What are the risks accompanying the reduced wear benefit of low-clearance hip resurfacing?

Background: Clearance is an important determinant of metal-metal bearing function. Tribologic theory and laboratory evidence suggest low clearance (LC) reduces wear but with a potential to increase friction and clinical reports show LC resurfacings have high implant failure rates. Thus, the role of LC is unclear.

Questions/purposes: We asked: is in vivo wear as reflected by cobalt (Co) and chromium (Cr) levels reduced in LC bearings, and if so, is this benefit offset by increased friction as assessed by implant-bone interface changes?

Methods: We retrospectively reviewed 26 patients with LC resurfacings. We assessed Co and Cr levels in blood and urine, hip function, and radiographic adverse features. These data were compared with those from 26 patients with a similar resurfacing but with conventional clearance (CC) from a previous study. Minimum followup was 4.0 years (mean, 4.1 years; range, 4.0-4.7 years).

Results: Co and Cr ion comparisons showed three phases: in the first 2 months, there was no difference between the cohorts; at 2 to 24 months, the CC group showed higher levels; and subsequently, levels in the two groups converged. A mean Oxford hip score of 13 and step activity of 1.9 million cycles per year in the LC group were similar to those of the CC group. Cup radiolucencies were seen in three patients in the LC group and none in the CC group.

Conclusions: Lower Co and Cr levels suggest lower wear in the LC resurfacings in the intermediate term, but the presence of radiolucencies raises the concern that higher bearing friction is affecting implant fixation. A larger clearance than the theoretically predicted ideal may be required to allow for minor manufacturing imperfections, component deformation, and progressive changes in the in vivo lubricant.

Level Of Evidence: Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.