Superior labral strain during the throwing motion. A cadaveric study.

We studied the strain on the superior labrum of 10 fresh-frozen cadaveric shoulders with the arm in simulated positions of a pitching motion. We used linear transducers to measure the strain in both the anterior and posterior superior labrum with the arm in various planes and rotations simulating the motions of pitching: early cocking, late cocking, acceleration, deceleration, and follow-through. Predetermined loads, according to the percent of maximum voluntary contraction of the biceps muscle during each phase of pitching, were calculated and applied to the long head of the biceps tendon using a spring device. Only during the late cocking phase, when the arm was in maximal external rotation, was the increase in strain statistically significant for the anterior and posterior portions and the strain on the posterior portion significantly greater than that on the anterior portion of the labrum. The increased strain in the posterior portion may be due to the anatomic orientation of the long head of the biceps tendon at the superior labrum. The increased strain in the late cocking phase may contribute to the detachment of the labrum with the eccentric contraction of the biceps muscle that occurs with rapid extension of the elbow.