Abstract
In total hip arthroplasty, the use of metal-on-metal bearings (MOM) had been increasing with expansion of demand for higher physical activities in young patients before clinical studies reported that metal ion release caused by wear leads to metal allergy and pseudotumour. Using ELID grinding system we have developed low friction and wear MOM to reduce these adverse reactions. The ELID system produces thick oxide layer on bearing surface that can inhibit metal ion release. The lubrication performance on friction and wear depends on geometric design and surface feature. Therefore, this study demonstrated the optimal clearance of the newly developed MOM for reducing metal ion release. We evaluated the influence of diametrical clearance on friction using a hip friction simulator. The nine φ36 mm femoral heads and 9 liners were prepared with 13 nm in roughness (Ra), and 2 μm in sphericity and were divided into three bearing groups based on the diametrical clearance (C_d = 20, 50, 70 μm). The frictional moment generated in each bearing under walking was measured and the frictional coefficient was calculated. The frictional coefficient decreased with reducing in the clearance but, maximum value was observed in C_d = 20 μm. In the clearance group of 20 μm, we expected the hydrodynamic lubrication because the Λ value defined as the ratio between lubricant thickness and average roughness is almost 3.0. Too small clearance induced the severe contact. Thus, our results suggest that the optimal clearance of new MOM is 50 μm to enhance lubrication performance.
