Patent ID: 7469474

Claim:
A method of making a component of an orthopaedic joint prosthesis which has a bearing surface whose shape corresponds approximately to a part of a sphere and is symmetrical about its polar axis, the method making use of a cutting tool which has a circular cutting edge and which can be rotated about an axis which is perpendicular to the plane containing said cutting edge, and comprising the steps of: a. rotating the component about its polar axis and rotating the cutting tool about its axis, with the cutting edge of the cutting tool in contact with the surface of the component, to cut the surface in a region between the equator and the polar region of the component, and b. moving the cutting tool (i) in a direction parallel to the polar axis of the component while leaving the angle between the axis of the cutting tool and the polar axis of the component unchanged, and (ii) along its axis, and c. repeating step (a), the movements (i) and (ii) of the cutting tool causing the radius of curvature of the bearing surface to change continuously and monotonically as the angle between the radius and the polar axis of the component changes in the region between the equator and the polar region of the component, so that the shape of the bearing surface deviates from that of a true sphere in such a way that discontinuities in the shape of the bearing surface as a result of individual movements are minimised.