Investigating the Failure of Metal on Metal Hips

Metal-on-metal hip replacement is a treatment for “wear and tear” arthritis (osteoarthritis) – one of the commonest diseases in the world. These hip replacements use cobalt chrome alloy for both bearing surfaces and are the only type that can be made thin enough to enable resurfacing of both sides of the worn hip joint, called a hip resurfacing. The most common type was developed 13 years ago in the UK and has been adopted world wide. One million people now have a metal-on-metal hip replacement. It was thought to be a solution to providing a longer lasting hip for active and young patients with hip osteoarthritis but unfortunately unexplained relatively high failure rates have been observed. A group of scientists and surgeons from Imperial College London, the Royal National Orthopaedic Hospital and the Medical Research Council have been working with Diamond Light Source to examine failed hips to try to establish the mechanism of failure.

This work has been published in the journal Acta Biomaterialia.

In Detail

A sample of Metal on Metal hip replacements.

Metal-on-metal (MOM) hip replacements make up about 35% of all hip replacements in the US. All mechanical devices fail occasionally, however MOM hips can fail in up to 13% of patients within 6 years as a result of unexplained inflammation in the tissues surrounding the hip joint. The likely cause is poor biocompatibility with metal debris from wear of the implant, but the exact mechanism of failure is not understood. The concentration of the metal is low, making it hard to analyse, while the preparation of tissue and x-ray exposure could affect the sample. Synchrotron microfocus X-ray spectroscopy can address these issues. In addition, samples do not have to be stained or placed in a vacuum, so the sample can be studied in an environment close to in vivo.

The group compared tissue from failed MOM hips with tissue from a different type of hip (Metal-on-Polyethylene), chemical standards and metal discs cut from MOM hips. They found that the tissue had a range of metallic species, but in patients with significant amounts of metal in the tissue the most abundant implant-derived metal was Cr(III) phosphate, and this was true for hip samples from four different manufacturers. No Cr(VI) was detected, which is of clinical relevance as Cr(VI) is known to cause cancer. Additionally they were able to identify other metallic species that may represent intermediate steps in the corrosion process of metal alloys in humans.

Identifying the abundance of Cr(III) phosphate and the other products of corrosion is useful as it provides a focus for future research on human synovial tissue response to biomaterials. Surgeon Alister Hart is from Imperial College and Imperial College Healthcare NHS Trust.

The chemical form of metallic debris in tissues surrounding metal-on-metal hips with unexplained failure, Alister J. Hart, Paul D. Quinn, Barry Sampson, Ann Sandison, Kirk D. Atkinson, John A. Skinner, Jonathan J. Powell J. Fred W. Mosselmans, Acta Biomaterialia
DOI: 10.1016/j.actbio.2010.06.006