AC Joint Injury: Understanding Shoulder Separation, Causes, Grades, and Treatment

What is an ACM injury?

An Acromioclavicular (AC) joint injury, often referred to as a "shoulder separation," is a sprain or tear of the ligaments that connect the collarbone (clavicle) to the shoulder blade (scapula) at the highest point of the shoulder (acromion).

Understanding the Acromioclavicular (AC) Joint

The shoulder is a complex structure, and the acromioclavicular (AC) joint is a critical component for its stability and function. Located at the very top of the shoulder, it forms the articulation between the outer end of the clavicle (collarbone) and the acromion, which is a bony projection of the scapula (shoulder blade).

This joint, while small, plays a vital role in shoulder movement, allowing the scapula to rotate on the clavicle, which is essential for arm elevation. Its stability is primarily provided by two sets of ligaments:

• Acromioclavicular (AC) Ligaments: These surround the joint capsule, connecting the clavicle directly to the acromion. They provide horizontal stability.
• Coracoclavicular (CC) Ligaments: These are stronger and more robust ligaments located slightly below the AC joint, connecting the clavicle to the coracoid process (another part of the scapula). They are comprised of two distinct bands, the conoid and trapezoid ligaments, and are crucial for vertical stability, preventing the clavicle from displacing upwards.

What is an AC Joint Injury?

An AC joint injury occurs when these stabilizing ligaments are stretched or torn. Unlike a "shoulder dislocation," which involves the ball-and-socket glenohumeral joint, an AC joint injury is more accurately termed a "shoulder separation." The severity of the injury depends on which ligaments are affected and to what extent, leading to varying degrees of displacement of the clavicle relative to the acromion.

Causes and Mechanisms of Injury

AC joint injuries most commonly result from direct trauma to the shoulder. The typical mechanisms include:

• Direct Fall onto the Shoulder: This is the most common cause, where an individual falls directly onto the top or outer aspect of the shoulder, driving the acromion downwards while the clavicle remains relatively stable. This "shearing" force can stretch or tear the AC and CC ligaments.
• Direct Blow to the Shoulder: Similar to a fall, a direct impact, such as during a collision in contact sports, can cause the injury.
• Fall onto an Outstretched Arm (less common): While more commonly associated with other shoulder injuries, a fall onto an outstretched hand can transmit force up the arm to the shoulder, potentially injuring the AC joint.

These injuries are prevalent in athletes participating in sports with a high risk of falls or direct contact, such as:

• Cycling (especially mountain biking)
• Football (American and Rugby)
• Hockey
• Skiing
• Wrestling
• Martial Arts

Classifying AC Joint Injuries (Grades)

AC joint injuries are classified into grades based on the severity of ligamentous damage and the degree of clavicular displacement. The Rockwood Classification is the most widely accepted system, categorizing injuries from Grade I to VI:

• Grade I:
  • Ligament Damage: Sprain or partial tear of the AC ligaments. CC ligaments are intact.
  • Displacement: No significant displacement of the clavicle. The joint appears normal on X-ray.
  • Appearance: Minimal swelling, localized tenderness.
• Grade II:
  • Ligament Damage: Complete tear of the AC ligaments, with a sprain or partial tear of the CC ligaments.
  • Displacement: Slight superior displacement of the clavicle (less than 25% of the clavicular width relative to the acromion).
  • Appearance: Noticeable bump, moderate pain, some instability.
• Grade III:
  • Ligament Damage: Complete tear of both the AC and CC ligaments.
  • Displacement: Significant superior displacement of the clavicle (25-100% of the clavicular width). The clavicle is no longer aligned with the acromion.
  • Appearance: Obvious "step-off" deformity (prominent clavicle), significant pain, marked instability.
• Grade IV:
  • Ligament Damage: Complete tear of AC and CC ligaments.
  • Displacement: The clavicle is displaced posteriorly (backwards) into or through the trapezius muscle. This is a severe and unstable injury.
• Grade V:
  • Ligament Damage: Complete tear of AC and CC ligaments, with disruption of the deltoid and trapezius muscle attachments from the clavicle.
  • Displacement: Extremely severe superior displacement of the clavicle, often exceeding 100% of the clavicular width, appearing very prominent.
• Grade VI:
  • Ligament Damage: Complete tear of AC and CC ligaments.
  • Displacement: The clavicle is displaced inferiorly (downwards) and lodges under the coracoid process. This is a very rare injury, typically resulting from high-energy trauma.

Signs and Symptoms

The symptoms of an AC joint injury vary with its severity:

• Pain: Localized pain directly over the AC joint, often worsening with arm movement, especially reaching across the body or overhead.
• Swelling and Bruising: Visible swelling and tenderness at the top of the shoulder. Bruising may develop over time.
• Deformity: A visible "bump" or "step-off" deformity at the top of the shoulder, particularly noticeable in Grade II and higher injuries due to the elevated clavicle.
• Limited Range of Motion: Difficulty and pain when moving the arm, especially lifting it overhead or across the body.
• Tenderness to Palpation: Significant pain when the AC joint is pressed.
• Popping or Clicking: Less common, but may be felt during certain movements.

Diagnosis

Accurate diagnosis of an AC joint injury involves a combination of physical examination and imaging studies:

• Physical Examination: A healthcare professional will assess the shoulder for tenderness, swelling, deformity, and perform specific tests to evaluate the stability of the AC joint and the range of motion of the shoulder.
• Imaging:
  • X-rays: Standard X-rays are crucial. AP (Anteroposterior) views and specialized Zanca views (angled X-rays) are often taken to best visualize the AC joint. Stress X-rays, where weights are held by the patient, may be used to exaggerate any instability and help differentiate between Grade II and III injuries by revealing greater displacement.
  • MRI (Magnetic Resonance Imaging): An MRI may be ordered to assess the extent of ligamentous damage (especially the CC ligaments), rule out other soft tissue injuries, or if the diagnosis is unclear.
  • CT Scan (Computed Tomography): Less common for AC joint injuries but may be used in complex cases to assess associated fractures or for pre-surgical planning.

Treatment Approaches

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

Conservative Management (Non-Surgical)

This is the primary treatment for most Grade I, II, and many Grade III injuries.

• Rest and Immobilization: Initially, the arm is rested, often in a sling for a period (e.g., 1-3 weeks) to support the arm, reduce pain, and allow the ligaments to heal.
• Ice: Application of ice packs to the affected area helps reduce swelling and pain.
• Pain Management: Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or naproxen are typically prescribed to manage pain and inflammation.
• Physical Therapy: A structured rehabilitation program is essential.
  • Early Phase: Focuses on pain control, protecting the joint, and gentle range-of-motion exercises to prevent stiffness.
  • Progressive Phase: As pain subsides, exercises progress to restore full range of motion, strengthen the muscles around the shoulder (rotator cuff, deltoid, scapular stabilizers), and improve proprioception (joint awareness).
  • Return to Activity: Gradual return to daily activities and sports, guided by the physical therapist and physician.

Surgical Management

Surgery is generally reserved for higher-grade injuries (Grade IV, V, VI) and sometimes for Grade III injuries in high-level athletes or individuals who experience persistent pain or functional limitations despite conservative treatment.

• Indications: Significant displacement, persistent instability, severe cosmetic deformity, failure of conservative treatment, or specific occupational/athletic demands.
• Surgical Goals: To restore the anatomical alignment of the clavicle and acromion and provide stability to the joint.
• Common Surgical Techniques:
  • Coracoclavicular (CC) Ligament Reconstruction/Repair: This involves reconstructing or repairing the torn CC ligaments, often using grafts (from the patient's own body or a donor) or synthetic materials, sometimes combined with a strong suture construct (e.g., "tightrope" technique) to pull the clavicle back into position.
  • AC Joint Fixation: Techniques such as hook plate fixation or screws are sometimes used to temporarily hold the joint in place while the ligaments heal, though these often require a second surgery for removal.
  • Distal Clavicle Excision (Mumford Procedure): In chronic cases with persistent pain and arthritis, a small portion of the end of the clavicle may be removed to prevent bone-on-bone rubbing.

Rehabilitation and Recovery

Rehabilitation is a critical component of recovery, whether treatment is conservative or surgical. It follows a progressive, multi-phase approach:

• Phase 1 (Protection & Early Motion): Focus on pain and swelling control, immobilization (if indicated), and very gentle, passive or active-assisted range of motion exercises to prevent stiffness without stressing the healing ligaments.
• Phase 2 (Mobility & Early Strength): As pain decreases, emphasis shifts to restoring full, pain-free range of motion and initiating isometric and light resistance exercises for the shoulder girdle muscles.
• Phase 3 (Strength & Endurance): Progressive resistance training to build strength and endurance in the deltoid, rotator cuff, and scapular stabilizers. Functional movements and sport-specific drills are introduced.
• Phase 4 (Return to Activity): Gradual return to full activities, including contact sports, guided by pain levels, strength, and functional ability. This phase typically includes plyometrics and higher-intensity drills relevant to the individual's sport or occupation.

The recovery timeline varies significantly:

• Grade I-II: Full recovery typically takes 2-6 weeks for most activities, with return to contact sports potentially taking longer.
• Grade III (Conservative): May take 6-12 weeks for initial recovery, with full return to strenuous activity potentially taking 3-6 months.
• Surgical Cases: Recovery is longer, often involving 4-6 months before a full return to demanding activities, and potentially up to a year for complete restoration of strength and function.

Prevention

While not all AC joint injuries can be prevented, especially those from unforeseen accidents, some measures can reduce risk:

• Awareness in Sports: Understanding the risks in contact sports and practicing proper falling techniques (e.g., in cycling, learning to roll rather than bracing with an outstretched arm or landing directly on the shoulder).
• Shoulder Girdle Strength and Stability: Engaging in a balanced strength training program that targets the rotator cuff, deltoid, and scapular stabilizing muscles can improve overall shoulder resilience.
• Protective Equipment: Wearing appropriate protective gear in sports, such as shoulder pads, can help dissipate impact forces.

Prognosis

The prognosis for AC joint injuries is generally good, particularly for lower-grade injuries. Most individuals recover full or near-full function.

• Grades I and II: Typically heal well with conservative management, though some may experience lingering minor pain or a slight cosmetic bump.
• Grade III: Conservative management often yields good functional outcomes, but a noticeable deformity (the "step-off") is common. Some individuals may experience chronic pain or instability, leading to consideration of delayed surgery.
• Higher Grades (IV-VI): Usually require surgical intervention to restore stability and function, and the recovery process is more extensive.

Long-term complications can include chronic pain, stiffness, or the development of post-traumatic osteoarthritis in the AC joint, especially in higher-grade injuries or those with a history of repetitive trauma. Regular follow-up with a healthcare professional is important to monitor recovery and address any persistent issues.