Glenoid labrum tears related to the long head of the biceps

Abstract

Tears of the glenoid labrum were observed in 73 base ball pitchers and other throwing athletes who under went arthroscopic examination of the dominant shoul der. Most of the tears were located over the anterosu perior portion of the glenoid labrum near the origin of the tendon of the long head of the biceps muscle into the glenoid.

At arthroscopy, the tendon of the long head of the biceps appeared to originate through and be continuous with the superior portion of the glenoid labrum. In many cases it appeared to have pulled the anterosuperior portion of the labrum off the glenoid. This observation was verified at arthroscopy by viewing the origin of the biceps tendon into the glenoid labrum as the muscle was electrically stimulated. With stimulation of the mus cle, the tendinous portion became quite taut, particu larly near its attachment to the glenoid labrum, and actually lifted the labrum off the glenoid.

Three-dimensional high-speed cinematography with computer analysis revealed that the moment acting about the elbow joint to extend the joint through an arc of about 50° was in excess of 600 inch-pounds. The extremely high velocity of elbow extension which is generated must be decelerated through the final 30° of elbow extension. Of the muscles of the arm that provide the large deceleration forces in the follow-through phase of throwing, only the biceps brachii traverses both the elbow joint and the shoulder joint. Additional forces are generated in the biceps tendon in its function as a "shunt" muscle to stabilize the glenohumeral joint during the throwing act.

On the basis of our observations that (1) the biceps tendon is subjected to large forces during throwing, (2) most tears of the glenoid labrum occur at the antero superior portion near the origin of the biceps tendon, and (3) the biceps tendon lifts the labrum off the glenoid when its muscle is stimulated, we believe that the tendon of the long head of the biceps may be a cause of tearing of the glenoid labrum in the throwing athlete.