Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints: II: A microscopical study of the surfaces of Charnley polyethylene acetabular sockets

Abstract In this paper attention is focused on the findings of a recent microscopical study of explanted polyethylene hip joints from Wrightington Hospital. The project was initiated some years ago in collaboration with the late Professor Sir John Charnley, but the present study is based on an extended study of acetabular cups which have been sectioned for direct examination in the electron microscope for the first time. The account of the appearance of worn polyethylene acetabular cups reported in this paper provides a useful basis for the quantitative assessment of wear in the current form of Charnley low friction arthroplasties reported in part III of the present trio of papers. It has been confirmed that worn polyethylene acetabular sockets exhibit three distinct regions: a relatively smooth high wear region in the superior half of the cup, a rougher low wear area in which the original machining marks are still visible and a low ridge separating the two. Rim wear associated with impingement of the femoral stem is also evident in some of the heavily worn sockets. Some five cups representing service lives ranging from 1 to 14 years were selected for detailed visual examination. Descriptions of the high and low wear regions are offered and it will be evident from the geometry of the intermediate ridge region that creep cannot have contributed significantly to the overall penetration of the femoral head into the acetabular cup. This point is discussed further in part III. Perhaps the most significant finding is that cement was found in all the cups examined. The adhesive wear process evident in the high wear region was supplemented by scratching or abrasive wear associated with cement ingress. It was concluded that cement in the explanted polyethylene cups of total replacement hip joints contributed to the appearance of the worn regions and, no doubt, to the wear process itself. It is expected that new designs of acetabular cups and improved surgical techniques will be of considerable benefit in this context.