Automatic external rotation during passive glenohumeral abduction.

A study of human cadaveric glenohumeral joints was undertaken to investigate the mechanism of external rotation during abduction. Previously explored extra-articular influences such as the coracoacromial arch and the rotator cuff muscles were removed from the specimens. A specially designed device was used to measure abduction in the scapular plane as well as any rotation of the humerus. The findings show that external rotation is a consistent feature of passive abduction in the plane of the scapula and could take place in the absence of extra-articular influences. The mechanism appears to lie either in the nature of the capsule or of the articular contours. A role for the intra-articular part of the long head of the biceps cannot be excluded.