Kate Tilley, third-year, investigates the effects of using metals in prostheses.
Research published by the Lancet has provided ‘unequivocal evidence’ that metal on metal stemmed prostheses are associated with a high rate of failure, particularly among the younger generation, women, and patients with larger implants. Since the 1960’s, metal on metal hip replacements have been used to provide treat- ment for symptomatic os- teoarthritis of the hip using a cobalt-chrome alloy as a stemmed replacement . Osteoarthritis is becoming increasingly prevalent, with the hip being the second most common large joint to be affected . Osteoarthritis is an important cause of pain and disability in the older generation with roughly 5 percent of people over 60 having the disorder .
It is thought that an aging population and obesity is positively correlated with the prevalence of osteoarthritis. This will be accompanied with an increase in the number of total hip replacement (THR) operations. THR involves removing a dam- aged hip joint and replac- ing it with an artificial one known as a prosthesis. The prosthetic parts are either cemented by securing to healthy bone using glue, or uncemented, where the prosthesis is made from permeable material where the bone can grow into it, holding it in place. In addition to metal on metal THRs, there are several other materials employed instead such as ceramic and plastic .
THR is extremely common and has been described as ‘the operation of the 20th century’. It revolutionised management of elderly patients with significant morbidity and restored quality of life with good long term results. However, there are increasing numbers of active and young patient that undergo THR. They have different expectations of management and quality of life, which need to be accommodated .
The main concern with long term THR is failure, resulting in revision surgery. In England and Wales, it is estimated that 5% of THR’s are revised within 7 years . The majority of THR failure is caused by aseptic loosening due to polyethylene wear , followed by dislocation affecting approximately 4% of THR’s within 6 months .
In order to address these problems, alternative bearing surfaces have been investigated to reduce wear and to produce larger head sizes. Metal on metal bearing surfaces have been reported to be highly resistant to wear and Healthcare Regulatory Agency (MHRA) raised alarm in the UK by advising patients, who have had large metal on metal THRs, to be monitored for life. Evidence showed that patients with failed metal on metal hip arthroplasty had higher concentrations of serum cobalt and chromium. This is caused by the ball and socket of the hip joint rubbing together, which result in the release of metal debris.
Elevated serum metals can enter the bloodstream, and seep into surrounding tissue to cause inflammation, destroy muscle and bone. Therefore, the MHRA has suggested that all patients in the UK with large head hip replacements should have blood tests to check for high metal concentrations. They may also require MRI scans if there are raised levels of metal ions or if patients show adverse symptoms. 
Although our understanding of the local and systemic pathological effects of raised metal ion concentrations are limited, analysis of the National Joint Registry has provided critical information about the high failure rates of metal on metal hip implants, particularly in the female population. Failure was related to head diameter with larger heads failing earlier. MHRA recommends that all patients with metal on metal THR’s have regular follow up to ensure there are no further complications in relation to increased metal ion concentrations. However, further research is required to understand the physiological consequences of exposure to orthopaedic metals.
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