ABSTRACT. The viscosity and shear stress versus shear rate relationship for pure bovine serum (BS) and its aqueous solutions with and without carboxymethyl cellulose (CMC) were investigated. BS and diluted BS without CMC showed pseudoplastic flow curves up to a critical shear rate of ∼100 s-1 above which a Newtonian flow with significant rise in shear stress was observed. The viscosity flow curve for the diluted BS+5g CMC showed only shear thinning up to a shear rate of 3000 s-1 whereas diluted BS+1g or +2g CMC showed similar flow curves to pure BS. The shear rate application modified the flow behaviour of BS from a pseudoplastic to a Newtonian flow depending on its purity and CMC content. Friction factor was dependent on viscosity and clearance with mixed lubrication as the dominant mode within the viscosity range 0.001-0.044 Pas. INTRODUCTION. Pure BS and diluted BS are used as in vitro lubricants for tribological studies. Boundary, mixed and fluid film lubrication are the mechanisms involved in the lubrication of both natural and artificial joints. Clearance and lubricant viscosity will influence the nature of contact between the articulating surfaces. The objectives of this work were to study the flow properties of serum-based lubricants with different viscosities and the correlation between the rheological properties and frictional (and lubrication) behaviour of large diameter Biomet ReCaps with various clearances. MATERIALS AND METHODS. Rheological analysis was performed using a cone-on-plate rheometer (RHEOPLUS/32 V3.40) with a gap of 0.049mm at a constant temperature of 25°C on pure BS as base, 25BS+75 distilled water (DW), and 25BS+75DW+1g, +2g, or +5g CMC. The viscosity and shear stress were measured within shear rates of 0.3-3000 s-1. Frictional measurements of all the joints were carried out at the University of Bradford using a Prosim Friction Simulator on four, as-cast, high carbon, cobalt-chrome resurfacing systems (supplied by Biomet UK Healthcare Ltd., Swindon) with a nominal diameter of 52 mm each and diametral clearances of 167-178 μm. Stribeck plots were used to determine the lubrication mode. RESULTS AND DISCUSSION. Pure BS and diluted BS without and with CMC (1g and 2g) showed similar flow curves with viscosity decreasing from ∼1 to ∼0.001 Pas as shear rate increased from 0.3 up to ∼100 s-1 indicating shear thinning behaviour. A Newtonian flow with a significant rise in shear stress (from ∼0.2 to ∼3.5 Pa) was then observed above ∼100 s-1. Diluted BS+5g CMC showed the pseudoplastic flow only with viscosity decreasing from ∼12 to ∼0.236 Pas as shear rate increased up to 3000 s-1 with a significant rise in shear stress from 3.84 to 708 Pa in the range 0.3-3000 s-1. The Biomet ReCap with a clearance of 167 μm had lower friction factors (0.07 at a viscosity of 0.044 Pas) as compared to that with 178 μm clearance (0.1 at 0.044 Pas). All the ReCaps showed a mixed lubrication up to a viscosity of 0.044 Pas, above which the friction factor increased to 0.13 at a viscosity of 0.236 Pas for the 178 μm clearance

Hip simulators have been used for ten years to determine the tribological performance of large-head metal-on-metal devices using traditional test conditions. However, the hip simulator protocols were originally developed to test metal-on-polyethylene devices. We have used patient activity data to develop a more physiologically relevant test protocol for metal-on-metal devices. This includes stop/start motion, a more appropriate walking frequency, and alternating kinetic and kinematic profiles.

There has been considerable discussion about the effect of heat treatments on the wear of metal-on-metal cobalt chromium molybdenum (CoCrMo) devices. Clinical studies have shown a higher rate of wear, levels of metal ions and rates of failure for the heat-treated metal compared to the as-cast metal CoCrMo devices. However, hip simulator studies in vitro under traditional testing conditions have thus far not been able to demonstrate a difference between the wear performance of these implants.

Using a physiologically relevant test protocol, we have shown that heat treatment of metal-on-metal CoCrMo devices adversely affects their wear performance and generates significantly higher wear rates and levels of metal ions than in as-cast metal implants.