Lateralizing the glenosphere and decreasing the humeral neck-shaft angles are implant design parameters that reduce the risk of scapular impingement. The effects of these parameters on joint stability remain unclear. This study evaluated the effect of glenosphere lateralization and humeral neck-shaft angle on joint stability by quantifying the anterior dislocation force in different arm positions.
Reverse shoulder arthroplasty was performed on 19 human shoulder specimens. Anterior dislocation force and maximum external rotation were evaluated using a robot-based shoulder simulator. By varying the neck-shaft angle and magnitudes of glenosphere lateralization, 12 configurations were analyzed with the glenohumeral joint in 30° and 60° of abduction, in neutral, and in 30° of external rotation.
At 30° of abduction, measurements showed significantly higher dislocation forces for the 9-mm and 6-mm lateralized glenosphere than for the 0-mm (P < .0001, P = .007) nonlateralized glenosphere. At 60° of abduction, measurements showed significantly higher dislocation forces for the 9-mm and 6-mm lateralized glenosphere than for the 0-mm (P < .0001, P = .0007) and 3-mm (P = .0003, P = .04) glenosphere. Configurations with a neck-shaft angle of 135° showed significantly higher dislocation forces than configurations with a neck-shaft angle of 145° (P = .02) or 155° (P = .02) at 30° of abduction in 30° of external rotation. Neck-shaft angle and glenosphere lateralization had no influence on maximum external rotation capability.
Glenosphere lateralization significantly increased anterior stability of the glenohumeral joint without influencing the range of passive external rotation. The humeral neck-shaft angle only had a minor effect on anterior stability.