Limits imposed on glenohumeral motion by joint geometry

Contact of the greater tuberosity against the glenoid raises the question of the limits imposed on motion by glenohumeral joint geometry. In 50 cadaveric shoulders the arcs of articular cartilage on the humeral head and glenoid in the frontal and axial planes were measured or calculated. In the coronal plane the humeral heads had an arc of 159° (±8.5°) covered by 96° (±8°) of glenoid, leaving 63° (±10°) of cartilage uncovered. In the transverse plane the humeral arc of 160° (±11°) is opposed by 74° (±6°) of glenoid, leaving 86° uncovered. The 159° of coronal humeral arc articulating with the transverse glenoid has 88° (±9°) of cartilage uncovered. The geometry of the glenohumeral joint limits motions that occur in simple arcs. The obligate external rotation that occurs in elevation is necessary to make more humeral cartilage available for articulation with the glenoid.