An implantable prosthesis for replacing the proximal head of a radius, the radius distal to a humerus and adjacent an ulna, the radius having an intramedullary canal, the prosthesis comprising: a stem portion for insertion into the intramedullary canal to anchor the prosthesis in the radius; and a head portion having a proximal end, a distal end, and a circumferential peripheral wall extending between the proximal end and the distal end, the head proximal end including a first bearing surface for articulation with the humerus, the circumferential peripheral wall including a second bearing surface for articulation with the ulna, the second bearing surface defining a proximal to distal profile between the proximal and distal ends of the head portion; characterised in that: the head portion is attachable to the stem portion the second bearing surface includes first and second convex portions (536, 538), a first concave portion, and a third convex portion, wherein the entire proximal to distal profile of the second bearing surface is either concavely or convexly curved; the stem portion has a proximal end and a distal end, wherein the stem proximal end includes an attachment portion and the head distal end includes a bore shaped to receive the attachment portion and a distal end of the attachment portion is flush with the head distal end when the head portion is properly attached to the stem portion. a stem portion for insertion into the intramedullary canal to anchor the prosthesis in the radius; and a head portion having a proximal end, a distal end, and a circumferential peripheral wall extending between the proximal end and the distal end, the head proximal end including a first bearing surface for articulation with the humerus, the circumferential peripheral wall including a second bearing surface for articulation with the ulna, the second bearing surface defining a proximal to distal profile between the proximal and distal ends of the head portion; characterised in that: the head portion is attachable to the stem portion the second bearing surface includes first and second convex portions (536, 538), a first concave portion, and a third convex portion, wherein the entire proximal to distal profile of the second bearing surface is either concavely or convexly curved; the stem portion has a proximal end and a distal end, wherein the stem proximal end includes an attachment portion and the head distal end includes a bore shaped to receive the attachment portion and a distal end of the attachment portion is flush with the head distal end when the head portion is properly attached to the stem portion. the head portion is attachable to the stem portion the second bearing surface includes first and second convex portions (536, 538), a first concave portion, and a third convex portion, wherein the entire proximal to distal profile of the second bearing surface is either concavely or convexly curved; the stem portion has a proximal end and a distal end, wherein the stem proximal end includes an attachment portion and the head distal end includes a bore shaped to receive the attachment portion and a distal end of the attachment portion is flush with the head distal end when the head portion is properly attached to the stem portion.