Normal and abnormal mechanics of the glenohumeral joint in the horizontal plane.

This study was performed to evaluate the relationship of the humeral head to the scapula in the horizontal plane of motion and to describe in detail a method of obtaining and interpreting modified axillary roentgenograms. Twenty normal subjects and twelve patients who had anterior instability of the shoulder were evaluated with this technique. In the control group, the humeral head was centered in the glenoid cavity throughout the horizontal plane of motion except when the arm was in maximum extension and external rotation. In this position, the cocked stage of the throwing motion, the center of the humeral head rested approximately four millimeters posterior to the center of the glenoid cavity. When the arm was flexed or rotated from this cocked position, the humeral head glided anteriorly, producing a shearing stress on the articular surface of the glenoid and labrum. In seven of the twelve patients who had anterior instability, abnormal mechanics were observed: anterior translation of the humeral head occurred. This indicates a significant disruption of the structures responsible for containing the humeral head within the glenoid fossa.