hip replacement

By the end of the twentieth century hip replacement had become the commonest major orthopaedic operation in the over-60 age group — and the most successful, by virtue of relief of pain, and restoration of mobility and independence for periods of up to 20 years.

Total prosthetic replacement of the hip joint involves substitution both of the socket in the pelvic bone and of the head of the femur, which rotates within it. This was first carried out in 1938, by Philip Wiles at the Middlesex Hospital, using all-metal replacement parts, secured by screws, as an advance on earlier procedures, which replaced only the head of the femur. This ‘metal-on-metal’ prosthesis was not successful because of the loosening of the metal parts, caused by high friction in the joint which the fixation screws were unable to withstand when the patient took full weight on the leg.

Following World War II, McKee of Norwich and Charnley of Wrightington developed further types of hip joint replacement. The major advance was the first use of methyl-methacrylate bone cement to fix the components into the bone of the pelvis and of the femur, based on pioneering work by Wiltse in the US. This prevented loosening of the components and gave immediate pain relief to the patient. The McKee-Farrar type was a chrome-cobalt metal replacement for both surfaces of the joint, whereas the Charnley type employed a high-density polyethylene cup for the socket and stainless steel for the femoral head. Charnley had demonstrated by wear-testing methods that friction was lower with a polyethylene cup articulating on metal. He predicted that it would not loosen and would last for 10-20 years.

If the technique for inserting the bone cement was not performed precisely, the cement was prone to fracture or to loosen in the bone, causing a recurrence of pain, and it was extremely difficult to remove. Hence, in the 1970s and 1980s, many experiments were directed towards hip replacements which did not require cement; instead they had a roughened surface which would encourage fixation by incorporation in the surrounding bone. A variety of porous coatings, employing metal beads or micromeshes, and more recently hydroxyapatite, were developed. Although these are being pursued in some centres still, the long-term results have not become comparable with those achieved by the original cemented hips.

As time passed it became apparent that the all-metal joint had some problems, in terms of friction and loosening, which were not present with the high-density polyethylene articulating on stainless steel. However, time has also shown that all prostheses are prone to wear. Moreover, the particles produced by wear from the polyethylene type of prosthesis eventually stimulate the formation of bone-destroying cells, which cause it to be loosened from the bone over a period of 10-20 years. Although it is possible to replace these hips a second time, the result of the operation is never so successful as the first.

Recently, new interest has been generated in improving the quality of the high-density polyethylene to reduce its wear by sterilizing it in an oxygen-free atmosphere to reduce cross-linking. At the same time, more precise engineering of the bearing surfaces of metal-on-metal hip joints has occurred, so as to maintain low friction with fluid-filled bearings; this may create a prosthesis which wears very little and will thus last longer than 20 years.

The results for relieving pain and improving quality of life in severe osteoarthritis of the hip have been dramatic, and the lives of many thousands of patients throughout the world have been transformed. Most patients walk with crutches on the day after operation and walk unaided 6-8 weeks later. In the late 1990s approximately 400 000 hip replacements were performed annually world wide: about 200 000 in the US, 50 000 in the UK, and 150 000 elsewhere). Between 5% and 10% require replacement of the opposite hip within 5 years — and that gives equally good results.

Prospective clinical trials on hip replacements have been few, although the establishment of the Swedish Hip Registry of all prostheses implanted has yielded extremely valuable information, from long-term follow-up studies, regarding the performance of the different types and the factors that contribute to success and failure. For example, out of 415 patients treated by cemented Stanmore and Charnley hip replacement at the Royal National Orthopaedic Hospital between 1982 and 1987, 96% of the replaced hips survived with good function for 10 years. Studies from other centres have indicated an advantage of cemented prostheses over uncemented designs.

The quest continues, however, for the everlasting hip. This may be achieved by the use of better articulating surfaces which do not wear, such as ceramic femoral heads articulating on high-density polyethylene or ceramic articulating on ceramic. The alternative may be the improvement of metal-on-metal articulations. However, it appears that all prostheses will wear in due course, and though improvement will be achieved, the everlasting joint seems unlikely.

— George Bentley