A TRIBOLOGICAL AND PARTICLE DEBRIS STUDY OF AS-CAST AND HEAT TREATED COCRMO ALLOY

Abstract

The generation of particle debris from ultra high molecular weight polyethylene (UHMWPE) against metal hip joints has been shown to cause osteolysis leading to joint loosening in the medium term. This is known as late aseptic loosening since infection is absent¹.

In an attempt to reduce the volume of wear debris, attention has moved to metal-on-metal prostheses as the total volume of wear debris is less. However, the size, shape and number of the particles are important as well as the total volume as these affect the biological response of the body leading to aseptic loosening.

The Durham Mk I Hip Joint Simulator was used to generate CoCrMo wear particles in a series of tests. Four simulator tests took place in succession, initially 50 mm Birmingham hip replacements were tested where both the head and the cup were as-cast CoCrMo alloy. A second test was conducted where the components were 38 mm and both head and cup were as-cast CoCrMo. A third test using 50 mm components was completed where both head and cup were double heat treated CoCrMo alloy and a final test took place where both components were 50 mm the head was as-cast and the cup was as-cast which had been pre-worn to 5 million cycles. Bovine serum with a concentration of 17 g/l of protein was used as a lubricant and particles were sampled every half million cycles. The volumetric wear was also obtained gravimetrically.

A double enzymatic protocol was used to cleave the proteins from the particles taking great care to minimise any effect on the particles. Finally the particles were suspended in distilled, de-ionised water to enable them to be analysed using a NanoSight LM10 particle analyser. This yielded the size distribution of the particles. This was then confirmed by placing an aliquot of the suspended particles onto a carbon coated copper grid and drying them under a lamp. These particles were then imaged in the TEM. Energy Dispersive X-ray analysis was also used to obtain the chemical composition of the particles.

The results indicated a strong correlation between the gravimetric wear and the number of particles. All the as-cast CoCrMo alloy tests showed a consistent particle modal average size. The double heat treated particles were shown to be smaller, with occasional large flake like particulates which were identified under the TEM. This particle data correlates extremely well with previous data from simple material testing pin on plate experiments.

Previous studies have used microscopy to investigate the size and morphology of the particulate debris², however these studies are limited due to the time taken to image the particles individually. This current method allows many more particles to be analysed, thus the data accumulated is more statistically significant and may be compared with the wear volumes calculated gravimetrically.