Control of crystalline orientation to enhance the wear resistance of ultra-high molecular weight polyethylene crystallization cups for artificial joints

Ultra-high molecular weight polyethylene (UHMWPE) has been used as a bearing surface for artificial joints and its wear is one of the limiting factors for long term use of such prosthetic implants. Previous studies have revealed that the oriented and crystallized UHMWPE, prepared by plane compression, enhanced the resistance of wear. In this paper, we studied the influence of compression by half-spherical shape on the preferred orientation and wear. UHMWPE with slight cross-linking was compressed at molten state by using a sphere. A half-spherical cavity was molded, and the molded shape was similar to a cup. Wide angle X-ray diffraction (WAXD) tests revealed that the (1 1 0) and the (2 0 0) crystalline planes were orientated along the half-spherical cavity surface (3D orientation). The melting point, the heat of fusion, and the density near the curved surface showed relatively higher than a conventional UHMWPE. These structure and physical properties can fit to the results of plane orientation. Wear tests were performed using hip simulator, and the weight loss was compared to that of a conventional cup. In conclusion, the compression with the 3D orientation enhances wear resistance, similarly to the plane orientation results, and is useful for molding an improved cup.