AC Joint Injuries: Understanding Subluxation, Separation, Symptoms, and Treatment

What is the Difference Between AC Joint Separation and Subluxation?

AC joint subluxation refers to a partial dislocation of the acromioclavicular joint, typically involving stretching or partial tearing of its stabilizing ligaments, whereas an AC joint separation signifies a complete dislocation with full tears of the acromioclavicular ligaments and often the critical coracoclavicular ligaments, leading to more pronounced instability and deformity.

Understanding the Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is a small but crucial articulation located at the top of the shoulder. It connects the acromion, a bony projection of the shoulder blade (scapula), to the clavicle, or collarbone. While small, this joint plays a vital role in shoulder movement, allowing the scapula to rotate relative to the clavicle, which is essential for full arm elevation and overhead activities.

The stability of the AC joint is primarily maintained by two sets of ligaments:

• Acromioclavicular (AC) Ligaments: These surround the joint capsule itself, providing direct stability.
• Coracoclavicular (CC) Ligaments: Comprising the trapezoid and conoid ligaments, these are located below the joint, connecting the clavicle to the coracoid process of the scapula. They are significantly stronger than the AC ligaments and provide the primary vertical stability to the joint.

Understanding AC Joint Injuries: A Spectrum

Injuries to the AC joint are common, particularly in contact sports, falls onto the shoulder, or direct impacts. These injuries exist on a spectrum of severity, from a mild sprain to a complete dislocation. The terms "subluxation" and "separation" describe different points along this spectrum, primarily distinguished by the extent of ligamentous damage and the degree of displacement of the clavicle relative to the acromion.

What is an AC Joint Subluxation?

An AC joint subluxation represents a partial dislocation of the joint. It is generally considered a less severe injury compared to a full separation.

• Definition: In an AC joint subluxation, the joint surfaces of the acromion and clavicle lose their normal alignment, but they are still partially in contact.
• Ligament Involvement: This injury typically involves stretching or partial tearing of the acromioclavicular (AC) ligaments. Crucially, the stronger coracoclavicular (CC) ligaments remain intact or are only minimally stretched, preventing significant upward displacement of the clavicle.
• Mechanism of Injury: Often results from a less forceful direct blow to the top of the shoulder or a fall onto an outstretched arm.
• Symptoms:
  • Localized pain and tenderness directly over the AC joint.
  • Mild swelling and bruising.
  • Pain with overhead movements or reaching across the body.
  • A slight, often barely noticeable, bump or prominence at the top of the shoulder.
• Rockwood Classification: Corresponds to a Type I or Type II AC joint injury.
  • Type I: A sprain of the AC ligaments with no displacement of the joint. The joint remains stable.
  • Type II: A complete tear of the AC ligaments, but the CC ligaments are intact. There is a slight superior displacement of the clavicle, but it remains within the confines of the acromion.
• Treatment: Typically managed conservatively with rest, ice, compression, elevation (RICE), pain medication, and a sling for comfort. Early rehabilitation focuses on restoring range of motion and strength.

What is an AC Joint Separation?

An AC joint separation, often referred to as a "separated shoulder," is a complete dislocation of the AC joint. It signifies a more severe injury with significant ligamentous disruption.

• Definition: In an AC joint separation, the joint surfaces of the acromion and clavicle are no longer in contact. The clavicle is significantly displaced upwards from the acromion.
• Ligament Involvement: This injury involves a complete tear of the acromioclavicular (AC) ligaments. Furthermore, it also involves a complete tear of the stronger coracoclavicular (CC) ligaments (trapezoid and conoid), which are essential for vertical stability.
• Mechanism of Injury: Usually caused by a high-force direct impact to the top of the shoulder, such as falling directly onto the shoulder or being tackled in sports.
• Symptoms:
  • Severe pain at the top of the shoulder.
  • Obvious deformity or "step-off" appearance, where the clavicle appears visibly elevated above the acromion.
  • Significant swelling and bruising.
  • Marked loss of shoulder function and difficulty with arm movement.
  • Pain with any attempt to lift the arm.
• Rockwood Classification: Corresponds to Type III through Type VI AC joint injuries, with Type III being the most common of the true separations.
  • Type III: Complete tears of both AC and CC ligaments. The clavicle is significantly displaced superiorly, but it remains relatively stable horizontally.
  • Type IV: Complete tears of AC and CC ligaments, with the clavicle displaced posteriorly into the trapezius muscle.
  • Type V: Complete tears of AC and CC ligaments, with extreme superior displacement of the clavicle, often through the deltotrapezial fascia.
  • Type VI: Complete tears of AC and CC ligaments, with the clavicle displaced inferiorly (rare).
• Treatment: While Type III injuries can sometimes be managed non-surgically, higher-grade separations (Types IV-VI) almost always require surgical intervention to reduce the joint and reconstruct or repair the torn ligaments, followed by extensive rehabilitation.

Key Differences Summarized

Diagnosis and Professional Evaluation

Accurate diagnosis of an AC joint injury requires a thorough clinical examination by a healthcare professional. This typically involves:

• Physical Examination: Assessing tenderness, swelling, range of motion, and checking for any visible deformity or "step-off."
• Imaging Studies:
  • X-rays: Standard X-rays of the shoulder are crucial to visualize the joint alignment and rule out fractures. Stress X-rays (taken while the patient holds weights) may be used to exaggerate any instability and better assess the degree of displacement.
  • MRI (Magnetic Resonance Imaging): May be used to assess the extent of soft tissue damage, including ligament tears, if the diagnosis is unclear or surgical planning is needed.

Treatment Approaches

Treatment for AC joint injuries depends significantly on the grade of the injury, the patient's activity level, and their specific goals.

• Conservative Management:
  • Rest and Immobilization: A sling is often used for comfort and to reduce stress on the joint during the initial healing phase (typically 1-3 weeks).
  • Ice and Pain Management: Applying ice packs and using over-the-counter or prescription pain relievers (NSAIDs) to reduce pain and inflammation.
  • Physical Therapy: Once initial pain subsides, a structured rehabilitation program is essential. This focuses on restoring pain-free range of motion, strengthening the surrounding shoulder and scapular muscles (deltoid, trapezius, rotator cuff), and improving stability. This is the primary treatment for Type I, Type II, and often Type III injuries.
• Surgical Intervention:
  • Surgery is generally reserved for higher-grade separations (Type IV, V, VI) or for Type III injuries in highly active individuals (e.g., overhead athletes, laborers) where long-term instability or cosmetic deformity is a concern.
  • Surgical procedures aim to reduce the clavicle back into its anatomical position and stabilize it, often by repairing or reconstructing the torn ligaments using grafts or synthetic materials. Various techniques exist, including direct repair, reconstruction with tendon grafts, or internal fixation with plates and screws.

Rehabilitation and Return to Activity

Regardless of whether the injury is treated conservatively or surgically, a comprehensive rehabilitation program is critical for optimal recovery.

• Phased Approach: Rehabilitation progresses through distinct phases, starting with pain control and gentle range of motion, advancing to strengthening exercises, and finally incorporating sport-specific or activity-specific drills.
• Focus Areas: Rehabilitation emphasizes restoring full shoulder mobility, rebuilding strength in the deltoid, trapezius, and rotator cuff muscles, and enhancing proprioception (joint awareness) and stability.
• Gradual Return: Return to full activity or sport is always gradual and guided by the physical therapist and physician, ensuring the joint has regained sufficient strength, stability, and pain-free function. Premature return can lead to re-injury or chronic issues.

Prevention Strategies

While not all AC joint injuries can be prevented, certain strategies can reduce the risk:

• Proper Falling Techniques: Learning how to fall safely, such as rolling to absorb impact rather than landing directly on the shoulder, can be beneficial in sports or activities with a high fall risk.
• Protective Gear: Athletes in contact sports should wear appropriate padding and protective gear to minimize direct impacts to the shoulder.
• Shoulder Strength and Stability: Engaging in a balanced strength training program that targets the entire shoulder girdle, including the rotator cuff, scapular stabilizers, and deltoid, can improve overall shoulder resilience.

Conclusion

The terms AC joint "subluxation" and "separation" describe a spectrum of injuries to the acromioclavicular joint, differentiated by the extent of ligamentous damage and the degree of clavicular displacement. A subluxation is a partial dislocation with intact or minimally stretched coracoclavicular ligaments, typically less severe. A separation is a complete dislocation involving full tears of both the AC and CC ligaments, leading to greater instability and often a visible deformity. Accurate diagnosis and appropriate treatment, followed by dedicated rehabilitation, are paramount for restoring shoulder function and ensuring a successful return to daily activities and sport.