Center of rotation affects abduction range of motion of reverse shoulder arthroplasty

Although clinical outcomes of the reverse shoulder replacement have noted improvements in pain and function, evaluation of these outcomes reveals concerns regarding progressive scapular notching and variability of functional improvements in range of motion. Therefore, an apparatus was designed to examine differences in abduction range of motion for seven configurations of reverse shoulder arthroplasty. An electronic goniometer was used to measure abduction range of motion, and digital video analysis was used to determine impingement points. Finally, a correlation analysis between range of motion and the effect of changing the center of rotation of the glenosphere was performed. As the center of rotation was moved more lateral from the glenoid, abduction range of motion increased. The greatest range of motion was 97 degrees +/- 0.9 degrees using a glenoid component with a center of rotation offset 10 mm +/- 0.4 mm from the glenoid. The smallest range of motion was 67 degrees +/- 1.8 degrees using a glenosphere with a center of rotation offset 0.5 mm +/- 0.1 mm from the glenoid surface. Range of motion always was limited by impingement points on the scapula. Inferiorly, adduction was limited by impingement on either the inferior scapular border or the glenoid. Superiorly, abduction was limited by impingement on the acromion. A positive linear correlation was found between abduction range of motion and center of rotation offset relative to the glenoid.