Wear of Hard-on-Hard Bearings in the Hip: The Influence of Loss of Conformity Under Edge Loading

Abstract

When hip joint replacements are correctly positioned, they form highly conforming ball and socket bearings with low wear. Variation in the translational position or the rotational position can result in the head contacting the rim of the cup, edge loading with a low conforming contact and in some cases increased wear. The nature and magnitude of the increase in wear with edge loading depends on the type of bearing as well as the nature of the variation in position (translational or rotational malposition). This chapter considers the effect of edge loading in hard-on-hard, metal-on-metal and ceramic-on-ceramic bearings, using novel advanced preclinical simulation methods.

In ceramic-on-ceramic bearings, rotational malposition of the cup did not increase the wear rate or produce stripe wear. Translational malposition of the head and cup, resulting in dynamic microseparation, produced stripe wear on the femoral head and a wear rate of greater than 1 mm³/million cycles for Biolox Forte and much less than 1 mm³/million cycles for Biolox Delta. This level of wear is not expected to cause adverse tissue reactions.

In larger-diameter metal-on-metal bearings, both rotational and translational malpositioning and edge loading produced substantial increases in wear rates. Translational malpositioning and resulting dynamic microseparation produced the highest wear rate of 9 mm³/million cycles, levels expected to produce elevated ion levels and adverse reactions in some patients.