AC Joint Dislocation: Understanding Causes, Symptoms, Diagnosis, and Treatment

What is a dislocated AC joint in the shoulder?

An AC joint dislocation, more accurately termed an AC joint separation, occurs when the ligaments connecting the collarbone (clavicle) to the shoulder blade (scapula) at the acromioclavicular joint are stretched or torn, leading to varying degrees of separation.

Understanding the Shoulder Joint

The shoulder is a complex and highly mobile joint, crucial for a vast range of upper limb movements. While often referred to as a single entity, it comprises several distinct joints that work in concert. One critical component is the acromioclavicular (AC) joint.

• Anatomy of the AC Joint: This joint is formed by the lateral (outer) end of the clavicle (collarbone) and the acromion, which is the highest point of the scapula (shoulder blade). It's a relatively small joint, but it plays a vital role in shoulder stability and motion, particularly in overhead activities.
• Ligamentous Support: The stability of the AC joint is primarily maintained by two sets of ligaments:
  • Acromioclavicular (AC) ligaments: These surround the joint capsule, connecting the clavicle directly to the acromion.
  • Coracoclavicular (CC) ligaments: These are much stronger and provide the primary vertical stability for the AC joint. They connect the clavicle to the coracoid process, a hook-like projection from the scapula.

What is an AC Joint Dislocation?

An AC joint dislocation, or separation, refers to an injury where the forces applied to the shoulder cause the clavicle to separate from the acromion. Unlike a glenohumeral dislocation (where the ball of the humerus comes out of the shoulder socket), an AC joint separation involves damage to the supporting ligaments rather than the main shoulder joint itself.

• Mechanism of Injury: The most common cause is a direct blow to the top or side of the shoulder, often with the arm tucked into the side. This can happen during:
  • Falls: Falling directly onto the shoulder (e.g., from a bicycle, during sports).
  • Contact Sports: Tackles in football, checks in hockey, or impacts in rugby.
  • Motor Vehicle Accidents: Direct impact to the shoulder from the seatbelt or dashboard.
• Common Causes: The injury typically occurs when the force drives the scapula downwards, while the clavicle remains relatively fixed, leading to a tearing or stretching of the AC and/or CC ligaments.

Symptoms of an AC Joint Dislocation

The signs and symptoms of an AC joint separation can vary significantly depending on the severity of the injury.

• Pain:
  • Immediate and often severe pain at the top of the shoulder, directly over the AC joint.
  • Pain is worsened by moving the arm, especially lifting it overhead or across the body.
  • Tenderness to touch over the joint.
• Deformity:
  • A visible bump or prominence may appear on top of the shoulder, caused by the clavicle being displaced upwards. This is often more noticeable in more severe separations.
  • The shoulder may appear "drooped" or lower than the uninjured side.
• Loss of Function:
  • Difficulty or inability to lift the arm above shoulder height.
  • Weakness in the affected arm.
  • Limited range of motion in the shoulder.
• Swelling and Bruising:
  • Localized swelling around the AC joint.
  • Bruising may develop over time, extending down the arm or chest in some cases.

Classifying AC Joint Separations (Grades)

AC joint separations are typically classified using the Rockwood Classification System, which categorizes the injury into six types based on the degree of ligamentous damage and displacement of the clavicle relative to the acromion. This classification guides treatment decisions.

• Type I: Mild sprain of the AC ligaments. No significant tearing, minimal pain, no deformity. CC ligaments are intact.
• Type II: Complete tear of the AC ligaments, but the CC ligaments are sprained or partially torn. Slight vertical displacement of the clavicle, creating a subtle bump.
• Type III: Complete tears of both the AC and CC ligaments. Significant vertical displacement of the clavicle, resulting in a noticeable bump on top of the shoulder.
• Type IV: Complete tears of both AC and CC ligaments, with the clavicle displaced posteriorly (backwards) into or through the trapezius muscle. This is a severe injury.
• Type V: Complete tears of both AC and CC ligaments, with extremely severe vertical displacement of the clavicle (often 100-300% more than normal). The skin over the clavicle may be tented.
• Type VI: Rare and severe. Complete tears of both AC and CC ligaments, with the clavicle displaced inferiorly (downwards) and lodged under the coracoid process or acromion.

Diagnosis

Accurate diagnosis is crucial for determining the appropriate course of treatment.

• Physical Examination: A healthcare professional will assess the shoulder for pain, tenderness, swelling, deformity, and range of motion. They may perform specific tests to evaluate the stability of the AC joint.
• Imaging:
  • X-rays: Standard X-rays are typically used to visualize the AC joint and determine the degree of clavicular displacement. Specific "stress views" (e.g., with weights held in the hands) may be taken to accentuate the separation.
  • MRI (Magnetic Resonance Imaging): While not always necessary for diagnosis, an MRI can provide detailed images of the soft tissues, including the AC and CC ligaments, and rule out other associated injuries.

Treatment and Management

Treatment for AC joint separations depends heavily on the grade of the injury, the patient's activity level, and their individual goals.

• Conservative Management (Non-Surgical):
  • Rest and Immobilization: Initially, a sling may be used to support the arm and reduce stress on the joint.
  • Pain Management: Over-the-counter pain relievers (NSAIDs like ibuprofen) or prescribed medication can help manage pain and inflammation.
  • Ice: Applying ice to the affected area helps reduce swelling and pain.
  • Physical Therapy: Once initial pain subsides, a structured rehabilitation program is essential. This includes:
    • Restoration of Range of Motion: Gentle exercises to regain shoulder mobility.
    • Strengthening: Exercises to strengthen the surrounding shoulder and scapular muscles (e.g., deltoid, trapezius, rotator cuff) to improve stability and support.
    • Proprioception and Balance: Exercises to improve the body's awareness of the joint's position.
  • Conservative management is typically recommended for Type I, II, and often Type III injuries, especially in less active individuals.
• Surgical Management:
  • Surgery is typically considered for more severe injuries (Type IV, V, VI), or for Type III injuries in high-level athletes or individuals whose occupations demand full overhead function, where non-surgical treatment may not provide adequate stability or functional recovery.
  • Surgical Goals: To reduce the clavicle back into its anatomical position and stabilize the AC joint, often by repairing or reconstructing the torn ligaments, particularly the CC ligaments.
  • Types of Surgery: Various techniques exist, including direct repair, reconstruction with grafts, or using synthetic materials to stabilize the joint while the ligaments heal.

Rehabilitation and Recovery

Regardless of whether treatment is surgical or conservative, a comprehensive rehabilitation program is critical for optimal recovery and return to function.

• Phases of Recovery:
  • Phase 1 (Acute/Protection): Focus on pain and swelling control, protecting the joint, and maintaining mobility of the hand, wrist, and elbow.
  • Phase 2 (Early Motion): Gradual introduction of passive and active-assisted range of motion exercises for the shoulder.
  • Phase 3 (Strengthening): Progressive strengthening of the shoulder girdle muscles, including the rotator cuff, deltoids, and scapular stabilizers.
  • Phase 4 (Return to Activity): Sport-specific or activity-specific drills, focusing on power, agility, and endurance, leading to a gradual return to full activity.
• Importance of Professional Guidance: Working with a physical therapist or athletic trainer is paramount. They can provide a tailored exercise program, monitor progress, and ensure safe progression through the recovery phases, minimizing the risk of re-injury.

Prevention

While not all AC joint injuries can be prevented, certain measures can reduce the risk.

• Injury Avoidance Strategies:
  • Proper Falling Techniques: Learning how to fall safely in sports or activities can reduce direct impact to the shoulder.
  • Protective Gear: Using appropriate padding or protective equipment in contact sports.
  • Awareness of Surroundings: Being mindful of environmental hazards that could lead to falls.
• Strengthening Surrounding Musculature: A strong and balanced shoulder girdle, including the rotator cuff, deltoids, and scapular stabilizers, can improve overall shoulder stability and potentially absorb some forces that might otherwise lead to AC joint injury. However, direct prevention of high-impact trauma to the AC joint through strength training is limited.