ANATOMICAL STUDY: The common pathway to failure in total hip replacements is loosening of the acetabular and femoral components. The reliable diagnosis of this loosening is difficult. A non-invasive technique has been developed which analyses a vibration signal transmitted through the femur. This can distinguish between a femoral component which is loose and one which is firmly fixed. Charnley femoral components were cemented securely into ten cadaver femurs and the transmitted vibration signal recorded. The prostheses were then loosened first at the cement-prosthesis and then at the cement-bone interface. The tests were then repeated. Consistent and distinct differences between output vibration signals were observed between the firmly implanted and the loss prostheses. PILOT CLINICAL STUDY: Seven patients admitted for revision surgery because of clinical and radiological evidence of femoral implant loosening were tested using this technique. A further four patients with secure femoral components were tested approximately two weeks after total hip replacement. The differences in output signal, which had been observed between firmly implanted and loose prostheses during the anatomical study, were again demonstrated.

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Interfacial shear strength between poly(methyl methacrylate) (PMMA) bone cement and cancellous bone was measured in bone samples from human proximal femora. Samples were prepared with fresh cement-bone, fresh cement inside a mantle of existing cement and with fresh cement-revised bone surfaces. Push-out tests to measure shear strength caused failure only at bone-cement interfaces; revised bone interfaces were 30 per cent weaker (P < 0.02) than primary interfaces. The clinical relevance is that revision of cemented joint arthroplasties may necessitate removal of components with sound cement-bone fixation. The practice of removing all traces of PMMA cement may not yield the optimal fixation; adhesion of fresh cement to freshly prepared surfaces of the existing cement might also be considered where circumstances are favourable.

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