Patent ID: 7815685

Claim:
An orthopaedic joint prosthesis, comprising: a generally rounded head portion with a rounded head bearing surface, the head bearing surface being symmetrical about a head axis and having a head pole; and a cup configured to receive the head portion and to articulate with the head portion when the cup is assembled with the head portion, the cup having a rounded cup bearing surface, the cup bearing surface being symmetrical about a cup axis and having a cup pole; wherein the cup bearing surface has a first cup bearing surface and a second cup bearing surface, the first cup bearing surface having a radius of curvature that continuously and monotonically decreases as the angle between the cup radius and the cup axis decreases in a direction towards the cup pole to a circumferential line, the first cup bearing surface being substantially linear or having substantially the shape of at least a part of a parabola, a hyperbola or an involute, when viewed in cross-section on the cup axis, the second cup bearing surface being that surface located between the circumferential line and the cup pole, the second cup bearing surface having a constant cup radius and a cup center defined by the constant cup radius; the cup and the head portion being configured such that when the cup is assembled with the head portion and the cup axis of symmetry and the head axis of symmetry are aligned, the distance between the second cup bearing surface and the head bearing surface is substantially uniform.