What is the Acromioclavicular Joint (AC Joint)?
The acromioclavicular joint, otherwise known as the AC joint, is the junction between the acromion (highest point of the shoulder) and the clavicle (collarbone). The AC joint is a main connection between the arm and skeleton, therefore, it is a critical structure to the shoulder. The acromioclavicular joint is quite small and has a limited range of motion, allowing the ability to raise the arm above the head. Dr. Jeff Padalecki, orthopedic shoulder specialist serving patients in Austin, Round Rock, Cedar Park, Texas and surrounding communities, is well versed at treating orthopedic injuries associated with the shoulder AC joint including shoulder separations.
What is the Anatomy of the AC Joint?
Stabilized by three ligaments and two sets of muscles, the AC joint is relatively strong. The acromioclavicular ligament runs horizontally from the acromion to the clavicle. The two coracoclavicular (CC) ligaments connect the coracoid process, a portion of the scapula, to the clavicle. The deltoid muscle and trapezius muscle are the two sets of muscles responsible for assisting with support of the shoulder AC joint.
The AC joint capsule is composed of a loose fibrous layer which encloses the two articular surfaces. As a plane type synovial joint, the joint capsule is lined by a synovial membrane that releases synovial fluid into the joint cavity.
Much like those of the sternoclavicular joint, the articular surfaces of the shoulder AC joint are lined with fibrocartilage. The cavity of the acromioclavicular joint is partially divided by an articular disc, a wedge of fibrocartilage suspended from the upper portion of the capsule.
What is the Function of the AC Joint?
The acromioclavicular joint is classified as a synovial joint. Located where the collarbone meets the shoulder blade, the AC joint allows the ability to raise the arm above the head. It also allows the transmission of forces from the upper arm to the rest of the skeleton. No muscles act directly on the joint so all movement is initiated by movement at other joints, and is considered passive movement.