Superior Labral Strain during the Throwing Motion

Abstract

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.

Electromyographic analysis of physical examination tests for type II superior labrum anterior-posterior lesions

“…Many investigators suspect that this lesion may develop through traction on the long head of the biceps. 1,3,5,12,13,20,25,26,30,32 This traction on the biceps likely has an active component (muscle firing) 26,32 and passive component (positioning) 3,5,13 that contribute to the pathogenesis of SLAP lesions.Several physical examination tests have been described for diagnosing SLAP lesions. 2,7,9,10,14,18,21,[28][29][30][31] They can be classified into 2 general categories: those that place a tensile force on the biceps tendon and those that compress the labrum between the humeral head and glenoid.…”

“…Many investigators suspect that this lesion may develop through traction on the long head of the biceps. 1,3,5,12,13,20,25,26,30,32 This traction on the biceps likely has an active component (muscle firing) 26,32 and passive component (positioning) 3,5,13 that contribute to the pathogenesis of SLAP lesions.…”

Current Concepts in the Recognition and Treatment of Superior Labral (SLAP) Lesions

“…They suggest that when the shoulder is placed in a position of abduction and maximal external rotation (ER), the rotation produces a twist at the base of the biceps, transmitting torsional force to the anchor (Figure 2). Pradham et al 41 recently measured superior labral strain in a cadaveric model during each phase of the throwing motion. They noted that increased superior labral strain occurred during the late cocking phase of throwing.…”

Development and Validation of a Finite Element Model of the Superior Glenoid Labrum

“…Strains in the superior labrum have been investigated under conditions that are experienced during throwing. 30,34 In these laboratory studies, labral strains were measured using differential variable reluctance transducers (DVRTs) that were affixed to the superior labrum at the anterior and posterior attachments of the LHB tendon.…”

“…Pradhan et al 30 found that the posterosuperior labrum experience significantly greater strains than the anterosuperior labrum. Rizio et al used a similar experimental model to Pradhan et al but induced instability by creating a labral lesion.…”