A composite bearing insert for a total knee joint or a unicondylar knee joint which minimizes or eliminates the production of wear debris resulting from relative motion at the interface between the endoskeleton and a tibial tray of a knee joint prosthesis. The composite bearing insert includes an endoskeleton and a polymer portion which is preferably molded into and locked within the endoskeleton. The endoskeleton is configured to be locked to a tibial component, such as a tibial tray or keel. The modularity of the assembly facilitates the interchangeability of various composite bearing inserts with various tibial components.