Acromioclavicular joint

• Most of the AC joint’s intrinsic stability arises from the coracoclavicular ligament, a structure extrinsic to the joint. Because of its direct connection to the scapula, the coracoclavicular ligament influences scapulohumeral motion.

• This ligament is divided into two distinct portions:

1. The lateral quadrilateral-shaped trapezoid ligament.

2. The medial triangular-shaped conoid ligament.

• Separated by a bursa, the trapezoid ligament limits lateral movement of the clavicle over the acromion. The conoid ligament restricts superior movement of the clavicle. Acting jointly, these ligaments limit rotation of the scapula and provide some degree of horizontal stability.

• The conoid portion of the ligament is critical for the passive posterior rotation of the clavicle that occurs during shoulder elevation. A horizontal dislocation of the AC joint can occur with the coracoclavicular ligament remaining intact.