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

Dynamic acetabular cup orientation during gait: a study of fast and slow walking total hip replacement patients

The dynamic orientation of total hip replacement acetabular cups during walking may vary sub-stantially from their assumed position at surgical implantation and may vary between individuals. The scale of this effect is of interest for both pre-clinical device testing and for pre-operative sur-gical planning. This work aimed to evaluate: 1) patient variation in dynamic cup orientation; 2) whether walking speed was a candidate proxy measure for the dynamic cup orientation; and 3) the relationships between dynamic cup orientation angles and planar pelvic angles. Pelvic movement data for patients with fast (n=20) and slow (n=19) self-selected walking speeds, was used to calcu-late acetabular cup inclination and version angles through gait. For aim 1, the range and extremes of acetabular cup orientation angles were analysed for all patients. Large patient to patient varia-tion was found in the range of both inclination angle (1° to 11°) and version angle (4° to 18°). The version angle was typically retroverted in comparison to the implantation position (greatest devi-ation 27o). This orientation is substantially different to the static, 0° version simplifying assump-tions in pre-clinical ‘edge loading’ testing. For aim 2, the cup orientation angles were compared between the fast and slow walking groups using statistical parametric mapping. The only signifi-cant differences observed were for cup version angle, for ~12% of the gait cycle, before toe-off (p < 0.05). Therefore, self-selected walking speed, in isolation, is not a sufficient proxy measure for dynamic acetabular orientation. For aim 3, correlations were recorded between the acetabular cup orientation angles and the planar pelvic angles. The cup inclination angle during gait was strongly correlated (Spearman’s coefficient -1) with pelvic obliquity alone, indicating that simple planar assessment could be used to anticipate range of inclination angle. The cup version angle was cor-related with both pelvic rotation and tilt (Spearman’s coefficient 0.8-1), indicating that cup version cannot be predicted directly from any single pelvic movement. This complexity, along with the interaction between inclination angle and range of version angle, supports the use of computa-tional tools to aid clinical understanding