AC Joint Injuries

The acromioclavicular joint is the small joint at the top of the shoulder where the outer end of the collarbone meets the acromion, the bony projection of the shoulder blade. It is roughly the size of a fingertip, and it is the only true bony connection between the arm and the rest of the skeleton.

Stability comes from two separate sets of ligaments, and the difference between them explains how these injuries are graded and treated. The acromioclavicular ligaments surround the joint itself and control horizontal stability, preventing the collarbone from shifting forward and backward. The coracoclavicular (CC) ligaments, the conoid and the trapezoid, run from the underside of the collarbone down to the coracoid process, and they control vertical stability, suspending the shoulder blade and arm from the collarbone. Overlying both is the deltotrapezial fascia, the sleeve of tissue into which the deltoid and trapezius insert, which contributes meaningfully to stability and to healing.

An AC joint injury, commonly called a shoulder separation, is a disruption of some or all of these ligaments. The collarbone stays where it is; the shoulder blade and the arm drop away from it under the weight of the limb, which is what produces the visible step at the top of the shoulder.

Injuries are classified into six types, and the classification drives treatment directly:

• Type I: A sprain of the AC ligaments without tearing. The joint is tender but stable, and there is no deformity.
• Type II: The AC ligaments are torn while the coracoclavicular ligaments remain intact but sprained or partially torn. There is some widening of the joint and a small step, with the collarbone remaining partially in contact.
• Type III: Both the AC and coracoclavicular ligaments are torn, with a clear step deformity as the shoulder blade drops. The deltotrapezial fascia remains largely intact. This is the type where treatment is genuinely debated, and where most of the clinical judgment lies.
• Type IV: The collarbone is displaced backward through the trapezius muscle. This is unstable and requires surgery.
• Type V: A more severe vertical displacement, with the collarbone markedly elevated and the deltotrapezial fascia stripped off. This requires surgery.
• Type VI: A rare pattern in which the collarbone is displaced downward beneath the coracoid. This requires surgery.

• A direct fall onto the point of the shoulder: The dominant mechanism, driving the acromion downward while the collarbone is held in place by its attachment to the chest.
• Contact and collision sport: Football, rugby, hockey, and lacrosse, where these are among the most common shoulder injuries sustained.
• Cycling and skiing crashes: Going over the handlebars or falling onto the shoulder at speed.
• High-energy trauma: Motor vehicle collisions, which more often produce the higher-grade injuries.

• Immediate pain at the top of the shoulder: Sharply localized over the joint, and patients can usually point to it with one finger.
• A visible bump or step: A prominence at the top of the shoulder, which reflects the shoulder blade dropping rather than the collarbone rising. The size of the step corresponds broadly to the grade.
• Pain with reaching across the body: Bringing the arm across the chest compresses the joint and reproduces the pain.
• Pain lifting the arm overhead: Particularly in the last part of the range.
• A sense of the arm feeling heavy: Common in higher-grade injuries, where the arm is no longer properly suspended.
• Pain with pressing and lifting: Bench press, push-ups, and dips are frequently limited for weeks.

Diagnosis begins with a thorough evaluation by Dr. Rynecki Baker. The history establishes the mechanism and the direction of the force. Examination assesses the size and direction of the step, tenderness over the joint and over the coracoclavicular interval, and, importantly, whether the collarbone can be reduced by supporting the elbow and lifting the arm. That last finding helps separate a type III, which reduces, from a type V, which does not. She also examines for horizontal instability by moving the collarbone forward and backward, which is increasingly recognized as a factor in which type III injuries do poorly without surgery.

X-rays are the mainstay of grading. A dedicated view of the AC joint, angled to avoid overlap with the rest of the shoulder, demonstrates the joint. Comparison views of both shoulders on a single film allow the coracoclavicular distance to be measured against the uninjured side, which is how types III and V are distinguished objectively. An axillary view is essential to identify a type IV, where the collarbone is displaced backward and the deformity is not apparent on a standard front-to-back film. MRI is not required for most injuries. It is useful when the grade is unclear, when a concurrent rotator cuff or labral injury is suspected, and in higher-energy injuries.

The consequential decisions here are made early. A type V mistaken for a type III is treated in a sling for six weeks and presents later as a chronic separation requiring reconstruction with a graft rather than a more straightforward repair. A type IV goes unrecognized because no axillary view was obtained. And at the other extreme, patients with type III injuries are sometimes steered toward surgery they do not need, when the evidence supports treating many of them without an operation.

Dr. Rynecki Baker’s fellowship training at NYU Langone Health and Hospital for Special Surgery, her dedicated focus on shoulder surgery and sports medicine, and her published research in shoulder surgery mean patients are evaluated by a specialist who grades the injury properly, obtains the views that actually distinguish the types, and is candid that a visible bump is a cosmetic finding rather than a reason to operate.