What Is an AC Joint?

The AC joint, also called the acromioclavicular joint, is the point at which the clavicle (collarbone) and scapula (shoulder blade) connect.

Anatomy

• The scapula is a mostly flat bone that has a projection on its outer upper point called the acromion, which you can feel if you place your fingers on the outside point of the top of the shoulder. The acromion is connected to the clavicle by ligaments. That's the AC joint.

Function

• When you move your scapula (for example, shrugging or raising your arm overhead), the AC joint lets the clavicle glide and rotate on the scapula.

AC Joint Injury

• A direct fall onto the shoulder or falling on an outstretched arm often injures the AC joint. When the ligaments holding the two bones together are torn, the scapula separates from the clavicle because of the weight of the arm. This is called a separated or dislocated shoulder.

Types of Injury

• AC joint injuries are classified from Type 1 (mild) to Type 6 (worst). According the Wake Forest School of Medicine, Types 1 and 2 usually respond to rest, ice, analgesics and rehab exercises. Types 4 to 6 require surgery, and there is a difference of opinion among experts on how to treat Type 3.

Arthritis

• The AC joint often develops overuse arthritis in people who do repetitive overhead work or do a lot of overhead lifting (such as weightlifters).