the inwardly facing support surfaces such that a compressive load applied to the outwardly facing surfaces is resisted by said at least one belleville washer; wherein said at least one belleville washer includes a central opening forming a curvate socket; and wherein at least one of said first and second plates includes on its inwardly facing support surface a semispherical protuberance that is rotatably and angulatably couplable to the curvate socket such that the plates are rotatable and angulatable relative to one another thereby.

59. The artificial intervertebral disc of claim 58, wherein the semispherical protrusion comprises a radially deflectable semispherical portion and the curvate socket has an interior volume and an opening leading to the interior volume, the curvate socket accommodating the semispherical portion for free rotation and angulation therein, the semispherical portion fitting through the opening only when radially deflected, the semispherical portion being adapted to receive a deflection preventing element that when applied to the semispherical portion prevents the semispherical portion from fitting through the opening.

60. The artificial intervertebral disc of claim 59, wherein the semispherical protuberance comprises at least one radial slot such that the semispherical protuberance is radially deflectable upon the application of a radially inwardly directed force.

61. The artificial intervertebral disc of claim 60, wherein the semispherical protuberance further comprises an axial bore into which the deflection preventing element is disposable to prevent the radial deflection of the semispherical protuberance.

62. The artificial intervertebral disc of claim 61, wherein the deflection preventing element comprises a rivet.

63. The artificial intervertebral disc of claim 58, wherein the semispherical protuberance is provided by a post structure extending outwardly from the inwardly facing support surface of the one of said first and second plates, which post structure includes a ball-shaped head.

64. The artificial intervertebral disc of claim 63, wherein the post structure further includes a bore that extends axially from the ball-shaped head toward the inwardly facing support surface of the one of said first and second plates, and which bore receives therein a deflection preventing element such that prior to an insertion of the deflection preventing element therein, the bore permits the ball-shaped head to compress radially inwardly, and such that after the insertion of the deflection preventing element the ball-shaped head is not readily radially compressible.

65. The artificial intervertebral disc of claim 64, wherein the deflection preventing element comprises a set screw and the bore is threaded to accept the set screw.

66. The artificial intervertebral disc of claim 64, wherein the curvate socket accommodates the ball-shaped head for free rotation and angulation within the curvate socket after receiving the deflection preventing element when disposed in the curvate socket.

67. The artificial intervertebral disc of claim 64, wherein the belleville washer having a radially varying thickness is thicker at the inner portion of the washer as compared with the outer portion.

68. The artificial intervertebral disc of claim 64, wherein the belleville washer having a radially varying thickness is thicker at the outer portion of the washer as compared with the inner portion.

69. The artificial intervertebral disc of claim 64, wherein the radially varying thickness varies continuously.

70. The artificial intervertebral disc of claim 64, wherein the grooved belleville washer has at least one concentric groove.

71. The artificial intervertebral disc of claim 70, wherein the at least one concentric groove in the grooved belleville washer comprises a plurality of radially spaced concentric grooves, and wherein at least one of the plurality of radially spaced concentric grooves in the grooved belleville washer has a length.

72. The artificial intervertebral disc of claim 70, wherein the at least one of the plurality of radially spaced concentric grooves in the grooved belleville washer has a depth and a width, and at least one of the width and the depth is uniform along the length.

73. The artificial intervertebral disc of claim 70, wherein the at least one of the plurality of radially spaced concentric grooves in the grooved belleville washer has a depth and a width, and at least one of the width and the depth varies along the length.

74. The artificial intervertebral disc of claim 70, wherein the length forms a closed loop.

75. The artificial intervertebral disc of claim 70, wherein the length forms an open loop.

76. The artificial intervertebral disc of claim 70, wherein the radial spacing of the plurality of radially spaced concentric grooves is uniform.

77. The artificial intervertebral disc of claim 70, wherein the radial spacing of the plurality of radially spaced concentric grooves is non-uniform.

78. The artificial intervertebral disc of claim 70, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer has a respective length and a respective depth along the respective length, and wherein at least one of the depths is different than at least one other of the depths.

79. The artificial intervertebral disc of claim 78, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance feom an outer edge of the grooved belleville washer, wherein the depths increase incrementally with decreasing the distances.

80. The artificial intervertebral disc of claim 70, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer has a respective length, a respective width along the respective length, and wherein at least one of the widths is different than at least one other of the widths.

81. The artificial intervertebral disc of claim 80, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance from an outer edge of the grooved belleville washer, wherein the widths increase incrementally with decreasing the distances.

82. The artificial intervertebral disc of claim 70, wherein the at least one concentric groove in the grooved belleville washer comprises a plurality of radially spaced concentric grooves, and wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer has a respective length, a respective depth along the respective length, and a respective width along the respective length, at least one of the respective depth and the respective width being uniform along the respective length.

83. The artificial intervertebral disc of claim 82, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance from an outer edge of the grooved belleville washer, wherein the depths differ with respect to one another depending on the distances.

84. The artificial intervertebral disc of claim 82, wherein each of the plurality of radially spaced concentric grooves in the grooved belleville washer is at a respective distance feom an outer edge of the grooved belleville washer, wherein the widths differ with respect to one another depending on the distances.

85. The artificial intervertebral disc of claim 58, wherein the grooved belleville washer has at least one radially extending groove.

86. The artificial intervertebral disc of claim 85, wherein the at least one radially extending groove in the grooved belleville washer comprises a linear groove having a length extending from a locus on a peripheral edge of the grooved belleville washer toward a locus that is radially in from the peripheral edge, a depth along the length, and a width along the length.

87. The artificial intervertebral disc of claim 86, wherein at least one of the width and the depth tapers along the length.

88. The artificial intervertebral disc of claim 86, wherein each of the width and the depth decreases along the length.

89. The artificial intervertebral disc of claim 85, wherein the at least one radially extending groove in the grooved belleville washer comprises a plurality of spaced apart radially extending grooves, each of which extends from a locus on a peripheral edge of the grooved belleville washer to a locus that is radially in from the peripheral edge.

90. The artificial intervertebral disc of claim 89, wherein each of the plurality of radially extending grooves in the grooved belleville washer comprises a linear groove having a length extending from the locus on the peripheral edge toward the locus that is radially in from the peripheral edge, a depth along the length, and a width along the length.

91. The artificial intervertebral disc of claim 90, wherein at least one of the width and the depth tapers along the length.

92. The artificial intervertebral disc of claim 90, wherein each of the width and the depth decreases along the length.

93. The artificial intervertebral disc of claim 58, wherein at least the other of said first and second plates comprises a retaining element that maintains a wide end of the at least one belleville washer adjacent the inwardly facing surface of the at least the other of said first and second plates.

94. The artificial intervertebral disc of claim 93, wherein the retaining element comprises a retaining wall behind which the wide end is disposed.

95. The artificial intervertebral disc of claim 94, further comprising a retaining ring between which retaining ring and the inwardly facing surface of the at least the other of said first and second plates the wide end is disposed.

96. The artificial intervertebral disc of claim 93, wherein the retaining element comprises a circular recess on the inwardly facing surface of the one of said first and second plates, within which circular recess the wide end of the at least one belleville washer is disposed.

97. The artificial intervertebral disc of claim 96, wherein the retaining element further comprises a retaining shield between which retaining shield and the inwardly facing surface of the at least the other of said first and second plates the wide end is disposed.

98. The artificial intervertebral disc of claim 58, further comprising on at least one of the outwardly facing surfaces an element that is deformably reshapeable under anatomical loads to securably engage a vertebral body endplate.

99. The artificial intervertebral disc of claim 98, wherein the element comprises a mesh.

100. The artificial intervertebral disc of claim 99, wherein the mesh has a resting shape in the shape of a dome convexly extending from the at least one of the outwardly facing surfaces.

101. The artificial intervertebral disc of claim 99, wherein the mesh is laser- welded to the at least one of the outwardly facing surfaces.

102. The artificial intervertebral disc of claim 99, wherein the mesh comprises titanium.

103. The artificial intervertebral disc of claim 99, further comprising an osteoconductive feature adjacent the mesh.

104. The artificial intervertebral disc of claim 103, wherein the osteoconductive feature adjacent the mesh comprises a porous area on the at least one of the outwardly facing surfaces.

105. The artificial intervertebral disc of claim 2, wherein at least one of the outwardly facing surfaces comprises an osteoconductive feature.

106. The artificial intervertebral disc of claim 105, wherein the osteoconductive feature comprises a porous coating.

107. The artificial intervertebral disc of claim 105, wherein the osteoconductive feature comprises a wire mesh mounted to the at least one of the outwardly facing surfaces.

108. The artificial intervertebral disc of claim 107, wherein the wire mesh is deflectable.

109. The artificial intervertebral disc of claim 107, wherein the wire mesh is convex.