Wear of hard-on-hard hip prostheses: influence of head size, surgical position, material and function

Edge loading in hip replacement bearings may occur due to rotational and translational mal-positioning. Rotational mal-positioning is easier to detect clinically which include steep inclination angle and excessive version/anteversion angles. Translational mal-positioning encompasses micro-separation of the centres of rotations of the head and the cup and could occur due to several clinical reasons, such as head offset deficiency, medialised cup, stem subsidence, impingement, subluxation and laxity of the joint/ soft tissue. Microseparation conditions were validated on the Physiological Anatomical Leeds Mark II Hip Joint Simulator against retrievals and shown to replicate stripe wear, and wear debris seen in vivo. The present thesis is focused on understanding the wear mechanisms of different sized ceramicon- ceramic and metal-on-metal bearings under the different edge loading conditions and determining the contributions of rotational and translational mal-positioning to the increase in wear. The wear of ceramic-on-ceramic bearings did not increase due to rotational mal-positioning, however stripe wear and increase in wear rates occurred under translational mal-positioning conditions. On the other hand, the wear of metal-on-metal bearings was influenced by both rotational and translational mal-positioning with the latter having the more severe effect. There was clinically no difference in the wear of the 36mm bearings when compared to the 28mm bearings under translational mal-positioning conditions. However, with the larger bearings, edge loading due to rotational mal-positioning occurred at a steeper cup inclination angle. Edge loading caused severe wear features and roughening of the surface of metal-on-metal bearings with indications of increased corrosion rates. The new validated geometric measurement and analysis technique have helped determine the volumetric wear of ceramic and metal components and show three-dimensional representations of the wear areas, which could be applied for determining wear volumes and understanding wear features on retrieved explants. This thesis has emphasised the need for a more accurate surgical positioning and shown the necessity for testing hip replacement bearings under a wider range of clinical conditions that go beyond the current ISO standards. This should also include conditions which generate rotational and translational mal-positioning in vivo.