ArticlesFailure rates of stemmed metal-on-metal hip replacements: analysis of data from the National Joint Registry of England and Wales
Introduction
Osteoarthritis is an increasingly prevalent disorder. Roughly 5% of people in France aged 70 years have symptomatic osteoarthritis of the hip,1 and, as populations age and become more obese, it is probable that the prevalence will increase further. Advanced hip osteoarthritis can be so successfully treated with total hip replacement (THR) that it has been described it as “the operation of the 20th century”.2 THR is now extremely common: about 270 000 replacements are done each year in the USA and this is projected to increase by 174% between 2005 and 2030.3 The National Joint Registry of England and Wales reported 68 907 primary operations in 2010.4
The long-term problem with THR is failure, resulting in revision surgery. In England and Wales, about 5% of THRs are revised within 7 years.4 However, this figure probably underestimates failure, because many hips with poor function and pain are not revised.5 In England and Wales, the mean age of a patient undergoing THR is 67 years; however, 12% of patients are younger than 55 years.4 Implant survival is particularly important in younger patients who have longer life expectancy and higher activity levels. The results in these patients are disappointing; the Finnish Arthroplasty Register reports 15 year implant survival of only 60% in patients younger than 55 years.6
The leading cause of THR failure is aseptic loosening secondary to polyethylene wear.7 A second major cause of failure is dislocation affecting about 4% of THRs, most within 6 months.8 Surgeons and engineers have tried to address these problems by investigating alternative bearing surfaces that have lower wear and allow larger head sizes. This would decrease the risk of dislocation by increasing the primary arc of movement (the movement before the neck impinges and begins to lever out the head) and the jump distance (the distance the head needs to travel to dislocate, which equals the radius of the head). However, there are anatomical limitations to the size of implant and the largest sizes can only be implanted in people with large acetabula. Ceramic bearing surfaces have the advantage of very low wear, but are expensive and there have been reports of ceramic fracture9 and audible squeaking.10 Metal-on-metal bearing surfaces have been extensively assessed in simulator tests and noted to be highly resistant to wear even when used in very large head sizes.11 In simulator tests, larger head sizes paradoxically give lower wear because the lubrication regime changes from less favourable boundary lubrication to more favourable mixed or fluid-film lubrication.12
Therefore, implantation of large diameter metal-on-metal bearing surfaces on stemmed prostheses has become popular, because they should result in lower failure rates. However, there are several potential dangers of exposure to metals such as chrome and cobalt, including genotoxicity and direct damage to end organs such as kidneys.13 Furthermore, major problems have been reported with the ASR, a brand of metal-on-metal prosthesis, which seems to fail early.14
We aimed to use the National Joint Registry of England and Wales to assess if metal-on-metal bearing surfaces lead to increased implant survival compared with other bearing surfaces in stemmed THRs. Because larger head sizes should result in lower dislocation rates, our second hypothesis is that larger head sizes will result in improved implant survival.
Section snippets
Data source
We assessed data from the National Joint Registry of England and Wales—established in 2003 and the largest arthroplasty database in the world. It records all primary and revision hip and knee replacements done in England and Wales. By April, 2011, 1 082 465 procedures had been recorded.
Our analysis is based on 402 051 (82%) of 491 505 primary stemmed THR procedures between April, 2003, and September, 2011, recorded in the National Joint Registry, which include patient identifiers that allow
Results
Overall, about 8% of stemmed THR procedures recorded in the National Joint Registry of England and Wales involve a metal-on-metal prosthesis (n=31 171). Stemmed metal-on-metal procedures involve either a modular cup (a metal cup with a metal liner) or a large diameter monobloc cup (as used in hip resurfacing). In England and Wales, the use of stemmed metal-on-metal implants escalated strongly after 2004 to a peak of more than 9000 operations in 2008, but then declined sharply (figure 1). The
Discussion
Our hypothesis that large-diameter metal-on-metal stemmed prostheses would have good implant survival rates has been refuted by use of the largest arthroplasty database in the world. Stemmed metal-on-metal THR has poorer implant survival in the medium term than other bearing surfaces, and larger head sizes are associated with higher, rather than lower, revision rates. Implant survival is particularly poor in women. By contrast, ceramic-on-ceramic articulations have better implant survival with
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