surface surrounds two ribs 4 which are arranged symmetrically with respect to the median plane 1 and enclose an angle alpha of approximately 20 u00b0 their more ventrally oriented side face 5 is steep namely approximately 90 u00b0 to the base surface 3 and the more dorsally oriented side face 6 running parallel thereto is inclined obliquely at an angle of approximately 45 u00b0 to the base surface 3 in a sectional plane fig2 extending perpendicular to the rib in a cross section extending parallel to the implantation direction fig3 the angle appears much flatter however it is arranged in such a way that during the implantation movement it slides gently onto the bone surface or cartilage surface of the vertebral body cover plate assigned to it this even applies when the vertebral bodies involved have not first been distracted to a spacing greater than the thickness of the prosthesis it is for this reason that it is designated as a slide on surface since the two side faces 5 and 6 enclose an angle of approximately 45 u00b0 or preferably even less with one another they form a sharp cutting edge 7 at the head of the rib when the force of the ligaments connecting the vertebral bodies and the load to be taken up by the spinal column move the vertebral bodies concerned toward one another after the implantation the rib penetrates completely or partially into the bone tissue and thereby anchors the prosthesis in the desired position it is not necessary.