All-metal hips ‘wear down faster’
“Experts are calling for controversial metal-on-metal hip implants to be banned,” according to The Guardian. The newspaper said that research has found “unequivocal evidence” of high failure rates of these implants, particularly among women.
In recent months there has great deal of scrutiny about the safety of some types of metal-on-metal hip replacements, with concerns they wear out much faster than implants featuring plastic and ceramic parts.
To examine the issue, researchers working on behalf of the National Joint Registry in England and Wales analysed data on 402,051 hip surgeries performed using implants attached to the thigh bone by a metal stem, including 31,171 metal-on-metal implants. Researchers found that these had higher failure rates than other types of hip replacement, with an overall five-year failure rate of 6.2%. Those with larger ‘heads’ (the part of the implant fitting into the hip joint socket) had a higher failure rate than those with smaller heads, as did hip implants in women.
This study provides more information on the longer-term performance of metal-on-metal hip replacements, and supports claims that they do not last as long as other types of hip implants.
The use of this type of implant in England and Wales is reported to have dropped dramatically since 2008, and the ongoing concerns seem likely to reduce its use further. Overall, this study supports the recent recommendations by UK health regulators. It states that people with large-headed metal-on-metal implants should be monitored carefully over time to identify whether their implants are wearing down at a faster rate.
Where did the story come from?
The study was carried out by researchers from the Universities of Bristol and Exeter, and the Centre for Hip Surgery at the Wrightington Hospital in Lancashire. It was performed on behalf of the National Joint Registry of England and Wales, which also funded the research.
The study was published in the peer-reviewed medical journal The Lancet.
This story was covered in a balanced way by the Guardian.
What kind of research was this?
In recent months there has been some concern over the use of certain all-metal hip implants, particularly over whether they wear down at a faster rate compared to other types of implants.
Hip implants come in a variety of different sizes and materials, but the debate has centred on large-headed ‘metal-on-metal’ implants. These implants are designed so that both the ball replacing the top of the thigh bone and the artificial socket placed in the pelvis are made from metal.
This research was a registry study looking at data on hip implants collected by the National Joint Registry of England and Wales, which records all hip and knee replacement surgeries. This includes the first operation to install the implant, and any revision operations carried out to replace or remove part of the original implant.
Just like with natural bone, metal hip implants experience wear and tear and can eventually deteriorate. This means any implant may eventually need revision surgery, although analysing the revision rate gives an estimate of how often and how soon implants fail early. While the revision rate is an important indicator of the outcomes of hip replacement, it should be noted that not all hip implants that do not function well or cause pain will be replaced.
The researchers say that due to their resistance to wear, large diameter metal-on-metal hip stemmed implants have become popular. ‘Large diameter’ refers to the size of the ‘head’ part of the implant that sits in the hip socket section of the implant. Stemmed means the head is attached to an elongated stem that sits inside the top of the high bone and holds the head in place.
The study’s authors note that there have been concerns about the high failure rate of one particular brand of metal-on-metal hip stemmed implant called ASR, which was withdrawn from use in 2010. Given this withdrawal and fresh concerns about other types of metal-on-metal implants, the researchers aimed to look at whether general metal-on-metal hip stemmed implants fail any more regularly than other hip implants (ceramic-on-ceramic or metal-on-polythene). They also looked at whether large diameter hip implants lasted any longer than implants with smaller diameter heads.
This type of registry analysis is useful for monitoring the long-term performance of devices once they are in use. It can help to identify any problems that are occurring with the implants. Ideally, data on the comparative performance of different implants would come from randomised controlled trials, but the researchers report that there are few such studies available.
What did the research involve?
The researchers looked at 402,051 first total-hip replacements using a stemmed implant carried out in England and Wales between April 2003 and September 2011. They then identified any revision operations carried out on these hip implants. This allowed them to determine how long it took before a revision operation was needed for each type.
The researchers did not include data on ASR implants in their analysis, because they are already known to have much higher revision rates than other brands and have already been withdrawn from the market. The researchers also only included data on hip replacements that had sufficient data recorded to allow them to identify which operations were revisions of which earlier hip replacements. This allowed them to analyse data on 82% of all first total-hip replacements using a stemmed implant performed in the study period.
The researchers tried to make sure the hip implant operations being compared were as similar as possible. For example, they only included those where the implants were not ‘cemented’ in, and where the operation was being performed in ‘typical’ patients.
Typical patients were defined as those whose hip replacement was needed due to osteoarthritis only, and who were generally healthy or with only mild illness at the time of primary surgery. This was defined using a recognised measure of pre-operative health. The researchers also took into account the age of the patient, and looked at men and women separately.
What were the basic results?
The researchers found that metal-on-metal hip implants were used in 8% of the 402,051 first total hip replacements using a stemmed implant. This equated to 31,171 replacements. Use of these types of implants peaked around 2008 but then reduced sharply after this.
Overall, metal-on-metal implants required revision due to failure more quickly than other implants, with a 6.2% needing revision within five years of implantation. The size of the head of the metal on metal implant affected the failure rate in men and women, with larger heads failing earlier. Overall, each 1mm increase in head size increased the risk of revision over time by about 2% (hazard ratio [HR] 1.020 in men, 95% confidence interval [CI] 1.004 to 1.037; HR in women 1.019, 95% CI 1.001 to 1.038).
In men aged 60 years, the five-year revision rate was 3.2% for 28mm head metal-on-metal implants, and 5.1% for 52mm head implants. In younger women, the five-year revision rate was 6.1% for 46mm head metal-on-metal implants, compared with 1.6% for 28mm head metal-on-polyethylene implants.
Revision rates for metal-on-metal implants were higher for women than men, even with implants with the same head size. For example, a 36mm head metal-on-metal implant in women aged 60 had a five-year revision rate of 5.1% compared to 3.7% among men of the same age and implant head size.
However, larger head sizes were more durable for ceramic-on-ceramic hip implants. In men aged 60 years, the five-year revision rate was 3.3% with 28mm head ceramic-on-ceramic implants, and 2.0% with 40mm head ceramic-on-ceramic hip implants.
Age also had an effect on hip implant survival for women, with younger women receiving hip implants more likely to have revision surgery.
The most common reasons for revisions were loosening and pain, and these were more common in people who had metal-on-metal implants.
How did the researchers interpret the results?
The researchers concluded that metal-on-metal stemmed hip implants have a higher failure rate than other options, and should no longer be used. They say all patients with these types of implants should be carefully monitored, particularly young women whose hip implants have large diameter heads. They say that their findings support the continued use of large diameter ceramic-on-ceramic bearings as they seem to perform well.
Metal-on-metal hip implants have been under intense scrutiny in recent months, and this analysis provides useful data on how often they require revision and how they compare to hip implants made of other materials. Overall, this research indicates that metal-on-metal hip implants have higher revision rates (rates of replacement) than other types of hip implants in England and Wales.
As with all such studies, there is the possibility that factors other than the implant type differed between the groups being compared, and that these other factors may influence the results. The researchers tried to minimise the risk of this by:
- comparing similar operations in similar patients
- looking at men and women separately
- looking at the effect of age and implant head size
However, there are other factors such as activity levels that could still be having an effect.
As the data used in this study came from a surgical registry, not a lot of information was available about factors such as body mass index (BMI) or activity levels. These two factors could potentially influence the stress implants are placed under and therefore the wear that they display. The researchers say that in their opinion there is no obvious reason to suppose that these factors would vary to a large extent between people receiving the different large head metal and ceramic hip implants.
Due to these inherent limitations with observational research it is difficult to conclude that the differences seen are definitely due to the implants alone. The advantage of this registry data is that a large number of people were assessed. Furthermore, this is not a selected subsample of people receiving hip implants but all patients from different surgeons and using different implants. The researchers say that these strengths and the consistency of their findings support the suggestion that these findings do represent the true effects of the implant types.
Overall, these findings do seem to suggest that metal-on-metal hips do require revisions more frequently than other types of hip implants. This study reports that since 2008 there has been a dramatic reduction in the use of metal-on-metal hip implants in England and Wales. Overall it found that most hip replacements analysed in the study period (92% between April 2003 and September 2011) did not use metal-on-metal implants.
It seems likely that based on this study their use may decline further. It is important to bear in mind that the overall five-year revision rates with metal-on-metal hips is 6.2%, so the majority of these implants have not needed revision in this time. This study supports the authors’ suggestion and MHRA recommendation that people with these implants should be monitored carefully over time, to identify when such revisions might be required.
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