Clinical trials involving metal-on-metal hip replacements have been inadequate to date, according to a review of the literature recently published in the Journal of Bone and Joint Surgery. Looking primarily at research published before 2005, the review sought to provide clinicians with an overview of evidence on the risks associated with increased metal ions and the durability of metal-on-metal hip replacements.
Hip-joint-simulator tests demonstrated a correlation between increased femoral head diameter and general durability in the short-term (“running-in wear”) and the long-term (“steady-state wear”). The review concludes that “large-diameter heads with the smallest practicable clearance lead to less running-in and steady-state wear.” Additionally, research has highlighted the benefits of coupling large diameter heads with proper component alignment to sustain good head coverage and fluid-film lubrication through steady-state wear.
The article also reviewed investigations into metal ion levels in patients with metal-on-metal hip arthroplasties, as wear debris continue to be an area of concern. One area of intense research is the possible carcinogenic effects of these ions, as metal ions are potentially mutagenic. While some studies have documented correlative effects between ion levels and chromosomal abnormalities, in 2000 The International Agency for Research on Cancer could not reach a conclusion on the cancer-risks associated with hip and knee implants, citing lack of available research. Metal ions have also been shown to cross the placental barrier, though further research is necessary to determine transfer rates and mechanisms. The review recommends that women of child-bearing age should not have metal-on-metal total hip arthroplasties until further data is available.
Finally, the authors recommended additional randomized controlled studies comparing metal-on-metal surfaces with other bearing surfaces such as metal-on-polyethylene. Large sample sizes are essential in future prospective studies to allow for follow-up over long-term studies, as many such studies in past were hindered by high drop-out rates among patients.
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