What is the AC joint and why is it susceptible to injury?

The acromioclavicular (AC) joint connects the acromion of the scapula to the clavicle, providing crucial stability and range of motion to the shoulder, making it vulnerable to injury due to its superficial location and role in force transmission.

What are the main causes of acromioclavicular joint injuries?

AC joint injuries are primarily caused by direct trauma, such as a fall onto the top of the shoulder, or less commonly, indirect trauma from a fall onto an outstretched arm.

How are acromioclavicular joint injuries classified?

The Rockwood classification system categorizes AC joint injuries into six types (I-VI) based on the extent of ligamentous damage to the AC and CC ligaments and the degree and direction of clavicular displacement.

What are the common symptoms of an AC joint injury?

Common symptoms include localized pain and tenderness at the top of the shoulder, swelling, bruising, a visible deformity (like a "step-off" or "piano key" sign in more severe types), limited range of motion, and weakness.

How are AC joint injuries diagnosed and treated?

Diagnosis involves physical examination and X-rays, sometimes with stress views, while MRI may be used for soft tissue assessment. Treatment ranges from conservative management (rest, ice, physical therapy) for milder types (I-III) to surgical intervention for more severe injuries (IV-VI) or active individuals with Type III injuries.

What are the types of acromioclavicular joint injuries?

What are the different types of acromioclavicular joint injuries?

Acromioclavicular (AC) joint injuries are classified primarily by the degree of ligamentous damage and displacement of the clavicle relative to the acromion, ranging from mild sprains (Type I) to severe dislocations (Types IV-VI) involving complete ligament tears and significant structural disruption.

Understanding the Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is a critical articulation in the shoulder complex, connecting the acromion process of the scapula (shoulder blade) to the distal end of the clavicle (collarbone). This joint is crucial for providing the shoulder with its wide range of motion and stability. Its stability is primarily maintained by two sets of ligaments:

Acromioclavicular (AC) Ligaments: These surround the joint capsule, offering horizontal stability.
Coracoclavicular (CC) Ligaments: Comprising the conoid and trapezoid ligaments, these provide robust vertical stability by connecting the clavicle to the coracoid process of the scapula.

Due to its superficial location and role in force transmission, the AC joint is particularly vulnerable to injury, especially from direct trauma.

Mechanisms of AC Joint Injuries

AC joint injuries typically result from two primary mechanisms:

Direct Trauma: The most common cause, often involving a direct fall onto the top of the shoulder with the arm tucked in. This compresses the acromion against the clavicle, driving the scapula downwards while the clavicle remains relatively fixed. This mechanism is common in contact sports like football, rugby, and cycling accidents.
Indirect Trauma: Less common, occurring from a fall onto an outstretched arm (FOOSH) that transmits force up the arm to the shoulder, or from a direct impact to the elbow.

Classification of AC Joint Injuries: The Rockwood System

The most widely accepted classification system for AC joint injuries is the Rockwood classification, which categorizes injuries into six types based on the extent of ligamentous damage and the degree and direction of clavicular displacement.

Type I: AC Joint Sprain

Ligament Damage: Stretching or minor tearing of the AC ligaments; CC ligaments are intact.
Displacement: No significant displacement of the clavicle.
Clinical Presentation: Localized pain and tenderness over the AC joint, minimal swelling, no visible deformity. Shoulder range of motion is usually preserved but may be painful.

Type II: Partial AC Joint Dislocation

Ligament Damage: Complete tear of the AC ligaments; the CC ligaments are sprained or partially torn but remain largely intact.
Displacement: Mild superior subluxation (partial dislocation) of the clavicle, often less than 25% of the clavicular width above the acromion.
Clinical Presentation: More significant pain, swelling, and tenderness. A palpable step-off or slight bump may be visible at the AC joint. Painful and restricted shoulder movements.

Type III: Complete AC Joint Dislocation

Ligament Damage: Complete tears of both the AC and CC ligaments.
Displacement: Significant superior displacement of the clavicle, often 25-100% of the clavicular width above the acromion. The clavicle is no longer in contact with the acromion.
Clinical Presentation: Obvious deformity (a prominent "step-off" or "piano key" sign where the clavicle can be pushed down but springs back up), severe pain, significant swelling and bruising. Marked limitation of shoulder movement.

Type IV: Posterior Clavicular Displacement

Ligament Damage: Complete tears of both AC and CC ligaments.
Displacement: The clavicle is displaced posteriorly (backward) and often embedded into the trapezius muscle. This type is rare.
Clinical Presentation: Severe pain, significant deformity that may be difficult to palpate due to muscle involvement, and potential neurological symptoms if nerves are compressed.

Type V: Severe Superior Clavicular Displacement

Ligament Damage: Complete tears of AC and CC ligaments, along with significant stripping of the deltoid and trapezius muscle attachments from the distal clavicle and acromion.
Displacement: Gross superior displacement of the clavicle, often more than 100% of the clavicular width above the acromion.
Clinical Presentation: Extremely painful, very prominent deformity with a large gap between the clavicle and acromion, extensive swelling and bruising, and severe functional limitation.

Type VI: Inferior Clavicular Displacement

Ligament Damage: Complete tears of AC and CC ligaments.
Displacement: The clavicle is displaced inferiorly (downward), either under the coracoid process or under the acromion. This is the rarest type of AC joint injury.
Clinical Presentation: Severe pain, unique deformity with a sunken appearance of the clavicle, and potential for neurovascular compromise due to the clavicle's abnormal position.

Symptoms of AC Joint Injuries

Common symptoms across all types of AC joint injuries include:

Pain: Localized at the top of the shoulder, often exacerbated by movement, lifting the arm, or sleeping on the affected side.
Swelling and Bruising: Varying degrees depending on the severity of the injury.
Deformity: A visible or palpable "bump" or "step-off" at the AC joint, particularly in Type II and higher injuries.
Tenderness: Acute sensitivity to touch over the AC joint.
Limited Range of Motion: Difficulty and pain with overhead movements, reaching across the body, or lifting objects.
Weakness: Due to pain and compromised biomechanics of the shoulder.

Diagnosis of AC Joint Injuries

Diagnosis typically involves:

Physical Examination: Assessment of pain, tenderness, deformity, and range of motion. The "cross-body adduction test" often elicits pain.
X-rays: Standard anterior-posterior (AP) views of the shoulder, often with stress views (where weights are held in each hand) to accentuate any clavicular displacement and better visualize CC ligament integrity.
MRI (Magnetic Resonance Imaging): May be used to assess the extent of soft tissue damage, including AC and CC ligament tears, and to rule out other associated injuries.

Treatment Approaches

Treatment depends heavily on the injury type and the patient's activity level and goals:

Conservative Management: Types I and II injuries, and often Type III in less active individuals, are typically managed non-surgically. This includes:
- Rest and Immobilization: Sling use for initial pain relief and protection.
- Ice and NSAIDs: To reduce pain and inflammation.
- Physical Therapy: Progressive rehabilitation focusing on pain reduction, restoring range of motion, strengthening the surrounding musculature (rotator cuff, deltoid, scapular stabilizers), and proprioception.
Surgical Intervention: Generally considered for Type IV, V, and VI injuries, and for Type III injuries in highly active individuals (e.g., athletes, manual laborers) or those who fail conservative treatment. Surgical goals are to reduce the clavicle back into its anatomical position and reconstruct or repair the torn ligaments, often using various fixation devices or ligament reconstruction techniques.

Rehabilitation and Return to Activity

Regardless of the treatment approach, a structured rehabilitation program is crucial for optimal recovery. This typically progresses through phases:

Pain and Inflammation Control / Protection: Initial phase focusing on rest, ice, and gentle range of motion.
Restoration of Range of Motion: Gradual exercises to regain full shoulder mobility.
Strength and Endurance: Progressive strengthening of the entire shoulder girdle, including rotator cuff, deltoid, and scapular stabilizers.
Proprioception and Sport-Specific Training: Advanced exercises to restore joint awareness and prepare for return to specific activities or sports.

Return to full activity or sport is gradual and guided by pain levels, strength, and functional capacity, often taking several weeks to months, especially after surgery.

Conclusion

AC joint injuries are common and can significantly impact shoulder function. Accurate diagnosis, primarily guided by the Rockwood classification system, is paramount for determining the appropriate course of treatment. Whether managed conservatively or surgically, a comprehensive rehabilitation program is essential to restore strength, stability, and function, enabling individuals to return to their desired activities safely and effectively.

Acromioclavicular (AC) Joint Injuries: Types, Causes, Symptoms, and Treatment