Normal Anatomy

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The shoulder is composed of two articulations: the glenohumeral joint and the acromioclavicular (AC) joint [2]. Glenohumeral articulation is maintained by the joint capsule, glenohumeral ligaments, rotator cuff musculature, and labrum. The labrum is a ring of fibrocartilage that is adherent to the glenoid rim. The intact labrum increases the concavity of the bony glenoid and the superior labrum serves as the anchor for the long head of the biceps tendon. The joint capsule may insert variably on the periphery of the labrum or on the neck of the scapula [3]. Distally, the capsule inserts on the anatomic neck of the humerus.

The glenohumeral ligaments are cordlike thickenings in the anterior and inferior joint capsule. They include the superior, middle, and inferior glenohum-eral ligaments. The superior and middle glenohumeral ligaments attach to the anterior labrum. The inferior glenohumeral ligament has anterior and posterior bands that attach to the anterior inferior and posterior inferior labrum, respectively. The size of glenohumeral ligaments varies from patient to patient.

The rotator cuff is comprised of tendons from the supraspinatus, infraspina-tus, teres minor, and subscapularis muscles. The supraspinatus, infraspinatus, and teres minor muscles arise from the posterior surface of the scapula, cross posterior to the humeral head, and insert on the greater tuberosity. The supra-spinatus insertion is most superior and the teres minor insertion most inferior on the tuberosity. The infraspinatus and teres minor tendons may appear fused, and a separate teres minor tendon may not be seen [4].

The subscapularis muscle arises from the anterior surface of the scapula, crosses anterior to the humeral head, and inserts on the lesser tuberosity. The deep fibers of the subscapularis tendon blend with the transverse humeral ligament across the bicipital groove and help maintain the normal position of the biceps tendon.

The supraspinatus and teres minor muscles have single muscle bellies and tendons. The subscapularis and infraspinatus are made up of multiple muscle bellies and small tendons that coalesce to form common tendon insertions.

The rotator cuff interval is the space between the supraspinatus and subsca-pularis tendons along the anterior superior humeral head. Through this space run the intracapsular portion of the biceps tendon, coracohumeral ligament, and the superior glenohumeral ligament.

On their course to their insertion sites on the humeral head the rotator cuff tendons pass under the coracoacromial arch and AC joint. The coracoacromial arch is made up of the coracoid process, coracoacromial ligament and the acro-mion. Hypertrophic abnormalities of the AC joint or arch structures may cause mechanical impingement on the underlying rotator cuff muscle or tendon, particularly the supraspinatus tendon.

Interposed between the coracoacromial arch and supraspinatus tendon lies the subacromial-subdeltoid bursa, which normally does not contain fluid. Fluid may be seen within the bursa when there is bursitis or when fluid leaks into it from the glenohumeral joint through a full-thickness cuff tear.

Because of normal openings in the joint capsule, the glenohumeral joint is in communication with the subscapular recess (beneath the subscapularis muscle) and the long head of the biceps tendon sheath. When a joint effusion is present fluid often is seen in the recess or tendon sheath and does not have pathologic significance.

The AC joint is a synovial joint surrounded by a fibrous capsule. This capsule is reinforced by fibers of the AC ligament. The coracoacromial and cora-coclavicular ligaments also are important in maintaining normal position of the clavicle and acromial process. Tearing of these ligaments results in various degrees of AC joint separation.

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