Primary osteoarthritis of the shoulder: are there differences in scapular morphology, orientation, and rotator cuff action lines between patients and healthy controls?

Background: There is currently no in depth description of glenoid vault morphology, the rotator cuff action lines, and scapulothoracic orientation in patients with shoulder osteoarthritis (OA). Therefore, the goal of this study was to provide a quantitative analysis of these parameters in Walch types A, B, and C, along with rotator cuff action lines, scapulothoracic orientation, and humeral head centering compared to healthy controls. Methods: Patients who were treated for primary shoulder OA in our hospital between 2010 and 2018 were included in this retrospective case-control study. The cases were categorized into type A, B, and C according to their glenoid morphology on computed tomography using the modified Walch classification and compared with a healthy control group in a matched-pair analysis (matching by age, gender, and affected side). The glenoid version, glenoid inclination, glenohumeral and scapulohumeral head centering, neck angle, glenoid and humeral offset, subscapularis (SSC) and infraspinatus (ISP) tendon traction vectors, and an overall rotator cuff (RC) vector were measured in a standardized axial plane. The protraction, internal rotation, upward rotation, translation, and tilt of the scapula were also measured in three-dimensional reconstructions. Results: A total of 59 shoulders in 47 patients were identified with the following distribution of Walch glenoid types: 24 type A, 30 type B, and 5 type C glenoids. Type A glenoids showed no differences compared to their control group except significantly higher SSC angle, higher resultant RC vector, and lower scapular tilt. Type B glenoids had higher glenoid version, lower glenoid inclination, higher SSC angle, lower ISP angle, higher resultant RC vector, and more posterior humeral offset in comparison with their control group. Scapulothoracic orientation measurements for type B glenoids indicated significantly reduced scapular internal and upward rotation, and lower scapular tilt compared to controls. Type C glenoids showed significantly higher glenoid version, lower glenoid inclination, higher posterior humeral offset, and lower ISP angle with no significant change in the resultant RC vector in comparison with their control group. Conclusion: Patients with primary OA and glenoid type A barely show differences in scapular morphology and scapulathoracic orientation compared to a healthy control group. In contrast, patients with glenoid type B or C showed significant differences of scapulohumeral centering in glenoid version, humeral offset, and glenoid inclination compared to controls. Furthermore, despite the changes of SSP and ISP angles in type B glenoids, the humeral head stayed centered to the glenoid fossa.