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.

ABSTRACT

The friction and lubrication behaviour of four Biomet ReCap components with a nominal diameter of 52 mm and diametral clearance ranging from 167-178 μm were investigated using a friction hip simulator. Friction testing was carried out using pure bovine serum and aqueous solutions of bovine serum (BS), with and without carboxymethyl cellulose (CMC), adjusted to a range of viscosities (0.001-0.236 Pas). The Stribeck analyses suggested mixed lubrication as the dominant mode with the lowest friction factor of 0.07 at a viscosity of 0.04 Pas.

Introduction: Cementless cup designs in metal-on-metal (MoM) hip resurfacing devices generally depend on a good primary press-fit fixation which stabilises the components in the early post-operative period. Pressfitting the cup into the acetabulum generates non-uniform compressive stresses on the cup and consequently causes non-uniform cup deformation. That in turn may result in equatorial contact, high frictional torque and femoral head seizure. It has been reported that high frictional torque has the potential to generate micromotion between the implant and its surrounding bone and as a result adversely affect the longevity of the implant. The aim of this study was to investigate the effects of cup deformation on friction between the articulating surfaces in MoM bearings with various clearances.

Materials and methods: Six Birmingham Hip Resurfacing (BHR) devices with various clearances (80 to 306 μm) were tested in a hip friction simulator to determine the friction between the bearing surfaces. The components were tested in clotted blood which is the primary lubricant during the early post-operative period. The joints were friction tested initially in their pristine conditions and subsequently the cups were deflected by 25– 35 μm using two points pinching action before further friction tests were carried out.

Results and Discussions: It has been reported that reduced clearance results in reduced friction. However, none of the previous studies have taken cup deflection into consideration nor have they used physiologically relevant lubricant. The results presented in this study show that for the reduced clearance components, friction was significantly increased when the cups were deflected by only 30 μm. However, for the components with higher clearance the friction did not change before and after deflection. It is postulated that the larger clearances can accommodate for the amount of distortion introduced to the cups in this study.

Introduction: modern cementless joints depend on bony ingrowth for durable long term fixation. Increased friction and micromotion in the early weeks can prevent ingrowth and affect long-term success.

Most friction studies are conducted in a bovine serum- carboxymethylcellulose (bs-cmc) medium. Following implantation however, the joint is bathed in blood which contains macromolecules and cells. The effect of these on friction is not fully understood.

A progressive radiolucent line (fig 1) observed in some low clearance resurfacings raises the concern that increased friction may be affecting component fixation. The purpose of this investigation was to study the effect of clearance on friction for a given bearing diameter in the presence of blood as lubricant.

Methods: Six Birmingham Hip Resurfacing devices with a nominal diameter of 50mm each and a range of diametral clearances (80, 135, 175, 200, 243 and 306μm) were used. Frictional measurements were carried out on a Prosim Hip Friction Simulator (Simsol Simulation Solutions, Stockport, UK). The test was conducted sequentially with whole blood (viscosity 0.009Pas) and a BS-CMC mixture as the lubricants (viscosity 0.01Pas).

Results: Low clearance devices (80–175μm) generated higher friction with blood than with BS-CMC (fig 2). With blood as the lubricant, low clearance devices generated much higher friction than higher clearance devices (200–306μm).

Discussion: Ongoing research into the in vitro performance of bearings is performed in hip simulators with lubricants that are believed to simulate joint fluid in terms of viscosity. However these lubricants are unable to simulate the friction effects of macromolecules.

The results of this study suggest that reduced clearance bearings have the potential to generate higher friction when blood is the lubricant. this higher friction in the low clearance bearings may produce micromotion in the early postoperative period and hamper bony ingrowth resulting in impaired fixation with long-term implications for survival.