Hip hemiarthroplasty (HA) is a conservative treatment for hip diseases (e.g. osteoporosis, femoral head necrosis) and injuries (e.g. femoral neck fractures) compared to total hip replacement (THR). HA is commonly used in elder patients who have low activity requirements and can be delivered with reduced blood loss, shorter operation time, and hospital stay. HA has been developed to unipolar and bipolar prostheses, and the range of motion has been improved with new designs of the bipolar prostheses. However, satisfaction of HA is not always positive due to the erosion, degradation of the acetabular cartilage, and the dislocation of femoral head which have reported in clinical studies. Hence, the mechanical factors which affect the tribological properties of articular cartilage in HA have been investigated experimentally in an in- vitro simulation of HA. In a simple geometry HA model, the articular cartilage coefficient of friction, deformation, and wear have been examined in a cartilage pin on metal plate model. The loading time, contact stress levels, contact areas, stroke lengths and sliding velocities were set as the input variables, and it was found that: 1) coefficient of friction was time- and load- dependent in short-term (1 hour) testing; 2) wear was load-dependent in long-term (24 hours) testing; 3) contact area only affected the coefficient of friction when both fluid and solid phase of cartilage supported the load due to the different aspect ratio; 4) the stroke length and sliding velocity affected the coefficient of friction only under low level of contact stress (≤2 MPa), but affected the cartilage wear under all levels of contact stresses. In an entire simulation of HA, the acetabular cartilage coefficient of friction, surface roughness, wear, and deformation have been studied with metal or ceramic heads articulating against acetabular cartilage in a pendulum friction simulator. The clearance between the head and acetabulum, loading types, and prosthesis design were set as the input variables, and it was demonstrated that: 1) a trend of decrease of the coefficient of friction was shown with increasing clearances under both constant and dynamic loading; 2) severe cartilage severe damage was observed when radial clearance was >1.8 mm under constant loading; 3) ceramic heads decreased the cartilage wear, and change of surface roughness compared to metal heads; 4) a novel bipolar design with a thin metal cup and metal head failed to reduce the coefficient of friction due to the cup locking in the acetabulum. The methodologies and findings in this research can currently adopted for related future studies, additionally the anatomic pendulum friction natural joint simulation can be used for the pre-clinical studies in less invasive procedures in the future.

Metal-on-metal hip resurfacing arthroplasty (MoMHRA) is an alternative treatment option to total hip arthroplasty (THA), especially in the young and active. Despite the early successful results, reports of adverse soft-tissue reactions around the MoMHRA have caused increased concern. These reactions have been termed pseudotumours and have been shown to be associated with increased wear. However, their incidence and aetiology is not known. The broad aims of this thesis were to 1) determine the clinical impact of pseudotumour; and 2) identify ways of reducing the risk of pseudotumour. The clinical impact was determined by examining the incidence and prevalence of pseudotumours needing revision and the subsequent outcome following revision. At the Nuffield Orthopaedic Centre, the 10-year survival of the BHR, commonest MoMHRA used, was 87.1% (95%CI: 83.0% to 91.2%) and the 10-year incidence of revisions due to pseudotumour was 7.5% (95%CI: 4.3% to 10.7%). In a prevalence study of two tertiary referral centres, pseudotumour was identified as the most common revision indication. An outcome based, case-control study characterised outcome following revision due to pseudotumour as inferior to that following revision for any other failure mode, and inferior to the outcome of matched primary THAs. The inferior outcome was associated with the extensive tissue damage at revision. Risk factors contributing to development of pseudotumour were found to be both patient and surgeon related. Patient factors significantly increasing risk included female gender and small component size. Case control studies demonstrated two surgical factors that were associated with significantly increased risk; cup malorientation and a reduction in head neck ratio (HNR) at operation particularly in females. Furthermore, evidence based recommendations were provided on optimum cup orientation and screening practice. Surgeons should aim for a radiographic cup inclination/anteversion zone of 45°/20° ± 10°. Patients with mal-positioned cups, those with high HNR prior to surgery, and those that had significant HNR reduction with the MoMHRA should be considered at increased risk of developing a pseudotumour. An ex vivo study demonstrated that the majority of pseudotumours occur due to high prosthesis wear, and result from an exaggerated innate response with an accompanying variable immune response. However a small but significant number of pseudotumours (20%) were observed with low wear and a prominent immune response. Furthermore, the cups oriented within the optimum zone were associated with less wear. Hence, subsequent in vivo research focused on providing evidence-based guidance on how to achieve the target cup orientation and factors to consider in minimising the scatter in cup orientation observed following hip arthroplasty. This great scatter was mostly due to two factors: 1) the variability between surgeons at orientating a component at implantation and 2) the difference in the pelvic position between implantation and radiographic assessment. The angular offset was 5° for inclination and 8° for anteversion because of the different definitions at impaction and X-Ray. Surgeons should need to be aware of these factors. Greater amount of pelvic movement occurs during MoMHRA, in comparison to THA. To achieve the target MoMHRA cup orientation surgeons should implant the cup with an intra-operative inclination/anteversion of 40°/30°. It is concluded that pseudotumour following MoMHRA has a significant clinical impact. In order to minimise it, surgeons should appropriately select patients and optimise surgical technique to achieve optimum cup orientation and try to avoid reducing the native head neck ratio. Appreciation of risk factors for pseudotumour development (female gender, small size component, cup orientation, HNR reduction), would aid detection prior to extensive soft-tissue damage and improve inferior outcome associated with revision.