Ongoing problems with metal-on-metal hip implants

Major advances in total hip replacement have occurred since the prosthesis was first developed by Charnley in the 1960s. Yet substantial concerns currently exist over newer hip implants as a result of rising revision rates and increased risk of cancer.

By the 1980s revision rates of total hip replacements were as low as 20% after 15 years of follow-up.1 Throughout this era, the main focus was reducing aseptic loosening and osteolysis, which were thought to reduce lifespan. Various combinations of materials have been used in attempts to improve the lifespan of prostheses, which comprise three main types: metal on plastic, metal on metal, and ceramic on ceramic. The first word in the name refers to the head and the second name the cup (metal on plastic is a metal femoral head against a plastic acetabular cup).

Hip resurfacing systems use metal to replace the surface of both joints, preserving the femoral head. Conventional total hip replacement requires complete removal of the femoral head with the prosthesis secured into the upper part of the femoral shaft. Hip resurfacing became popular for younger patients, in whom a quicker return to function and a more active lifestyle are priorities,2 but it is a challenging operation requiring specialised training and practice.3

In 2000, the National Institute for Health and Clinical Excellence (NICE) guidance on selection of prostheses for primary hip replacement and resurfacing set a benchmark revision rate for conventional hip replacement of 10% or less at 10 years.4 At the time NICE stated, “Surgeons should ensure that patients considering MoM [metal-on-metal] hips resurfacing arthroplasty understand that less is known about the medium-to-long term safety and reliability of these devices or the likely outcome of revision surgery than for conventional hip replacements.”5

However, over a decade later, evidence on safety is still lacking. A 2011 systematic review of 29 studies of hip resurfacing found no studies fulfilled the NICE 10 year benchmark of 10% .6 Indeed, a 2007 technology assessment of hip resurfacing concluded “the peer-reviewed literature had not kept pace with changes in hip resurfacing technology.”7

Problems with hip devices emerged in July 2008 when the Zimmer Durom acetabular component was voluntarily recalled because of much higher failure rate than expected. And in 2010 DePuy had to voluntarily recall its ASR hip prostheses (one for resurfacing and one for total hip replacement) because of failure rates of about one in eight.8 A year later the UK regulator, the Medicines and Healthcare Products Regulatory Agency (MHRA), had to send letters to surgeons to notify them “that some of the 40 000 patients who received the metal-on-metal DePuy ASR hip implants never received the recall notice.”9 At the British Orthopaedic Association’s 2011conference, further concern was expressed that large diameter metal-on-metal devices from other manufacturers were also showing higher than expected failure rates, especially in women.10 To add to all this, complaints of “metal poisoning” are growing among patients with DePuy Pinnacle metal-on-metal hip implants.