Patent ID: 8491663

Claim:
A joint part prosthesis comprising: (a) a proximal or distal component; (b) a shaft to be mounted in a bone, said shaft having a longitudinal axis extending away from the proximal or distal component and comprising a free end and a mantle surface, the shaft being configured to be round in cross-section to have an outer diameter in the cross-section and being configured to be spherical at the free end, the outer diameter of the cross-section being smaller in a mid-region of the shaft than at both the free end of the shaft and at an end of the shaft adjacent the proximal or distal component, the outer diameter continually increasing from said mid-region to said free end of the shaft and from said mid-region to said end of the shaft adjacent the proximal or distal component; and (c) a fin-like anti-twist device comprising at least two fin-like anti-twist elements disposed on the mantle surface with rotational symmetry and extending radially from said mantle surface in a longitudinal direction of the shaft to a radial distance from the mantle surface, the radial distance decreasing slightly in a direction toward the free end of the shaft, the at least two fin-like anti-twist elements starting at the free end of the shaft; wherein the at least two fin-like anti-twist elements have a first thickness at a first location toward a free lateral side of the fin-like anti-twist elements and a second thickness at a second location toward the shaft, the first location being disposed along a first line perpendicular to the longitudinal axis of the shaft, the second location being disposed along the same first line perpendicular to the longitudinal axis of the shaft, and the second thickness being greater than the first thickness; and wherein during insertion of the shaft into a medullary space of a bone, rotational movements of the shaft clear the medullary space and the at least two fin-like anti-twist elements fix the joint part prosthesis in place in the medullary space so as to prevent rotation.