Working principle of the dual mobility (total hip replacement): wear mechanisms and design optimization

Abstract: The dual mobility concept for hip implants was invented in the 70s to address the issue of postoperative dislocations. The idea was to mix the Charnley “Low Friction Principle” that recommended a small head, i.e. decreasing of wear due to small diameter, and the benefit on the dislocation rate of a big head closer to the native one. Thus a polyethylene insert was added between the metallic metal back and the femoral head but some concerns remained, particularly the cup wear.To keep on improving the dual mobility cup, the first point was to understand how it is working. Therefore explants analysis and studies on simulators were carried out on several series of samples. Analysis techniques like surface profilometry or 3D profiling with a Coordinate Measuring Machine (CMM) were then performed to measure roughness and wear. It was of interest to be able to compare the results given by different techniques to draw more reliable conclusions from the comparison. Indeed it was possible to conclude that wear was significant for some explants and not for the others which gave information about the wear mechanisms. It was also possible to give more reliable ranges for the average volumetric wear.Given observations made during the previous studies, an ellipsoidal shape was thought to improve the dual mobility longevity and reduce wear. Indeed the polyethylene deformed under loading with time which resulted in a blocking of the insert at its equator and an increase in wear. The idea was to increase the thickness at the average load application point which corresponded to a reduced clearance thus a higher contact area during loading. It resulted in a more uniform contact pressure distribution and lower maxima in stress and pressure. The final effect would be a slower deformation and the blocking effect would be avoided. Obviously it would go together with a reduced wear which was the major goal of that shape optimization.