A technique for locating an axis of a bone, which extends through the center of rotation of the bone around a joint between that bone and another bone, involves a. moving the bone in a plane about the said joint, between a first point and a second point, b. locating a third point which is equidistant from the first and second points, and c. locating the axis which passes through the third point, perpendicular to a line which connects the first and second points. The invention provides an instrument for performing this technique.
