Fixed and functional decentering of the head of the humerus in patients with omarthrosis

Aim: To determine whether in patients with specific types of osteoarthritis of the shoulder not only a fixed but also a functional decentering of the humeral head exist. Method: The shoulder joints of 10 healthy volunteers and of 16 patients with osteoarthritis of the shoulder were examined in various arm-positions, using an open MR scanner. After segmentation, 3D reconstruction of the scapula and humerus were performed and the position of the midpoint of the humeral head calculated relative to the center of mass of the glenoid cavity. Results: At 30° of abduction, 4 of 16 patients demonstrated a fixed posterior (12.9±2.8 mm) position and 8 (all patients with cuff-arthropathy) a fixed superior (6.6±2.6 mm) position of the humeral head. At 90° of abduction the patients showed a significant (p<0.001) combined decentering in the superior and posterior direction as compared to the healthy shoulders (functional decentering). Conclusions: This study demonstrates, that in most of the patients with osteoarthritis of the shoulder, a significant functional decentering occurred during abduction and external rotation, even if they showed no fixed decentering of the humeral head at 30° of abduction.