MICROTRAUMATIC GLENOHUMERAL LAXITY: EFFECT OF NUMBER OF CYCLES AND MAGNITUDE OF TORQUE ON EXTERNAL ROTATION IN A CADAVER MODEL

Participants: J.E. Kuhn, L.J. Huston, R.B. Blasier, R.E. Hughes

Keywords: shoulder, microtrauma, instability, athlete

Introduction

Microtraumatic laxity is considered a major source of glenohumeral pathology in the athlete’s shoulder, however scientific studies investigating this form of laxity are lacking. The purpose of this study was to develop a model for microtraumatic laxity in a cadaver shoulder mimicking the late-cocking phase of the throwing motion, employing cyclic loading of the glenohumeral joint with sub-failure external rotation torque. Our hypothesis was that the number of cycles and the magnitude of the torque have an effect on increasing the amount of external rotation in the shoulder. Furthermore, the damage to the capsule was not expected to recover with time.

Methods

Thirty-two cadaver shoulders without obvious disease were dissected retaining the rotator cuff tendons, glenohumeral joint capsule, coracoacromial ligament, humerus and scapula. The scapula of each specimen was rigidly fixed to a custom testing apparatus with the humerus in 60 degrees of glenohumeral abduction in the plane of the scapula. The humerus was mounted to an MTS torsion-testing machine in series with a sliding universal joint. The biceps tendon and rotator cuff tendons (supraspinatus, subscapularis, and combined infraspinatus and teres minor) were each loaded with 5 lb. of force using pneumatic cylinders in order to maintain joint congruity. Five additional specimens were loaded to failure in external rotation and 95% confidence intervals were derived for the torque at failure 158+35 inlbs 95%CI (123, 193). The lower boundary was chosen as the maximal torque (Lmax) for testing. Testing groups consisted of 100% Lmax, 75% Lmax, 50% Lmax, and 25% Lmax. Eight randomly chosen specimens were tested in each group following a similar protocol. All external rotation measures were performed using a nominal torque (10inlbs). For each group, external rotation was measured before the experiment, after 10 cycles, after a 1-hour delay, after 100 cycles, after a 1-hour delay, after 500 cycles, and after a 1-hour delay. During the recovery period, specimens were returned to the starting position. External rotation data was analyzed statistically, and significance will be set at p<0.05.

Results

For the specimens tested to date, both the magnitude of the sub-failure torque and the number of cycles employed resulted in significant increases in the amount of external rotation. The increases did not change appreciably over one hour.