Computational testing of patient-specific gait features and pelvic motion effects on the risk of edge contact in total hip replacements

Although total hip replacement (THR) surgery is considered one of the most successful orthopaedic interventions, failures which require revision still occur. One of the known contributors to the failure of THR is edge contact, where the acetabular cup and the femoral head remain concentric but contact falls partially on the cup's rim. Failures associated with edge contact include rim damage, osteolysis and cup dissociation due to altered in vivo loading and torques. The current structural and tribological pre-clinical testing protocols fail to capture the spread of pelvic movement and joint contact force directions, which can be seen in a patient-specific analysis. Therefore these tests cannot always predict the success of the THR while in vivo.

The broad aim of the PhD project presented in this thesis was to bridge the gap between pre-clinical testing and biomechanical THR studies with a focus on risk of edge contact. The effect of pelvic motion exclusion (common in in vitro studies) on the risk of edge contact was assessed from patient-specific perspective.

In this work a computational approach was used to achieve the aim. The data for the analysis was gained from previous experimental biomechanical studies, including a conventional force platform and motions marker study, an instrumented implant study and a dual video-fluoroscopy study. The developed computational algorithms identified the relative position of THR bearing components based on the motions of femur and pelvis. The results of two central studies within this PhD showed that the exclusion of pelvic motions substantially affects the risk of edge contact. However, the effect of pelvic motions on the risk of edge contact was shown to be patient-specific. It was found that pelvic sagittal tilt, coronal obliquity and internal-external rotation all contribute to the overall effect of pelvic motions on the risk of edge contact. In addition, the studies within this project revealed that static orientations of the acetabular cup during standing are not representative of the orientation during dynamic activities. The use of dual video-fluoroscopy techniques were shown to have potential to eliminate uncertainty in variability between static acetabular cup orientation and while THR is in motion.

The work presented in this thesis, showed the importance of considering the dynamic activity effects on the success of THR device, which potentially applies to other artificial joints. The methods used can be applied to both pre-clinical testing and preoperative planning, as well as postoperative THR success management. Further studies on larger and more diverse patient cohorts are required to estimate, and in some cases predict, the patient-specific characteristics which affect the risk of edge contact in vivo.