FAILURE OF BICEPS-SUPERIOR LABRAL COMPLEX IN THE THROWING ATHLETE: A BIOMECHANICAL MODEL COMPARING MAXIMAL COCKING TO EARLY DECELERATION

Participants: J.E. Kuhn, S.R. Lindholm, L.J. Huston, L.J. Soslowsky, R.B. Blasier

Keywords: shoulder, ligaments, biomechanical model

Introduction

Lesions of the biceps anchor and superior labrum are thought to occur during the early deceleration phase of throwing. Recent observations suggest that maximal cocking may contribute to their development as well. The purpose of this experiment was to evaluate the potential for biceps anchor and superior labral lesions to develop by comparing these positions using a cadaveric biomechanical model.

Materials and Methods

Ten paired cadaver shoulders (age 55.4 + 21 years; 5 male and 5 female) were dissected leaving the rotator cuff tendons, long head of biceps tendon, glenohumeral ligaments and capsule, coracohumeral ligament, coracoacromial ligament, humerus, and scapula intact. Specimens were mounted in a custom testing apparatus in 60° of glenohumeral abduction with the humerus in the plane of the scapula. Using an MTS torsion testing machine, specimens underwent preconditioning with 100 cycles of sub-failure external rotation torque (7.9 Nm) with loads of 31N applied to the subscapularis, supraspinatus, external rotators, and long head of the biceps tendon. One of each pair of specimens was randomly assigned to be tested in a maximal cocking position for throwing (in excess of 125° of external rotation, with the arm at 60° glenohumeral abduction, and in the plane of the scapula). The other of the pair was tested in a position of early deceleration (10° of internal rotation, 60° of glenohumeral abduction, and 16° of horizontal adduction). In these positions, 31N of load were applied to the rotator cuff tendons, and the position of the humeral shaft was fixed. The long head of the biceps was held with a cryogenic clamp which was connected to the MTS load cell via a cable and pulley. The long head of the biceps was loaded to failure at a rate of 12.7 cm/second. Specimens were then dissected and the presence of a Type II SLAP lesion was independently identified by two experienced shoulder surgeons blinded to the test protocol. The load to failure was compared between the two test positions using a paired t-test. Differences in the mode of failure between the two test positions were analyzed with a two-tailed binomial test.

Results

The biceps labral complex failed under significantly less force for the maximal cocking position (289+39N) than for the early deceleration position (346+40N, p=0.004). Failure of the biceps occurred at the superior glenoid in 9/10 specimens in maximal cocking and only 2/10 in early deceleration (p=0.11). Type II SLAP lesions were created in 5/20 specimens overall, with 4 in the maximal cocking position and 1 in the early deceleration position.

Conclusions

This data suggests that maximal cocking may be an important position for the development of lesions of the biceps-superior labral complex.