Do Articular Surfaces of Dual Mobility Hips Have More Wear and Friction? An In Vitro Investigation

Background: Larger head-to-neck ratio of dual mobility (DM) hip arthroplasties provide greater range of motion/less risk of dislocation, but raise concerns for high wear and friction. We measured in vitro, the wear rates of contemporary DM hips with highly cross-linked ultra high molecular weight polyethylene (UHWMPE), where it came from, and their frictional torques. Methods: Hip simulators were used to compare the wear of DM to fixed-bearing (FB) designs of 2 different implants. Each of 8 different configurations underwent millions of simulated walking cycle tests, some as full DM, some as FB controls, some DM with the outer-articulation deliberately immobilized, and some the inner. Wear and 3-dimensional-frictional torques were measured and friction independent of size was deduced. Results: The DM hips produced lower wear and friction-torque than the FB hips. The DM wear during walking gait comes mostly from the smaller inner articular surface. If the outer surface was immobilized, the wear and torque of the inner alone would be small, but the full DM (inner and outer free-to-move) wear and torque were smallest of all. Friction measurements expectedly showed larger hips having higher frictional torques, but the DM showed the lowest, again because its motion was mostly the smaller inner articulation; smaller than even a modern fixed-bearing hip. Conclusion: The DM hips appear to combine the benefits of greater range of motion and less impingement of larger hips, with the lower wear and friction of smaller FB hips, with some benefits compromised if the outer or inner articulations are immobilized.