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Metal-on-metal Bearing Surfaces

Joshua J. Jacobs, MD, Robert M. Urban, Nadim J. Hallab, PhD, Anastasia K. Skipor, MS, Alfons Fischer, PhD, Markus A. Wimmer, PhD

Journal of the American Academy of Orthopaedic Surgeons; February 2009, Vol 17, No 2  [download here]


Metal-on-metal bearing couples remain a popular option in total hip arthroplasty and are the only currently available option for surface replacement arthroplasty. In general, the intermediate-term clinical performance of metal-on-metal bearings has been favorable. There are, however, lingering concerns about the biologic consequences of metal release from these bearings in terms of both local tissue effects, including delayed-type hypersensitivity reactions in a subset of patients, and potential systemic effects as a consequence of chronic elevations in serum cobalt and chromium content. Advances in the understanding of the operant wear mechanisms in these bearings provide strategies for reducing the burden of metal released into the periprosthetic milieu, which in turn will mitigate the concerns about the biologic response to the metal debris. Continued surveillance of patients with these bearings is warranted to determine whether metal-on-metal bearing couples provide a longterm survivorship advantage over other bearing couple options and to evaluate whether chronic elevations in the body burden of cobalt
and chromium is well tolerated over the long term.

Discussion and Summary

Even though volumetric wear rates are substantially reduced compared with those of conventional bearing couples, metal-on-metal bearings actually produce a greater number of particles because the debris generated is in the nanometer-size range, likely as a consequence of wear-induced metallurgical transformations (recrystallization and martensite formation) at or near the surface. Although the bioreactivity of nanometer- sized cobalt alloy debris is not well-characterized, large numbers of such minute particles yield a relatively high specific surface area (surface area/mass) available for dissolution (ie, corrosion) of these particles. This may account, at least in part, for the substantial elevations in serum and urine Co and Cr concentration that have been documented by numerous investigators. The effect of head size on serum metal levels is not clear; conflicting data are reported in the literature. Thus, it is unclear at this time whether metal-on-metal resurfacing arthroplasty offers any advantage in this regard over metal-on-metal THA.

In the midst of the current enthusiasm over alternative bearings, the present findings emphasize that the actual wear conditions encountered in vivo, the degradation products generated, and the tissue responses they evoke have not yet been fully characterized for metal-on-metal bearings. For these reasons, patients receiving such devices should be prospectively monitored to measure the clinical significance of these phenomena and for clinical signs associated with failure of metal-on-metal bearings.

In the high-demand patient, metal-on-metal bearings remain a reasonable option for THA and the only currently available option for surface arthroplasty. Continued surveillance of populations with metal-on-metal devices is warranted to determine whether there is a demonstrable long-term survivorship advantage associated with the use of these bearings and to determine whether chronic elevations in systemic metal content are tolerated in the long term.