Along with the sternoclavicular joint, the AC joint provides the only articulation between the upper extremity and the axial skeleton. This joint dissipates forces placed on the upper extremity and the axial skeleton. It is diarthrodial, consisting of the medial aspect of the acromion and the lateral aspect of the clavicle. Its unique anatomy allows movement between the clavicle and the scapula. During its development, the ends of the clavicle and acromion are fibrocartilage, creating an intra-artic-ular disk that is subject to degeneration with use and aging. The distal end of the clavicle does not ossify until around the age of 19, allowing for the possibility of a periosteal sleeve fracture in younger patients.
The stability of the joint is provided by two major ligamen-tous structures (Fig. 26-1). The AC ligaments are critical for anteroposterior stability.1 The coracoclavicular (CC) ligament provides restraint of superior translation of the distal clavicle. The CC ligament consists of the trapezoid ligament and the conoid ligament. The trapezoid ligament is attached anterolat-erally to the conoid ligament on the coracoid process, extending superiorly to the inferior aspect of the clavicle. The conoid ligament extends from the posteromedial margin of the coracoid process to the posterior inferior clavicular curve on the conoid tubercle.
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