Graham Isaac PhD CEng FIMechE, Distinguished Engineering Fellow
While we respect the BMJ’s right to rigorously investigate issues, this article is unnecessarily alarmist and in a number of instances factually incorrect. We submit this letter to highlight some of the factual errors and omissions regarding metal-on-metal hip implants generally, and, in particular, DePuy’s ULTAMET® metal-on-metal implant, which is one of the available options used with the Pinnacle® Acetabular Cup System.
Adverse Reactions to Metal Debris and Ions:
It is well documented that all hip implants, no matter what materials are used, experience wear over time and generate wear debris. Indeed, it was the widely published concerns that wear debris from metal-on-polyethylene hip replacements used in the 1990’s caused bone damage to a significant number of patients that re-kindled the orthopaedic community’s interest in developing a new generation of metal-on-metal implants. Contrary to the suggestion in the article, the potential for reactions to wear debris from metal-on-metal hip implants has been known and studied for over two decades, and the reported incidence of such adverse reactions is very low. According to the U.S. Food and Drug Administration, “Based on the limited number of case reports in published literature, the true incidence or prevalence of adverse systemic effects from MoM hip implants is not known at this time, but is believed to be rare.” Similarly, the MHRA, in releasing updated guidelines last month for monitoring patients with metal-on-metal hip implants, stated: “The majority of patients implanted with MoM hip replacements have well functioning hips and are thought to be at a low risk of developing serious problems.”
Metal Ion Levels:
The article states that DePuy had been aware since 2008 of raised metal ion levels in people with ULTAMET metal-on-metal implants and implies that DePuy concealed this information. This assertion is misleading as it has long been known that patients with metal-on-metal hip implants – and also occasionally hip implants with other bearing surfaces – have “raised levels of ions.” In fact, an article published by Antoniou et al. in a widely read orthpaedic journal and referred to in your article cited seven references on this subject, which date back as far as 1996. When metal-on-metal hip implants were introduced, it was recognized that one of the effects of having metal bearing surfaces would be that constituents of cobalt and chromium would be released locally and systemically. However, data for prior generations of metal implants did not indicate clinical problems from these releases in well functioning implants, some of which had been in place more than 15 years.
The article expresses considerable alarm about the levels of metal ions associated with the ULTAMET metal-on-metal implant, appearing to claim that 20% of patients with ‘DePuy’s flagship Pinnacle hip system’ had blood metal ions above the 7 parts per billion (ppb) level in the MHRA guidance. The three studies cited for this proposition do not support this. The one peer-reviewed publication amongst them found that 5% of patients with ULTAMET implants had levels above 7ppb, with mean cobalt levels at or below 2.3 ppb at all time points measured.
In addition, the article does not establish that the level of metal ion release has clinical significance. Indeed, the U.S. FDA recently stated that there is insufficient evidence to correlate the presence of localized lesions, clinical outcomes, and/or the need for revision with specific metal ion levels for individual patients.
Although the article focuses on the potential risk of cancer from metal-on-metal implants, the connection between metal-on-metal devices and any form of cancer remains unproven, as the article itself concedes. The British Orthopaedic Association/British Hip Society, in response to the media attention generated by the article’s reference to cancer risks, stated that “Currently there is no verified evidence that having a metal on metal hip replacement increases cancer risk.” The BOA/BHS went on to warn that “It would be unwise to suggest removing these joint replacements and taking additional risks of surgery just on the possible cancer risk.” The BOA/BHS statement also noted that while the media reports mentioned one study that found abnormalities in bladder cells of patients with metal-on-metal hip implant, those same reports failed to mention a larger, longer term study that did not show any increased risk of bladder cancer in metal-on-metal hip implant patients. Furthermore an article in the current volume of BMJ comprising 40,576 patients with metal-on-metal hip implants showed that “…there was no evidence that metal on metal bearing surfaces were associated with an increased risk of any cancer diagnosis in the seven years after surgery”.
Observations of taper corrosion have been reported over the years in modular total hip replacement systems, regardless of what bearing surface is used. The clinical significance of these observations has not been established. Contrary to the assertion in the article, DePuy’s most commonly used tapers (‘12/14’) have employed the same angles and surface finishes since they were first introduced. The article also mistakenly asserts that current taper lengths were first introduced in 2004. In fact, the current taper length was first used in 2001. Since 2001, DePuy has gradually adopted this taper length on other stems.
The company investigates any specific complaints it receives related to taper corrosion. For example, as noted in the article, DePuy received complaints about taper corrosion from some surgeons in Japan. However, the article fails to mention that the stems at issue used a particular taper first introduced in the mid 1980’s, which is referred to as a ‘9/10’ taper, and that those stems, when used with metal-on-metal implants, were only available in Japan. The article also omits that in connection with its investigation, DePuy communicated with doctors in Japan in June 2011 and advised them not to use those particular stems in combination with certain metal-on-metal implants.
Introduction of the current generation of metal-on-metal implants:
The article suggests that the current generation of metal-on-metal hip implants was introduced despite known risks and without a clear clinical need. This is not true. Early failures in the first generation of metal-on-metal implants, largely attributed to limitations in the manufacturing process, led to the widespread adoption of polyethylene as the bearing of choice. However a subsequent review of these early metal-on-metal implants showed that many of them exhibited long-term success with much lower wear than polyethylene systems. As a result, when concerns emerged over the damage caused by polyethylene debris, and since manufacturing techniques had advanced, the orthopaedic community had renewed interest in metal-on-metal bearings once again. The other alternatives at this stage were newer polyethylenes that did not yet have long term clinical track records, and ceramics which had also experienced early failures in prior generations. An additional benefit of metal-on-metal implants was that they allowed for larger femoral heads to be used which provide greater stability and lower risk of dislocation. Viewed in context, the re-introduction of metal-on-metal hip implants was a well-considered response to the patient safety concerns being voiced about traditional polyethylene bearing technology.
Revision rate data:
DePuy continues to monitor on a monthly basis data about the ULTAMET metal-on-metal implants from a variety of sources, including published and unpublished data from national joint registries, published literature, company-sponsored clinical trials and internal complaint data. The ULTAMET metal-on-metal implant is performing in a manner consistent with or better than other metal-on-metal products and consistent with benchmarks set by the UK National Institute of Health and Clinical Excellence (NICE). Further, DePuy has conducted four separate studies that include patients with ULTAMET metal-on-metal implants with a head size equal to or greater than 36mm. The pooled results from those studies comprising 1076 hips (779 with a greater than 2 year follow-up), which show a 5 year survival rate of 97%, are consistent with the data from the national joint registries. A femoral stem with a shortened taper was used in 59% of these cases.
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As shown with the examples above, the article fails to provide a balanced presentation of the facts surrounding metal-on-metal implants in general, and DePuy’s ULTAMET metal-on-metal implants in particular. Given the serious nature of the issues raised by the article, and the potential, as noted by the BOA/BHS in their statement, that patients may be scared into having unnecessary surgical procedures, a more balanced presentation would have been appropriate and more in keeping with the BMJ’s stated aim of ‘helping doctors make better decisions’.
ok now look at Debbie Cohen's response to this commentary above: