Failure of Polyethylene Acetabular Cups Two Case Reports

Abstract

Fatigue fracture involving the metallic femoral stem is well recognized after total hip arthroplasty. Two cases of Charnley-Mueller polyethylene acetabular cup failure, in a 54-year-old-woman and a 77-year-old woman with abnormal wear patterns, were diagnosed prior to operation. The fracture of the acetabular cup was recognized by the fragmented, crushed appearance of the cup. The arthrogram clearly showed the radiographic contrast agent passing through the substance of the cup. The wear of the cups was measured by micrometer calipers which have an accuracy of 0.001 mm. The polyethylene acetabular cup was analyzed in four zones formed by three circumferential grooves. Markedly increased wear of the components occurred in the superolateral dome area. This wear rate was as high as 0.8 mm/year, which is four times the average wear of 0.2 mm/year. This marked wear associated with repetitive cyclic fatigue or static load resulted in fracture of the polyethylene acetabular cup. Improper machining of the cup using low- rather than high-density polyethylene in the manufacturing process and the heavy weight of these patients may have been factors in the marked fissures and crack lines revealed by microscopic study of the fractured area after sputter coating with gold palladium. Fracture of the polyethylene acetabular component, although rare, may be encountered more often in longer follow-up studies of patients who have undergone total hip arthroplasty. Proper selection of patients, acetabular spacers, and pressure injection techniques may prevent early loosening of the acetabular polyethylene components.