Abstract: A joint prosthesis for replacing a joint between two elongated bones of the hand. A first member is provided with an elongated stem for reception in one of the elongated bones and includes a head remote from the stem, the head having a central arcuate dovetail rib arising from a spheroidal convex articulating surface. A second member has a stem for reception in the other bone and a base remote from the stem, the base having a central arcuate dovetail slot formed in a spheroidal concave surface shaped to articulate with the convex articular surface. The central arcuate dovetail rib of the first member and arcuate dovetail slot of the second member are assembled to fully engage and articulate with each other to restrain separation of the members at all degrees of flexion ranging from 0 degrees extension through to 110 degrees of flexion, the force between the members being borne primarily by said spheroidal articular surfaces throughout the range of motion.