Abstract
The subscapularis muscle is an important mover and stabilizer of the glenohumeral joint. The purpose of this study was to measure regional variations in the structural properties of the subscapularis tendon in two joint positions. Subscapularis tendons from cadaveric shoulders were divided into four sections superiorly to inferiorly and tested to failure at 0 or 60Β° of glenohumeral abduction. Arm position had a significant influence on stiffness in the inferior and superior portions (p < 0.05). The inferior region showed a higher stiffness in the hangingβarm position (0Β°) than at 60Β° of abduction (27.4 Β± 17.7 compared with 9.5 Β± 5.9 N/mm). Meanwhile, stiffness of the superior portion was higher at 60Β° of abduction than in the hangingβarm position (208.7 Β± 60.9 compared with 147.2 Β± 32.3 N/mm). In the hangingβarm position (0Β°) and at 60Β° of abduction, the superior and midsuperior portions failed at significantly higher loads (superior: 623.2 Β± 198.6 and 478.2 Β± 206.6 N at 0 and 60Β° of abduction, respectively; midsuperior: 706.2 Β± 164.6 and 598.4 Β± 268.4 N, respectively) than did the inferior portion (75.1 Β± 54.2 and 30.3 Β± 13.0 N, respectively). Likewise, stiffness of the superior and midsuperior portions was significantly higher than that of the inferior region in both positions. Higher stiffness and ultimate load in the superior tendon region may explain the infrequent extension of rotator cuff tears into the subscapularis tendon. Conversely, the significantly lower ultimate load and stiffness in the inferior tendon region could facilitate anterior dislocation of the humeral head when this portion stabilizes the joint in a dislocated position. Therefore, repair of torn inferior portions of the subscapularis tendon should be considered in surgery for glenohumeral instability.