A method of determining the offset of the head part of an orthopaedic joint prosthesis stem component, relative to the neck of the stem part on which the head part can be fitted, involves obtaining a first and second images of the patient's joint on a non-coincident first and second planes. Template parts are fitted to the first and second images of the bone, which have markings on them which can be placed relative to the head part and the stem part respectively of the patient's bone, and scales for measuring first and second displacements which are the distances between the axis of the head part and the axis of the neck of the patient's bone measured in the first and second planes.