Abstract:
A total disc replacement system for use in spine surgery involving a pair of endplates, a pair of bearing surfaces, and an intradiscal element composed of an elastomeric polymer.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application is a nonprovisional patent application claiming benefit under 35 U.S.C. § 119(e) from U.S. Provisional Application Ser. No. 60/515,247, filed on Oct. 28, 2003, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth fully herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     I. Field of the Invention  
         [0003]     The present invention relates generally to orthopedic surgery and, more particularly, to a total disc replacement system for use in spine surgery involving a pair of endplates, a pair of bearing surfaces, and an intradiscal element composed of an elastomeric polymer.  
         [0004]     II. Discussion of the Prior Art  
         [0005]     The human spine is a very delicate apparatus. The vertebral bone is separated and cushioned by intervertebral discs. When these discs rupture or otherwise deteriorate, back pain develops. Back pain can be minor or very serious, depending on the extent of injury to, or deterioration of, the disc.  
         [0006]     Numerous U.S. patents disclose disc replacement inventions of different types intended to solve the problem. When simplified, most of these inventions are comprised of two endplates (or anchor plates) and an intradiscal element. In some variations, the two endplates are connected by a hinging mechanism to form one unit. The endplates function to attach the apparatus to the vertebral bone, while the intradiscal element functions to stabilize and cushion the adjacent vertebrae. With regard to injuries involving rupture of spinal ligaments, the intradiscal element can also serve the function of connecting adjacent vertebrae to each other.  
         [0007]     Variations in prior inventions have occurred in two main ways: (1) means of attachment of the apparatus to the vertebrae and (2) the composition of the intradiscal element. To date, disc replacement apparatuses are attached to the vertebrae primarily through employment of a number of spikes on the surface of the mechanism. Another attachment method is use of a screw drilled directly into the bone. Screws will maintain their hold, but they tend to be larger in size and at least two are required to hold each plate in place.  
         [0008]     To date, the types of intradiscal elements have varied widely between inventions. Some previous disc prostheses contained rigid assemblies primarily for the purpose of fusing the vertebrae together. Others use a hinging, or ball and socket mechanism to allow for limited motion. Some prior prostheses contained springs as an intradiscal element to act as a shock absorber. Still others seek to employ a polyolefin rubber material or a polyethylene plastic material as an intradiscal element. The idea behind this type is to allow for a greater array of motion while also maintaining some load bearing and shock absorbing functionality.  
         [0009]     However, all of these previous attempts at invertebral disc prostheses are flawed in one way or another. Using spikes on the surface of the endplate to attach the prosthesis to the bone only prevents lateral movement of the device within the spinal column. Screws are better at securing the prosthesis to the bone, but are considerably bulkier and, as a result of boring deep holes and threads into the bone, may make it difficult to replace the prosthesis in the future, if such action is necessary.  
         [0010]     Similarly, the various intradiscal elements previously employed can be easily improved upon. Rigid elements that serve no purpose other than to fuse adjacent vertebrae together are no longer desirable in most cases because they do not allow for the retention of motion. Semi-rigid elements such as a hinge or ball and socket mechanism allow for greater motion but are limited in the motion that they allow. The primary flaw is the absence of vertical elasticity: The mechanisms neither stretch nor provide any shock absorption if compressed. Rubber and plastic intradiscal elements seem to address this problem through their elasticity, but the problem with these devices is that the elements are firmly attached to the endplates. While they succeed at providing vertical elasticity as well as lateral bending motions, these elements must also resist torsional forces when the patient twists his/her body. In theory, a great torsional force could cause the intradiscal element to become detached from the endplate, and thus become detached from the vertebra.  
         [0011]     The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention accomplishes this goal by providing a total disc replacement (TDR) mechanism that allows for vertical and lateral motion, provides a shock absorption function, and does not resist torsional forces.  
         [0013]     According to one broad aspect of the present invention, the system comprises a total disc replacement mechanism for use in the human spine. The invention consists of a pair of endplates, a pair of bearing surfaces, and an intradiscal element composed of an elastomeric polymer.  
         [0014]     The endplates may include any number of components capable of providing structural support and anchoring the mechanism to adjacent vertebrae. By way of example only, the endplates may be composed of a metal or ceramic material. The endplates may also be form-fitted to the size of the adjacent vertebrae. The anchor may include any number of components capable of anchoring the prosthesis to the vertebrae. By way of example only, the anchor may include a diametric structure protruding from the endplates at a generally perpendicular angle, optionally containing teeth to secure the prosthesis in place.  
         [0015]     The bearing surfaces may include any number of components capable of providing a motile connection between the endplates and the intradiscal element. By way of example only, the bearing surfaces may include discs of approximately the same size as the intradiscal element, but smaller in diameter than the endplates. The bearing surfaces may be composed of a metal or ceramic material.  
         [0016]     The intradiscal element may include any number of components capable of providing sufficient support yet also capable of flexing, bending, twisting, or elongating as the need may arise. By way of example only, the intradiscal element may include a naturally occurring or synthetic polymer that is nonreactive with the body, such as a hydrogel. Hydrogels may be cross-linked, for example by radiation or chemical treatment, to provide greater strength and thus improved functionality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein:  
         [0018]      FIG. 1  is a perspective view of the total disc replacement system of the present invention, showing the showing two endplates with toothed anchors, two bearing surfaces, and the intradiscal element;  
         [0019]      FIG. 2  is an exploded elevated perspective sectional view of the present invention, illustrating the knob-recess connection method of the endplates and bearing surfaces;  
         [0020]      FIG. 3  is an elevated side view of the prosthesis of the present invention, showing the single perpendicular toothed anchor on each endplate.  
         [0021]      FIG. 4  is the view of  FIG. 3  rotated 90°, showing the diametrical orientation of the toothed anchor structure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The total disc replacement system and related methods disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination.  
         [0023]      FIG. 1  illustrates a total disc replacement system  5  of the present invention. The total disc replacement system  5  includes two endplates  10 , each endplate  10  containing one anchor structure  20 , protruding in a generally perpendicular direction therefrom. The anchor  20  is comprised of a structure situated along the diameter of the endplate  10 , extending in a generally perpendicular fashion and containing several rows of teeth  25 . On the interior side of each endplate  10  is a bearing surface  30 , which functions as an intermediate between the endplate  10  and the intradiscal element  40 , allowing for greater mobility and a reduction in resistance to torsional forces.  
         [0024]      FIG. 2  is an exploded perspective view of the present invention, illustrating the connection between the endplates  10  and bearing surfaces  30 . The center of the interior face of each endplate  10  contains a knob  15  which fits into the center hole  35  of the bearing surface  30 . This arrangement, once the device is fully assembled, will prevent the intradiscal element  40  from sliding laterally out of position.  
         [0025]      FIG. 3  demonstrates the location and orientation of the anchor mechanism  20  on the exterior side of each endplate  10 , as well as the presence of teeth  25  on the anchor  20 . The anchor  20  is situated along the diameter of the endplate  10  and extends in a perpendicular manner from the surface of the endplate  10 .  FIG. 3  shows four rows of teeth  25  on each anchor  20 , but this configuration may be varied depending on the length of the anchor  20 . The teeth  25  function to prevent displacement of the device once placed in the vertebrae.  
         [0026]      FIG. 4  illustrates the elongated structure of the anchor  20 , which extends perpendicularly from the surface of the endplate  10 . The length of the base of the anchor  20  is equal to the diameter of the endplate  10 . The anchor  20  is tapered as it extends from the endplate, such that the length of the side opposite the endplate  10  is somewhat less than the diameter of the endplate  10 . The elongated structure and orientation of the anchor  20  functions to prevent lateral movement of the prosthesis once inserted into the vertebra. Such an orientation of the anchor  20  also prevents any twisting or rotating of the endplates that may have occurred in the absence of such a design.