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.