A prosthesis for partial replacement of a long bone in humans or animals, the prosthesis including: a proximal or distal end component forming one component of a prosthetic joint; a shaft on or assembled with the distal or proximal end component in use; a stem for engagement in a resected bone; and an initially separate collar assembled to the shaft or an extension thereof in use, the collar being dimensioned to abut the resected end face of the bone into which the prosthesis is to be fitted, wherein the end face of the collar facing the resected face of the bone is sized and shaped to correspond to the resected end face of the bone.