Abstract
Aim: Lower friction in metal on metal (MOM) hip joints can reduce the wear, production of metal ions and loosening of acetabular cups. The effect of the fluid viscosity on the friction, especially in the relation to the physiological range, is still not fully investigated. The aim of this paper was to study the frictional behaviour of MOM hip joint within the human physiological fluid viscosities.
Materials and Methods: Friction measurement was carried out using a friction simulator on CoCrMo hip prostheses of 50 mm diameter with 100 and 200 micron diametral clearances. The samples were taken from a wear simulator test at 1.4, 2.3 and 3.2 million cycles. A dynamic loading of 100–2000N was applied to the femoral head with a movement of 1 Hz and +/−23 degree amplitude. Tests were performed using 25% new born calf serum which consisted of different ratios of serum and carboxymethyl cellulose with viscosities ranged from 0.0011 to 1.1 Pa s determined by a viscometer at a shear rate of 300/s.
Results: The friction of both clearances reduced with the progress of wear. In the lower range of viscosity, the friction of 100 micron clearance was lower than that of 200 micron clearance. However, when the viscosity reached the range of 0.01–0.06 Pa s, the friction of 100 micron clearance surpassed that of 200 micron clearance and this difference became wider with the increase of viscosity. During one measurement, the curves of 100 micron clearance started from low friction (≈0.05), but increased rapidly for all viscosities except the lowest of 0.001 Pa s. For the 200 micron clearance, the curves were stabilized when the viscosity was below 0.36 Pa s and low friction was observed as the viscosity increased. The friction started to increase only from 0.36 Pa s, but the gradient was less stiff compared to that of 100 micron clearance.
Discussion: Normal synovial fluid is non-Newtonian in nature with shear-rate dependent viscosity. Researchers have reported 300/s shear rate viscosities for normal, osteoarthritis, and inflammatory synovial fluids as 0.010.4, 0.0025–0.2 and 0.001–0.07 Pa s respectively. In this study, the 200 micron clearance had lower friction than that of 100 micron clearance in the majority of physiological viscosity range. Theoretical studies have suggested that smaller clearance and higher viscosity can benefit the lubrication in MOM bearings. However, this theory is valid if continuous and complete lubrication film is achieved. Small clearance and high viscosity may prevent the recovery of lubricant between cycles and cause depletion of lubricant, which can lead to direct contact of bearings and increase of friction. It is therefore concluded that the selection of clearance for MOM components should consider the human physiological fluid viscosities so that an optimal tribological performance can be achieved.
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