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Acromioclavicular Joint – AC Joint Conditions

The acromioclavicular joint connects the shoulder blade to the collarbone. Injuries or conditions affecting this joint cause shoulder pain and restricted movement.

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The acromioclavicular joint connects the shoulder blade to the collarbone. Injuries or conditions affecting this joint cause shoulder pain and restricted movement.

What is the Acromioclavicular Joint?

The acromioclavicular joint (commonly abbreviated as the AC joint) is a small but functionally important joint in the shoulder region. It connects the acromion -- a bony projection of the shoulder blade (scapula) -- with the outer end of the collarbone (clavicle). This joint plays a key role in the mobility and stability of the entire shoulder, particularly when lifting the arm above shoulder height.

Anatomy and Function

The AC joint is a plane joint with limited intrinsic movement, but it efficiently transfers forces between the arm and the trunk. It is stabilized by several ligaments:

Acromioclavicular ligament: a direct connection between the acromion and the clavicle
Coracoclavicular ligaments (conoid and trapezoid ligaments): connect the coracoid process of the scapula to the clavicle, providing vertical stability

An intra-articular disc may be present within the joint, helping to absorb mechanical stress.

Common Conditions and Injuries

Acromioclavicular Joint Separation

The most common injury to the AC joint is an AC joint separation, also called a shoulder separation. It typically results from a direct fall onto the shoulder, such as during sports or after an accident. Injuries are classified according to the Rockwood classification into six grades (Type I–VI):

Type I: sprain without ligament rupture
Type II: partial tear of the acromioclavicular ligament
Type III: complete rupture of both ligament systems with superior displacement of the clavicle
Types IV–VI: severe displacements with associated soft tissue injuries

Acromioclavicular Osteoarthritis

Over time, the AC joint can degenerate through wear and tear. AC joint osteoarthritis presents as chronic shoulder pain, especially during overhead movements or when crossing the arm across the body. It commonly affects older individuals and athletes with long-term repetitive loading, such as weightlifters.

Shoulder Impingement

A narrowed or arthritic AC joint can contribute to shoulder impingement syndrome, in which tendons of the rotator cuff become compressed. This leads to pain when lifting the arm.

Symptoms

Pain at the top of the shoulder (over the AC joint)
Tenderness directly over the joint on palpation
Visible or palpable elevation of the collarbone
Restricted shoulder range of motion, especially when raising the arm
Swelling or bruising following an acute injury

Diagnosis

Diagnosis is confirmed through clinical examination and imaging studies:

X-ray: the standard method to assess joint space, clavicular elevation, and bony changes; often performed as a stress view with weight held in the hand
Ultrasound: useful for evaluating ligaments and soft tissue structures
MRI (Magnetic Resonance Imaging): indicated when findings are unclear or when associated rotator cuff injuries are suspected

Treatment

Conservative Treatment

Type I and II injuries and mild osteoarthritis are generally managed conservatively:

Immobilization using a sling or shoulder harness for several days
Pain management with non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen or diclofenac
Ice application in the acute phase
Physiotherapy to strengthen the shoulder musculature and restore range of motion
Corticosteroid injection into the joint for persistent arthritic pain

Surgical Treatment

More severe injuries (Types III–VI) or treatment-resistant osteoarthritis may require surgery:

Ligament reconstruction: restoration of ligament stability, often performed arthroscopically or via minimally invasive techniques
Hook plate fixation: temporary stabilization of the clavicle with a metal plate
Lateral clavicle resection: removal of the outer end of the clavicle to eliminate joint friction in cases of arthritis

Prognosis and Outlook

Minor AC joint injuries generally heal completely with appropriate care. Most patients can return to full activity within weeks to months. Severe injuries left untreated may result in chronic pain and lasting functional impairment.

References

Rockwood CA, Williams GR, Young DC. Disorders of the acromioclavicular joint. In: Rockwood CA, Matsen FA (eds.): The Shoulder. Saunders, Philadelphia 1998.
Mazzocca AD, Arciero RA, Bicos J. Evaluation and treatment of acromioclavicular joint injuries. Am J Sports Med. 2007;35(2):316-329. PubMed PMID: 17251175.
Warth RJ, Martetschlaeger F, Gaskill TR, Millett PJ. Acromioclavicular joint separation. Curr Rev Musculoskelet Med. 2013;6(1):71-78. doi:10.1007/s12178-012-9144-9.

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