Other than muscular attachments, the scapula is supported only by the acromioclavicular joint and the coracoclavicular ligaments. Motion between the scapula and the thorax-the scapulothoracic articulation-is an integral part of normal shoulder function. However, a recent cadaveric study found that the average contribution of the labrum to glenohumeral stability through concavity compression was only 10% ( 46). Lippett and co-workers have shown that removal of the labrum decreases the joint’s stability to shear stress by 20% ( 14). Howell and Galinat ( 45) showed that the labrum deepened the glenoid socket by nearly 50%. Several investigators have suggested that the labrum may act as a load-bearing structure for the humeral head and serve to increase the surface area of the glenoid ( 31, 32). The remaining labrum is composed of dense fibrous connective tissue with a small fibrocartilaginous transition zone at the anterior inferior attachment of the osseous glenoid rim ( 38, 43, 44). Huber and Putz ( 42) showed that the tendon fibers of the long head of the biceps continued posteriorly as periarticular fiber bundles, thereby constituting the “labrum” in the posterior superior quadrant. The labrum is located along the periphery of the glenoid and is the site of attachment of the capsuloligamentous structures and the long head of the biceps brachii tendon. However, with repetitive loading of the inferior glenohumeral ligament, the ligament becomes lax with an unrecoverable increase in length of the ligament ( 41). In a cadaveric study simulating a first time anterior shoulder dislocation and Bankart lesion, it was found that there was little unrecoverable stretching of the IGHL ( 40). Furthermore, cutting the anterior band of the IGHL alone caused little increase in external rotation ( 39). In a cadaveric study simulating the late cocking phase of throwing, it was found that cutting the entire IGHL resulted in the greatest increase in external rotation however, this increase in external rotation was not significantly different from sectioning the coracohumeral ligament. The labrum with some fibers extending to the glenoid neck however, in some shoulders, the anterior band has its origin solely on the glenoid neck ( 38). In most people, the anterior band of the IGHL has its origin from The anterior band of the IGHL arises between the 2 o’clock and the 4 o’clock position on the glenoid. However, in atraumatic instability, the head remains decentralized despite muscle activity ( 18).
With traumatic instability, there is increased translation in functionally important arm positions, and dynamic stabilizers are often able to re-center the humeral head.
Glenohumeral joint compression through muscle contraction was also found to be responsible for resisting inferior translation of the humeral head ligament tension and negative intraarticular pressure were less important ( 17).
Likewise, simulated inactivity of the supraspinatus and subscapularis muscles yielded an 18% and 17% decrease, respectively, in the force needed to dislocate the joint ( 16). Labeled “concavity-compression” by Lippett and co-workers ( 14), this maintains anterior joint stability over a large range of shoulder motion ( 13, 14, 15). The orientation of the shoulder muscles to the glenohumeral joint is such that the joint reaction force has a large component that acts perpendicular to the glenoid fossa ( 13) to compress the concave humeral head into the glenoid fossa. A recent animal study supports this theory, demonstrating an increase in cellularity, a loss of normal collagen fiber organization, and a lower modulus of elasticity and maximum stress to failure ( 9). Overuse activity has been thought to be a major contributing factor in the development of supraspinatus tendinopathy. Simulated supraspinatus retraction in a cadaveric study involving one third, two thirds, and the entire tendon resulted in losses of torque measuring 19%, 36%, and 58% respectively ( 8). Paralysis of the suprascapular nerve results in an approximately 50% loss of abduction torque ( 7). Intratendinous strain increases as the shoulder is abducted from 15 to 60 degrees ( 6). The supraspinatus is active during the entire arc of scapular plane abduction. Through the supraspinatus outlet, and inserts on the greater tuberosity. The supraspinatus has its origin on the posterosuperior scapula, superior to the scapular spine. The rotator cuff is composed of four muscles: the supraspinatus, subscapularis, infraspinatus, and teres minor.