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 °. 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 . 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 . 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 . 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 . 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 . 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 fig3 , through which bore a fastening screw extends . 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 .
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 direction of the prosthesis in its implanted position and are arranged at an angle to one another .