Limitation of the Lubricating Ability of Total Hip Prostheses with Hard on Hard Sliding Material

Recently, metal on metal hip prostheses have been revived following excellent clinical results. However, the risk of wear diseases has not been removed yet. Fluid film formation is necessary for long term reliability of a total hip prosthesis because solid contact of the prosthesis induces serious wear. However, the fluid film can be only formed under limited conditions together with an appropriate shape design. Radial clearance is also an important factor because the radius has an upper limit in the joint space. A radial clearance that is too small induces severe contact, whereas decreasing radial clearance enhances squeezed film formation. The most appropriate radial clearance currently remains unclear. We prepared some specimens with high accuracy of polishing, which were balls and cups of the Co-Cr-Mo alloy with a common radius of 16 mm and several radial clearances, 0.15-95.8 μm. Their lubricating ability was evaluated by a friction test using a pendulum machine. The frictional coefficient was a local minimum where the radial clearance was 20-30 μm. It was found that appropriate radial clearance of a total hip prosthesis with hard on hard material must exist within this range.