No cancer risk with metal on metal hip replacements: Study

By Dr. Ananya Mandal, MDApr 4 2012

A new study by researchers - from the universities of Bristol and Exeter found that there is no evidence that metal-on-metal hip replacements increase the risk of cancer.

Cancer risk was just one of the fears linked to the implants. Regulators have already called for them to be monitored closely following reports of high failure rates. Tiny metal ions made up of cobalt and chromium are thought to break off from the implants and leak into the blood, with fears that this leads to muscle and bone damage, and neurological issues.

The new analysis found no link in the seven years following surgery, the British Medical Journal website reported. The data for this study, based on the National Joint Registry of England and Wales, covered 40,576 patients with metal-on-metal hip implants and 248,995 who had other types. The study found no evidence of an increased risk of any type of cancer in the patients. The investigation was commissioned by the National Joint Registry of England and Wales, which contains over one million procedures from more than 97% of orthopedic units.

The authors decided to compare cancer rates in patients with metal-on-metal hip replacements with patients with other types of hip implants, as well as with the general population. 40,576 registered patients in total, i.e. 14% had some kind of metal-on-metal bearing surface, whilst 21,264 patients or 7% had a stemmed metal-on-metal prosthesis and 19,312 patients or 7% had a resurfacing procedure.

The authors compared patient outcomes by using mathematical models that included the patient's age and sex, as well as three measures of the patient's general health at the time of hip surgery, i.e. the American Society of Anesthesiologists grade, which scores the patient's other serious illnesses, the number of NHS funded hospital admissions within the previous five years as well as the number of distinct diagnostic codes recorded at time of surgery.
According to the authors, a comparison between hip replacement patients with people in the general population is not really straightforward, given that hip replacement patients “tend to be healthier than others of the same gender and age group”.

The outcome of the study demonstrated that the likelihood of a 60 year old man of moderate health and a metal-on-metal stemmed hip replacement will be diagnosed with cancer five years after surgery is 6.2% as compared with a likelihood of 6.7% in men with hip replacements that contain other bearing surfaces. In women, these figures were 4.0% and 4.4% for other bearing surfaces respectively. The findings furthermore reveal that the incidence of diagnosed cancers after hip replacements is low, whilst that predicted for the age and sex of the matched general population is lower.

Last month, experts writing in The Lancet called for all metal-on-metal implants to be banned because of evidence of high failure rates. In February, the Medicines and Healthcare Products Regulatory Agency (MHRA) issued guidance on all metal-on-metal implants, saying 49,000 patients in the UK would need annual blood or MRI checks. But it insisted there was a “small risk” that the implants could cause complications in patients. The MHRA has said the clinical evidence is mixed and does not support their removal from the market.

The authors believe that their study will assist clinicians in reassuring their patients that the “risk of cancer for hip replacement patients is relatively low” and that there is no evidence that metal-on-metal hip replacements pose a higher risk of cancer.

However, researchers said “as some cancers have a long latency period it is important that we study the longer-term outcomes and continue to investigate the effects of exposure to orthopedic metals”.