Abstract
Introduction
Modularity allows surgeons to use femoral heads of various materials, diameters and offsets to achieve the best possible outcome, nevertheless the fretting corrosion behaviour of modular junctions can be significantly affected. The aim of this study was to assess physiological friction moment and lubrication ratio in order to compare various tribological materials against different bearing sizes. This data is important as lubrication will affect the friction, wear and torque generated which may lead directly to the production of debris or to enhanced corrosion at modular junctions.
Materials and methods
Hip joints were tested in lubricant condition on a hip simulator following the ISO14242-3 configuration. Three samples for each combinations were examined: 1) 36mm metal-on-metal made in CoCrMo 2) 36mm ceramic-on-ceramic made in ZTA 3) 58mm resurfacing metal-on-metal made in CoCrMo 4) 57mm resurfacing ceramic-on-ceramic made in ZTA. Preconditioning and dynamic loading steps were spaced out by rest periods (Fig. 1) and the entire series was repeated three times for each combination. Strains were measured on the Ti6Al4V neck's femoral stem with three couples of biaxial strain gauges and were converted into friction moments by means of analytical formulas. Mean maximum moment M and lubrication ratio λ were calculated. MSTART-UP and MTURN-OFF were respectively the first three and last three peak moment sampled for each consecutive step.
Results
Fig. 2 reports the obtained results. It can be observed that MoM large bearings showed a mean maximum friction moment lower than MoM smaller bearings (p=0,001), whereas no effect of bearing diameter on friction moment was observed for CoC (p=0,162). There is no statistically significant difference on friction moment between Ø57mm ceramic-on-ceramic resurfacing bearing and Ø58mm metal-on-metal resurfacing (p=0,805). However the CoC Ø36 friction moment was significantly lower than with MoM Ø36 (p=0,001). The calculated lubrication ratio λ gave information on lubrication regime: in the case of standard bearings mixed lubrication (1≤λ≤3) occurred, while resurfacing bearings were in full film lubrication (λ>3). Correlating lubrication ratio λ with MSTART-UP and MTURN-OFF (Fig. 2) it can be observed that the peak friction moment increased during the dynamic step for bearing in mixed lubrication, while a decrease was observed for bearing in full film lubrication. The breaking point cycle between downward and flat peak friction moment trend decreased more than half with longer dynamic steps (Fig. 3), leading to a stable maximum friction moment.
Discussion
Ceramic bearing friction moment monotonically increased with bearing diameter, this trend being not observed on metal couplings because the tested standard diameter was in mixed lubrication lower limit. Mixed lubrication regime energized the bearing by increasing the friction moment during the dynamic step such as a self-powered system, on the other side full film lubrication reduced friction moment to a lower asymptote, such as a self-stabilized system.
Conclusion
Modern ceramic hip resurfacing was designed in full film lubrication and its friction moment was equivalent to metal-on-metal hip resurfacing. Modern ceramic resurfacing may reduce fretting corrosion compared to traditional metal resurfacing while keeping the same biomechanical advantages.
For figures/tables, please contact authors directly.
