Abstract
Introduction
PEEK-OPTIMA™ has been considered as an alternative bearing material to cobalt chrome in the femoral component of total knee replacements. To better understand the tribology of UHMWPE-on-PEEK-OPTIMA™ and to find the most appropriate environmental conditions under which to test this novel bearing material combination, a series of tests under different protein lubricant concentrations at rig (∼24°C) and elevated temperature (∼35°C) were carried out in simple geometry wear and friction rigs. Under all conditions, the wear of UHMWPE-on-PEEK-OPTIMA™ was compared to UHMWPE-on-cobalt chrome (CoCr).
Methods
The pins used were GUR1020 UHMWPE (conventional, non-sterile) and the plate material was either polished CoCr (Ra<0.01µm) or PEEK-OPTIMA (Ra∼0.03µm) provided by Invibio Ltd, UK. The wear simulation was carried out in a six station reciprocating rig. The kinematic conditions were consistent for all tests and reflected the average cross shear and contact pressure (3.2MPa) in a total knee replacement. Tests were carried out at either rig running temperature (∼24°C) or at elevated temperature (∼35°C) and in varying protein lubricant concentrations (0, 2, 5, 25 and 90%). Wear of the UHMWPE pins was determined by gravimetric analysis. The pin-on-plate friction rig study was carried out at rig temperature in 0, 2, 5, 25 and 90% serum and reflected the contact pressure used in the wear tests. Measurements were taken using a piezoelectric sensor and the steady state friction derived. At least 3 repeats were taken for each study, statistical analysis carried out using ANOVA with significance taken at p<0.05.
Results and Discussion
The influence of protein lubricant concentration and temperature on the wear of UHMWPE was different for the two bearing couples tested. In low serum concentrations (≤5%), polymer transfer was evident on the surface of the plates suggesting insufficient boundary lubrication and a non-clinically relevant wear mechanism. In 25% serum at rig temperature, the wear factor of UHMWPE-on-PEEK-OPTIMA™ was similar (p>0.05) to UHMWPE-on-CoCr at 2.00×10−7±1.08×10−7mm3/Nm and 2.15×10−7±7.44×10−8mm3/Nm respectively. Increasing the temperature of the lubricant lowered the wear factor of UHMWPE-on-PEEK-OPTIMA (9.93×10−8±2.96×10−8mm3/Nm); there was no influence of temperature on UHMWPE-on-CoCr (1.87×10−7±6.14×10−8mm3/Nm). The lower wear rate of the all-polymer couple was attributed to the elevated test temperature coupled with the higher friction of the all-polymer combination causing protein in the lubricant to come out of solution, adhere to the articulating surfaces and protect them from wear. In high serum concentrations (90%), protein deposition was visible on the surface of the plates and protein precipitation was visible in the lubricant.
Under all protein lubricant concentrations, the coefficient of friction was higher for the all-polymer bearing couple than for UHMWPE-on-CoCr, at serum concentrations ≥2%, this difference was significant (p>0.05).
Conclusion
Environmental conditions such as protein concentration and lubricant temperature influence wear and the effects of these variables can differ for different material combinations. For UHMWPE-on-PEEK-OPTIMA™, testing in 25% bovine serum at rig temperature minimised test artefacts such as polymer transfer, protein deposition and protein precipitation which suggests that these may be appropriate test conditions for this material combination.
