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External rotation of the glenohumeral joint is important in a variety of pathologic states, yet the ligamentous restraints to external rotation have not been thoroughly investigated and the muscle effects have received even less attention. The purpose of this study was to investigate the ligamentous restraints and muscle effects limiting external rotation of the glenohumeral joint in a biomechanical cadaveric model. External rotation torque was applied to the humeri of 15 shoulders mounted in the supine position in a custom fixture while varying rotator cuff and biceps loads in 15 degrees and 60 degrees of glenohumeral abduction. A randomly chosen ligament (coracohumeral ligament [CHL], combined superior and middle glenohumeral ligaments [S+M], anterior band of the inferior glenohumeral ligament [AB], entire inferior glenohumeral ligament [IGHL], or posterior capsule [PC]) was cut, and testing was repeated (n = 3 for each ligament). Torque data were collected every 3 degrees through the entire range of external rotation. Individual muscles were loaded with 22.2 N (designated as a standard state) and were compared with loads of 0, 11.1, and 44.5 N. Alterations in muscle loads were analyzed with a residual maximum likelihood-based repeated-measures model. Ligament effects were analyzed by use of analysis of variance with Tukey correction. In the neutral position, each ligament except the PC significantly affected the torque required for external rotation, with IGHLCHLABS+M. In this position, loading the subscapularis to 44.5 N significantly increased the torque required to externally rotate the shoulder whereas unloading it significantly decreased the torque required. In the 60 degrees abducted position, each ligament except the PC significantly affected the torque required for external rotation, with IGHLABS+MCHL. In this position, loading the biceps or subscapularis to 44.5 N significantly increased the torque required to externally rotate the shoulder as rotation increased whereas unloading it significantly decreased the torque required as rotation increased. This work demonstrates that the glenohumeral joint capsule behaves as a cylinder with many regions serving as restraints to external rotation. In addition, the long head of the biceps has an important role as a dynamic restraint to external rotation in the abducted shoulder. This is a new function attributed to the biceps and reinforces the role of external rotation in the generation of biceps and superior labral pathology.

This experiment was conducted to determine the primary ligamentous restraints to anterior and posterior translation of the sternoclavicular joint. Twenty-four unpaired cadaver specimens were mounted in a custom fixture. Anterior and posterior translations were measured under a sub-failure load in the intact specimen and again after transecting one randomly chosen ligament (anterior capsule, posterior capsule, interclavicular ligament, and costoclavicular ligament; n = 6 for each group). Cutting the posterior capsule resulted in significant increases in anterior translation and posterior translation. Cutting the anterior capsule produced significant increases in anterior translation. Cutting the costoclavicular and interclavicular ligaments had little effect on sternoclavicular joint translation. The posterior capsule is the most important restraint for anterior and posterior translation of the sternoclavicular joint. The anterior capsule is another important restraint for anterior translation. The costoclavicular and interclavicular ligaments have little effect on anterior or posterior translation of the sternoclavicular joint.

Lesions of the superior glenoid labrum are a source of shoulder disease. However, the mechanisms of injury to this region are unknown, and controversy exists regarding the role of shoulder instability in creation of this lesion. With a cadaver model that simulates physiologic rotator cuff forces and produces traction on the biceps tendon, the creation of type II superior labrum, anterior, and posterior (SLAP) lesions and the role of glenohumeral subluxation were investigated: Left and right shoulders from each of 8 paired cadavers (age 62 +/- 7.2 years, 5 male and 3 female) were randomized to be tested in either a 20 mm inferiorly subluxed position or in a reduced position. The long head of the biceps tendon was held near the musculotendinous junction with a novel cryogenic clamp. Traction on the long head of the biceps tendon was applied at a fast rate of 12.7 cm/sec with a servohydraulic testing machine. A load cell was used to monitor the biceps tendon load. After testing to failure, the presence or absence of a type II SLAP lesion was determined by 2 experienced shoulder surgeons masked to the test group. The production of type II SLAP lesions differed significantly (P = .03) between reduced shoulders (2 SLAP lesions out of 8 tests) and the shoulders with inferior subluxation (7 SLAP lesions out of 8 tests). This experiment has shown that traction on the biceps tendon in this biomechanical model can reproducibly create type II SLAP lesions, and inferior subluxation facilitates the generation of such lesions.