“There is no evidence that metal-on-metal hip replacements increase the risk of cancer,” the BBC reported today.
The story is based on a study that found that patients with metal-on-metal hip replacements did not have a higher risk than the general population of developing cancer up to seven years after surgery, or than patients with hip replacements made of other materials.
The study comes in the wake of recent concerns about metal-on-metal hip implants, including high failure rates and the possible risks of small amounts of metal (ions) being released into the body. While the findings are reassuring, this type of study has limitations. In particular, it only looked at the risk of cancer within a few years of having hip replacement surgery. Given that several cancers can take many years to develop, a study of longer-term outcomes of metal-on-metal implants is required and recommended by the researchers.
Recommendations from UK health regulators state that people with large metal-on-metal implants should be monitored annually. If they have any concerns, they can consult their doctor for patient-specific guidance.
Where did the story come from?
The study was carried out by researchers from the University of Bristol, University of Exeter and Wrightington Hospital, Wigan. It was funded by the National Joint Registry for England and Wales.
The study was published in the peer-reviewed British Medical Journal, which has recently published a number of pieces on the wear rate and safety of metal-on-metal hip implants. Most notably, it conducted a joint investigation with BBC’s Newsnight.
The research was reported fairly by the media. Both the BBC and The Daily Telegraph pointed out that the study only looked at cancer rates up to seven years after surgery, and that ongoing monitoring may need to be performed.
What kind of research was this?
The authors point out that metal-on-metal hip implants have become popular over the past decade. These consist of:
- resurfacing implants – where only the articulated surfaces of the existing hip joint are replaced with metal
- 'stemmed' implants – in which the ball replacing the top of the thigh bone and the artificial socket placed in the pelvis are made of metal. As their name suggests, stemmed implants feature an elongated metal stem that surgeons slide down into the thighbone to secure the implant in place
However, recent data show that all-metal stemmed implants have significantly higher failure rates and that metal-on-metal resurfacing implants have an above-average failure rate compared with implants made of other materials (such as ceramic or plastic).
In addition, the researchers say that little is known about the biological effects of the metals – predominantly cobalt, chromium and molybdenum – which are released into the body as the surface of the implants wears down. The researchers say that traces of these metals can be found in many organs, including the marrow, blood, liver, kidneys and bladder. They also say there is evidence that patients who have had joint replacements show a higher than normal incidence of DNA (genetic) damage, although there is no proven link between this and an increased risk of cancer.
To assess any raised risk of cancer, this study compared rates of cancer in patients with metal-on-metal hip implants to rates in patients with hip replacements made of alternative material, within seven years of surgery. It also compared cancer rates in patients undergoing hip replacements with those of a section of the general population, with predicted cancer rates matched for age and sex.
What did the research involve?
The researchers used data on hip replacements from the National Joint Registry of England and Wales, a database that holds records on over 1 million joint replacement procedures undertaken since its establishment in 2003. Researchers consulted all relevant data up until April 2011. The Registry is also linked annually to national hospital episode statistics data, in order to routinely monitor health information on patients who have had joint replacements. The hospital episode statistics data collection contains details of all admissions to NHS hospitals in England. It includes private patients treated in NHS hospitals, patients who were resident outside England and care delivered by treatment centres (including those in the independent sector) funded by the NHS.
For their study, the researchers used data from 289,571 patients in England who underwent hip replacements from 2003 to 2010, for whom joint registry data could be linked to hospital episode statistics. This comprised 40,576 patients who had metal-on-metal hip replacements and 248,995 who had hip implants made of other materials.
The researchers looked at hospital episode statistics data on these patients between 1997 and 2010, including any diagnoses of cancer (other than non-melanoma skin cancers) in the years after hip replacement. They also looked separately at specific cancers suspected of being related to metal ions, including blood cancers (such as leukaemia), malignant melanoma, prostate cancer and renal tract cancer (cancer of the bladder, ureter or kidney). They excluded from their analysis any patient who had a recorded diagnosis of the specific cancer before or at the time of their hip replacement.
They compared the outcomes in patients with metal-on-metal hip replacements (both stemmed and resurfaced) with patients who had hip implants made of other materials. They separated patients into three groups: those with stemmed metal implants, those with metal resurfacing, and total hip replacement with other materials. They adjusted their results for other factors that might affect the risk of cancer such as age, sex and general health.
In addition, they compared cancer rates in patients undergoing any type of hip replacement with those in the general population, using age- and sex-matched predicted incidence rates derived from national data.
What were the basic results?
The study found that, compared with patients who had hip implants made of other materials, there was no evidence that metal-on-metal implants were associated with an increased risk of any cancer diagnosis in the seven years after surgery. This was based on an average (mean) follow-up of three years, with 23% of patients observed for five years or more. Similarly, there was no increase in the risk of malignant melanoma or cancers of the blood, prostate and renal tract.
For men aged 60, the risk of being diagnosed with any cancer in five years after surgery was:
- 4.8% (4.4% to 5.3%) after metal-on-metal resurfacing
- 6.2% (5.7% to 6.7%) after a stemmed metal-on-metal implant
- 6.7% (6.5% to 7.0%) after a hip implant made of other materials
For women aged 60 the rates were lower:
- 3.1% (2.8% to 3.4%) after resurfacing
- 4.0% (3.7% to 4.3%) after a stemmed metal-on-metal implant
- 4.4% (4.2% to 4.5%) after other types of material
The researchers also found that one year after hip replacement, the incidence of new diagnoses of cancer was 1.25% (95% confidence interval [CI] 1.21% to 1.30%). This was lower than the predicted incidence of 1.65% (95% CI 1.60% to 1.70%) for the age- and sex-matched general population.
How did the researchers interpret the results?
The researchers said the findings are reassuring and pointed out that, compared with the general population, the risk of cancer for hip replacement patients is low. However, they also said that a study of longer-term outcomes is needed.
This study’s strength lies in its large sample of patients who have had hip replacements. However, it should be noted that:
- The study only shows results for up to seven years after surgery. Since some cancers take time to develop, an analysis of longer-term data is required.
- All hip implants produce some metal 'debris', even if the surfaces are not metal. It would, therefore, be better to compare the cancer rates of patients having metal implants with a control group of patients with osteoarthritis without any implants. The registry used does not include data on these people.
- The fact that the study found lower cancer rates after one year in patients with metal-on-metal implants compared with the age- and sex-matched 'normal' population is not easy to explain. It may indicate the influence of other factors (confounders) because patients undergoing hip replacements are checked to ensure they are healthy before surgery. People of the same sex and age in the comparison group may not be as healthy. In the comparison of different hip replacement types, those selected for resurfacing may also be younger and fitter as this is one of the reasons these devices are fitted. These confounders could have explained some of the effect reported.
- Using hospital statistics to identify cancers may underestimate cancer risk. This is because some patients are diagnosed and treated without hospital admission, for example only as outpatients.
While these findings offer some reassurance about the potential carcinogenic effect of hip replacement implants, further longer-term study of the effects of metal-on-metal implants is required. Given the various concerns about these types of implants, it is likely that their use will be reduced in future and that monitoring of any cancer risks will continue.