In-depth morphological changes and embrittlement near the wear surface of UHMWPE inserts from uncemented hip systems

Polyethylene hip cups were examined with optical and electron microscopy after permanganic etching, a technique that allows in-depth examination from the articulating surface downward. In addition to wear features present on the surface, novel defects were revealed in implants after retrieval from the body but not in as-manufactured controls. They were incipient cracks that indicated the existence of an embrittled layer extending 10 microm or more into the implant from the wear surface after exposure to the body environment. The lengths of the cracks, which were perpendicular to the tensile stresses responsible for their formation, were mostly more or less parallel to the wear surface. The embrittlement and cracking revealed are probably major contributors to the wear of polyethylene implants in the body. Poor particle consolidation may be a contributory factor, but it was not observed to be the primary cause of implant wear within the body.