The recently published reports from the National Joint Registry for England and Wales (UK NJR) and the Australian National Joint Replacement Registry (NJRR) reveal increasing failure rates and other significant safety issues affecting all metal-on-metal hip replacements (MOMs). The increase in the failure rates is particularly striking for the DePuy's ASR XL and ASR resurfacing devices.
I. ASR XL
The 2013 UK NJR report offers figures for the ASR XL total hip replacement device in combination with the most widely used Corail femoral stem. The cited failure rates are revised upwards compared to the 2012 report - 7.47% at 3 years post-implantation (vs. 6.93% in 2012) and 23.28% at 5 years (vs. 21.93% in 2012). For the first time, long-term survivorship data is available, which shows a rapid acceleration of the failure rate past 5 years. Thus, at 8 years, the ASR XL failure rate is 43.3%. By contrast, the generally accepted failure rate for a well-performing hip implant is at most 1% per year, or 8% or less at 8 years.
The 2013 NJRR report cites similar figures for the ASR XL system: 9.5-9.7% at 3 years, 23.4-23.7% at 5 years, and 39.1% at 7 years.
II. ASR Resurfacing
Similar to the exceptionally poor performance of the ASR XL system, the reported failure rates for the ASR resurfacing system are much higher than those for any other resurfacing or total hip replacement metal-on-metal device.
The numbers reported by the UK NJR for the ASR resurfacing are 5.91% at 3 years post-implantation, 13.71% at 5 years, and 24.07% at 7 years. The failure rate at the 9-year mark is as high as 36.40%. As explained above, the generally acceptable maximum rate at 9 years is only 9%. And the ASR failure rate is more than double of the rate for the second worst performer (Stryker Cormet), which is 16.34% at 9 years.
The respective numbers for ASR resurfacing from the 2013 NJRR report show a comparable performance at 7 years and even worse survivorship at shorter follow-ups. The figures are: 7.1% at 3 years, 15.4% at 5 years, and 23.9% at 7 years.
III. Metal-on-Metal Hip Replacements in General
The 2013 reports from the UK NJR and the NJRR portray a grim picture for all metal-on-metal hip replacement implants both in terms of high failure rates and other serious safety issues.
The UK NJR report contains new 9-year survivorship data that show further widening of the gap between MOMs and other bearings. The failure rate for hybrid MOMs (uncemented acetabular component with cemented femoral components) is 14.86% at 9 years post-implantation, compared with 2.99% for hybrid Metal-on-Polyethylene devices (MOPs). The respective figures at 9 years for fully cemented MOMs and MOPs are 21.43% and 2.62%. The NJRR report cites comparable figures for MOMs in general at 10 years post-implantation - 15.5% (up from 13.4% in 2012). The newly reported long-term figure (12 years post-implantation) is even higher - 18.6%.
Both the UK NJR and the NJRR reports note the rapid decline in the use of MOMs as hip replacement devices. The UK NJR report in particular "note[s] that metal-on-metal stemmed hip replacement and hip resurfacing [which are also MOMs] have virtually ceased with fewer than one in one thousand hip replacements performed in 2012 belonging to each of these classes of implant." The report links this usage trend with the trend of increasing revision rates: "The failure rates [for MOMs] continue to be markedly higher than the alternatives. Other bearing surfaces continue to have very low failure rates regardless of fixation, especially for ceramic-on-polyethylene bearings."
However, the significantly higher comparative failure rate of MOMs is not the only issue contributing to their negative image relative to other bearing types. The 2013 UK NJR and NJRR reports both provide new insight about the higher incidence and greater scope of complications arising from the use of MOMs relative to other hip devices. One of the conclusions in the NJR report is that "...the metal-on-metal sub-groups had the highest incidences of pain and aseptic loosening and also appeared to have the highest incidences of adverse soft tissue reactions."
The NJRR report goes even further. The Australian Registry replaces the term "metal sensitivity" with "metal related pathology" (MRP) in order to "refer to the entire spectrum of surgeon identified... pathology" related to MOMs. The report then notes that "[i]n the last three years, MRP as a reason for revision [among all bearing types] increased from 1.2% to 7.5%. Almost all revisions for MRP are secondary to the use of metal/metal bearings... There have only been 15 revision procedures reported to the Registry with a diagnosis of MRP when non metal/metal bearings have been used." Therefore, while MOMs can lead to complications common to all types of bearings, they can also lead to potentially serious adverse effects peculiar to them - metal related pathology.
Further, in addition to its main 2013 NJRR report, the Australian Registry produced a Supplementary Report specifically devoted to MOMs used for conventional hip replacement. This Supplementary Report observes that "[t]he main reasons for revision are metal related pathology (30.5%), loosening/lysis (28.3%) and infection (11.7%)..." In other words, MRP, one of the most problematic complications stemming from hip devices, is the single most common revision reason for MOMs.
The Supplementary Report goes into more details surrounding MRP. While its finding that MRP is associated with MOMs but not with MOPs is not surprising, the incidence of loosening and infection is also higher for MoMs:
"The cumulative incidence for [loosening/lysis, MRP and infection] for [MOMs] at twelve years is 4.5%, 6.7% and 1.8% respectively compared to [MOPs] 2.3%, 0.0% and 0.8% respectively... It is uncertain why there is a higher incidence of reported infection [with MOMs]. The diagnosis of infection is reported to the Registry at the time of surgery and has not been confirmed by linking to results of microbiological investigation... The Registry has also identified that lysis, as the sole diagnosis for revision, is reported with the highest frequency in [MOMs]."
The Supplementary Report further observes a striking difference between the incidence of MRP for MOMs with smaller head sizes and that for MOMs with larger head sizes (0.2% at 10 years for head sizes <=32 mm and 7.2% for head sizes >32 mm, or 36 times higher incidence; for reference, the smallest ASR XL head size is 39mm).
Also, several charts in the Supplementary Report show a dramatic increase in the incidence of MRP with large-diameter MoMs as a function of time - with no signs of abating even at 12 years post-implantation. This suggests MRP might become an even bigger problem. In fact, the Supplementary Report warns of another reason why the magnitude of the MRP issue might have been underestimated:
"The incidence of metal related pathology is potentially higher as it is possible that undiagnosed metal related pathology contributes to the increased rate of loosening/lysis and infection reported in [MOMs] with larger head sizes..."
Finally, the UK NJR reports that a thorough investigation of MOM resurfacing devices shows that women are at a significantly higher risk for revision than men. More specifically, "[t]he in-depth analysis of hip resurfacing... demonstrates that failure rate is related to both gender and head size, with smaller femoral heads and female gender independently associated with higher rates of implant revision."