An endoprosthetic joint implant includes a fixation element and a hydrogel hinge portion. The fixation element supports the hydrogel hinge portion on a resected bone-end, for example, for use in surgical joint replacement. The fixation element of one embodiment features a stem made of titanium. The stem is threaded for insertion distally into an intramedullary canal. The hydrogel hinge portion connects to the stem using a snap-fit arrangement. The hydrogel is available in various surface configurations and can articulate against a natural joint surface or a synthetic joint surface. A total joint-replacement implant is provided by attaching a second stem to an opposite side of the hinge for insertion into bone on the opposed side of the articulation.