In all of the joints of the shoulder, both the ligamentous and musculo-tendonous connections between bones play irreplacable roles in maintaining shoulder structure and function. Just before the outer end of the clavicle attaches to the scapula at the acromion, two ligaments connect the clavicle to the coracoid process, providing additional attachments between the clavicle and scapula to form a sturdy "roof" for the shoulder. These coracoclavicular ligaments (Figure 6) are crucial for maintaining the alignment of the acromion and clavicle at the acromioclavicular joint. Additional ligaments attach the clavicle to the acromion (acromioclavicular ligament) and the acromion to the coracoid process (coracoacromial ligament), forming a dense web of connective tissues that maintain attachment and alignment of the clavicle and scapula. Injuries to these ligaments result in the condition sometimes referred to as a "separated shoulder".
The glenohumeral joint connecting the head of the humerus to the glenoid fossa of the scapula is notable for its mobility and freedom from constraint. The soft tissues surrounding the glenohumeral joint give it stability due to both the dynamic actions of the muscles plus the relatively static effects of ligaments connecting the joint. They also add to the complexity of the shoulder's structure (Figure 7).
The shoulder joint is totally surrounded by a relatively loose-fitting joint capsule (Figure 7B & 7E) whose area is almost twice the surface area of the humoral head (facilitating joint mobility). It surrounds the glenoid fossa and extends across to just below the humeral head to fully enclose the glenohumeral joint (Figure 7B &7E). Within the capsule, less than a milliliter of joint fluid is normally present, and the pressure within the capsule is negative. Also within the capsule, a tough, fibrous ring of tissue, the glenoid labrum (Figure 8), encircles the glenoid fossa and increases the surface area of the glenoid fossa in contact with the humeral head by 50-75%. The upper edge of the labrum in intimately associated with the tendon of the long head of the biceps muscle (Figure 7D) and also gives rise to the superior glenohumoralligament (Figure 7C & Figure 8), a fold in the front wall of the capsule that connects directly to the humoral head. On the front edge of the glenoid labrum, a second ligament (middle glenohumeral ligament) that also arises from a fold in the anterior capsule wall attaches to the humerus, as does a third ligament (anterior inferior glenohumeral ligament) that attaches the inferior surface of the humeral head to the glenoid labrum (Figure 7C, 7D & Figure 8). The function of these short ligamentous attachments between the labrum and the head of the humerus is to maintain the humoral head in the glenoid fossa and limit glenohumeral joint movement in certain anatomical planes. The ligamentous attachments outside the glenohumeral joint that are crucial to shoulder structure and function are also illustrated in Figure 7F & 7G: the coracoclavicular, acromioclavicular and coracoacromial ligaments (Figure 7F), and the coracohumeral ligaments that cross the glenohumeral joint outside of the joint capsule (Figure 7G).