The acromioclavicular ligament is a band of fibrous connective tissue linking the acromion of the scapula to the clavicle or collar bone at the acromioclavicular (AC) joint. Actually two ligaments known as the superior acromioclavicular ligament and inferior acromioclavicular ligament, it encapsulates and stabilizes the joint as well as holds the bones together. It is this stabilization that makes it possible to lift the arms above the head.
A synovial gliding joint known as a planar joint, the AC joint is characterized by the flat surfaces of the adjacent bones sliding past each other. Here, the lateral or outside end of the long and narrow clavicle adjoins with the acromion, a club-shaped bony projection off the shoulder blade found at the top of the shoulder. These two surfaces are held together above and below the joint by the acromioclavicular ligament, which allows the bones to slide slightly against one another as the arm is lifted overhead or rotated inside of the glenohumeral or shoulder joint.
The acromioclavicular ligament is divided into two sections. As its name suggests, the superior acromioclavicular ligament covers the top of the joint, stretching from the upper outermost surface of the clavicle to the upper innermost surface of the acromion. Likewise, the inferior acromioclavicular ligament runs across the underside of the joint from the lower outside aspect of the clavicle to the lower inside aspect of the acromion.
With fibers running lengthwise between the two bones, the superior acromioclavicular ligament is larger and broader than the inferior ligament. It is continuous on its upper surface with the aponeuroses of the trapezius and deltoid muscles of the upper back and shoulder, respectively. An aponeurosis is the white tendon-like casing that envelops each muscle and attaches it to the joint it crosses, made of a fibrous substance not unlike that of a ligament. Below the superior ligament is the joint capsule, which contains the synovial fluid and disk that lubricate the joint and cushion the adjacent bony surfaces.
The inferior acromioclavicular ligament is found just below the joint capsule, separated from it by a double membrane. Beneath the inferior ligament is the supraspinatus tendon, running horizontally under the acromion process and over the top of the humerus bone, where it attaches in the upper arm. The supraspinatus is the uppermost muscle of the rotator cuff and is involved in raising the arm above the head while also stabilizing the shoulder joint.