Abstract
Metal-on-metal (MoM) bearings have shown low-wear rates under standard hip simulator conditions; however, retrieval studies have shown large variations in wear rates and mechanisms. High-wear in vivo has caused catastrophic complications and has been associated with steep cup-inclination angle (rotational malpositioning). However, increasing the cup-inclination angle in vitro has not replicated the increases in wear to the same extent as those observed in retrievals. Clinically relevant wear rates, patterns, and particles were observed in vitro for ceramic-on-ceramic bearings when microseparation (translational malpositioning) conditions were introduced into the gait cycle. In the present study, 28 and 36-mm MoM bearings were investigated under adverse conditions. Increasing the cup angle from 45° to 65° resulted in a significant increase in the wear rate of the 28 mm bearings. However, for the 36 mm bearings, head-rim contact did not occur under the steep cup-angle condition, and the wear rate did not increase. The introduction of microseparation to the gait cycle significantly increased the wear rate of the MoM bearings. Cup angle and head size did not influence the wear rate under microseparation conditions. This study indicated that high-in vivo wear rates were associated with edge loading due to rotational malpositioning such as high-cup-inclination angle and translational malpositioning that could occur due to several surgical factors. Translational malpositioning had a more dominant effect on the wear rate. Preclinical simulation testing should be undertaken with translational and rotational malpositioning conditions as well as standard walking cycle conditions defined by the ISO standard. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2013.
Highlights
Metal-on-metal (MoM) bearings in total hip replacements (THRs) have been used as an alternative to metal-on-polyethylene due to the polyethylene particles inducing osteolysis.[1]
For the 36-mm bearings, the introduction of microseparation conditions to the gait cycle produced a large increase in the wear rate to 5.47 mm3/million cycles for a cup-inclination angle of 45 (p < 0.01) and to 4.14 mm3/ million cycles for a cup-inclination angle of 65 (p < 0.01; Figure 1)
In vitro studies with CoC bearings where microseparation conditions were introduced to the gait cycle leading to edge loading have replicated in vivo wear rates, wear mechanisms, and bimodal nano- and micron-sized wear particles.[16,18,19,20]
Summary
Metal-on-metal (MoM) bearings in total hip replacements (THRs) have been used as an alternative to metal-on-polyethylene due to the polyethylene particles inducing osteolysis.[1]. MoM bearings in THRs have shown low-in vitro wear rates under standard hip simulator conditions, which correlate with well-positioned prostheses.[7,8,9] Under these standard conditions, the centers of rotation of the femoral head and the acetabular cup are matched, and the inclination angle of the acetabular cup is below a clinical equivalent of 55. With these conditions, the contact area (wear patch) is within the intended bearing surface, and mixed lubrication regimes are dominant. The wear is split into two phases, an initial bedding in phase and a steady-state phase with a lower wear rate
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