Abstract
There is currently much interest in the wear of metal-on-metal THRs and potential concerns about elevated metal ion levels. Generally, wear of metal-on-metal THR’s has been low in simulator studies. Slightly higher and more variable wear has been found clinically. Variations in surgical approach, technique and fixation method may influence the level of force applied across the prosthesis during gait. It is hypothesised that increased joint tensioning may increase loading of THR’s during the swing-phase; leading to elevated wear and friction due to depleted fluid film lubrication. This study aimed to assess the effect of swing-phase load on the friction, lubrication and wear of metal-on-metal THR’s.
Cobalt-chrome 28mm metal-on-metal THR’s were tested in a physiological hip simulator, loading was modified to provide; (1) ISO swing-phase load (280N, as per ISO 14242-1) and (2) low swing-phase load (< 100N). Friction testing was conducted using a pendulum friction simulator, with 280N and 100N swing-phase loads. Theoretical lubrication modelling was carried out using elastohydrodynamic lubrication theory.
The overall mean volumetric wear rates was 10-times greater in THR’s tested with an ISO swing-phase load in comparison to THR’s tested with low swing-phase loads (0.58±0.49 compared to 0.06±0.039mm3/million cycles). The friction factors were 0.129 and 0.173 respectively under low and ISO swing-phase conditions. A decrease in the predicted lubricant film thickness when the swing-phase load was increased was observed; at the start of stance phase this was 0.12microns and 0.07microns under low and ISO swing-phase conditions respectively.
The results demonstrate that the performance of metal-on-metal THR’s is highly dependent on swing-phase load conditions. It is postulated that fixation method and surgical technique can affect the swing-phase load. This study has demonstrated that over-tensioning of the tissues may also accelerate wear. These observations may explain some of the variations reported clinically.
Editoral Secretary Mr Peter Howard. Correspondence should be addressed to BHS at the Royal College of Surgeons, 35 - 43 Lincoln’s Inn Fields, London WC2A 3PN.
