Abstract:
An intervertebral disc prosthesis has two cover plates and a prosthesis core which is connected to one of the cover plates by connection profiles on one of the cover plates which are undercut in a complementary manner and include at least one pair of profile sections which are symmetrical with respect to the AP (anteroposterior) direction of the prosthesis in its implanted position and are arranged at an angle to one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 10/623,803 filed Jul. 23, 2003, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth fully therein. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION 
       [0002]    A known type of intervertebral disc prosthesis (WO 01/01893) has two metal cover plates and a polyethylene prosthesis core which forms a pair of slide surfaces with one of the two cover plates and is fixed on the other cover plate. For this fixing, the cover plate is provided with edge ridges whose undercut connection profile cooperates with a complementary connection profile on the underside of the prosthesis core. On the right and left lateral edges of the cover plate and of the prosthesis core, such connection profiles are provided parallel to one another. In addition, there is one such profile running in a straight line on the dorsal edge of the cover plate and of the prosthesis core. There is no such profile on the ventral edge, with the result that the prosthesis core can be pushed into the connection profiles of the cover plate from the ventral direction. The prosthesis core is secured in the assembled position by a pair of locking elements provided on the ventral edge. Although the connection profiles are produced almost free of play, an important concern in prosthesis production is to ensure that, even in the case of production tolerances, it is possible to rule out any movement of the prosthesis core relative to the cover plate holding it. In particular, rotation movements of the prosthesis core relative to the cover plate holding it must be ruled out. The invention has recognized that this can be improved not only by precision production by also by a special arrangement of the connection profiles. 
       SUMMARY OF THE INVENTION 
       [0003]    According to the invention, provision is made that the connection profiles include at least one pair of connection profile sections which are symmetrical with respect to the AP direction and are arranged at an angle to one another. AP direction signifies the anteroposterior direction. The profile sections arranged at an angle to one another according to the invention create a triangular configuration which counteracts twisting of the prosthesis core relative to the cover plate if play exists between the connection profiles of the cover plate and of the prosthesis core. The connection profiles can basically be made up exclusively of such profile sections arranged at an angle to one another. However, these can also be provided additionally to connection profiles arranged in parallel on the lateral edge of the cover plate and of the prosthesis core. They are intended to run in a substantially straight line and are preferably arranged at least on the dorsal side. The angle between the profile sections arranged at an angle to one another should not be greater than 150°. The limit preferably lies for example at 140°. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in more detail below with reference to the drawing which depicts an advantageous illustrative embodiment and in which: 
           [0005]      FIG. 1  shows a perspective view of the end plate with the prosthesis core secured on it, 
           [0006]      FIGS. 2 to 4  show different views of the same end plate without the prosthesis core, and 
           [0007]      FIGS. 5 and 6  show perspective views of the prosthesis core. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    The end plate  1 , which is normally used as the lower end plate, has an approximately oval contour and comprises a toothed fastening surface  2  for bearing on the vertebral body and a flange  3  which comes to lie on the ventral side of the vertebral body and can be screwed to the latter via screw holes  4 . On that side opposite the fastening surface  2 , the one normally lying to the top, the cover plate  1  forms a support for the prosthesis core  5 . This support consists of a plane surface  6 , and surrounding the latter, an edge  7  which is composed of two parallel sections  7 ′ on the lateral sides of the plate and of a dorsal edge  7 ″. The outer contour of the edge  7  corresponds to the oval contour of the cover plate. On its side facing toward the surface  6 , the edge is provided with a profile ridge  8  and, below the latter, with an undercut  8 ′. The connection profile thereby formed on the inner side of the edge  7  runs in a straight line in the lateral section  7  and  7 ′ and parallel on both sides. In the dorsal section  7 ″, the connection profile is composed of two sections  9 . In each of these sections, the connection profile runs in a straight line. The profiles of both sections are arranged symmetrically with respect to the AP axis  10  of the implant and enclosed an angle α which, in the example shown, is 140°. 
         [0009]    The top of the edge  7  forms a plane surface  6 ′ which runs parallel to the support surface  6  and likewise forms a support surface for the prosthesis core. 
         [0010]    The flange  3  of the cover plate has a slit  11  which runs perpendicular to the connection profile  6  and whose function is explained later. 
         [0011]    On its top  12 , the prosthesis core  5  forms a slide surface which cooperates with the second cover plate (not shown) to form a hinge. The contour of the prosthesis core is identical to that of the cover plate  1 . On its underside, it has a plane surface  13  which is intended to bear on the support surface  6  of the end plate  1  and has substantially the same contour shape. It is limited by a connection profile which is formed by a profile ridge  14  and an undercut  14 ′. This connection profile corresponds in a complementary manner to the connection profile  8 ,  9  of the edge ridge  7  of the cover plate  1 . In particular, the connection profile is composed of two lateral sections  15  and two dorsal sections  16  which correspond respectively to the lateral and dorsal sections of the edge  7  of the cover plate  1  and can cooperate with these in a form-fitting manner. 
         [0012]    Outside the connection profile  14 ,  14 ′, on the underside of the prosthesis core  5  there is a plane surface  17  which corresponds to the support surface  6 ′ on the top of the edge  7  of the cover plate  1  and cooperates with this when the prosthesis core is mounted on the cover plate  1 . 
         [0013]    The prosthesis is assembled by means of the prosthesis core being pushed with its connection profile  14 ,  14 ′ into the corresponding connection profiles  8 ,  8 ′ from the direction of the open ventral side. When the prosthesis core has been pushed in fully, its profile ridge  14  fits, not only in the area of the lateral sections  7 ′ but also in the area of the dorsal section  7 ″, into the undercut  8 ′ of the edge  7  of the cover plate, the connection profiles lying on one another with substantially full-surface contact. 
         [0014]    At the location where the slit  11  is situated in the gap  3 , the prosthesis core has a groove  18  which is flush with the gap  11  when the prosthesis core has reached its end position in which the dorsal connection profiles abut one another. A plate (not shown) is pushed into the gap  11  and the groove  18  and secures the prosthesis core in this position relative to the cover plate  1 . The plate is in turn secured by means of also having a bore at the location of the screw hole shown on the left in  FIG. 3 , through which bore a fastening screw extends. 
         [0015]    By virtue of the fact that the connection profiles of the prosthesis core and of the cover plate are equipped, respectively, with the sections  9  and  16  which each form a triangle, it is possible, in comparison with known prostheses, to obtain increased stability of the position of the prosthesis core relative to the cover plate  1 . In particular, its stability with respect to relative movements in the sense of rotation is thereby increased.