Biomechanical relevance of glenoid component positioning in the reverse Delta III total shoulder prosthesis

The presence of a notch at the inferior part of the scapular neck is a common radiographic finding in patients treated with a reverse Delta III shoulder prosthesis. It is thought that this notch is a result of mechanical contact between the polyethylene cup of the humeral implant and the inferior glenoid pole during adduction of the arm. This in vitro study assessed the effect of glenoid component positioning on glenohumeral range of motion in 8 shoulder specimens. Four different positions of the glenosphere were tested: glenosphere centered on the glenoid, leaving the inferior glenoid rim uncovered (configuration A); glenosphere flush with the inferior glenoid rim (configuration B); glenosphere extending beyond the inferior glenoid rim (configuration C); and glenosphere tilted downward 15 degrees (configuration D). The respective mean adduction and abduction angles in the scapular plane were -25 degrees and 67 degrees for configuration A, -14 degrees and 68 degrees for configuration B, -1 degrees and 81 degrees for configuration C, and -9 degrees and 75 degrees for configuration D. Placing the glenosphere distally (test configuration C) significantly improved adduction and abduction angles compared with all other test configurations (P < .001).