Influence of additional weight carrying on load-induced changes in glenohumeral translation in patients with rotator cuff tear - an experimental glenohumeral simulator study

The rotator cuff muscles are critical in providing the necessary compressive forces in the joint to maintain stability during glenohumeral motion. In shoulders with rotator cuff tears, this mechanism may be compromised, and unfortunately, this is a common shoulder pathology in the general population. Untreated rotator cuff tears can lead to secondary pathologies such as osteoarthritis and fatty infiltration of the torn muscle. To study glenohumeral biomechanics, in vivo studies can provide physiological data, but often require invasive or ionizing radiation measurements, while ex vivo studies allow an in-depth study of biomechanics while facing the challenge of reproducing physiological behavior.
The purpose of this thesis was twofold: first, to develop a control strategy for a glenohumeral simulator to simulate the abduction motion of shoulders with rotator cuff tears, and second, to study the glenohumeral biomechanics of weight-bearing shoulders with rotator cuff tears during 30° glenohumeral abduction.
We thoroughly reviewed the state-of-the-art of glenohumeral simulators and their limitations. We then developed a musculoskeletal model-based simulator to simulate abduction motion in weight-bearing shoulders with rotator cuff tears. Compared to in vivo measurements, we showed that our simulators replicated similar muscle activation and joint reaction forces during 30° abduction. Furthermore, our experiments showed that glenohumeral translations were affected by weight-bearing and critical shoulder angle, but not by rotator cuff tears. In addition, we found that the center of force in the glenohumeral joint was located in areas where glenoid erosion has been reported in patients with osteoarthritis following massive rotator cuff tears.
The remaining intact tendons of the rotator cuff muscles were able to compensate for torn rotator cuff tendons and maintain glenohumeral translation comparable to intact shoulders. Our findings suggest that patients with multiple rotator cuff tears are at risk for developing secondary osteoarthritis and that strengthening the remaining rotator cuff muscles with intact tendons may be beneficial for these patients.