Abstract:
A modular hydraulic spinal intervertebral prosthetic device offering individualized optimization of an implantable disc prosthesis by having selectable crown plates modules with differing lordosis angles and differing cross-sectional profiles, and selectable bellows cartridges having differing load-bearing capabilities. The device offers substantially full physiological degrees of motion, and by the incorporation of both a dashpot mechanism and a biasing element within reversibly displaceable and tiltable bellows provides hydraulic load bearing capability. The bellows assembly is advantageously pre-loaded to sub-atmospheric pressure. The dashpot assembly further increases resistance to lateral shear loading beyond the bellows convolutions acting alone. Rotational coupling of the upper crown plate and center bearings plate permits normal twisting movements, and spinal flexural freedom is provided by the bellows interposed between the center bearings plate and the lower end plate. The bellows also provides a hermetic seal which prevents any wear debris from migrating to the surrounding body tissue.

Description:
RELATED PATENTS 
       [0001]    This application is a Continuation-in-Part of co-pending application Ser. No. 11/110,893 that was filed on 21 Apr. 2005, which is a continuation in part of patent application Ser. No. 10/419,899 filed on 22 Apr. 2003, now U.S. Pat. No. 6,981,989. 
     
    
     FIELD OF INVENTION 
       [0002]    The subject invention relates to a compressible, rotatable and tiltable spinal hydraulic prosthesis device and system for implantation of an individualized spinal disc prosthesis assembly to replace a degenerated disc in an intervertebral level of the spine. In particular, the present invention directs itself to a modular spinal prosthesis assembly formed of selectable components, the components allowing optimal individualization of the spinal disc prosthesis for a particular patient. Furthermore, the present invention is directed to a spinal disc prosthesis comprising a bio-compatible metallic bellows formed from a plurality of rigid washer-like members to minimize shear and lateral movements, filled with a non-compressible fluid and a compressible fluid, and permitting compression and tilting movements. Additionally, the bellows contain other axial load dampening mechanisms, including a dash pot mechanism and at least one biasing member. 
         [0003]    More particularly, the invention directs itself to a modular spinal prosthesis assembly comprising a pair of opposing selectable crown plates having a selectable bellows cartridge interposed, with the assembly permitting rotation of the two vertebrae adjacent to the prosthesis relative to each other. 
         [0004]    The invention further directs itself to an implantable modular spinal prosthetic device having selectable cross-sectional profiles, selectable angles of lordosis, and selectable load-bearing capacities. By the appropriate selection of modular components, the compressible, rotatable and tiltable spinal hydraulic prosthetic device assembly implanted in a particular patient at a particular spinal level is optimized. 
       BACKGROUND OF THE INVENTION 
       [0005]    Implantable spinal prosthetic devices are well known in the art. Presently, the primary method used to remediate severe disc disease, spinal instability, discogenic pain, and/or spinal stenosis, is by surgical spinal fusion. In the spinal fusion procedure, two or more adjacent vertebrae are displaced, the spinal discs in between the vertebrae are removed by dissection, and crushed bone material is inserted between the two vertebrae; the bony material promotes the growth of new bone in the intervertebral space. The bony fusion material may be harvested intra-operatively from the patient&#39;s iliac crest or, alternatively, banked bone may be used. Since the fusion depends upon the ingrowth of new bone which takes months, mechanical means are necessarily incorporated at the time of surgery to maintain the stability and proper spacing between the vertebrae so as to permit the patient to carry normal loads imposed on the patient&#39;s spine during normal activities. Once the affected vertebrae are fused; that spinal segment will no longer take part in normal flexing, extending and twisting movements; higher stress loads will subsequently be imposed on discs and vertebra above and below the fused vertebral segment, often leading to the patient developing transition syndrome. 
         [0006]    An important goal of spinal disc prosthesis implantations is to obviate the loss of normal biomechanics and range of motion associated with surgical fusion of a diseased spinal segment. Lordosis is an important element of the biomechanics of the spine, especially in the lumbar spine. While the lumbar vertebrae could be articulated in such a way that they form a straight vertebral column, this is not the shape assumed by the normal lumbar spine when a person is in the upright posture. This is because the sacrum, on which the lumbar spine rests, tilts forward so that its upper surface is inclined downwards and forwards. The size of this angle, with respect to a horizontal plane of the body, has a value in the range of about 40-45 degrees and increases by about 8 degrees upon standing. A straight lumbar spine would have to be inclined forward to articulate with the sacrum. In order to restore a normal upward orientation and to compensate for the normal inclination of the sacrum, the intact lumbar spine must assume a curve that is known as the lumbar lordosis. The shape of lumbar lordosis is achieved as a result of several factors. One of the main factors is the shape of the lumbar discs, and particularly the L5-S1 lumbosacral intervertebral disc. The L5-S1 lumbosacral disc, more than other lumbar intervertebral discs, is substantially wedge-shaped. Typically, the posterior disc height is about 6 or 7 mm less than its anterior height. The angle formed between the bottom of the L5 vertebrae and the top of the sacrum (S1) is found to vary from person to person in a range of roughly 5 to 30 degrees, with an average value of about 16 degrees. 
         [0007]    One important advantage that derives from the lumbar lordosis is resilience to compressive forces and shocks. In a straight lumbar spine, axial compressive forces would be transmitted through the vertebral bodies and intervertebral discs and the only mechanism to protect the lumbar vertebra would be the shock-absorbing capacity of the intervertebral discs. 
         [0008]    In a normally curved lumbar spine, compressive forces are transmitted through the posterior ends of the intervertebral discs while the anterior ends of the vertebral bodies tend to separate. Compression tends to accentuate the lumbar lordosis, which tendency tenses the anterior ligaments, which in turn resists the accentuation. Thus some of the energy of the axial compressive force is diverted into the stretching of the associated ligaments instead of being transmitted directly to the next vertebral body. In order to restore relatively normal biomechanical relationships to the vertebral column having structural derangements severe enough to require prosthetic spinal disc implantation, the prosthesis ought to provide for and replicate—as much as possible—the normal lordosis found in the healthy spine. 
         [0009]    Axial compression is the movement that occurs during weight-bearing in the upright posture, or as a result of contraction of the longitudinal back muscles. During compression, intervertebral discs undergo an initial period of rapid creep, deforming about 1.5 mm in the first 2 to 10 minutes depending on the size of the applied axial load. Subsequently, a much slower but definite creep continues at about 1 mm/hour. Depending on age, a plateau is attained by about 90 minutes beyond which no further creep occurs. It is therefore important to incorporate this gradual accommodating compression—this cushioning—of the intervertebral disc to axial loads as part of the effort to restore and replicate normal vertebral biomechanics as much as possible. 
         [0010]    During the axial rotation of an intervertebral joint inherent in twisting movements, the normal intervertebral disc resists torsion more than bending. Normally, the stress-strain curves for torsion rise steeply in the range of 0 to 3 degrees of rotation; beyond 3 degrees very large forces have to be applied to rotate the disc further. The risk of disc element failure increases substantially as the amount of rotation approaches 12 degrees, suggesting that 12 degrees is normally the maximal range of rotation. Thus, in order to replicate normal spine movements, an implanted prosthetic spinal disc ought to permit at least 3 degrees of rotation and preferably between 8 and 12 degrees of maximal rotation. None of the currently available disc prostheses provide for anything close to this amount of rotation. 
         [0011]    Commonly used implantable spinal prosthetic devices include semi-rigid elastomeric filler materials that are sandwiched between two layers of some bio-compatible metal. The upper and lower plate surfaces typically have multiple spikes for their fixation to the vertebral end plates. Other similar devices offer means to screw the upper and lower plates to the co-joining vertebrae and some also include plates treated to promote bone growth into them. A few of the newer devices permit a small amount of articulation between the vertebrae but the extent of flexing and twisting is quite limited; furthermore, the elastomeric materials and their bonding agents in these devices have a disappointingly limited longevity. Ideally, a spinal disc prosthesis should last 30 to 40 years and be able to withstand approximately two million compression cycles per year. 
         [0012]    It is a purpose of this subject invention to provide an implantable spinal disc prosthesis assembly comprising a combination of selectable modular components that has a long life expectancy, a negligible rate of failure and/or complications, and provides for maximal articulation in all normal physiological planes of movement within the spine. More particularly, the subject spinal disc prosthesis allows for tilting from side-to-side, rotation such as with twisting movements, and compression along a primary axial direction to absorb and transmit axial loads typical for normal activities. 
       PRIOR ART 
       [0013]    Among the prior art spinal prosthesis is the device in U.S. Pat. No. 5,002,576. The patent reference is directed to an intervertebral disc prosthesis. This reference teaches a prosthetic disc device provided with a central elastomeric layer sandwiched between two cover plates. This particular prosthetic disc device offers neither rotation between the vertebra nor does it provide for any significant amount of bending in the forward, backward, or side directions. 
         [0014]    Another prior art prosthetic disc implant is shown in U.S. Pat. No. 4,932,975. This reference patent is directed to a vertebral prosthesis. The prosthetic device disclosed in this patent includes a flexible bellows but the bellows here do not allow for rotation between the two adjacent vertebrae. 
         [0015]    U.S. Pat. No. 3,875,595 discloses and claims intervertebral disc prosthesis along with instruments for positioning the same. The prosthesis is a hollow, bladder-like member with an expanded shape having the appearance of a natural nucleus of a normal spinal disc. The device does not provide for rotation between adjacent vertebrae, thus failing to provide the patient will full articulated movement. 
         [0016]    U.S. Pat. No. 5,571,189 is directed to an expandable fabric implant for stabilizing a spinal motion segment. The implant is in the form of an inflatable bag positioned within a cavity artificially formed intervertebrally within the spine. The inflatable bag does not allow for rotation of that spinal motion segment. 
         [0017]    Yet another prior art prosthesis is disclosed in U.S. Pat. No. 5,755,807. This patent is directed to an implant module unit and rotating seal for a prosthetic joint. The implant includes a ball-and-socket joint surrounded by a flexible metallic bellows. The system has certain limitations, namely, that it is subject to wear and premature failure as a result of friction and the buildup of particle debris. 
         [0018]    U.S. Pat. No. 5,401,269 discloses an intervertebral disc prosthesis, the Charite&#39; disc prosthesis. This prosthesis does allow some minimal rotational movement, as well as a small amount of tilting and bending movement, by providing for an articular surface with surface forms curved with different average radii in the median section and the frontal section. Unlike the present invention, the Charite&#39; prosthesis does not provide for axial compression. In addition, the Charite&#39; prosthesis allows for some translational movements, which while mimicking normal physiological movements to some extent may not be well tolerated in the context of the multi-level spine degeneration typical of patients requiring such prosthetic implants. Furthermore, the components of the Charite&#39; prosthesis do not allow for individualization of the device&#39;s axial load-bearing capability; in contradistinction to the present invention, Charite&#39; prostheses offer negligible shock absorption and seems to permit progression of the Transition Syndrome whereby spinal levels above and below the implanted level suffer progressive disc and joint degeneration. 
         [0019]    The present subject application device provides for improvements to the rotatable, compressible and tilting functions of the parent application device. The crown plate members as disclosed and claimed herein inventively provide for both selectable lordosis angles and selectable cross-sectional profiles. The selectable rotatable, compressible and tiltable cartridges have pre-loaded bellows that extend the device&#39;s functional lifetime indefinitely. The cartridges also provide enhanced axial load-bearing and shear-resisting capabilities as a result of the axial load bearing mechanism comprised of biasing members—a dashpot and springs in the preferred embodiment—positioned within the cartridge&#39;s bellows assembly. The selectable modular components of the present spinal disc prosthesis assembly permit optimization of the disc prosthesis assembly on a patient-by-patient basis. 
         [0020]    The prior art does not include a combination of elements forming a modular compressible, rotatable and tiltable spinal hydraulic prosthesis assembly that is optimizable on a patient-by-patient basis. The present invention solves the problematic unavailability in the prior art of individually optimizable spinal disc prostheses by providing for crown plates selectable according to a best cross-sectional profile and lordosis angle, where selected crown plates are best for a particular patient&#39;s needs; and for selectable cartridges with pre-loaded metallic bellows having redundant biasing elements to augment axial load-bearing and shear-resistance. The selectability of a spinal prosthesis components—as provided by the present invention—according to selectable lordosis angles, selectable crown plate shapes and sizes, and selectable load-bearing capabilities, permits optimization of the spinal disc implant assembly that is tailored to an individual patient. Optimization is accomplished by selecting the specific disc prosthesis components according to important relevant factors that may include the particular patient&#39;s gender, age and body habitus, the extent of co-existing spinal degeneration at nearby spinal levels, the patient&#39;s level of activity and general condition, as well as the particular spinal level(s) in need of prosthetic spinal disc replacement. 
       SUMMARY OF THE INVENTION 
       [0021]    The present invention provides for a compressible, rotatable, and tiltable hydraulic spinal disc prosthesis system with selectable modular components. The system comprises an individually optimizable compressible, rotatable, and tiltable hydraulic spinal disc prosthesis assembly for the surgical replacement of a severely diseased or missing intervertebral disc, as well as at least one insertion instrument to facilitate the surgical implantation of this hydraulic disc prosthesis assembly. 
         [0022]    The present modular hydraulic spinal disc prosthesis assembly further provides for selectable crown plate modules that sandwich between them at least one selectable compressible, rotatable and tiltable hydraulic disc prosthesis cartridge, the crown plates being affixed to the corresponding opposed vertebral end plates. 
         [0023]    The selectable cartridge interposed between the crown plate modules is comprised of pre-loaded flexible bellows preferably capped at the cephalad end of the bellows by a center bearings plate, and at the caudal end by an endcap having formed therein a sealable fluid conduit to facilitate filling and pressurizing the bellows and dashpot chambers with a mixture of compressible and non-compressible fluids. 
         [0024]    The axial thrust elements are preferentially ceramic components slidingly juxtaposed and lubricated, thereby providing for substantially normal articulated axial movement at that spinal motion segment. The cartridge bellows assembly is hermetically sealed at the ends to provide a fluid-tight chamber containing an axial load-bearing mechanism. The axial loads are absorbed, transmitted and dispersed by a combination of a dash pot located centrally in the bellows assembly, and at least one biasing member such as a coil spring in close proximity to the dashpot to augment the axial load bearing capability provided by the bellows assembly and the dashpot. The selectable nature of the spring and load bearing elements allows the optimal choice among selectable cartridges to best accommodate anticipated demands as presented by particular clinical situations. Furthermore, the dashpot piston is provided with a spherical ball bearing—preferably made of a ceramic material or the like—through which the piston extends, which ball bearing is constrained by a shear-resisting retainer ring seated within the bellows assembly. 
         [0025]    Additionally, the lifespan of the bellows element, which is subjected to repeated axial loading and unloading under a spectrum of tilting and bending movements, is extended indefinitely by preloading the bellows to a sub-atmospheric pressure as described herein. 
         [0026]    It is a principle objective of the subject invention to provide a hydraulic spinal prosthesis for replacement of a missing or diseased intervertebral spinal disc and annulus. 
         [0027]    It is a further objective of the subject hydraulic spinal prosthetic device to provide a hydraulic spinal prosthesis with dimensions and load-bearing capabilities that can be optimized for individual patients. 
         [0028]    It is yet a further objective of the subject hydraulic spinal prosthesis device to provide a spinal disc replacement that permits substantially physiologic range of motion—including rotation—between the vertebrae adjacent to the implanted prosthesis. 
         [0029]    It is also an objective of the subject inventive concept to provide a hydraulic spinal prosthetic device that resists translation and shear movements in the horizontal, coronal and sagittal planes. 
         [0030]    It is also an important objective of the present invention to provide a hydraulic spinal prosthetic device provided with a selectable cartridge having a set of bellows filled with a mixture of compressible and incompressible fluids and preloaded to a sub-atmospheric pressure so as to very substantially prolong the functional lifespan of the implanted hydraulic spinal disc prosthesis. 
         [0031]    It is yet another objective of the present invention to provide a hydraulic spinal prosthetic device having selectable load bearing capability afforded through the bellows, the dash pot mechanism, as well as by at least one further biasing member, such as coil spring(s) and Belleville washers. 
         [0032]    It is a further important objective of the present invention to provide a hydraulic spinal prosthesis with fluid-filled bellows that have a washer-convoluted design, and which permits a substantially physiologic range of movements. 
         [0033]    An important objective of the present inventive device is to provide a prosthetic hydraulic spinal disc replacement that reduces the incidence of Transition Syndrome. 
         [0034]    The present inventive device takes as an important objective to provide a hydraulic prosthetic spinal disc replacement that does not migrate from its initial implantation position. 
         [0035]    It is an important objective of the present inventive device to provide a hydraulic prosthetic spinal disc replacement that improves the patient&#39;s spinal stability at the affected motion segment as well as at nearby spinal levels. 
         [0036]    It is a further objective of the present invention to provide a modular hydraulic spinal prosthesis device and corresponding insertion instrument(s) that does not demand unusual or extraordinary surgical skills to implant an individually-optimized spinal prosthesis assembly properly. The terms “insertion instrument” and “insertion tool” are used interchangeably and synonymously throughout the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1A  is an exploded perspective view of the hydraulic spinal disc prosthesis system; 
           [0038]      FIG. 1B  is a perspective view of the insertion tool coupled with the hydraulic spinal disc prosthesis assembly; 
           [0039]      FIG. 1C  is a cross-sectional view through line  1 C- 1 C in  FIG. 1B  showing the mechanical coupling of the insertion tool with the spinal disc prosthesis assembly; 
           [0040]      FIG. 1D  is an exploded perspective view of the insertion instrument and the spinal disc prosthesis assembly in situ; 
           [0041]      FIG. 2  is a cross-sectional view of an alternative preferred embodiment of the bellows cartridge of the hydraulic spinal disc prosthesis assembly; 
           [0042]      FIG. 3A  is a cross-sectional view of a preferred embodiment of the hydraulic spinal disc prosthesis assembly in an untilted condition; 
           [0043]      FIG. 3B  is a cross-sectional view of a preferred embodiment of the hydraulic spinal disc prosthesis assembly in a tilted condition; 
           [0044]      FIG. 4A  is a cross-sectional elevational view of the hydraulic spinal disc prosthesis assembly through the line  4 A- 4 A in  FIG. 4C ; 
           [0045]      FIG. 4B  is a left lateral side-view of the hydraulic spinal disc prosthesis assembly; 
           [0046]      FIG. 4C  is a posterior—anterior side-view of the hydraulic spinal disc prosthesis assembly; 
           [0047]      FIGS. 5A and 5B  are lateral side-views of the crown plates and bellows cartridge respectively, illustrating their assembly; 
           [0048]      FIG. 5C  is a rear posterior—anterior side view of the top crown plate member; 
           [0049]      FIG. 6A  is a cross-sectional view of the preferred embodiment of the bellows cartridge in an untilted condition; 
           [0050]      FIG. 6B  is a cross-sectional view of the preferred embodiment of the bellows cartridge in a tilted condition; 
           [0051]      FIG. 7  is a cross-sectional view of an alternative non-modular embodiment of a hydraulic spinal disc prosthesis assembly. 
           [0052]      FIG. 8  is a cross-sectional view of an embodiment of a hydraulic spinal disc prosthesis assembly. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    Referring now to the figures, there is shown a compressible, rotatable, and tiltable hydraulic spinal disc prosthesis system with selectable modular components. As seen in  FIG. 1A , Prosthetic modular assembly  1  includes an opposed pair of crown plate modules  10 A and  10 B,  10 A′ with  10 B and  10 ′A and  10 ′B,  10 ′ A′ with  10 ′B which make up crown plates  10  and  10 ′ respectively, and a bellows cartridge  100  sandwiched between the crown plates  10  and  10 ′ (seen in  FIG. 4C ). The bellows cartridge  100  as seen in  FIG. 2  is comprised of a cephalad end cap member  20 , a caudal end cap member  20 ′, and an assembly of compressible and tiltable bellows assembly  50  between them. 
         [0054]    The bellows assembly  50  is rotatably coupled at the cephalad end to the cephalad end cap member  20  by at least two sets of ball-bearings  35 ,  32 . The first rotatable coupling is a radial thrust bearing assembly  28  comprising a first race  36 A and B and ball bearings  35  and positioned to resist shearing forces. Axial rotation is further supported by a second bearing assembly with a plurality of ball-bearings  32  maintained in regular spacing in a respective race by a bearing retainer seal member  30  interposed between the cephalad end cap member  20  and the center bearings plate  40 . The second race, in which the ball-bearings  32  travel, is formed at its the top and bottom by a pair of opposed annular recesses defined in the opposing surfaces of the cephalad end cap member  20  and the center bearings plate  40 , and laterally by the arcuate edges formed in the retainer seal  30 . The second bearing assembly is positioned to rotationally transmit axial loads placed on the system. The two races are offset longitudinally relative to each other, adding to the system&#39;s stability and resistance to shearing forces. 
         [0055]    The caudal end, or synonymously, inferior endcap of the bellows cartridge  100  is fixedly attached to the caudal crown plate  10 ′. The caudal end cap member  20 ′ of  FIG. 3A  has formed therethrough a fluid channel  65  in fluidic communication with the fluid-filled bellows chamber  52  by means of aperture  67 , and in further fluid communication with the dashpot chamber  70 —by means of opening  63 . Seen within the bellows assembly  50  is the dashpot mechanism comprising a piston  45  formed from the center bearings plate  40  as a cylindrical protrusion received into the dashpot chamber  70  which is defined by extensions of the caudal end cap member  20 ′ at the other end to the cephalad end cap member and a retaining ring  44  defining the cephalad limit of the dashpot chamber  70 . 
         [0056]    Fixedly juxtaposed onto those end cap extensions, the interior wall of the retaining ring  44  in the preferred embodiment, assumes the shape of a sphere with a substantial flattening of the opposing poles of the sphere and defining a cylindrical shape for its outer surface. The retaining ring  44  is juxtaposed with the bearing  74  which assumes the shape of a cored sphere defining a cylindrical shape for its inner surface, and likewise, the dashpot piston  45  is slidably mounted through the bearing  74 , so that the dashpot piston  45 , the bearing  74 , and the retaining ring  44  are all coaxial with the bellows assembly  50 . As in  FIG. 2 , at least one biasing mechanism  80 ,  80 ′ is incorporated within the bellows assembly  50  and, in a preferred embodiment, is coil spring  80  positioned within recess  73  formed centrally within the dashpot piston  45 . In the preferred embodiment illustrated in  FIG. 2 , coil spring  80  extends in a caudal direction to be received into recess  73  formed centrally within dash pot piston  45 , which in this embodiment protrudes caudally from center bearings plate  40 . 
         [0057]    Referring to  FIG. 4C , crown plate  10  is seen in cross-sectional side view demonstrating certain of its important component elements. The crown plates  10 ,  10 ′ are formed by the joining of two vertebral engaging lordosis half-plates  10 A,  10 ′A onto U-shaped cartridge engaging plates  10 B and  10 ′B; in the preferred embodiment the two half-plates  10 A,  10 ′A are machined separately and then spot welded to  10 B and  10 ′B thereby forming a unitary crown plate  10 . As seen in  FIG. 4A , the inner surface of the U-shaped cartridge engaging plate  10 ′B has two substantially parallel straight edges  7  that are continuous with a substantially circularly shaped edge  6  connecting the straight edges  7  at the posterior aspect. Both the straight and circular sections of the inner edge of cartridge engaging plates  10 ′B and  10 B are beveled or chined (as seen in  FIG. 4C ) to matingly receive the complementarily beveled or chined edges of the bellows cartridge end caps  20  and  20 ′ respectively. 
         [0058]    The selectable lordosis angle refers to the angle formed between the vertebral engaging surface of the crown plate top  10 A and the bottom surface of crown plate  10 B fixedly connected to the cephalad end cap  20  of bellows cartridge  100 . The selectable crown plates  10 ,  10 ′ have a cross sectional profile that may be further chosen so as to best match in size and shape the patient&#39;s vertebral end plate to which the prosthetic modular assembly  1  is to be fixedly attached. 
         [0059]    On the vertebral engaging surfaces of the crown plates  10  and  10 ′ there are a plurality of spikes  12 ,  12 ′ respectively, formed and protruding from the vertebral engaging crown plate surfaces at the periphery, and designed to secure the prosthetic modular assembly  1  in position between the adjacent vertebrae. In the preferred embodiment each vertebral engaging crown plate surface has six spikes  12 , but the number of such spikes are preferably in the range of two to eight spikes. Further apparent from  FIG. 4C  is a pair of threaded through-holes  95 A,  95 B and  95 ′A,  95 ′B spaced equidistant from the device&#39;s center of axial rotation. In  FIG. 5A , the pairs of threaded through-holes  95 A and  95 ′A are positioned so as to align with a corresponding pair of set screw recesses  22 A,  22 ′A formed on the top and bottom surfaces respectively of the bellows cartridge  100  (as seen in  FIG. 3A ,  3 B) for receiving set screw pairs  96 A,  96 ′A of  FIG. 5A  inserted therein for locking together cartridge  100  and crown plates  10 ,  10 ′. 
         [0060]    Further evident in  FIG. 1C  are the paired opposing pawl recesses  91 A,  91 ′A formed in the anterior aspect of opposed inner surfaces of crown plates modules  10 A and  10 ′A and adapted to receive therein a securing element or pawl  369 A,  369 ′B of insertion instrument  300  for purposes of intra-operative device placement. As may be further appreciated from the perspective views of the prosthetic modular assembly  1 , as seen in  FIG. 4C , a through channels  14 ,  14 ′ are formed in the vertebral engaging surface of crown plate members  10  and  10 ′, in between  10 A and  10 A′; and  10 ′A and  10 ′A′ respectively. The through channels  14  are centrally located, extend between the anterior and posterior edges of the crown plate members  10  and  10 ′ and, in the preferred embodiment, have a chined or beveled cross-sectional profile. The through channels  14  are important for the proper stereotactic positioning of the prosthetic device  1  during surgical implantation. 
         [0061]    Additionally, the vertebral engaging surfaces of the crown plates  10 ,  10 ′ are formed with a roughened irregular surface, having a sintered or otherwise textured surface so as to facilitate the permanent fixation of the prosthetic modular assembly  1  subsequent to surgical placement and implantation. The cross-sectional profile of the through channels  14  matingly complement the distraction bars  320  of insertion instrument  300  as seen in  FIG. 1A , thereby permitting prosthetic modular assembly  1  to be slidingly advanced—preferably from anterior to posterior—along the previously positioned distraction bars  320  to a preferred position relative to the vertebrae and associated spinal structures. In the preferred embodiment, the through channels  14 ,  14 ′ have parallel lengths, but tapering sides or other functionally equivalent shapes are within the contemplation and scope of this invention. 
         [0062]    As seen in  FIG. 6A , the bellows cartridge  100  is comprised of a compressible and tiltable bellows assembly  50  that, together with center bearings plate  40  and caudal end cap  20 ′ which cover the top and bottom bellows openings, define a bellows chamber  52 . In another embodiment, the bellows assembly  50  is compression biased by a plurality of Belleville washers  80 . The bellows assembly  50  is preferably comprised of titanium but other like materials are also contemplated. Bellows chamber  52  is filled with a mixture of compressible and non-compressible fluids and fluidically communicates with the dash pot chamber  70 ,  73  by fluid conduits  63 ,  65 , and  67  as illustrated in  FIGS. 2 ,  3 A,  3 B,  6 A and  6 B. 
         [0063]    The 360° rotation afforded by the prosthetic modular assembly  1  is provided structurally by a bearing retainer ring  30  formed with regularly-spaced through-holes to accept a plurality of ball-bearings  32  rollably positioned between center bearings plate  40  and cephalad end cap  20 . The ball-bearings  32  roll around in the race defined by the partly circular channels formed on opposing surfaces of center bearings plate  40  and end cap  20  respectively, with the bearing retainer seal member  30  maintaining the ball-bearings  32  in a preferred spacing. As may be seen in  FIGS. 2 ,  3 A,  3 B,  6 A and  6 B, the arcuate lateral edges formed in the spaces of the bearing retaining seal member  30  further define the race in which the ball-bearings  32  make rotatable contact. 
         [0064]    Additionally, in  FIGS. 2 and 3A , a radial thrust bearing assembly  28  is centrally placed in the bellows cartridge  100  and comprises a caudal cylindrical protrusion  25  formed from and extending down from cephalad end cap  20 ; inner and outer encircling race members  36 A,  36 B; and a plurality of ball bearings  35  rotatably seated in the race formed by the opposing inner and outer race members  36 A,  36 B respectively. This axially formed caudal cylindrical protrusion  25  is received in a cephalad recess of the center bearings plate  40  and is positioned coaxial with the axis of rotation of the bellows cartridge  100 . Inner encircling race member  36 A is fixedly coupled with the caudal cylindrical protrusion  25  of end cap  20 , and outer encircling race member  36 B is fixedly positioned within the centrally positioned cephalad recess of center bearings plate  40 . 
         [0065]    The dashpot mechanism is coaxial with and centrally positioned within the bellows assembly  50  and comprises a central axial dashpot piston  45  formed from an inner horizontal surface, which dashpot piston  45  has formed within it a fluid-filled recess  73  that is a fluidic extension of dashpot chamber  70 . The dashpot piston  45  is slip fitted in and through the central axial bore of spherical bearing  74 ; preferably, spherical bearing  74  is formed with the shape of a cored sphere. The substantially spherical lateral sides are in sliding juxtaposition with the dashpot walls. In a preferred embodiment shown in  FIG. 8 , both the spherical bearing  74 ′ and the spherical annular rings  12  are composed of a ceramic material for which water or similar aqueous solutions are an acceptable working fluid. The use of Si 3 N 4  or other similar ceramics such as Al 2 O 3 , or like materials, for the spherical bearing  74 ′ as well as ball bearings  32 , annular rings  12 , and piston sleeve bearing  46  is within the contemplation and scope of the subject inventive concept, and is the preferred embodiment. 
         [0066]    At least one biasing member, which in the preferred embodiment is a Belleville spring stack member  80 , is coaxially mounted within and/or around dashpot piston  45  as seen in  FIGS. 6A ,  6 B and  8 . The Belleville spring stack represents a preferred embodiment of biasing members that can be used to augment the load-bearing capabilities of the prosthetic modular assembly  1 . Different types of springs, as well as different spring constants for the chosen springs, allow for selectability of the bellows cartridge  100  according to the needs of a particular patient. Without intending to be bound by particular examples, the spring load for cervical implantation of the prosthetic modular assembly  1  is typically about 375 pounds per square inch; for lumbar implantations the spring load is typically about 575 pounds per square inch. 
         [0067]    In  FIG. 3A , the caudal end cap  20 ′ has formed within it a fluid channel  65  that fluidly connects the dashpot chamber  70 , the bellows chamber  52  and an internal aperture  67 . Channel  65  is adapted to receive a sealing plug or screw  60  that creates a watertight closed chamber  65 ,  70 ,  73 , and  52  when in place. The dashpot chamber  70  communicates with the fluid channel  65  by means of aperture  63 ; the bellows chamber  52  is in fluidic communication with the fluid channel  65  by means of opening  67 . By suitable factory adjustment and design selection of the diameter(s) of aperture  63  and/or opening  67 , the dampening function of the dashpot may be adapted to control how rapidly fluid can flow through those apertures during a down stroke of the piston  45 ; and then during the subsequent recovery upstroke of the piston  45 . 
         [0068]    The bellows chamber  52  and the dashpot chamber  70  that are fluidly connected by the fluid channels  63 ,  65  and  67  as discussed above, contain a mixture of compressible and non-compressible fluids so as to resist axial loads while providing some cushioning or yielding to those axial loads. It has been found that the functional life span of the device is substantially extended when the bellows chamber  52  is preloaded with fluid at a sub-atmospheric pressure. Preloading the bellows chamber  52  to a predetermined sub-atmospheric pressure is important and preferred for the present subject hydraulic spinal disc prosthesis assembly. 
         [0069]    The method of preloading the compressible, rotatable and tiltable bellows cartridge  100  is accomplished by first providing a compressible, tiltable bellows assembly  50  and then compressing the biasing member(s) to solid height—which is to say to the full stroke excursion-typically in the range of 0.05 to 0.06 inches. This compressed condition of bellows cartridge  100  is maintained by applying a constraining member (such as a clamp or the like) to the compressed bellows cartridge  100 . Subsequently, the compressed bellows cartridge  100  with the fluid channel  65  unplugged is placed into a vacuum chamber and the air is evacuated down to about 17-18 Torr at 20° C. while the compressed cartridge  100  is fully immersed in a fill fluid. While immersed, the cartridge  100  is exposed to sub-atmospheric pressure in the vacuum chamber so as to evacuate substantially all the air from the inside of bellows chamber  52 , dashpot chamber  70  and the associated fluid conduits  63 ,  65 ,  67 . Once this has been accomplished, the pressure in the vacuum chamber is adjusted back to atmospheric pressure which causes the immersed bellows assembly  50  to fill with the fluid. At this point the cartridge  100  with its compressed bellows assembly  50 , now filled with the fluid, is taken out of the vacuum chamber and fluid conduit  65  is sealed with the sealing member  60  such as a plug or screw or similar sort of elements. 
         [0070]    Once the fluid conduit  65  is sealed, thereby closing the cartridge&#39;s bellows chamber  52  and dashpot chamber  70 , the cartridge  100  is allowed to re-expand by removing any constraining member such as a clamp or the like, thereby permitting the biasing member(s)  80 —such as the coil spring in proximity with the dashpot piston  45 —to force apart the opposing end caps  20  and  20 ′ and connected structures with approximately 250 to 325 pounds of compressed spring force for a lumbar prosthesis, thereby re-expanding the bellows cartridge  100  to its virtual uncompressed condition. 
         [0071]    The biasing member  80  within the cartridge  100  exerts a distracting force, preferably in the range of 250 to 325 pounds for a lumbar prosthesis that tends to separate the end caps  20  and  20 ′ and connected structures. By uncompressing the bellows cartridge  100  containing the fluid that had been introduced at standard temperature and pressure, the bellows and dashpot chambers internal volumes,  52  and  70  respectively, expand. With the bellows cartridge  100  in the expanded condition, the fluid mixture occupies a proportionately larger volume thereby causing a concomitant lowering of pressure therein to sub-atmospheric levels; this accomplishes the pre-loading of the cartridge  100  by providing a sub-atmospheric fluid pressure within the fluid compartments of uncompressed bellows cartridge  100 . 
         [0072]    Per  FIG. 1B , the insertion tool assembly  300  comprises a handle assembly  310  formed proximally at one end, and at the opposite end, the distal end, by a prosthesis engaging effector head  360 ; the handle assembly  310  further comprises an elongate tubular member  330  connecting the prosthesis engaging effector head  360  to handle grip member  340  and knob control mechanism  350 . As illustrated in  FIG. 4A , the proximal surface of the prosthesis engaging effector head  360  has a circularly arcuate profile with substantially the same radius of curvature as the circularly arcuate profile  6  of crown module  10 B,  10 ′B and opposing end caps  20 ,  20 ′ of the prosthetic modular assembly  1 . Furthermore, the upper and lower edges of this proximal arcuate surface have chined or beveled contours that matingly complement the beveled or chined profile of the disc prosthesis cartridge end caps  20 ,  20 ′. Illustrated in  FIG. 5A , are the set screws  96 A,  96 ′A by which the cartridge  100  is fixedly secured to the respective crown plates  10 ,  10 ′. Pawl openings  91 ′A,  91 ′B of  FIG. 1A ,  1 C,  1 D, and  91 A and  91 ′A of  FIG. 4B  and particularly  4 A and  5 A are respectively symmetrically placed lateral to through channel  14  and  14 ′, and are aligned with the paired pawls  369 A,  369 B and pair  369 ′ (as seen in  FIGS. 1C and 1D ) of the insertion instrument  300 . When the insertion instrument  300  is slidingly juxtaposed with the chined upper and lower surfaces of cartridge  100 , and pairs of pawl openings  91 A and  91 ′B respectively receive therein the pairs of pawls  369  and  369 ′ so as to reversibly connect the insertion instrument  300  to the prosthetic modular assembly  1 . 
         [0073]    Per  FIG. 1B , the insertion tool handle  340  has at its proximal end a threaded knob  350  that is axially rotatable. By rotating the threaded knob  350 , a connecting rod  355  is displaced longitudinally either forward or backward according to the direction in which the threaded knob  350  is turned. By rotating the threaded knob  350  so as to thereby move connecting rod  355  proximally—which is to say away from the effector head  360 —the distal end of the connecting rod  355 , formed with at least one cam member  356  at its distal end, withdraws from pushing against at least one pair of hinged and spring-biased pawl-displacing lever arm members  370 ,  370 ′. As may be further appreciated by viewing  FIG. 1C , withdrawing the connecting rod  355  displaces tapered cam member  356  away from the lever arms  370  and thereby allows biasing member  380  to push the pairs of pawls  369 A and  369 ′B (as in  FIGS. 1C ,  1 D, and particularly  4 A) into pairs of upper and lower pawl recesses  91  respectively. The reversible engagement of the pawls  369  with the respective recesses  91  effects a reversible connecting capture of prosthetic modular assembly  1  by the insertion tool  300 ; the assembled system as shown in  FIG. 1B  permits the surgeon to insert and position the prosthesis modular assembly  1  given the usual operative exposure. 
         [0074]    As seen in  FIG. 1C , the distal part of the insertion tool  360  has a height substantially equal to the height of the spinal disc prosthesis cartridge  100 . Further illustrated in  FIG. 4A  is the profile of the crown plate  10 ′B, having a substantially U-shaped inner profile provided with beveled or chined edges that matingly complement the corresponding beveling of the prosthesis end caps  20 . While the preferred embodiment shows the biasing member  380  tending to displace the pawls  369  apart, other arrangements are within the contemplation and scope of this invention, including biasing members  380  tending to pull the pawls  369  together. 
         [0075]    Per  FIGS. 1A and 1D , the insertion tool  300  further comprises a pair of distraction bars  320  whose dimensions matingly complement the through channels  14  so that distraction bars  320  act as guide rails for the prosthetic modular assembly  1  to be slidingly advanced into position along the distraction bars  320  during the implantation procedure. 
         [0076]    As shown in  FIGS. 2 ,  3 A,  3 B,  6 A, and  6 B, the retaining ring  44  is fixedly fitted onto the axial protrusion that surrounds piston  45 , in sliding juxtaposition with the spherical dashpot bearing  74 . 
         [0077]      FIGS. 6A and 6B  show an alternative embodiment of the subject hydraulic spinal disc prosthesis, having the filling port positioned on the caudal surface of endcap  20 ′ of the bellows cartridge  100 . In this alternative embodiment, as axial forces cause compression of the disc assembly and concomitant downward movement of the dashpot elements, the spherical bearing  74  comes to completely occlude the bellows chamber opening  67  formed through the cephalad protrusion of the caudal end cap  20 ′, thereby impeding fluid flowing from the dashpot chamber recess  73  formed in the caudal protrusion  23  of center bearings plate  40 , as seen in  FIGS. 6A and 6B , into the bellows chamber  52  through conduits  63 ,  65 , and  67 . 
         [0078]    By variably impeding the equilibrating flow of fluid from the dashpot chamber recess  73  to the bellows chamber  52 , the dashpot offers greater resistance to the imposed axial load than if unimpeded equilibration of the fluid pressure were permitted. Keeping constant all the other fluid conduit specifications, the total cross-sectional area of the bellows chamber openings  63  and  67  is the primary determinant of the impeded fluid equilibration that augments the device&#39;s resistance to axial forces, rather than the actual number of such openings. 
         [0079]    The embodiment depicted in  FIG. 7  of a non-modular prosthetic assembly  2  is intended primarily for cervical disc replacements. In place of the spike-like vertebral engaging members in other embodiments of the present invention, the cervical disc prosthesis has non-planar vertebra engaging surfaces, comprising a convex surface that can be seated into the relatively concave central area of the central vertebral endplates. This convex surface has a roughened texture, such as with the sintered surfaces of the flat vertebra engaging surfaces of the crown plates of the preferred embodiments. The adaptation of the top and bottom outer surfaces to a concave profile complementary to the anatomic vertebral endplate profile, a snug fit is achieved that maintains the prosthesis in proper position as bony ingrowth solidifies the connection between the disc prosthesis and opposing vertebrae. 
         [0080]    To illustrate the use of ceramic materials as described above,  FIG. 8  shows an embodiment of the prosthetic assembly which is the same as drawings  6 A and  6 B, but using ceramic parts which are designated as item  12 , of which there are two ceramic rings. They are identical back to back. Spherical bearing  74 ′ is also ceramic, and there is a piston sleeve  46  which is also ceramic. These parts are slidingly interposed, and include the dashpot assembly, as discussed above. Operation is identical to that which is disclosed with regard to  FIGS. 1-6B . These parts are arranged in a manner that loads are in compression, not tension. The ceramics are very well suited for compression loads due to the fact that they are very hard. They are also well suited for water lubrication. 
         [0081]    The dashpot piston, which is a titanium alloy, is machined to accept the ceramic piston sleeve  46  which is inserted onto the piston in the position shown. Then the piston is press fitted into the caudal end plate  20 ′ thereby fixing the piston in place. The ceramic spherical bearing  74 ′ is inserted into the upper bearings plate  40 , and the bearing sleeves  12  are dropped around the spherical bearing  74 ′ and put into position as shown. Finally, the retaining ring  44 ′ is press fitted onto the bearings plate  40  capturing the bearing rings  12  and spherical bearing  74 ′. The piston can be put in and taken out at this point, and is finally captured when the final weld is made where the bellows assembly  50  is welded to the bearings plate  40  and the caudal end plate  20 ′. 
         [0082]    Surgical Procedure 
         [0083]    The particular crown plates  10 ,  10 ′ and the particularly selected bellows cartridge  100  to be implanted in a specific patient are chosen by the surgical team so as to best accommodate the anticipated biomechanical demands of that particular patient&#39;s spinal disc replacement. Once the disc prosthesis components have been selected, the crown plates  10 ,  10 ′ are connected to the bellows cartridge  100  by aligning and advancing cartridge  100  along the chined edges  6 ,  7  (as best seen in  FIG. 4A ) of the crown plates  10 ,  10 ′ that complement the chined edges of the cartridge&#39;s end caps  20 ,  20 ′. This coupling is similar to dove-tail joinery used in cabinet making, for example, in a cabinet drawer. Once the assembly is accomplished for both opposing crown plates, the juxtaposition of the crown plates  10 ,  10 ′ with cartridge  100  is fixedly secured by advancing pairs of set screws  96 A and  96 ′A (as in  FIGS. 5A and 5C ) into the threaded through holes  95 A and  95 ′A until the set screws  96  are seated in respective recess pairs  22 A,  22 ′A of end caps  20  and  20 ′ as shown in  FIG. 3A . 
         [0084]    With the spinal disc prosthetic modular assembly  1  thus assembled with selectable components chosen to optimize the biomechanics of the effected intervertebral space and joint, the prosthetic modular assembly  1  is attached to the insertion tool  300 . Per  FIGS. 4A and 1C , this is done by sliding the insertion tool&#39;s distal effector head  360  between the crown plates  10  so that the beveled edges of the insertion tool&#39;s distal surface matingly complement the beveled inner edges of the crown plate  6 ,  7 ; with the insertion tool effector head  360  juxtaposed against the opposed crown plates  10 ,  10 ′ but not in contact with the bellows assembly  50 . Threaded knob  350  is rotated so as to displace pawl pairs  369 A,  369 ′B into the respective recesses  91 A,  91 ′B in the crown plates  10 . When the pawls are seated in their respective recesses, the insertion instrument  300  is reversibly coupled with the spinal disc prosthetic modular assembly  1 . 
         [0085]    In the preferred embodiment, the diseased spinal disc is surgically approached from the patient&#39;s anterior and the diseased disc is removed by surgical techniques well known to those skilled in surgical arts. A standard vertebral distracting instrument is used to sufficiently spread apart the adjacent vertebrae so as to accommodate the spinal disc prosthetic modular assembly  1 . This standard vertebral distracting instrument has paired, opposing, detachable, interchangeable end pieces and the vertebrae are typically separated using a pair of rounded paddle attachments attached to the distracting instrument so that whatever force is necessary is applied over the largest possible surface area, thereby minimizing the risk of causing a vertebral endplate fracture or other trauma. 
         [0086]    The space between the vertebral endplates accomplished by distracting the vertebrae should be wide enough for the crown plates  10 ,  10 ′ to slide into, but not so the spikes  12  can clear the endplates. The spikes  12 ,  12 ′ protruding from the vertebral engaging surfaces of the crown plates  10 ,  10 ′ cuttingly engage the vertebral end plates during intra-operative positioning and placement of the prosthesis, thereby creating bony channels in the vertebral endplates, each channel extending from anterior to posterior. 
         [0087]    Once the vertebrae are sufficiently distracted as described above, the distracting tool may be removed without fear of the vertebrae suddenly re-approximating; the vertebrae do not simply spring back into juxtaposition, vertebral re-approximation being a very slow process. At this point, the distracting paddles are detached and the upper and lower (cephalad and caudal) distraction bars  320  are fitted onto the standard distracting instrument. Each distracting bar  320 ,  320 ′ is positioned in the midline of the respective vertebral end plates within the intervertebral space. The proper positioning and alignment of the prosthetic modular assembly  1  in the intervertebral space is crucial for the subsequent functioning of the spinal disc prosthetic modular assembly  1 . Proper surgical placement of the distraction bars  320 ,  320 ′ is confirmed by methods well known to the surgical arts, including but not limited to palpation, visual inspection, fluoroscopic and x-ray confirmation, and other stereotactic guiding measures well known to those skilled in the surgical arts. 
         [0088]    The vertebral engaging surfaces of the distraction bars  320 ,  320 ′ are roughened in the same or similar way as are the vertebral engaging surfaces of the crown plates  10 ,  10 ′ thereby allowing the distraction bars  320 ,  320 ′ to better maintain their preferred positioning as determined by the surgical team. With the distraction bars  320 ,  320 ′ in place and still connected to the standard distracting tool with its angled extension arm seen in  FIGS. 1A and 1D , the spinal disc prosthetic modular assembly  1 , coupled to insertion tool  300 , is advanced along the distraction bars  320 ,  320 ′ as the bars  320 ,  320 ′ are slidingly received into through channels  14 ,  14 ′. 
         [0089]    The advancement of the disc prosthetic modular assembly  1  into the intervertebral space requires some considerable force, as may be applied with a mallet or hammer or other such instrument. To avoid damaging the bellows cartridge assembly  100 , the insertion tool  300  connectedly contacts the crown plates  10 ,  10 ′ without making any contact with the bellows assembly  50 ; this is seen in the side view of the insertion tool  300  coupling with prosthetic modular assembly  1  in  FIG. 1C . The forceful advancing of the prosthetic modular assembly  1  from anterior to posterior into position between the vertebrae causes the protruding spikes  12 ,  12 ′ to carve a channel through the vertebral end plates as they are advanced posteriorly to the desired location. The apparently traumatic effect of the spikes  12 ,  12 ′ creating their own channel in the bony surface of the vertebral end plates is considered surgically desirable insofar as heterotopic bone growth is thereby stimulated, promoting the eventual solid bony fixation of the prosthetic modular assembly  1  in place. 
         [0090]    Once the spinal disc prosthetic modular assembly  1  is in the desired position, the insertion tool  300  is disconnected by reversing the connection of the pairs of pawls  369 A and  369 ′B in the recesses  91  and  91 ′ respectively. Retraction of the pawls  369  from recesses  91 , effected by rotation of the threaded knob  350 , permits the insertion tool  300  to be withdrawn while leaving the prosthetic modular assembly  1  in place. The distraction bars  320  are then slidingly withdrawn along channels  14  and out of the intervertebral space, thereby leaving the spinal disc prosthetic modular assembly  1  as the only hardware left in place between the opposing vertebrae. The rest of the implantation, involving the surgical wound closure and so forth, is well known to those skilled in the surgical arts. 
         [0091]    Although this invention has been described in connection with specific forms and embodiments thereof, it should be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular locations of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.