Patent ID: 8529632

Claim:
A hemi-arthroplasty implant for the base of the first metacarpal of the thumb CMC joint, said implant including a head section, a flat collar and a stem section, said head section comprising a spherical head and a neck section which connects said head to said flat collar, said neck section comprising an arcuate surface of circular cross section which extends between said spherical head and said flat collar, said neck section having a diameter (N) at its narrowest point of at least about 5% less than a diameter (H) of said spherical head, and said flat collar being of noncircular, shape having a dorsal edge, a volar edge and two lateral edges, with its dorsal width (DW) and its lateral width (LW) proportioned so that the dorsal width (DW) is within the range of 95% of the lateral width (LW) to 130% of the lateral width (LW), with the dorsal width (DW) being 15% to 60% greater than the diameter of said spherical head, and with both said dorsal edge and said volar edge of said flat collar being curved and said dorsal edge having a radius of curvature which is greater than the radius of curvature of the volar edge by at least about 20%.