The role of backside polishing, cup angle, and polyethylene thickness on the contact stresses in metal-backed acetabular components.

Mechanical interactions between the polyethylene liner and the metal-backing play an important role in the load transfer and debris-generation mechanisms of an acetabular component. Insert thickness, cup orientation, and insert-shell interface conditions affect the resulting contact stresses at the articulating and backside surfaces of the polyethylene component. The objective of this study was to determine the variation in contact stresses on a hemispherical acetabular component as a function of the friction coefficient of the line-shell interface, the thickness of the insert, and the load application angle. Three-dimensional finite element models of a metal-backed acetabular component with liner thicknesses of 3-12 mm were developed. The insert-shell interface was modeled as either matte or highly polished, and the load angle of the joint reaction force was changed from 36 to 63 degrees with respect to the dome. We found that the contact stresses at the articulating and backside surfaces of the insert were relatively insensitive to changes in the coefficient of friction at the insert-shell interface (resulting in approximately 1-10% variation in contact stress), when compared to the effect of changing the insert's thickness (approximately 80% variation in contact stress) or changing the direction of the joint reaction force (approximately 20% variation in contact stress). The results of this study suggest that polishing the metal at the insert-shell interface does not substantially change the contact stresses at either surface of the component. Of the design variables available for selective modification by either the surgeon or the engineer, insert thickness and shell orientation play a greater role in determining the magnitude of the resulting contact stresses.