while the present 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 which follow are to be understood as illustrative and exemplary of specific structures , aspects and features within the broad scope of the present invention and not as limiting of such broad scope , which scope shall be determined only by the claims appended hereto . like numbers refer to similar features of like elements throughout . referring now to fig3 a and 3 b , side cross - section views of the top and bottom plate members 100 , 200 of a first embodiment of the present invention is shown . as the device is designed to be positioned between the facing surfaces of adjacent vertebral bodies , the plates include substantially flat surface portions 102 , 202 which 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 . it is , therefore , preferred that the plates should 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 base plate does not extend across the intervertebral space .) referring now also to fig8 a and 8 b , plate 100 further includes a circumferential skirt 106 which serves as a retaining wall , into which the large end of a belleville washer ( see fig4 – 7 ) may be seated . the diameter of the retaining wall 106 is preferably slightly 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 106 includes an annular recess into which a retaining ring may be provided for holding the belleville washer in place ( see the assembled fig9 – 12 ). referring now also to fig8 c , plate 200 further includes a central post 206 which rises out of the interior face 208 at a nearly perpendicular angle . the top of this short post member 206 includes a ball - shaped head 210 . the head 210 includes a series of slots 212 which render it compressible and expandable in correspondence with a radial pressure ( or a radial component of a pressure applied thereto ). there is a central threaded axial bore 214 which extends down the post 206 . this threaded bore 214 is designed to receive a small set screw 216 . prior to the insertion of the set screw 216 , the ball - shaped head 210 can deflect radially inward ( so that the ball - shaped head contracts ) permitting the belleville washer to be flexibly mounted thereon . the insertion of the set screw 216 eliminates ( or greatly reduces ) the capacity for this deflection . referring now to fig4 – 7 , side cross - section views of four separate embodiments of the belleville washers are provided . in general , these belleville washers 130 comprise a domed circular shape ( a section of a sphere or three dimensional paraboloid would be an appropriately corresponding shape ), having a central opening 132 and an outer edge 134 . as a compressive load is applied to a belleville washer , the forces are directed into a hoop stress which 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 . in the present invention , the thickness ( the distance from the concave surface to the convex surface ) of the material which comprises the washer varies from the edge of the central opening 132 to the outer edge 134 of the element . more particularly with respect to the washer in fig4 ( and shown within the circumferential ring of plate 100 in fig8 b ), the belleville washer 130 a has a greater thickness at the outer edge 134 a than at the edge of the central opening . 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 central opening 132 a 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 . more particularly with respect to the washer in fig5 ( and shown within the circumferential ring of plate 100 in fig1 ), the belleville washer 130 b also has a greater thickness at the outer edge 134 b than at the edge of the central opening . however , the washer further includes a series of spiral slots 138 b extending from the outer edge l 34 b toward the central opening 132 b . the slots 138 b extend from the outer diameter of the belleville washer , inward along arcs generally directed toward the central opening 132 b of the element . the slots 138 b do not extend fully to the center of the device . in preferred embodiments , the slots may extend anywhere from a quarter to three quarters of the overall radius of the washer , depending upon the requirements of the patient , and the anatomical requirements of the device . as the restoring force of a belleville washer is proportional to both the geometry of the material being loaded and its elastic properties , the varying thickness combined with the radial slots 138 b 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 . more particularly with respect to the washer in fig6 ( and shown within the circumferential ring of plate 100 in fig1 ), the belleville washer 130 c has a smaller thickness at the outer edge 134 c than it is at the inner edge 132 c . 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 134 c permits a load profile in which the load which 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 . more particularly with respect to the washer in fig7 ( and shown within the circumferential ring of plate 100 in fig1 ), the belleville washer 130 d has a smaller thickness at the outer edge 134 d than at the edge of the central opening . however , the washer further includes a series of spiral slots 138 d extending from the outer edge 134 d toward the central opening . the slots 138 d extend from the outer diameter of the belleville washer , inward along arcs generally directed toward the central opening of the element . the slots 138 d do not extend fully to the center of the device . in preferred embodiments , the slots may extend anywhere from a quarter to three quarters of the overall radius of the washer , depending upon the requirements of the patient , and the anatomical requirements of the device . as the restoring force of a belleville washer is proportional to both the geometry of the material being loaded and its elastic properties , the varying thickness combined with the radial slots 138 d 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 . in addition , the central openings of each of the belleville washer embodiments described hereinabove further includes a curvate volume for receiving therein the ball - shaped head 210 of the post 206 of the lower plate 200 described above . referring now to fig9 – 12 , side cross - sectional views of the fully assembled embodiments of the intervertebral spacers which comprises the present invention are provided . each structure includes the belleville washer ( selected from the corresponding ones illustrated in fig4 – 7 ). each further includes the following common features : two opposing plates 100 , 200 having their flat surfaces 102 , 202 , respectively , directed away from one another ( to be seated against the adjacent bone ); a retaining ring 110 is seated in the annular groove of the retaining wall 106 ; and a ball - shaped - headed central post 206 extending into the central opening 132 of the corresponding belleville washer 130 , rotatably secured in place by set screw 216 . the deflectability of the ball - shaped head of the post 206 , prior to the insertion of the set screw 216 , permits the head to be inserted into the interior volume at the center of the belleville washer 130 . subsequent introduction of the set screw 216 into the axial bore of the post 206 prevents the ball - shaped head from deflecting . thereby , the washer 130 can be secured to the ball - shaped head so that it can rotate thereon through a range of proper lordotic angles . while not in this preferred embodiment , it should be noted that in other embodiments , a tightening of the set screw can lock the washer 130 on the ball - shaped head at one of the lordotic angles .
an intervertebral spacer device having a pair of opposing plates for seating against opposing vertebral bone surfaces , separated by a subassembly including a ball captured in a curvate volume , at least a portion of the curvate volume being formed in a convex element that is maintained against a first plate by a retaining wall and a retaining ring .