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 degrees (+/- 8.5 degrees) covered by 96 degrees (+/- 8 degrees) of glenoid, leaving 63 degrees (+/- 10 degrees) of cartilage uncovered. In the transverse plane the humeral arc of 160 degrees (+/- 11 degrees) is opposed by 74 degrees (+/- 6 degrees) of glenoid, leaving 86 degrees uncovered. The 159 degrees of coronal humeral arc articulating with the transverse glenoid has 88 degrees (+/- 9 degrees) 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.