Abstract: A surgically implantable prosthesis for replacing a bone joint, specifically the knuckle, of an individual is disclosed herein. The prosthesis includes a one-piece main body of flexible, inert material including first and second spaced apart end sections and a narrower web extending between and joining the sections together for flexing movement about a particular flexing axis through the web, whereby the main body functions as a hinge. First and second elongated stems connected to and extend out from these end sections in directions normal to the flexing axis and away from one another. In accordance with one feature of the prosthesis disclosed herein, a specific one of the stems is laterally offset in a specific way with respect to the flexing axis. In accordance with another feature, the end sections and web of the main body together define the key-hole shaped groove across one side of the web between the end sections.