Contact mechanics of metal on polyethylene hip replacements

Metal-on-ultra high molecular weight polyethylene (UHMWPE) total hip replacement (THR) has been the most popular and clinically successful hip prosthesis to date. The long-term performance of THR depends on both the tribological characteristics and biomechanical behaviour of the prosthesis. This project focused on understanding the contact mechanics and mechanical behaviour of cemented and cementless metal-on-UHMWPE THRs under different conditions using a computational approach.
Three-dimensional (3D) computational models of THRs with realistic pelvic bone were developed. Two typical bearings, the Charnley hip and the Pinnacle cup system, were investigated. The effect of different factors on the contact mechanics and cement stresses for Charnley THR were examined. Additionally, the contact mechanics and mechanical behaviour of Pinnacle THR under daily activities, standard and microseparation conditions were analysed.
The cup angles and penetration depths in the cup, and the sizes of the components were found to have a significant effect on the contact mechanics and cement stresses for Charnley THR. The stresses at the bone-cement interface for the Charnley THR with outer diameter of 40 mm were predicted to be higher than that of 43 mm, the difference was found to be consistent with the clinical observation of different aseptic loosening rates.
The cup angles and radial clearances were found to have a synergistic effect on the contact mechanics of Pinnacle THR. Edge loading on both articulating surface and backside surface of the liner was observed during some daily activities due to steep cup inclination angles and smaller radial clearance. The introduction of microseparation into the gait cycle, especially when combined with steep cup inclination angles, resulted in concentrated stresses and plastic deformation in the liner, which would cause potential damage to the liner. Therefore, it is critically important to reduce the levels of rotational and translational mal-positioning of the components clinically.