| Summary: | Total shoulder arthroplasty (TSA) is used to treat patients with glenohumeral joint osteoarthritis. Despite high success rates, complications occur and many patients undergo revision surgeries. The top three most frequent complications are: instability/dislocation of the joint, glenoid loosening and rotator cuff insufficiency. It is believed that these complications occur in most part due to alignment of the implant. In this thesis a two part investigation was conducted to test the effects of joint replacement humeral head version (-15° to 15°) and tilt (-10° to 10°) and glenoid version and tilt (-15° to 15°). The first part investigates the effect of the humeral head alignment on range of motion and activities of daily living using a collision detection modelling method. The second part investigates the effect of both humeral head and glenoid variations on the joint reaction and muscle forces to describe the risk of the three most frequent complications using the United Kingdom National Shoulder Model. This thesis shows that increasing humeral head posterior version decreased the ability to perform activities of daily living (up to 32% at 15°) mostly due to bone-implant collision, increased the risk of the rocking-horse mechanism (by up to 37% at 15°) and increased subscapularis activity (by up to 14% at 15°). Similarly, increasing inferior tilt of the glenoid to 10° produced the best outcomes: vertical rocking-horse mechanism decreased by 19% and no significant differences in muscle forces were observed. In conclusion, normal alignment of the humeral head following surgical guidelines is recommended to increase the chances of implant survival. Posterior versions of the humeral head should be avoided more so than other small mal-alignments. Increasing glenoid inferior tilt to 10° produced favourable results but after combining the results from this thesis and from the literature, it is concluded that all glenoid mal-alignments should be avoided and highlights the need for more effective surgical tools to accurately position the shoulder replacement.
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