Effects of biceps loading and arm rotation on the superior labrum in the cadaveric shoulder.

Effects of loading the long head of the biceps brachii (LHB) and arm rotation on the strain of the superior labrum (anterior and posterior) in 10 fresh frozen cadaveric shoulder joints were studied. Loads were applied to the rotator cuff muscles to stabilize the humeral head. The strain of the anterior and posterior portions of the labrum with the biceps loaded with 0.42 kg, 1.36 kg, and 2.31 kg were measured using linear transducers. The humerus was rotated externally (30, 45, 60, and 90 degrees) and internally (30, 45, and 60 degrees) with the arm elevated 60 degrees at glenohumeral joint (simulated 90 degrees elevation of arm to the trunk). The strain increased with an increase in the weight of the load to LHB and with increase in rotation angle both internally and externally. Since the strain in the posterior portion was larger than that of the anterior portion it seems likely that the labrum, especially the posterior portion, is subject to large strain during biceps loading and arm rotation.