Tribological and Structural Characterisation of Ceramic on Metal Hip Replacements

Abstract

A tribological investigation of ceramic on metal hip replacements was carried out using in vitro wear testing methods. Two ceramic materials, pure alumina and an alumina matrix compound (zirconia toughened alumina (ZTA)), were articulated against as-cast CoCrMo alloy cups. The diameter was changed from 38 mm to 60 mm to explore the effect of diameter on the tribological performance. Three distinct wear tests were undertaken to allow a direct comparison between materials and sizes. These were; wear testing using standard loading and motion profiles, microseparation (edge loading) in which the loading and motion profiles were modified to allow medial-lateral and inferior-superior displacement of the head, and third body particle tests which incorporated 0.5 g of <1 μm alumina particle in the lubricant.

The Durham Mark I Hip Wear Simulator was used to simulate the standard walking cycle, and was further modified to incorporate microseparation during the swing phase of the walking cycle. All simulator tests were gravimetrically analysed and linear regression analysis was used to determine running in and steady state wear rates. In addition, surface analytical techniques including non contacting profilometery, atomic force, scanning electron and optical microscopy were used to identify changes in surface topography throughout the wear tests. Parameters such as the root mean squared roughness and skewness were monitored to provide quantitative changes in the surface features. The roughness was also used to calculate the ratio of minimum film thickness to equivalent roughness known as the lambda ratio. This provided an theoretical indication of the lubrication regime. Dynamic friction
measurements were undertaken on the Durham Friction Simulator, using water based bovine serum based lubricants, which allowed the lubrication mechanism to be identified through the generation of a Stribeck Curve. The results showed low wear rates for all materials and sizes tested, compared with standard metal on polyethylene and metal on metal components. The wear of the ceramic heads was unmeasurable using the gravimetric method, as the volume
change of the heads fluctuated with a similar trend and magnitude to the control head which did not experience wear. Wear was detected for the softer metallic cups
in all tests.

The standard wear test produced the lowest cup wear rates, compared with microseparation and third body tests which showed increased wear rates through extensive abrasive and adhesive wear mechanisms. After microseparation testing,
characteristic stripe wear patterns were found on the ceramic heads and a flattened lip on the metallic cups. Metal transfer was also identified, which was thought to be due to impact during dislocation of the head during the swing phase of the walking cycle. Third body tests resulted in significant grain loss from the ceramic components compared with both standard or microseparation testing. Low friction factors were recorded for all ceramic on metal components, generally showing the joints to be working close to full fluid film lubrication during the high load stance phase of the walking cycle.