acromioclavicular

The acromioclavicular joint, often abbreviated AC joint, is the articulation between the lateral end of the clavicle and the acromion of the scapula. It is a diarthrodial plane synovial joint that permits small gliding and rotational movements, contributing to the overall range of motion of the shoulder as the arm moves.

Anatomy and ligaments: The joint surfaces involve the clavicle’s lateral end and the acromion’s inferior surface. The joint capsule is relatively thin and reinforced by intrinsic ligaments, including the acromioclavicular ligaments (superior and inferior) spanning the clavicle to the acromion. The primary stabilizers are the coracoclavicular ligaments—the conoid and trapezoid—which extend from the coracoid process to the clavicle and provide most of the vertical stability. The coracoacromial ligament forms part of the coracoacromial arch but is not a direct stabilizer of the AC joint.

Function: The AC joint allows small translational and rotational movements that facilitate scapular rotation during arm elevation and help stabilize the shoulder girdle during overhead activities.

Clinical significance: Trauma to the AC joint is common in contact sports and can lead to acromioclavicular separation. Injuries are commonly classified by the Rockwood system, which ranges from simple sprains to ligament disruptions with clavicular displacement (types I–VI). Evaluation includes clinical history, physical tests such as the piano key sign and cross-body adduction test, and radiographs (often with dedicated views). Treatment is typically nonoperative for lower-grade injuries, while higher-grade injuries or those with persistent instability may require surgical reconstruction or fixation. Imaging and management strategies aim to restore alignment and function while minimizing stiffness.