Dissociation of the metal inlay from the polyethylene liner in an uncemented threaded cup.

We describe a case of spontaneous dissociation of the metal inlay from the polyethylene cup of a sandwiched metal-on-metal total hip prosthesis manufactured from Co-28Cr-6Mo SM 21 alloy. The patient, a 51-year-old active woman, started to feel groin pain 50 months postoperatively. The pain progressed after a slight trauma and led to final revision after 66 months in situ. No signs of impingement were observed. The polyethylene cup showed an approximately 2 mm-deep groove in the superior wall, and the head was heavily worn on the lateral side. In addition to the typically observed abrasive wear patterns, several types of severe wear defects were noticed. Extensive metallosis and necrosis were observed histologically. This unusual case of substantial deformation of the head and the cup was presumed to have occurred as a result of the increased friction and consequent high wear of the metal head.