Abstract:
An improved spinal intervertebral prosthetic device that offers all degrees of motion afforded by the anatomic spinal intervertebral disc, and by virtue of the incorporation of a dashpot assembly mechanism within reversibly displaceable bellows, provides hydraulic load bearing capability as well as limiting the maximum pressure that can be exerted on the bellows assembly by limiting the compression stroke of the device. The dashpot assembly also increases resistance to lateral shear loading in excess of the bellows convolutions acting alone. The spinal intervertebral prosthetic device absorbs varying axial loads via hydraulic pressure transfer to the bellows convolutions which can flex in a spring-like fashion. Rotational coupling of the upper crown plate and center bearings plate permits normal twisting movements, and spinal flexural freedom is provided by the bellows that are interposed between the center bearings plate and the lower crown plate.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The subject invention is directed to the field of prosthetic devices which may be surgically placed internal to the human or other mammalian body. 
   The subject invention is directed in particular to spinal hydraulic prosthetic devices which may be implanted within a diseased spinal segment of the mammalian body in place of a nucleus pulposus or alternatively may be used to replace a diseased disc in totality. 
   More particularly, the present invention relates to a rotatable as well as reversibly expandable spinal hydraulic prosthetic device for insertion within the vertebral column of a mammalian body. 
   Still further, the subject invention directs itself to the field of a spinal disc prosthesis which incorporates some type of biocompatible metallic bellows being filled with a substantially incompressible fluid. 
   Further, the subject invention relates to a spinal prosthesis having the ability to permit rotation between a pair of vertebrae which are positionally located adjacent to the prosthesis. 
   Additionally, the subject invention is directed to a prosthetic device which substantially replicates the normal spinal range of motion and tolerates forces in excess of normal ambulatory loads which may be applied to the mammalian body. 
   The subject invention in particular is directed to a prosthetic device which includes at least one bellows member formed from a multiplicity of rigid washer-like members that minimize shear movement in the lateral direction and in combination resist axial compression by a spring-like action of the bellow convolutions. 
   Still further, the subject invention directs itself to a prosthetic device which uses and includes bellow members that resist and re-direct axial force loads through the use of a substantially non-compressible fluid for translation of the axial forces to pressure forces for the intent of expanding and flexing the bellows convolutions. 
   Still further, the subject invention directs itself to a prosthetic device which includes a bellows member and further contains a dashpot assembly mechanism being mounted coaxial with a radial bearing assembly for absorbing and dispersing axial force loads and resisting radial shear forces. Without the dashpot, sudden excessive force loads might cause the bellows to deform, to leak, or even to rupture, with all the attendant risks of major neurologic and/or vascular injury. 
   Additionally, the subject invention pertains to an implantable spinal prosthetic device having an intervertebral span which is adjustable either pre-insertion during surgical implanting, or subsequent to surgery, where the height is adjustable to individualize and optimize the intervertebral spacing of the affected spinal segment. 
   PRIOR ART 
   Implantable spinal prosthetic devices are well known in the art. At present, the primary method employed to remediate degenerative disc disease, discogenic pain, or spinal stenosis is by means of surgical spinal fusion. In this procedure, two or more vertebrae are displaced, the spinal discs between the vertebrae are removed, and crushed bone material taken from the patient&#39;s pelvis is inserted between the two vertebrae. This bone material promotes the growth of new bone in the space between the vertebrae. Since this growth takes time, some mechanical means must be incorporated during the surgery to rigidly maintain the proper spacing between the vertebrae as well as carry normal ambulatory loads imposed on the spine by the patient. Insofar as fused vertebrae no longer take part in normal flexing and dispersion of force loads, higher stress loads will now be imposed on the discs and the vertebrae above and below the fused vertebrae thereby accelerating degenerative changes in those nearby spinal levels. 
   Implantable devices that have been used include semi-rigid elastomeric filler materials sandwiched between two layers of biocompatible metal. The upper and lower plate surfaces generally have multiple spikes for connection to the vertebrae. Other similar devices offer means to screw the upper and lower plates into their conjoining vertebrae and include treated plates to promote bone growth into them. These devices generally permit a small amount of articulation between the vertebrae, but the functional and structural longevity of the elastomeric materials and their bonding agents are often quite short. An ideal prosthesis would last thirty to forty years and withstand about two million cycles per year. 
   It is the purpose of the subject invention to provide a combination of elements making up an implantable spinal prosthesis having both a long life expectancy and providing for normal articulation within the spine. More particularly, the subject spinal prosthesis allows for tilting from side to side, front to back, minute elongation and compression along a main axial direction and also rotation of the prosthesis about the main axis between the two vertebrae. In addition, the subject spinal prosthesis provides for a means both to dampen sudden excessive axial forces and to resist shearing forces that may accompany activities more vigorous than normal ambulation. 
   U.S. Pat. No. 6,375,682 is the closest prior art known to applicants, patented by all the inventors of the present patent application and assigned to the same assignee, of which the present patent application is an improvement. The patent is directed at a collapsible, rotatable and expandable spinal hydraulic prosthetic device, which is hydraulically expandable or retractable. The spinal prosthetic device of that invention incorporates a pair of bellows as well as rotationally coupled members that, in combination, affords the patient full and normal range of motion as regards spinal motion-segment articulation. The device as taught is satisfactory for normal ambulatory loads but may under certain circumstances not provide sufficient means for safely accommodating higher than normal force loads. The present subject patent application overcomes the limitation of this prior art by including a dashpot element which allows further stability and tolerance of high force loads in both axial and radial dimensions associated with vigorous activities such as jumping, leaping, running, and so on. 
   One prior art spinal prosthesis is taught in U.S. Pat. No. 5,002,576, directed to an intervertebral disc prosthesis. This reference is directed to a prosthetic device having a central elastomeric layer sandwiched between two cover plates. The prosthetic device is neither rotatable between the vertebrae, nor does it provide for sufficient bending in the forward, backward or lateral directions. 
   A recent prior art publication, U.S. Pat. No. 6,527,806, is directed to an intervertebral spacer having a pair of opposing plates in apposition to adjacent vertebrae, and containing at least one spiral-shaped radially diminishing amplitude wave washer as a force restoring element. This device permits only limited rotation and flexural freedom. Furthermore, that non-hydraulic device does not employ a fluid-filled bellows and dashpot mechanism as does the present invention, nor does it utilize ball bearings to facilitate nearly normal twisting movements. In addition, that device does not permit individualized adjustment of the intervertebral spacing as does the present invention. 
   Another prior art prosthetic implant is shown in U.S. Pat. No. 4,932,975, directed to a vertebral prosthesis. This invention includes an initially flexible bellows which is made inflexible by injection of a fluid that solidifies and, notably, does not permit rotation between the two vertebrae. 
   U.S. Pat. No. 6,395,032 is directed to an intervertebral disc prosthesis with a liquid chamber. This invention includes bellows containing a liquid and an internal body that absorbs some of the liquid when axial stress is applied to the prosthesis; the prosthesis is attached to the adjacent vertebral bodies by screws lateral to the vertebral bodies. Unlike the present invention, this disc prosthesis lacks a dashpot mechanism and bellows formed from washers, which are designed to obviate lateral displacement of the prosthesis that may result from shearing forces. 
   U.S. Pat. No. 3,875,595 is directed to an intervertebral disc prosthesis as well as instruments for positioning the same. The prosthesis is a hollow, bladder-like member having the appearance of the natural nucleus of a natural spinal disc (when in an expanded shape). The device does not allow for rotation between vertebrae, thus failing to give the user fully articulated movement. 
   U.S. Pat. No. 5,571,189 involves an expandable fabric implant for stabilizing the spinal motion segment. The implant is in the form of an inflatable bag positioned within a cavity artificially formed within the spine. The inflatable bag does not permit rotation between the vertebrae. 
   Another prior art prosthesis is taught in U.S. Pat. No. 5,755,807, being directed to an implantable modular unit and rotating seal for a prosthetic joint. This implant involves a ball-and-socket joint surrounded by a flexible metallic bellows. This system is subject to wear and premature failure due to friction and debris particle buildup associated with the continual rubbing of component parts against each other. 
   None of the prior art provides for a combination of elements forming a rotatable and reversibly expandable spinal hydraulic prosthetic device that includes a flexible metallic bellows and a dashpot assembly mechanism, which together prevent shear-movement in a lateral direction and tolerate both normal and excessive axial force loads. Additionally, none of the prior art Patents by other inventors provide for a spinal prosthetic device having a ball-bearing mechanism in combination with a bellows and dashpot assembly mechanism, which has the important quality of being flexible and rotatable even under force loads greater than associated with normal ambulation, thereby allowing for substantially normal articulation and range of motion between the two vertebrae. 
   SUMMARY OF THE INVENTION 
   The present invention provides for a rotatable and reversibly expandable spinal hydraulic prosthetic device to be implanted between two vertebrae. The spinal prosthetic device includes a flexible metallic bellows positioned between a center bearings plate and a lower crown plate, the center bearings plate being rotationally coupled to an upper crown plate. The upper crown plate and lower crown plate are affixed in a secure fashion to the vertebral bodies above and below the implanted prosthetic device. The center bearings plate is rotationally coupled to the upper crown plate with interposed ball bearings held in position by the bearing retainer, thus allowing for full rotational articulated movement within the spine. 
   The bellows member(s) is formed from a plurality of rigid washer-like members which help to prevent shear-movement along a lateral direction. The bellows is filled with a biocompatible substantially non-compressible fluid, and may be adjusted to the desired intervertebral distance optimal for an individual patient either prior to, during, or subsequent to the surgical implantation. 
   It is a principal objective of the subject rotatable and reversibly expandable spinal hydraulic prosthetic device to provide a spinal prosthesis for replacement of a spinal disc. 
   It is a further objective of the subject spinal prosthetic device to provide a spinal prosthesis having an adjustable axial height. 
   It is a further objective of the subject invention to provide a spinal prosthesis which permits rotation between the two adjacent vertebrae. 
   It is a further objective of the subject invention to provide a spinal prosthetic device with means to prevent shear-movement along a lateral direction. 
   It is an important objective of the present invention to provide a spinal prosthetic device having a threaded fill port or valve member allowing for the variable filling of the prosthesis with incompressible fluids, thus allowing for a variable height of the prosthesis between the two vertebrae and also allowing for variable axial compression-movement within the prosthetic segment. 
   It is a further important objective of the present invention to provide a spinal prosthetic device having a load bearing capability afforded by a bellows as well as a dashpot assembly mechanism contained therein. 
   It is an important objective of the present invention to provide a spinal disc prosthetic device having a fluid-containing bellows, formed from convoluted conjoined metallic washer members designed to permit flexural movement and to resist parallel shear movement, although a preformed bellows can also be used. 
   It is a further important objective of the present invention to provide a spinal prosthetic device having a dashpot with a substantially spherical dashpot element that further resists parallel shear movement and augments the load-bearing tolerances of the invention, the dashpot being centrally located within the fluid-containing dashpot recess formed in the lower crown plate, and the recess being in fluidic continuity with the bellows interior. 
   It is a further objective of the present invention to provide a bellows of metallic or like composition, that permit flexing of the convolution elements which mimic natural body movements. 
   It is an objective of the present invention to provide a convolutional bellows design permitting axial height adjustment either pre- or post-implantation with a mechanism to make adjustments to individually fit the patient&#39;s intervertebral space requirement. 
   It is a further objective of the present spinal implant to provide an extension tubing which allows for post implantation intervertebral gap adjustments. 
   It is an additional objective of the present invention to provide a spinal prosthetic implant device which can be implanted in the space formerly occupied by the nucleus pulposus whereby the annulus fibrosus is left intact. 
   It is a further objective of the present spinal implant device to provide a system which can be substituted in place of the entire anatomical intervertebral disc, in which case the crown plates are to have a kidney-shaped contour that conforms to the outer edges of the bony vertebral end-plates so as to thereby reduce the heterotopic ossification that may interfere with the range of motion of the affected spinal motion segment. 
   It is an important objective of the present invention to provide a spinal prosthetic implant with an extended life expectancy, and with no chance of rejection by the body. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional, partially cut-away view of the subject spinal prosthetic device  10  positioned between two vertebrae  14 ; 
       FIG. 2  is a detailed cross-sectional view of the subject spinal prosthetic device  10 ; 
       FIG. 3A  is a cross-sectional view of a preferred embodiment of the substantially spherical dashpot assembly mechanism  72  of the subject spinal prosthetic device  10 ; 
       FIG. 3B  is a cross-sectional view of an alternative embodiment of the dashpot assembly mechanism  72  of the subject spinal prosthetic device  10  containing two joined bellows  60 , in which a cylindrical piston  73  is crimped onto the substantially spherical dashpot element  72 , and the dashpot recess  74 ′ is formed to a shape so as to accommodate the cylindrical piston  73  while allowing for fluidic continuity between the dashpot recess  74 ′ and the fluid  64  contained in the bellows chamber; 
       FIG. 4A  is a top view of a preferred embodiment of the upper crown plate  11  and the lower crown plate  80  in which the crown plates  80 ,  11  have a substantial kidney-shape that conforms to the cross-sectional shape of the adjoining vertebral end-plates; 
       FIG. 4B  is an embodiment of the upper crown plate  11  and the lower crown plate  80 , in which the crown plates  11 ,  80  have a more circular shape that conforms to the cross-sectional shape of the excised nucleus pulposus; 
       FIG. 5  shows one of the preferred embodiments that includes the valve mechanism  84  with the optional remote valve access components. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the preferred embodiment of the spinal prosthetic device  10  implanted between two lumbar vertebrae  14 . The preferred embodiment of the device with one bellows  60  is shown in  FIG. 1  tilted to its maximum inclination, which may be adjustably varied. The prosthetic device  10  may be used to replace an entire diseased spinal intervertebral disc, or it may be positioned within the nucleus pulposus space of a spinal disc wherein the nucleus pulposus material has been removed. The device  10  is shown in  FIG. 1  as it would be located if the annulus fibrosus were left intact. A larger version of this device configuration seen in  FIGS. 1 and 4A  may be similarly implanted within the space formerly occupied by the entire intervertebral disc, wherein both the nucleus pulposus and the annulus fibrosus are completely removed. 
   Referring now to  FIG. 1 , there is shown the rotatable and reversibly expandable spinal hydraulic prosthetic device  10 . As shown in  FIG. 1 , the prosthetic device  10  includes a pair of crown plates, an upper crown plate  11  and a lower crown plate  80 , which sandwich between them a center bearings plate  50  which is rotationally coupled by a radial thrust bearing assembly  20  to a bearing retainer seal  40 , and at least one bellows member  60 . A dashpot mechanism  70  is formed between the center bearings plate  50  and the lower crown plate  80 , and is seated in a recess  82  defined in the second surface  19  of the lower crown plate  80 . 
   The upper crown plate  11  incorporates a sintered titanium or other like metallic planar surface  15  as seen in  FIGS. 4A and 4B , with vertebral engaging members or spikes  17  projecting therefrom as seen in  FIGS. 1 and 2 . Opposite upper surface  18  is lower surface  19  defining a second surface which has an annular groove  22  formed therein, as illustrated in  FIG. 1 . 
   The lower crown plate  80  incorporates a mechanism for filling and bleeding fluids into or out of the assembly. In one preferred embodiment, seen in  FIGS. 1 and 2 , a threaded fill port  84  is contained in the lower crown plate  80  and a fluid channel  85  is formed therethrough. The threaded fill port  84  can be incorporated within the lower crown plate  80 , as shown in the embodiment of  FIGS. 1 and 2 . 
   Alternatively the fill port  84  may be accessed remotely from the lower crown plate  80 , as shown in  FIG. 5 . As shown, a capillary tube  88  fluidly connects the valve  89  with the lower crown plate member  80 . The capillary tube  88  may be formed from either ductile titanium metal, flexible braid reinforced silicone rubber tube, or like material, as shown in  FIG. 5 . Alternatively, a threaded fill port  84  or a valve and fluid channel may be located in the center bearings plate  50 . 
   As shown in the embodiment illustrated in  FIG. 4A , the crown plate members incorporate a layer of sintered titanium  15  on the vertebral engaging surfaces  18  and may be substantially kidney-shaped. The vertebral engaging surfaces  18  have a plurality of spikes  17  projecting therefrom for engaging the vertebral endplates. The spikes  17  laterally affix or set the vertebra engaging members  17  to their respective vertebrae. 
   The spikes  17  are shown in  FIGS. 1 and 2  as having a substantially triangular cross-section. However, the spikes  17  may be of any suitable size or shape for engaging the vertebral endplates, as shown in  FIG. 1 . The spikes  17  may be formed from sintered titanium, solid titanium, or any other suitable material providing biocompatiblity and both compressive and shear strength. Due to the fact that the spinal cord is especially sensitive to injury and damage, it is necessary that the spikes  17 , as well as the other elements forming a spinal prosthetic device  10 , such as the center bearings plate and the bellows  60 , be formed from strong, resilient and biocompatible materials such as titanium and other like metals or plastics compositions. Sintered titanium, for example, allows for longevity, strength, and no potential for rejection of the elements forming the spinal prosthetic device  10  by the body. Further, the prosthetic element formed from sintered titanium promotes bone growth in and around that projecting element. 
   Further shown in  FIGS. 4A and 4B  is the irregular surface  18 . The porous surface  18  forms a mesh or sintered surface  15  for permitting the upper crown plate  11  and the lower crown plate  80  to be joined to the vertebrae. In one embodiment illustrated in  FIG. 4A , showing a kidney-shaped crown plate vertebral surface, the outer surfaces  18  of crown plates  11 ,  80  are shaped and sized to substantially match the adjacent vertebral end plate cross-sections, thus reducing the occurrence of heterotopic ossification, also known as lipping, by new bone growth into the intervertebral space that could interfere with the full range of motion of the affected spinal motion segment. 
   Self-tapping screws (not shown) are passed through the vertebral fixation openings  16  in the crown plates, also illustrated in  FIG. 4A , into the contiguous vertebral endplates, the crown plates being further held to the vertebrae  14  by subsequent growth of the bone into the porous surfaces  15 . The self-tapping screws can be used to secure the larger kidney-shaped upper crown plate  11  and lower crown plate  80  to their conjoining vertebrae  14 . The surgeon, with the use of a separate drill insert bushing, can use bores to first pre-drill a screw diameter hole into the vertebrae prior to installing the self-tapping screw. This configuration can be used when the annulus fibrosus has been removed. The screws provide further stability and strength for implantation of the spinal prosthetic device. The screws may be used in conjunction with spikes  17 , as shown in  FIG. 4A . A porous surface  18  is also formed on the more circular crown plate members of the embodiment without the vertebral fixation openings and screws, shown in  FIG. 4B . 
   The flexible bellows  60  can be formed from a plurality of washers, formed of titanium or like composition, securely joined together through laser welding, electron beam welding, resistance welding, or some other suitable technique. Similarly, the flexible bellows  60  are joined to the outer surfaces of the center bearings plate  50  and the lower crown plate  80  through laser welding, electron beam welding, or any other suitable method. 
   The plurality of titanium washers forming the flexible bellows  60  allow for vertical flexibility and axial lengthening and shortening of the bellows, and also act to prevent lateral shearing displacement of the bellows. The sensitivity of the spinal cord to damage requires that the spinal prosthetic device  10  be both flexible along the main vertical axis and very resistant to lateral shear movement, that is, movement that would bend the bellows into a parallelogram shape. A rocking motion of the prosthetic device  10  is desired for more natural movement. 
   The bellows members  60  permit forward, backward and lateral motion within the spine, and afford axial height flexing about the longitudinal axis by means of the liquid contained therein. The bellows  60  absorbs the imposed stresses by flexing, bending and distention of its convolutions. There is very little rubbing of one component against another, thus eliminating wear on these mechanical parts, and giving the device  10  an extended working life. 
   Furthermore, the bellows  60  offers both spring-like action while also lending itself as a container to house a fill fluid  64 . In bellows systems utilizing Belleville type conical washers made of titanum, hundreds of millions of cycles are routinely imposed without failure. The bellows design incorporates the torsional stability of a coil spring with the tension and compression stability of a leaf spring. Regardless of how the loads are applied, deflection is effected without any of the flexing parts destructively rubbing against one another. 
   The joined washers forming the bellows  60  provide for not only flexibility along the main axis of the spinal prosthetic device  10 , but prevent shear movement in the radial direction. The spinal cord is especially susceptible to injury and damage, and the bellows assembly  60  containing the hydraulic fill fluid  64  prevents the device  10  from being crushed or shifted in the radial direction, thus preventing injury to the aorta or nerves of the spinal cord. 
   As shown in  FIGS. 1 and 2 , reversibly displaceable bellows  60  are sealed on one end to center bearings plate  50 , and on the other end to the second surface  19  of the lower crown plate  80 . Center bearings plate  50  may be made of titanium or any other suitable, strong and rigid metal material, which is weld compatible with bellows  60 . Center bearings plate  50  is bonded to the reversibly extendable bellows  60  through welding or any other suitable means. As shown, center bearings plate  50  has a bearing recess  54  and an annular groove  22  formed therein. Furthermore, center bearings plate  50  has formed on its inferior aspect a centrally located dashpot mechanism  70 . 
   In order to allow torsional motion between the conjoined vertebrae, simulating natural spinal movement, the upper crown plate  11  and the center bearings plate  50  rotate around the central axis, rolling on ball bearings  30 . There is little friction, heat, or wear produced in the rolling contact between the upper crown plate  11 , the center bearings plate  50 , and the ball bearings  30  as long as the yield stress of neither the ball bearings  30  nor the annular grooves  22  is exceeded. A silicone grease or other biocompatible lubricant may be present on either surface of the bearing retainer seal  40  in order to add lubrication to the center bearing plate  50  and the upper crown plate  11  with respect to the bearing retainer seal  40 . 
   Bearing retainer seal  40 , as shown in  FIGS. 1 and 2 , incorporates multiple hardened steel ball bearings  30  captured within a biocompatible washer seal material. As shown in  FIGS. 1 and 2 , a radial thrust bearing assembly  20  is received within the bearing recess  54  of the center bearings plate  50 . The ball bearing projections of the bearing retainer seal  40  are received within the annular grooves  22  formed both on the inner second surface of the upper crown plate  19 , as well as in the superior surface of the center bearings plate  50 . 
   Ball bearing members  30  of the bearing retainer seal  40  are received within the annular grooves  22  of both the upper crown plate  11  and the center bearings plate  50 . The upper crown plate  11  and lower crown plate  80  are assembled together with the bearing retainer seal  40 , the center bearings plate  50  and the bellows  60  as seen in  FIGS. 1 and 2 . The radial thrust bearing assembly forms the rotational coupling between the upper crown plate  11  and the center bearings plate  50 , the reversibly displaceable bellows  60 , the lower crown plate  80 , as well as the center bearings plate  50  are, together, free to rotate with respect to the vertebral upper crown plate  11 . 
   As shown in  FIGS. 1 and 2 , spikes  17  engage the bone of the vertebrae  14 , holding the spinal prosthetic device  10  in place with respect to the spine. The rotation of the lower crown plate  80 , the bellows  60  and the center bearings plate  50  with respect to the upper crown plate  11 , allows for a fully rotating and articulating motion of the prosthetic device  10  with respect to the adjoining vertebrae  14 . Thus, the spinal prosthetic device  10  provides for natural torsional movement of the spine and decreased risk of injury and dislocation of the crown plate members with respect to the vertebrae. 
   The bellows  60  are comprised of a plurality of conical washers formed of titanium or similar composition. This configuration is set to approximate the final desired distance between the vertebrae  14  and relies on the surgeon to distract the vertebrae  14  with a distractor tool prior to implantation of the device. The device  10  can be sized to fit into the space formerly occupied by the nucleus pulposus when the annulus fibrosus is left intact, or sized with a larger kidney shaped surface  18 , illustrated in  FIG. 4A , to fit between the vertebrae  14  when both the annulus fibrosus and the nucleus pulposus have been removed. 
   Furthermore, the bellows assembly  60  are comprised of paired conical washers formed of titanium or like composition. This configuration is pre-expanded to approximate the final desired distance between the vertebrae  14  and relies on the surgeon to distract the vertebrae with a distractor tool prior to positioning and implantation of the device. The device  10  can be sized to fit into the space formerly occupied by the nucleus pulposus when the annulus fibrosus is left intact, or sized with a larger, kidney-shaped contour to fit between the vertebrae  14  when the annulus fibrosus has been removed as part of a total discectomy. When the bellows member  60  is made from two Belleville type washers, as shown in  FIGS. 1 and 2 , the total number of degrees of spinal motion segment bending/tilting excursion allowed is what is considered normal, approximately 10 degrees. 
   The spinal prosthetic device  10  may be pre-filled with a substantially incompressible biocompatible fluid at the time of manufacture, wherein the bellows  60  may be pre-expanded to be near the desired final height and, in this embodiment, the surgeon would use a separate distracting tool to spread the two adjacent vertebrae apart before inserting the device  10  into final position. 
   A dashpot mechanism  70  is incorporated between the center bearings plate  50  and the lower crown plate  80  wherein the dashpot  70  and the spinal prosthetic device  10  are coaxial. The substantially spherical dashpot element  72  extends from the inferior surface of the center bearings plate  50  into the dashpot recess  82  formed on the second, inner surface  19  of the lower crown plate  80 . The substantially spherical dashpot element  72  when seated in the dashpot recess  82  is surrounded by fluid in the dashpot cavity  74  formed by the space between the spherical dashpot element  72  and the recess  82 . This dashpot cavity fluid is in continuity with the fluid  64  contained within the bellows  60 . 
   As seen in  FIG. 3A , the substantially spherical dashpot element  72  is fixedly seated on a dashpot mounting post  52 , which is formed from the inferior surface of the center bearings plate  50 . 
   As may be seen in  FIG. 3B , an alternative preferred embodiment includes a cylindrical piston  73  crimped onto the substantially spherical dashpot element  72 , with the dashpot recess  74 ′ formed in the inner surface  19  of the lower crown plate with a shape that accommodates the cylindrical piston  73  while allowing fluidic connection between the dashpot recess  74 ′ and the fluid  64  contained in the chamber formed by the bellows  60 . 
   The incorporation of a dashpot mechanism  70  adds further tolerance to sudden and excessive axial force loads, and also constitutes a further means for the device to safely resist shearing forces in the radial direction. By incorporation of a dashpot mechanism  70 , the safety and functionality of the spinal prosthetic device  10  is augmented, permitting more vigorous activities which are associated with greater force loads in an axial and radial direction, and further provides an increased margin of safety for shearing forces, thereby further protecting against injury to the spinal cord or nearby vascular structures. The use of the hydraulic dashpot permits dampening of high shock loads, without which the bellows could rupture. 
     FIGS. 1 and 2  show the preferred embodiment of the prosthetic device  10 , wherein the threaded fill port  84  is integral with the lower crown plate  80 . Once the threaded fill port  84  is opened by unscrewing and removing a seal screw  69 , fill fluid  64  may be injected, pumped or otherwise forced into the device, causing it to expand. When the surgeon wishes to detract the device  10 , a relief knob (not shown) is opened and the fill fluid  64  will be removed from the spinal prosthesis device chamber. 
   An alternative embodiment is shown in  FIG. 5 , wherein the valve insert  89  is provided with an outer thread and includes a ball check  90  which is biased in the closed position by spring  91 . Seal screw  69  is provided as a sealing means after all filling adjustments have been completed. Valve insert  89  has a threaded recess formed therein for receiving the threads of seal screw  69 . Also within the recess of valve insert  89  is an O-ring seal  93  for sealingly receiving the seal screw  69 , and sealing around the hypodermic needle  96 . 
   Valve insert  89  is provided for the filling of the reversibly extendable bellows  60  with an incompressible liquid, the fill fluid  64 . Fluid flows through fluid channel  85  to fill the bellows assembly  60  to a predetermined height, depending upon the desired intervertebral spacing. 
     FIG. 5  shows the spinal prosthetic device  10  having an alternate filling attachment means. Installed in the threaded port  84 , valve insert  89  is integral with the lower crown plate  80  as shown. Hypodermic needle charge fitting  95  is received within valve insert  89 . The charge fitting includes the hypodermic needle  96  which passes through O-ring seal  93 , effecting a seal. As the fitting  95  is further inserted into valve connector  89 , needle  96  pushes the ball check  90  away from its seat and further compresses the spring behind it. Once the ball check  90  is unseated, fill fluid may be injected or pumped into the device  10  thereby causing it to expand. When the surgeon wishes to detract the device  10 , the relief knob on the pump (not shown) is opened, and the fill fluid will bleed back into the pump. 
     FIG. 5  further illustrates extension tubing  88 . Extension connector  62  is received within valve insert  89 , allowing remote valve insert  89 ′ to be used remote to the valve insert  89 . Thus, the prosthetic device  10  may be filled with a substantially incompressible biocompatible fluid either through the threaded fill port  84  formed within the lower crown plate  80  or through the remote extension tubing  88 . Extension tubing  88  is preferable for filling and bleeding of fluids from the reversibly extendable bellows  60  when the spinal prosthetic device  10  has already been implanted within the spine of the patient. 
   When the final intervertebral distance setting has been achieved, the surgeon unscrews the extension connector  62  and, as needle  96  is withdrawn, the ball check  90  closes, effecting a seal before the needle  96  passes out beyond the O-ring  93 . In order to make the final seal, the surgeon screws seal screw  69  into the valve insert  89 , or the remote valve insert  89 ′ if the extension tubing  88  is employed. 
   If the surgeon needs to adjust the height of the device  10 , he or she may fill or drain fluid from the device  10 , that is, either evacuate fluid out or pump extra fluid in, by connecting a hand pump or similarly effective means to the device. 
   In cases where the surgeon desires to keep the annulus fibrosus intact, but cannot distract the vertebrae to their final position at the time of the surgery, the surgeon can attach a short length of tubing  88 , as shown in  FIG. 5  to the device  10  and tuck the entire tubing extension  88  inside the incision prior to closing the patient up. 
   After a clinically appropriate time, the patient may have an incision made on an adjacent area of the flank or back to temporarily externalize the remote filling element  89 ′. The tubing  88  can be removed after expanding the bellows  60  of device  10  to the final intervertebral distance desired. This could be performed with the aid of real time fluoroscopy. When the final intervertebral span distance has been achieved, the surgeon seals the valve  89 , thereby hermetically sealing the fluid  64  contained within the prosthetic device  10 . 
   The spinal prosthetic device  10  may be sized so as to make it adaptable to fit in other regions of the spine, such as the cervical and thoracic regions. Just as the cross-sections of the vertebrae increase in area from the cervical region down to the sacrum, the normal loading encountered increases proportionally. Thus, it follows that the larger is the cross-sectional area of the bellows  60 , the lower the imposed stresses will be on the bellows convolutions. The bearing load imposed between the crown plates&#39; vertebral engaging surfaces  18  and the vertebrae  14  will also be lower. 
   Although this invention has been described in connection with specific forms and embodiments thereof, it will 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.