Abstract and Introduction
Objectives To compare 10 year mortality rates among patients undergoing metal-on-metal hip resurfacing and total hip replacement in England.
Design Retrospective cohort study.
Setting English hospital episode statistics database linked to mortality records from the Office for National Statistics.
Population All adults who underwent primary elective hip replacement for osteoarthritis from April 1999 to March 2012. The exposure of interest was prosthesis type: cemented total hip replacement, uncemented total hip replacement, and metal-on-metal hip resurfacing. Confounding variables included age, sex, Charlson comorbidity index, rurality, area deprivation, surgical volume, and year of operation.
Main outcome measures All cause mortality. Propensity score matching was used to minimise confounding by indication. Kaplan-Meier plots estimated the probability of survival up to 10 years after surgery. Multilevel Cox regression modelling, stratified on matched sets, described the association between prosthesis type and time to death, accounting for variation across hospital trusts.
Results 7,437 patients undergoing metal-on-metal hip resurfacing were matched to 22,311 undergoing cemented total hip replacement; 8,101 patients undergoing metal-on-metal hip resurfacing were matched to 24,303 undergoing uncemented total hip replacement. 10 year rates of cumulative mortality were 271 (3.6%) for metal-on-metal hip resurfacing versus 1,363 (6.1%) for cemented total hip replacement, and 239 (3.0%) for metal-on-metal hip resurfacing versus 999 (4.1%) for uncemented total hip replacement. Patients undergoing metal-on-metal hip resurfacing had an increased survival probability (hazard ratio 0.51 (95% confidence interval 0.45 to 0.59) for cemented hip replacement; 0.55 (0.47 to 0.65) for uncemented hip replacement). There was no evidence for an interaction with age or sex.
Conclusions Patients with hip osteoarthritis undergoing metal-on-metal hip resurfacing have reduced mortality in the long term compared with those undergoing cemented or uncemented total hip replacement. This difference persisted after extensive adjustment for confounding factors available in our data. The study results can be applied to matched populations, which exclude patients who are very old and have had complex total hip replacements. Although residual confounding is possible, the observed effect size is large. These findings require validation in external cohorts and randomised clinical trials.
Total hip replacement was introduced in the 1960s and has developed into one of the most successful treatments in modern medicine. Historically, the operation involves replacing the arthritic joint surfaces with a hard metal head and a softer polyethylene cup, both of which are cemented for secure fixation to bone. In comparison to their longevity in patients older than 65 years, cemented metal and polyethylene prostheses are associated with high rates of early failure in more active patients; particularly men under the age of 55 years. Implant failure is commonly due to early wear and localised osteolysis. Metal-on-metal hip resurfacing was introduced in the 1990s with the hope that wear rates would be lower for the harder bearing surfaces, that larger head sizes would reduce the rate of dislocation, and that subsequent revision surgery would benefit from bone preservation of the femoral neck.
Functional outcomes after metal-on-metal hip resurfacing in the medium term were good and encouraged more widespread use, reaching a peak in 2006 when it constituted 10% of all primary total hip replacements in England and Wales. Subsequent reports of neck of femur fractures, local pseudotumour formation in response to metal wear particles, and the systemic accumulation of high levels of cobalt and chromium ions raised concerns over the long term safety of these devices. Unacceptably high early failure rates led to the withdrawal of the Articular Surface Replacement component, and have been reported in women and anyone receiving a small femoral head component (<50 mm). Although several groups have published findings supporting metal-on-metal hip resurfacing when performed by experienced centres on young men with appropriate anatomy, several influential commentaries reacted by calling for an abolition of the prosthesis.
Understanding the effect that different types of primary total hip replacements—especially metal-on-metal hip resurfacing—have on long term mortality is an essential part of treating patients with debilitating osteoarthritis. The establishment of national databases has allowed the analysis of population based mortality in the long term. McMinn and colleagues recently examined the National Joint Registry database of England and Wales and found a significantly decreased risk of death in patients undergoing uncemented total hip replacement compared with cemented total hip replacement. The greatest survival advantage was observed in patients undergoing a metal-on-metal Birmingham hip resurfacing component. The global effect of this finding was limited by the potential influence of unknown confounding factors, as well as the exclusion of all metal-on-metal hip resurfacings that were not Birmingham hip resurfacings of defined dimensions performed on men only.
We obtained data from the English hospital episode statistics database, which is linked to mortality records between 1999 and 2012 from the Office for National Statistics. This study aimed to account for confounding by indication, using propensity score matching to allow meaningful comparison of 10 year rates of all cause mortality in patients undergoing metal-on-metal hip resurfacing, cemented total hip replacement, and uncemented total hip replacement.