The current status of metal-on-metal hip replacement

Total joint replacement is an effective method for treating advanced arthritis of the hip. However, over time traditional metal on polyethylene (MoP) hip replacements develop aseptic loosening due to wear and osteolysis. Metal-on-metal (MoM) total hip replacements (THR) were designed with the aim of achieving lower wear rates, reduced dislocation rates and increased range of movement secondary to larger bearings. Hip resurfacing (HR) offered a bone conserving option on the femoral side for young patients. However, evidence from randomised controlled trials (RCTs), and data from national joint registries, have shown significantly higher failure rates of MoM THR and HR compared to conventional MoP implants. The Medicines and Healthcare products Regulatory Authority (MHRA) issued a series of alerts about specific implants, and a number of devices have been voluntarily withdrawn. Now MoM THR and HR represent a much smaller proportion of hip replacements implanted. All hip replacements disseminate metal particles but little is known about their local and systemic effects. MoM wear generates nanoparticles that cause inflammatory changes in peri-prosthetic tissues, so called Adverse Reaction to Metal Debris (ARMD). With over 70,000 MoM hip replacements implanted in England and Wales alone, with many more worldwide, it is important to decide how to monitor these patients for ARMD, but guidance from national bodies has resulted in varied recommendations. This review will consider Level I and II studies of MoM hip replacements, as well as evidence from established joint replacement registries, then compare the advice from a number of different national bodies for follow up of patients with MoM bearings.