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Study on failed MoM hips finds no correlation between femoral size and wear rate
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One of the largest clinical and retrieval analysis of current generation, large diameter metal-on-metal hip components revealed a strong correlation between head and cup wear rate, as well as a lack of relationship between femoral head size and wear rate.
The findings were presented by Alister Hart, FRCS, at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons.
The multivariate analysis also made positive correlations between wear rate and cup inclination, as well as edge loading and wear rate.
An extremely strong relationship
Hart’s group investigated a consecutive series of 138 head and cup couples of explanted metal-on-metal hips. They collected pre-, intra- and post-operative data to create categories for failure. These categories were: infection, loosening, unexplained — for strongly fixed implants with satisfactory alignment in patients without infection — and other. Age, gender, head size, hip type, cup inclination angle, blood cobalt levels, blood chromium levels, presence of edge loading, head wear rate and cup wear rate were also recorded.
Ninety-two failures, Hart noted, were unexplained and 43 out 138 (31%) of hips had a cup inclination angle greater than 55°. Sixty-four percent had relatively low wear, with a median cup wear rate of 4.51 microns per year. The study included 92 women and 46 men.
“The relationship between head and cup wear rate was extremely strong,” Hart said, adding his group also found a positive correlation between inclination and cup wear rate.
Edge loading
Hart reported that his team found that the most important factor responsible for the variation in wear rate was the presence or absence of edge loading, even when adjusted for cup inclination angle. Other factors, such as head size and implant type were less important. Strong positive correlations were found between cup and head wear rates, and blood metal ions levels and wear rate.
The proportion that had edge loading (64%) was more common than expected. The other group, wherein the cause of failure centered around technical issues such as dislocation, size mismatch, impingement and severe malalignment, represented 14 cases in the cohort.
Hart said that his team was first to report not only a strong correlation between head and cup wear rate, but also a lack of a relationship between femoral size and wear rate.
“I have presented you a large number of hips,” he concluded. “Many of them are low-wearing, many of them have a good cup inclination angle, many of them have a large head size. We looked at nine factors in total and performed a multivariate analysis. Many of the factors that people talk about were not significant.”
“Edge loading was the most important, so we need to avoid edge loading; but we still cannot explain the large number of failures with satisfactory cup inclination angles,” he added. “Overall, there seems to be doubt cast on the simple hypothesis that malposition causes excessive wear and failure.” – by Robert Press
Reference:
- Hart A, Underwood R, Cann P, et al. Clinical and wear analysis of 276 failed large diameter metal-on-metal hip components. Paper #67. Presented at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons. Feb. 15-19. San Diego.
- Alister Hart, FRCS, can be reached at the Department of Orthopaedics, Charing Cross Hospital, Fulham Palace Rd, London W6 8RF, England. +44 (0)20 3311 7332; email: a.hart@imperial.ac.uk.
- Disclosure: Hart has received consultancy fees from Depuy, has a family member with stock in Corin, and receives research or institutional support from JRI, Stryker, Biomet, Corin, DePuy, Finsbury, Mathys Ltd., Smith & Nephew and Zimmer.
