AC Joint Injuries

An AC joint injury refers to a sprain or separation of the connection between the collarbone and shoulder blade, commonly called a “shoulder separation”. This injury is distinct from a shoulder dislocation, which involves the ball-and-socket joint (glenohumeral joint).

When these ligaments are partially or completely torn—often from a direct blow or fall—the joint loses stability, causing the clavicle to rise above its normal position. The severity of an AC joint injury is classified from Grade I to Grade VI, depending on how much displacement and ligament damage has occurred. Lower-grade injuries involve mild sprains, while higher grades may cause visible deformity and require surgical repair.

The most frequent cause of AC joint injury is a direct fall onto the point of the shoulder. This is common in contact and high-impact sports such as football, hockey, rugby, snowboarding, skiing and cycling and martial arts that pose high risks due to collisions and falls. The impact of the fall drives the shoulder blade downward while the clavicle remains fixed, straining or rupturing the supporting ligaments.

Less commonly, AC joint injuries can occur from falls onto an outstretched hand or a heavy lifting incident, particularly among weightlifters and manual laborers. Repetitive stress and chronic overuse, such as from throwing sports or frequent bench pressing can, over time can cause cartilage wear or repeated microtrauma that can lead to AC joint arthritis, reducing joint stability and increasing pain.

Symptoms vary based on the severity of the injury but typically include localized pain and tenderness directly on top of the shoulder. Patients often describe a sharp pain immediately after impact, followed by swelling and difficulty lifting the arm. Other symptoms may include:

• A visible bump or deformity over the top of the shoulder
• Pain when reaching across the body or lifting objects overhead
• Bruising or swelling over the joint
• Limited range of motion
• A clicking or grinding sensation with movement

In mild sprains, pain may be the only symptom, with little to no visible deformity. In moderate to severe injuries, the end of the clavicle may appear elevated or “popped up,” creating a noticeable step-off at the top of the shoulder.

Dr. Collon will review your medical history including any previous shoulder injuries, ask about your symptoms and how the injury occurred, feel the joint for tenderness, and assess the degree of instability or deformity. Diagnostic imaging—typically X-rays—is used to confirm the diagnosis and classify the injury by measuring the space between the clavicle and coracoid process.

In cases where additional shoulder structures may be injured, such as the rotator cuff or labrum, magnetic resonance imaging (MRI) may be performed to provide a more comprehensive.

AC joint injuries are categorized into six grades:

• Grade I: Mild sprain of the acromioclavicular ligaments without joint displacement.
• Grade II: Complete tear of the AC ligaments with partial damage to the coracoclavicular ligaments; mild elevation of the clavicle.
• Grade III: Complete disruption of both ligament groups, resulting in obvious clavicle displacement.
• Grades IV–VI: More severe injuries with significant displacement, often requiring surgical stabilization due to torn muscles and displaced bone.

The grade of injury helps guide treatment, which may range from rest and physical therapy to surgical reconstruction for high-grade separations.