Abstract:
A spinal prosthetic device ( 10 ) which is hydraulically expanded or retracted, either pre-implantation or in-situ, thereby being adjustable for the exact intravertebral axial spacing required for the patient. Thrust bearing members ( 22 ) are positioned between the vertebra engaging members ( 12 ) and a pair of bellows ( 26 ), allowing the collapsible/expandable bellows ( 26 ) to be rotated with respect to the vertebra engaging members ( 12 ). The device ( 10 ) can be readjusted, on an out-patient basis, months after initial implantation. The spinal prosthetic device ( 10 ) offers all degrees of motion afforded by the anatomical spinal disc and by virtue of incorporating no rubbing contact of any parts, exhibits and infinite working life. The spinal prosthetic device ( 10 ) reproduces the same hydraulic load bearing capability as the nucleus pulposis and flexural freedom of movement similar to the annulus fibrosis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to a collapsible, rotatable and expandable spinal hydraulic prosthetic device which may be either implanted within the center of a diseased spinal disk in place of the nucleus pulposis or may replace a degenerated disk altogether. In particular, the present invention directs itself to a spinal prosthesis formed of a bio-compatible metallic bellows, the bellows being filled with a non-compressible fluid. More particularly, this invention directs itself to a spinal prosthesis having the capability of rotating between the two vertebrae adjacent the prosthesis. 
     Further, the bellows is formed from a plurality of rigid washer-like members, minimizing shear and movement in the lateral direction. Additionally, this invention directs itself to an implantable spinal prosthetic device having an adjustable height, either in vivo or pre-insertion, the height being adjusted to the desired intravertebral spacing of the spine. 
     2. Prior Art 
     Implantable spinal prosthetic devices are known in the art. Presently, the primary method employed to remediate degenerative disk disease, discogenic pain or spinal stenosis is through spinal fusion. In this procedure, two or more vertebrae are displaced, the spinal disks 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 interstitial space between the vertebrae. Since this growth takes time, some mechanical means must be incorporated at the time of 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. Since the fused vertebrae will no longer take part in normal flexing, higher stress loads will now be imposed on the disks and the vertebrae directly above and below the fused vertebrae. 
     Commonly used implantable devices 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 cojoining vertebrae and include treated plates to promote bone growth into them. These devices generally permit a small amount of articulation between the vertebrae, and the longevity of the elastomeric materials and their bonding agents are often quite short. The ideal prosthesis would last 30 to 40 years and withstand two million cycles per year. 
     It is a 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 total 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 between the two vertebrae about the main axis. 
     One prior art spinal prosthesis is shown in U.S. Pat. No. 5,002,576. This reference is directed to an intervertebral disk endoprosthesis. This reference teaches a prosthetic device having a central elastomeric layer sandwiched between two cover plates. The prosthetic device is neither rotatable between vertebrae, nor does it provide for sufficient bending in the forward, backward or lateral directions. 
     Another prior art prosthetic implant is shown in U.S. Pat. No. 4,932,975. This reference is directed to a vertebral prosthesis. The device includes an initially flexible bellows and is made inflexible by injection of a fluid which solidifies and, further, does not allow for rotation between the two vertebrae. 
     U.S. Pat. No. 3,875,595 is directed to an intervertebral disk prosthesis and instruments for locating the same. The prosthesis is a hollow, bladder-like member having in expanded shape the appearance of the natural nucleus of a natural spinal disk. The device does not allow for rotation between vertebrae, thus not giving the user full articulated movement. 
     U.S. Pat. No. 5,571,189 is directed to 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 provide for rotation between the vertebrae. 
     Another prior art prosthesis is shown in U.S. Pat. No. 5,755,807. This reference is directed to an implant module unit and rotating seal for a prosthetic joint. This implant includes a ball-and-socket joint surrounded by a flexible metallic bellows. However, the system is subject to wear and premature failure due to friction and debris particle build-up. 
     None of the prior art provides for a combination of elements forming a collapsible, rotatable and expandable spinal hydraulic prosthetic device including a flexible metallic bellows which prevents shear-movement in a lateral direction. Additionally, none of the prior art Patents provide for a spinal implant device having a bellows/roller-bearing combination which is rotatable, thus allowing full articulation, between the two vertebrae. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a collapsible, rotatable and expandable spinal hydraulic prosthetic device which is adapted to be implanted between two vertebrae. The spinal prosthetic device includes a flexible bellows positioned between two roller-bearing assemblies. The radial-thrust bearing assemblies are affixed to vertebra engaging members, respectively, which contact and set the prosthetic device to the vertebrae. The radial-thrust bearing assemblies allow for the bellows to be rotated between the two vertebrae, allowing for rotational articulated movement within the spine. 
     Further, the bellows is formed from a plurality of rigid washer-like members which prevent shear-movement along a lateral direction. The bellows is filled with a non-compressible fluid and may be adjusted to the desired height either pre-insertion or in vivo. 
     It is a principle objective of the subject collapsible, rotatable and expandable spinal hydraulic prosthetic device to provide a spinal prosthesis for replacement of a spinal disk. 
     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 rotates between the two adjacent vertebrae. 
     It is a further objective of the subject invention concept to provide a spinal prosthetic device which prevents shear-movement along a lateral direction. 
     It is an important objective of the present invention to provide a spinal prosthetic device having a valve assembly allowing for the variable filling of the prosthesis with a mixture of incompressible and compressible 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 objective of the present invention to provide a spinal prosthetic device having load bearing capability afforded via a bellows configurational device. 
     It is an objective of the present invention to provide a spinal prosthetic device having a liquid filled metallic washer convoluted design to permit flexural movement but resist parallel shear movement. 
     It is a further objective of the present invention to provide a metallic bellows which affords flexing of the convolution elements which mimic natural body movements. 
     It is an objective of the present invention to provide a bellows design permitting axial height adjustment either pre or post implantation which can be adjusted to precisely fit the patient&#39;s intravertebral space requirement. 
     It is a further objective of the present spinal implant to provide an extension hose which allows for post operative intravertebral gap adjustments. 
     It is an additional objective of the present invention to provide a device which can be implanted in the space formerly occupied by the nucleus pulposis whereby the annulus fibrosis 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 disc. 
     It is an important objective of the present invention to provide a spinal implant with a virtually infinite life expectancy and with no chance of rejection by the body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a top view of the subject spinal prosthetic device; 
     FIG. 1 b  is a cross-sectional view of the subject spinal prosthetic device; 
     FIG. 1 c  is an exploded view of the top half of the subject spinal prosthetic device; 
     FIG. 2 a  is a top view of the preferred embodiment of the spinal prosthetic device; 
     FIG. 2 b  is a cross-sectional side view of the subject spinal prosthetic device; 
     FIG. 2 c  is a side view of extension tubing used in conjunction with the spinal prosthetic device; 
     FIG. 2 d  is a cross-sectional view of a charge fitting device used in conjunction with the spinal prosthetic device; 
     FIG. 3 is a cross-sectional side view of the spinal prosthetic device implanted between two vertebrae; 
     FIG. 4 a  is a top view of an alternate embodiment of the spinal prosthetic device; 
     FIG. 4 b  is a side view of the alternate embodiment of the spinal prosthetic device; and, 
     FIG. 5 is a graph showing a typical Stress vs. Strain curve for a typical metal material. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 a - 1   c , there is shown a collapsible, rotatable and expandable spinal hydraulic prosthetic device  10 . As shown in FIG. 1 b , the prosthetic device  10  includes a pair of collapsible, expandable bellows  26 , a pair of first vertebra engaging members  12  and a base plate member  32  having a fluid channel  34  formed therethrough. 
     Base plate member  32  incorporates a means for filling and bleeding fluids into or out of the assembly. The valve connector  36  can be incorporated within the base plate, as shown in the embodiment of FIG. 2 b , or remotely, as shown in FIG. 1 b . As shown, a capillary tube  38  fluidly connects valve connector  36  with base plate member  32 . The capillary tube  38  may be formed from either ductile titanium metal or may be replaced by a flexible braid reinforced silicone rubber tube, as shown in FIG. 2 c  of the Drawings. 
     As shown in FIG. 1 a , the vertebra engaging members  12  are substantially circular and may incorporate a layer of sintered titanium on upper vertebra engaging surface  16 . The first vertebra engaging surface  16  has a plurality of projections  20  projecting therefrom. The projections, or spikes,  20  laterally affix or set the vertebra engaging members  12  to their respective vertebrae. 
     The projections or spikes  20  are shown in FIGS. 1B,  1 C, and  2 B as having a triangular cross-section. The spikes  20  may be of any suitable size or shape for engaging the vertebra  14 , as shown in FIG.  3 . 
     The spikes  20  may be formed from sintered titanium, solid titanium, or any other suitable material providing biocompatibility 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  20 , as well as the other elements forming the spinal prosthetic device  10 , such as the base plate member  32  and the collapsible bellows  26 , 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, a prosthetic element formed from sintered titanium promotes bone growth in and around the element. 
     FIG. 1 b  is a cross-sectional view of the spinal prosthetic device  10 , taken along line  1   b  of FIG. 1 a . As shown in FIG. 1 b , the base plate member  32  includes a pair of opposing outer planar surfaces  40  to which the pair of flexible bellows  26  are fixedly secured. The flexible bellows  26  are formed from a plurality of titanium washers securely joined together through laser welding, electron beam welding, resistance welding, or some other suitable method. Similarly, the flexible bellows  26  may be joined to the outer planar surfaces  40  of base plate member  32  through laser welding, electron beam welding, or any other suitable method. 
     The plurality of titanium washers forming the flexible bellows  26  allow for vertical flexibility and collapsibility/extendability of the bellows and also act to prevent lateral movement 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 resistant to lateral shear movement; i.e., movement that would bend the bellows into a parallelogram shape. A rocking motion of the prosthetic device  10  is desired for natural movement. 
     As shown in FIG. 1 c , the collapsible bellows  26  are sealed on one end by end cap  28 . End cap  28  may be made of titanium or any other suitable, strong and rigid metal material, which is weld compatible with bellows  26 . End cap  28  is bonded to collapsible, extendible bellows  26  through welding or any other suitable means. As shown, end cap  28  is formed with bearing recess  30  formed therein. Additionally, end cap  28  has an annular groove  42  formed therein. 
     Thrust bearing member  22 , as best shown in FIG. 1 c , incorporates multiple hardened steel ball bearings captured within a biocompatible washer seal material. As shown in FIGS. 1 b  and  1   c , a radial bearing assembly  24  is positioned in the center of the substantially annular thrust bearing member  22 . The radial bearing assembly  24  is received within the bearing recess  30  of end cap  28 . The ball bearing projections of the thrust bearing member  22  are received within the annular groove  42  formed on the end cap  28 . 
     The vertebra engaging member  12  incorporates a sintered titanium planar surface  16  with projections  20  projecting therefrom. Opposite upper surface  16  is lower surface  18  which has an annular groove formed therein. The vertebra engaging members  12  each have a through hole  46  formed through the center of the annular plate. 
     Ball bearing members  44  of thrust bearing member  22  are received within the annular groove  48  of the vertebra engaging member  12 . The vertebra engaging member  12  is fixed to the thrust bearing member  22  by screw  50 , which is received within the radial bearing assembly  24 . 
     When the vertebra engaging members  12 , the thrust bearing members  22 , and the collapsible, extendible bellows  26  are assembled, as shown in FIG. 1 b , the collapsible, extendable bellows and the base plate member  32  are free to rotate with respect to the vertebra engaging members  12 . FIG. 3 illustrates the spinal prosthetic device  10  implanted between two vertebrae  14 . The prosthetic device  10  may replace an entire diseased spinal disc or it may be positioned within the nucleus pulposis space of a spinal disc wherein the nucleus pulposis material is removed. 
     As shown in FIG. 3, projections  20  engage the bone of the vertebrae, holding the spinal prosthetic device in place with respect to the spine. The rotation of the collapsible, extendable bellows  26  and the base plate member  32  with respect to the vertebra engaging members  12  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 movement of the spine. 
     A silicone grease or other biocompatible lubricant may be present on either surface of the thrust bearing member  22  in order to add lubrication to end cap  28  and vertebra engaging member  12  with respect to the thrust bearing member  22 . In order to provide full articulated movement within the spine, it is necessary that the collapsible, extendable bellows  26  be rotatable with respect to the two vertebrae  14 . This rotation provides both natural rotating movement of the spine and also decreases the risk of injury and dislocation of the vertebra engaging members  12  with respect to the vertebrae  14 . 
     As shown in FIG. 1 b , valve connector  36  is provided with an outer thread and includes ball check  52  which is biased in the closed position by spring  54 . End cap  56  is provided as a back-up sealing means after all filling adjustments have been completed. End cap  56  has a threaded recess formed therein for receiving the threads of valve connector  36 . Also received within the recess of end cap  56  is an O-ring seal  58 . 
     Valve connector  36  is provided for the filling of the collapsible, extendable bellows  26  with either an incompressible liquid or a mixture of a liquid and a gas. Fluid flows through fluid channel  34  of capillary tube  38  to fill the bellows assembly  26  to a predetermined height, depending upon the desired intervertebral spacing. 
     FIG. 2 b  shows the preferred embodiment of the spinal prosthetic device  10 . Valve connector  36  is integral with base plate member  32 , as shown. Further, the bellows assembly  26  are each comprised of two conical washers formed of titanium or like composition. This configuration is pre-expanded to approximate the final desired gap between the vertebrae  14  and relies on the surgeon to distract the vertebrae with a distractor tool prior to implantation of the device. The device can be sized to fit into the space formerly occupied by the nucleus pulposis when the annulus fibrosus is left intact or sized with a larger diameter to fit between the vertebrae  14  when the annulus fibrosus has been removed. Using only four or fewer flexible Belleville type washers, as shown in FIG. 2 b , the total number of degrees of bending are limited to approximately 20°, which approximates normal spinal disk movement. 
     FIG. 2 d  illustrates a charge fitting  60  adapted for use with valve  36 . Charge fitting  60  is received within valve connector  36 . The charge fitting includes a hypodermic needle  64  which passes through O-ring seal  58 , effecting a seal. As the fitting  60  is further inserted into valve connector  36 , needle  64  pushes the ball check  52  away from its seat and further compresses the spring behind it. 
     Once the ball check  52  is unseated, liquid or gas may be pumped into the device, causing it to expand. When the surgeon wishes to detract the device, the relief knob on the pump (not shown) is opened and the liquid or gas will bleed back into the pump. 
     FIG. 2 c  illustrates extension tubing  68 . Extension connector  66  is received within valve connector  36 , allowing remote valve connector  36 ′ to be used in place of the valve connector  36 . Thus, the prosthetic device  10  may be filled with liquid and gas either through the valve positioned on the base plate member  32  or through the remote extension tubing  68 . Extension tubing  68  is preferable for filling and bleeding of fluids from the collapsible, extendable bellows  26  when the spinal prosthetic device  10  has already been implanted within the spine of the patient. 
     When the final gap setting has been achieved, the surgeon unscrews the charge fitting  60  and, as needle  64  withdraws, the ball check  52  closes, effecting a seal before the needle  64  passes out beyond O-ring  58 . In order to make the final seal, the surgeon screws plug  62  into the valve connector  36 , or the remote valve connector  36 ′ if the extension tubing  68  is employed. 
     FIG. 3 illustrates the preferred embodiment of the spinal prosthetic device  10 , shown in FIG. 2 b , implanted in the lumbar region of the spine. The device is shown tilted to its maximum of approximately 20° which may be varied to suit. The device is shown as it would be situated if the annulus fibrosus were completely removed. A smaller diameter version of this device configuration similar to FIG. 1 or  2  may be similarly implanted within the space formerly occupied by the nucleus pulposus, wherein the annulus fibrosus is left intact. 
     FIG. 4 a  is a top view of the spinal prosethetic device  10  with expanded vertebra engaging members  72 . The vertebra engaging members  72  have been modified in this embodiment to incorporate bores  74  for receiving screws  76 , illustrated in FIG. 4 b . The self-taping screws  76  can be used to secure the expanded vertebra engaging members  72  to their cojoining vertebrae. The surgeon, with the use of a separate drill insert bushing, can use the bores  74  to first pre-drill a screw diameter hole into the vertebrae prior to installing the self-taping screw  76 . This configuration can be used when the annulus fibrosus has been removed. The screws  76  provide further stability and strength for implantation of the spinal prosthetic device  10 . The screws  76  may be used in conjunction with projections  20 , as shown in FIG. 4 b.    
     Further shown in FIG. 4 a  is irregular surface  78 . The porous surface  78  forms a mesh or sintered surface for allowing vetebra engaging members  72  to easily join to the vertebrae. In addition to the screws  76 , the vetebra engaging members  72  are held to the bone of the vetebrae by actual growth of the bone into the porous surface  78 . A porous surface  78  is also formed on the vertebra engaging members  12  of the embodiment shown in FIG. 2 a.    
     FIG. 5 shows a typical Stress vs. Strain curve showing the behavior of most metal materials. The spinal prosthetic device  10  is designed such that the maximum anticipated load, with a built-in factor of safety, would never exceed the yield point stress allowed for the materials of construction. 
     FIG. 5 is a typical graph of the Stress vs. Strain or the Stress vs. Deflection of most metal materials. The linear portion of the curve defines what is commonly called the “elastic” portion. If the applied stress never exceeds the linear part of the curve, once the stress is removed, the material will revert back to its original shape. If the stress exceeds the “yield” point for the material, then “plastic” deformation occurs and the material will not revert back to its original shape. If the stress level increases to the “ultimate” value, the material will fail as a support. 
     The collapsible, extendable bellows  26  are designed such that the “yield point” is never reached and, theoretically, an infinite number of compression/extension and bending cycles can be expected. In bellows systems utilizing Belleville type conical washers made of titanium, 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 parts destructively rubbing against one another. 
     Further, the bellows assembly  26  offers both spring-like action while also lending itself as a container to house a fluid. If this fluid is an incompressible liquid, the bellows will only tilt laterally, not deflect axially. If partially filled with both a gas and a liquid, some axial deflection, as well, will be afforded due to the compressibility of the gaseous portion. 
     The bellows assembly  26  allow forward, backward and lateral motion within the spine while maintaining axial height rigidity due to the incompressibility of the liquid contained therein. The bellows  26  absorbs the imposed stresses via bending of its convolutions. There is no rubbing of one component against another, thus eliminating wear on the mechanical parts. 
     In order to afford a torsional degree of motion, simulating natural spinal movement, between the cojoining vertebrae, the upper and lower vertebra engaging members  12  roll on ball bearings  44 . There is virtually no friction, heat or wear produced in the rolling contact between the vertebra engaging members  12  and the ball bearings  44  as long as the yield stress of neither the ball bearing  44  nor the race  48  or  28  is exceeded. 
     The device 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 the cross-sectional area of the bellows  26 , the lower the imposed stresses will be on the bellows convolutions. The bearing load imposed between the vertebra engaging members  12  and the vertebrae  14  will also be lower. 
     Titanium which is the preferred metal, is not only bio-compatible, it is one of the strongest metals available. It lends itself to handling the pressures and flex-loading stresses encountered in a bellows configuration design. 
     The spinal prosthetic device  10  may be pre-filled with sterile saline solution at the time of manufacture. The device may, alternatively, be filled with 80%-90% liquid, the remaining volume being gas. This would allow the device to afford some spring action in the axial direction similar to a normal vertebral disk. Alternatively, the bellows  26  may be pre-expanded to be near the final height desired and, in this embodiment, the surgeon would use a separate distracting tool to spread the effected vertebrae apart before inserting the device  10  into final position. 
     If the surgeon needs to adjust the height of the device  10 , he or she may connect a hand pump to the fill/drain fitting  36  of the device  10  and either bleed fluid out or pump extra fluid in. 
     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 capillary braid reinforced tubing  68 , shown in FIG. 2C, to the bellows device  10  and tuck the entire tubing extension  68  inside the incision prior to closing the patient up. 
     After a predetermined time, the patient may have an incision formed on an adjacent area of the back. The tubing  68  can be removed for expanding the bellows  10  to the final gap height desired. This could be performed with the aid of real time fluoroscopy. 
     The radial bearing assembly  24  positioned centrally within the thrust bearing member  22  allows for rotation of the bellows with respect to the vertebra engaging members  12 . Thus, spinal prosthetic device  10  provides lateral tilting, minimal axial compression, and permits rotational movement, thus simulating a natural spinal disk. 
     Further, the individual washers forming the flexible bellows  26  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 liquid hydraulic fluid within the bellows assembly  26  prevents the device from being crushed or shifted in the radial direction, thus preventing injury to the sensitive nerves of the spinal cord or aorta. 
     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, functionally equivalent elements may be substituted for those specifically shown and described without departing from the spirit or scope of the invention as defined in the appended claims.