Stability and Trunnion Wear Potential in Large-diameter Metal-on-Metal Total Hips: A Finite Element Analysis

Large-diameter femoral heads for metal-on-metal THA hold theoretical advantages of joint stability and low bearing surface wear. However, recent reports have indicated an unacceptably high rate of wear-associated failure with large-diameter bearings, possibly due in part to increased wear at the trunnion interface. Thus, the deleterious consequences of using large heads may outweigh their theoretical advantages. We investigated (1) to what extent femoral head size influenced stability in THA for several dislocation-prone motions; and the biomechanics of wear at the trunnion interface by considering the relationship between (2) wear potential and head size and (3) wear potential and other factors, including cup orientation, type of hip motion, and assembly/impaction load. Computational simulations were executed using a previously validated nonlinear contact finite element model. Stability was determined at 36 cup orientations for five distinct dislocation challenges. Wear at the trunnion interface was calculated for three separate cup orientations subjected to gait, stooping, and sit-to-stand motions. Seven head diameters were investigated: 32 to 56 mm, in 4-mm increments. Stability improved with increased diameter, although diminishing benefit was seen for sizes of greater than 40 mm. By contrast, contact stress and computed wear at the trunnion interface all increased unabatedly with increasing head size. Increased impaction forces resulted in only small decreases in trunnion wear generation. These data suggest that the theoretical advantages of large-diameter femoral heads have a limit. Diameters of greater than 40 mm demonstrated only modest improvement in terms of joint stability yet incurred substantial increase in wear potential at the trunnion. Our model has potential to help investigators and designers of hip implants to better understand the optimization of trunnion design for long-term durability.