Impingement in total hip replacements

Dislocation is the most common reason for early failure in total hip replacements (THRs) and occurs following impingement. Impingement is the unwanted contact between either the component and/or bone around the THR. This thesis investigates the effect of factors such as patient bony anatomy, patient activity and cup orientation on impingement likelihood and severity to provide an understanding of how these factors and their interplay may be used to reduce impingement in THRs.
Geometric models were developed of bony geometries from different subjects so that variation in anatomy affecting impingement could be investigated, these were implanted with THRs. Kinematic datasets from different subjects were applied to the models to investigate the effects of subject activity and cup orientation. An in vitro simulator study was carried out which developed kinematic activity data for use in a hip simulator to investigate the consequences of impingement damage.
Variation in bony anatomy, patient activity and cup orientation affected the likelihood and severity of impingement. The anterior inferior iliac spine and anteversion angle of the natural acetabulum significantly affected impingement outcomes and could be used to predict bone-on-bone impingement. The variation in subject’s kinematic data resulted in varying impingement occurrences at different cup orientations. This suggests that clinically recommended implantation positions should be patient-specific. A hip simulator test was developed which applied clinically-relevant motions and loads to generate impingement damage and compared three subject’s squat activities. Findings correlated with geometric model predictions.
The variation in individual subject bone and motion suggest a subject-specific impingement-free range of motion, therefore to minimise impingement and potential dislocation, implantation targets for component orientation should be patient-specific. The methods developed in this thesis could be developed for a preoperative planning tool and aid the design of new implants by providing clinically relevant tools to assess for impingement.