While favorable tribological properties and allowance for larger femoral head sizes have made metal-on-metal bearings an increasingly popular choice for total hip arthroplasty, concerns have mounted regarding adverse reactions to metal wear debris and ions. Metal-on-metal cups differ from conventional polyethylene cups in terms of edge profile design and reductions from full hemisphericity, suggesting differences in loading at or near the cup edge, especially during subluxation events. Finite element analysis was used to investigate the effects of cup orientation and lip edge curvature on damage propensity for edge or near-edge loading during subluxation. Increased cup lip radius (resulting in reduced articular arc) had a detrimental effect upon subluxation-free hip range of motion and upon dislocation resistance. Contact stresses near the cup edge demonstrated complex relationships between edge radius and cup orientation, with peak stresses being influenced by both variables. The tendency for scraping wear at the egress site demonstrated similarly complex dependencies. These data indicate that acetabular cup design is an important determinant of edge and near-edge loading damage propensity.

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Damage to hard bearing surfaces of total joint replacement components typically includes both thin discrete scratches and broader areas of more diffuse scraping. Traditional surface metrology parameters such as average roughness (R a) or peak asperity height (R p) are not well suited to quantifying those counterface damage features in a manner allowing their incorporation into models predictive of polyethylene wear. A diffused lighting technique, which had been previously developed to visualize these microscopic damage features on a global implant level, also allows damaged regions to be automatically segmented. These global-level segmentations in turn provide a basis for performing high-resolution optical profilometry (OP) areal scans, to quantify the microscopic-level damage features. Algorithms are here reported by means of which those imaged damage features can be encoded for input into finite element (FE) wear simulations. A series of retrieved clinically failed implant femoral heads analyzed in this manner exhibited a wide range of numbers and severity of damage features. Illustrative results from corresponding polyethylene wear computations are also presented.

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