Wear of cast metal-metal pairs for total replacement hip prostheses

The present work evaluates the influence of dimensional and microstructural parameters upon the wear behaviour of metal-metal hip implants of a Co-Cr cast alloy by means of laboratory simulation. A total of 10 pairs (acetabular cup and femoral hemisphere) were manufactured with varying diametral clearances, carbon content (0.21 and 0.31 wt.%), and microstructures (as-cast, partial and complete carbide solutions) achieved by heat treatment. The pair specimens were subsequently tested in a newly developed hip simulator under severe reciprocating sliding conditions at a frequency of 1.5 Hz, a constant load of 2 kN and bovine serum solution as lubricant. It was found that pair specimens with large diametral clearances exhibited higher amounts of wear compared to those samples with smaller diametral clearances. In terms of microstructural parameters and carbon content, pair specimens with the as-cast and partial solution microstructures (with carbide volume fractions of about 10%) exhibited less amounts of wear than those with complete carbide solution microstructures (with carbide volume fractions of about 5%). This implies that a higher content of carbon enhances wear resistance only if the carbon is precipitated as carbides and not as solution within the matrix.