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Nine uncemented hip prostheses of modular design were revised because of late infection (2 cases), femoral stem loosening or fracture (2), loosening of threaded cups (3) and protrusion of bipolar cups (2). At surgery we found tissue discoloration and macroscopic corrosion in four of the nine prostheses, located at the head-neckjunction. Histological examination in most cases showed extensive necrosis associated with metal particles, indicating metal toxicity. Metallographic examination ofthe prosthetic heads revealed structural imperfections and inhomogeneity of the metal. We suggest that the crevice between the head and neck is a potential site of corrosion in modular Table I. Details of nine patients, reoperated for infection or aseptic loosening of uncemented Lord prostheses

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A series of 237 total hip replacements were analysed by means of a computer program. High body weight and postoperative fever, as well as several indications of a technically demanding operation, occurred significantly more often in the group of patients which developed paraarticular ossification postoperatively. The results point to a causal relation between tissue trauma and heterotopic bone formation. We wish to underline the importance of a gentle handling of tissues in the performance of a total hip replacement if the rate of ectopic bone is to be reduced to a minimum.

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