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

Abstract

The first metal–metal prostheses were implanted during the 1960s decade. Many of these implants showed unsatisfactory short-term results due to poor designs and inappropriate manufacturing methods leading to high friction and wear. However, in the cases where the implants lasted for 20 years or longer, it was found that they exhibited a highly polished surface finish, very small changes in dimensions and low wear rates. These findings brought a revival for the metal–metal implants, which at that time, were being progressively taken over by the metal–polymer, metal–ceramic and ceramic–ceramic pair materials. 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.