Function and failure mechanisms of dual mobility bearings for total hip replacement

Dual Mobility (DM) Total Hip Replacements (THRs) were introduced to combat the complex challenge of hip dislocation for at-risk patients. These are characterised by an unconstrained polyethylene liner therefore introducing a secondary articulating surface between the liner and acetabular component. Despite their emerging use, the in-vivo function of these implants is not well understood. Early evidence suggests that DM THRs may perform poorer than conventional designs, and failure mechanisms unique to these implants remain a concern. Therefore, the aim of this study was to improve the current understanding of DM function and failure mechanisms. It was clear that current characterisation and in-vitro testing methodologies reported in the literature are not suitable for the novel geometry and function of DM THRs. Therefore, two novel methods were developed in this Thesis: a geometric characterisation method to assess surface damage across the articulating surfaces of DM polyethylene liners, and an in-vitro motion tracking method to investigate the mechanisms of DM liner motion. These methods may be used to improve next generation implant designs and identify best- and worst-case operating conditions thus providing clinicians with more informed surgical guidelines (e.g., optimal component positions) and patient demographics for which these implants should be used. In addition, the function and failure mechanisms of DM THRs were investigated through a multi-method retrieval analysis (n=20 implants) and in-vitro motion tracking, which was the first study to directly observe the behaviour of DM liners under physiologically relevant loading, displacement, and lubrication conditions. These studies identified a primary articulation site (i.e., between the femoral head and liner) and rotational capabilities of the liner within the acetabular shell. Additionally, highly variable damage patterns were observed across the collection of retrieved components and poor intra- and inter-component repeatability of the liner motion was identified in-vitro. This provides evidence that DM motion is not driven purely by implant design but instead may be sensitive to a variety of other factors (e.g., component position, soft tissue interactions). Therefore, it is possible that small changes to the in-vivo environment may have a significant impact on the behaviour and thus long-term performance of DM THRs.