The nucleus may be composed of a low friction elastomer such as polyurethane, polycarbonate-polyurethane, a polymer such as polyethylene (particularly ultra-high molecular weight polyethylene), a suitable ceramic, metals or metal alloys such as titanium or a titanium alloy, chrome-cobalt-molybdenum (CoCrMo), cobalt 28 chromium molybdenum, cobalt chrome, stainless steel, or other suitable materials