AC JOINT INJURIES I INSTABILITY

Acromioclavicular joint (AC joint): Causes, separation, arthrosis and modern treatment options

What is the acromioclavicular joint?

The acromioclavicular joint (AC joint) connects the collarbone to the acromion. It is the only bony connection between the arm and the rib cage and must therefore withstand enormous loads . It is stabilized by a rigid joint capsule and strong ligaments.

In everyday life, the AC joint plays a rather inconspicuous role – until it is injured after a fall or becomes painful due to osteoarthritis .

What causes a fracture in the AC joint?

An AC joint dislocation often occurs as a result of direct impact trauma, such as a fall onto the shoulder while cycling, playing soccer, or skiing . This can cause the joint capsule and the coracoclavicular ligaments (CC ligaments) to become overstretched or torn.

Depending on the extent of the injury, different types of Rockwood tendon fractures are distinguished . While mild forms (types I and II) usually heal without surgery, severe fractures (types III–VI) often require surgical stabilization.

What is a shoulder joint dislocation?

An acromioclavicular joint dislocation is an injury to the AC joint in which the stabilizing ligaments tear. This causes the collarbone to protrude upward, resulting in shoulder instability. Depending on the severity, different Rockwood types are distinguished, ranging from a mild strain to a complete dislocation.

Symptoms of a shoulder joint dislocation

Typical signs are:

• Acute pain directly above the joint

• Swelling and bruising

• Pressure pain at the lateral end of the clavicle

• Instability or displacement of the clavicle

• Limited mobility and loss of strength

In the case of a complete rupture, the collarbone protrudes upwards – the so-called piano key phenomenon .

Piano key phenomenon – what does it mean?

In a complete dislocation, the clavicle lifts significantly from the acromion. When pressure is applied, it springs back up like a piano key—hence the term.

Strictly speaking, however, it's not the clavicle that "jumps" upward, but rather the acromion, which sinks downward and forward. The "piano key" image is therefore a rather simplified model—but it's easy to understand in practice.

Rockwood classification: What types are there?

The Rockwood classification describes the severity of an AC joint injury:

• Type I: Strain of the joint capsule, no instability

• Type II: Partial tear of the ligaments, slight dislocation

• Type III: Complete tear of the capsule and CC ligaments, complete dislocation upwards

• Type IV: Collarbone shifts backward and can become caught in the muscle

• Type V: Severe elevation of the clavicle with muscle detachment

• Type VI: Rare dislocation downwards under the coracoid process

Osteoarthritis in the acromioclavicular joint

In addition to acute injuries, the AC joint can also cause discomfort due to wear and tear (osteoarthritis) . The joint cartilage wears away, particularly in people who have lifted heavy loads for years or participate in sports involving a lot of overhead movement.

Not all cases of osteoarthritis cause symptoms. However, when symptoms do occur, they manifest as pressure pain, pain on exertion, and limitations in overhead movements .

Diagnosis: How is an AC joint dislocation diagnosed?

The diagnosis is based on:

• Physical examination (swelling, instability, piano key phenomenon)

• X-rays , if necessary with load

• MRI if additional ligament or rotator cuff injuries are suspected

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This makes it possible to determine exactly which type of Rockwood is present and what treatment is necessary.

Treatment options: Conservative or surgical?

✅ Conservative therapy

For minor injuries (Types I and II), as well as some Type III cases, rest in an arm sling , pain management, and physical therapy are usually sufficient. The goal is to relieve pain and restore mobility .

✅ Surgical therapy

From type III onwards with severe instability, and from types IV–VI onwards, surgery is often advisable. This involves reconstructing the torn ligaments and stabilizing the joint. Modern methods utilize minimally invasive techniques that enable faster healing.

Rehabilitation and aftercare

Rehabilitation after an AC joint injury includes:

1. Immobilization in the first few weeks to ensure healing

2. Early functional movement exercises to prevent shoulder stiffness

3. Strengthening the muscles , especially the rotator cuff

4. Gradual return to everyday life, work and sport

Depending on the severity, complete healing takes 6 weeks to 6 months .

Prevention: How to protect the AC joint

Not every injury can be avoided, but you can significantly reduce the risk through preventative measures :

• Warming up before exercise

• Strengthening exercises for shoulder muscles and shoulder blade stability

• Correct technique for overhead movements

• Ergonomic working in everyday life

• For contact sports: use protective clothing if necessary

Conclusion: The small joint with great importance

The acromioclavicular joint is small but crucial for shoulder mobility and stability. Injuries such as a dislocation after a fall or osteoarthritis due to wear and tear can cause significant discomfort – but are easily treatable with modern diagnostics and individually tailored therapy.

👉 Make an appointment now – we will help you make your shoulder stable and pain-free again.