The Acromioclavicular Ligament

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The acromioclavicular ligament is a band of fibrous connective tissue that connects the acromion of the scapula to the lateral end of the clavicle. It plays an important role in the function of the shoulder by holding the acromioclavicular joint together. The strength and flexibility of the acromioclavicular ligament are especially important when reaching up in the air and shrugging one’s shoulders.

Anatomy

The acromioclavicular ligament is a ligament of the shoulder that surrounds the acromioclavicular (AC) joint between the acromion on the top of the scapula and the clavicle, or collarbone. It is divided into two parts: the superior acromioclavicular ligament and the inferior acromioclavicular ligament. Both parts of the ligament are made of dense regular fibrous connective tissue containing a parallel bundle of strong collagen fibers and the fibroblast cells that produce the fibers.

The larger of the two parts — the superior acromioclavicular ligament — arises from the superior surface of the lateral end of the clavicle. It crosses the acromioclavicular joint superior to the joint capsule and inserts on the medial side of the acromion. The superior acromioclavicular ligament is roughly rectangular, with its fibers arranged to run horizontally across the joint. Tendon fibers of the trapezius and the deltoid muscles fuse into the edges of the ligament.

The inferior acromioclavicular ligament arises from the inferior surface of the clavicle. It passes under the joint capsule of the acromioclavicular joint to insert on the inferior surface of the acromion. Overall, it is both narrower and thinner than the superior acromioclavicular ligament.

Physiology

The acromioclavicular ligament plays an essential role in the shoulder by reinforcing and aligning the acromioclavicular joint. Gliding movements of the scapula relative to the clavicle at the planar AC joint allow the entire shoulder to elevate, depress, and rotate. Elevation and depression allow us to shrug our shoulders, which is useful for lifting heavy weights. The shoulder rotation is critical as it greatly increases the vertical range of motion when abducting the arm at the shoulder. By itself, the glenohumeral portion of the shoulder (between the humerus and glenoid cavity of the scapula) only permits the arm to abduct to the horizontal position; further abduction requires the rotation of the scapula at the AC joint.

Shoulder separation — another name for dislocation of the AC joint — is a common stress injury resulting from contact sports and falls. People often land on their shoulder or extend their arm to break their fall, resulting in energy transfer from the falling body weight to the shoulder. The sudden impact pushes the arm forcefully toward the trunk, elevating the clavicle, dislocating the AC joint, and straining or tearing the acromioclavicular ligament.