Rockwood Classification System: Your Diagnostic Framework
Developed by Dr. Charles Rockwood Jr. in 1984, this six-type classification system remains the gold standard for AC joint injury assessment. Understanding this system is crucial for appropriate treatment decisions and referral patterns.
Type I: Mild AC Ligament Sprain
Pathoanatomy: Sprain of AC ligaments with microscopic fiber disruption. AC joint capsule remains intact. CC ligaments completely uninjured. Joint mechanics preserved with minimal functional impairment.
Detailed Radiographic Features:
- Normal joint space width (1-3mm)
- No clavicular elevation or displacement
- Coracoclavicular distance unchanged from contralateral side
- Joint congruity maintained
- Diagnostic challenge: Often appears radiographically normal
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Clinical Pearls: Diagnosis relies heavily on mechanism of injury and point tenderness over AC joint. Consider stress views if clinical suspicion high with normal standard radiographs.
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Type II: AC Ligament Rupture with CC Sprain
Pathoanatomy: Complete disruption of AC ligaments and joint capsule. Partial injury to CC ligament complex (typically conoid ligament affected more than trapezoid). Deltoid and trapezius fascial attachments remain intact.
Detailed Radiographic Features:
- AC joint space widening >7mm (normal: 1-3mm)
- Minimal superior clavicular displacement (<25% elevation)
- Slight increase in CC distance (typically <25% compared to contralateral)
- Loss of joint congruity but maintained overall alignment
- Zanca view: Essential to visualize subtle joint space changes
Biomechanical implications: Horizontal stability lost but vertical stability partially maintained through remaining CC ligament fibers.
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Type III: Complete AC and CC Ligament Disruption
Pathoanatomy: Complete rupture of both AC and CC ligament complexes. Joint capsule completely disrupted. Variable degree of deltoid and trapezius muscle detachment from distal clavicle. This represents the classic “complete AC separation.”
Detailed Radiographic Features:
- Clavicle elevated above superior border of acromion
- CC distance increased 25-100% compared to contralateral side
- Absolute CC distance typically 15-20mm (normal: 11-13mm)
- “Step-off” deformity: Visible discontinuity between acromion and clavicle
- Joint space completely disrupted
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Clinical significance: This grade represents the controversial “gray zone” where treatment decisions are most challenging and patient-specific factors become paramount.
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Type IV: Posterior Clavicular Displacement
Pathoanatomy: Complete disruption of AC and CC ligaments with posterior displacement of distal clavicle. Clavicle displaces through or into trapezius muscle belly. This injury often involves significant soft tissue trauma and may be associated with brachial plexus injury.
Detailed Radiographic Features:
- AP view: May appear deceptively similar to Type II or III
- Axillary lateral view: Essential for diagnosis – shows posterior displacement
- Clavicle positioned posterior to acromion
- CT imaging: May be necessary to fully appreciate spatial relationships
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Clinical importance: Highest risk for neurovascular complications due to proximity to brachial plexus and subclavian vessels.
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Type V: Severe Superior Displacement
Pathoanatomy: Represents an extreme form of Type III injury with extensive soft tissue disruption. Complete avulsion of deltoid and trapezius muscle attachments from distal clavicle. Marked superior displacement creates significant cosmetic deformity and functional impairment.
Detailed Radiographic Features:
- Markedly elevated clavicle with dramatic “step-off”
- CC distance >25mm or increased 100-300% vs. contralateral
- Significant superior displacement often with skin “tenting”
- Visual impact: Deformity often visible through clothing
Associated findings: High incidence of concomitant injuries including rib fractures, pneumothorax, and brachial plexus injuries.
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Type VI: Inferior Displacement (Rare)
Pathoanatomy: Complete ligament disruption with inferior displacement of distal clavicle below acromion or coracoid process. Clavicle may become trapped behind coracobrachialis and short head of biceps. Usually associated with severe trauma and often has concomitant injuries.
Detailed Radiographic Features:
- Clavicle positioned inferior to acromion (subacromial position)
- May be positioned behind coracoid process (subcoracoid)
- AP view: Clavicle appears shortened or absent
- Axillary view: Shows true position of displaced clavicle
Mechanism: Often associated with severe trauma such as motor vehicle accidents or high-energy sports injuries.