What is the AC Joint?
The AC joint, or acromioclavicular joint, involves the joining of the clavicle -the collarbone- and the acromion, the highest part of the shoulder blade. The AC joint stabilizes the shoulder and allows for motions such as moving the arm away from the body as well as raising the arm above the head. Cartilage lubricates the joint and allows smooth movement of the bones, and three ligaments stabilize the joint. The acromioclavicular ligament connects the clavicle to the acromion, and the coracoclavicular ligaments (CC) connect the underside of the clavicle to the shoulder blade.
Injuries to the AC Joint:
Common conditions that affect the AC joint include but are not limited to arthritis and sprains or separations. Injuries such as AC joint sprains and separations commonly result from a blunt force sustained to the side of the shoulder or after falling onto the outstretched hand. Results of injury may range from mild, such as a sprain to one of the joint ligaments, to severe where both the AC and CC ligaments are torn. The severity of the injury is classified as I to VI, with VI being the most severe:
I: the AC ligament is sprained (stretched but still intact) and the CC ligament is intact. The collarbone is in place.
II: the AC ligament is torn and the CC ligament is sprained. The collarbone is slightly elevated or out of alignment but is not above the acromion or shoulder blade.
III: both the AC and CC ligaments are torn. The collarbone is above the shoulder.
IV-VI: both the AC and CC ligaments are torn, and the collarbone is displaced into the back muscle (IV), the collarbone is displaced upwards by over 25% (V), or the collarbone is displaced below the acromion (VI).
Symptoms of AC Joint Injuries:
Patients who have sustained an AC joint injury may complain of pain over the AC joint that is made worse with the movement of the shoulder or arm. There may be swelling, bruising, and tenderness to the touch over the affected area. A small bump may be visible at the top of the shoulder, while more serious injuries will display a larger one. The shoulder may appear to droop.
How AC Joint Injuries are Diagnosed:
Physicians may look for the “piano key sign”, which is determined by applying pressure to the end of the collarbone, pushing it downward. If the collarbone springs upwards, with the release of pressure, the test will be indicative of an injury. Tests that physicians may use include the following:
The cross-arm test: the patient crosses the affected arm across the body.
The shear test: the physician simultaneously presses the scapula from the back of the shoulder with one hand and presses the collarbone from the front of the shoulder with another hand.
Pain while performing any of these tests may be indicative of an AC joint injury. The joint will also be radiographically evaluated using x-rays, followed by an ultrasound or MRI test if there is uncertainty.
Treatments vary depending on the severity of the injury. Low grade injuries are more common than high grade, or severe, injuries. While injuries grade I-III may be treated conservatively, injuries grades IV-VI will likely require surgical treatment. Grade I, II, and III injuries are treated with rest, an arm sling, icing the affected area, taking pain medications, physical therapy, and a gradual return to normal activities. Individuals will regain their range of motion by 6 weeks and can resume normal activities by 12 weeks. Grade III injuries may require surgical intervention depending on the occupation of the patient. Surgical treatment of the more severe injuries may involve reconstruction of the CC ligaments, or insertion of a metal plate to realign the AC joint. Unlike conservative treatment, surgical treatments require a longer recovery period (6 weeks of immobilization followed by a 6 month recovery period of a gradual return to regular activities).
- Wong M, Kiel J. Anatomy, Shoulder and Upper Limb, Acromioclavicular Joint. [Updated 2021 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499858/
- Kiel J, Kaiser K. Acromioclavicular Joint Injury. [Updated 2021 Aug 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493188/