A radial head assembly comprising: a stem comprising a convex articular head on one end thereof; a collar comprising a collar wall defining a first collar opening, the collar wall having an interior collar surface comprising an angular portion proximate to the first collar opening; a locking ring comprising a ring wall defining a ring opening , the ring wall having an angular outer surface , the angular outer surface engaging the angular portion of the interior collar surface; an articular member (116; 116') comprising: a base having an outer rim and a first concave surface ; and a projection extending from the base, the projection comprising a second concave surface disposed between the first concave surface and an end of the articular member opposite the first concave surface; wherein the articular member and the locking ring define an articular space within the collar, the articular space configured to receive the convex articular head, characterised in that the collar wall defines a second collar opening and a passage through the collar, wherein the ring wall has a slot therethrough, wherein the slot is configured to permit the ring wall to radially expand, and the projection has a peripheral surface configured to engage the interior collar surface. a stem comprising a convex articular head on one end thereof; a collar comprising a collar wall defining a first collar opening, the collar wall having an interior collar surface comprising an angular portion proximate to the first collar opening; a locking ring comprising a ring wall defining a ring opening , the ring wall having an angular outer surface , the angular outer surface engaging the angular portion of the interior collar surface; an articular member (116; 116') comprising: a base having an outer rim and a first concave surface ; and a projection extending from the base, the projection comprising a second concave surface disposed between the first concave surface and an end of the articular member opposite the first concave surface; wherein the articular member and the locking ring define an articular space within the collar, the articular space configured to receive the convex articular head, characterised in that the collar wall defines a second collar opening and a passage through the collar, wherein the ring wall has a slot therethrough, wherein the slot is configured to permit the ring wall to radially expand, and the projection has a peripheral surface configured to engage the interior collar surface. a base having an outer rim and a first concave surface ; and a projection extending from the base, the projection comprising a second concave surface disposed between the first concave surface and an end of the articular member opposite the first concave surface; wherein the articular member and the locking ring define an articular space within the collar, the articular space configured to receive the convex articular head, characterised in that the collar wall defines a second collar opening and a passage through the collar, wherein the ring wall has a slot therethrough, wherein the slot is configured to permit the ring wall to radially expand, and the projection has a peripheral surface configured to engage the interior collar surface. <EOS>