Complete Guide to Acromioclavicular (AC) Joint Dislocations: A Radiologist's Perspective

Master the diagnosis and classification of shoulder separations to enhance your clinical expertise and patient outcomes

As a practicing chiropractor or medical provider, you understand that accurate diagnosis is the cornerstone of effective treatment. When it comes to AC joint injuries, commonly known as “shoulder separations”. Your ability to properly identify, classify, and manage these injuries can significantly impact your patient’s recovery trajectory and your practice’s reputation.

AC joint dislocations represent one of the most frequently misdiagnosed shoulder injuries in primary care settings. However, with the right knowledge and diagnostic approach, you can confidently differentiate between a simple sprain and a complex dislocation requiring immediate surgical referral.

What is an Acromioclavicular (AC) Joint Dislocation?

An acromioclavicular joint dislocation occurs when the stabilizing ligaments connecting the distal clavicle to the acromion of the scapula are disrupted. This injury involves two critical ligament complexes:

  • Acromioclavicular (AC) ligaments – Provide horizontal stability
  • Coracoclavicular (CC) ligaments – Provide vertical stability and prevent superior displacement

 

Unlike glenohumeral dislocations, AC joint injuries don’t involve the ball-and-socket joint itself, making them distinct clinical entities requiring different management approaches.

Common Mechanisms

AC joint separations typically result from:

  • Direct blow to the superolateral shoulder (most common)
  • Fall onto outstretched hand with superior force transmission
  • Contact sports injuries (football, hockey, rugby)
  • Cycling accidents with shoulder impact

 

Patient Demographics

Your typical AC joint injury patient is often:

  • Young athletic male (20s-30s)
  • Contact sport participant
  • Active individual with high functional demands

 

Clinical Signs and Symptoms

Patients present with:

  • Localized pain over the AC joint
  • Point tenderness at the joint line
  • Swelling and deformity (severity dependent)
  • “Step-off” deformity in higher grades
  • Pain with cross-body adduction (positive cross-body test)

Difficulty with overhead activities

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

 

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.

 

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.

 

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

 

Clinical significance: This grade represents the controversial “gray zone” where treatment decisions are most challenging and patient-specific factors become paramount.

 

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

 

Clinical importance: Highest risk for neurovascular complications due to proximity to brachial plexus and subclavian vessels.

 

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.

 

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.

Standard Radiographic Protocol

The cornerstone of AC joint evaluation begins with proper radiographic technique. Understanding the nuances of each view is crucial for accurate diagnosis:

Bilateral Anteroposterior (AP) Views:

  • Technique: Patient upright, arms at sides, beam perpendicular to AC joint
  • Key measurements: Normal AC joint space is 1-3mm; widening >7mm suggests Type II injury
  • Critical assessment: Compare coracoclavicular (CC) distances bilaterally
  • Normal CC distance: 11-13mm in men, 9-11mm in women
  • Pathological findings: 25% increase suggests Type III; >100% increase indicates Type V

 

Zanca View (Modified AP):

  • Technique: 15° cephalic angulation with 50% decreased penetration
  • Purpose: Eliminates rib and spine superimposition over AC joint
  • Clinical significance: Unmasks subtle AC joint space widening
  • When to use: Should be standard in all suspected AC joint injuries

 

Axillary Lateral View:

  • Technique: Patient supine, arm abducted 90°, beam shot through axilla
  • Critical for: Detecting posterior displacement (Type IV injuries)
  • Anatomical landmarks: Coracoid process, acromion, and clavicle relationship
  • Pathological findings: Posterior clavicular displacement behind acromion

 

Stress Radiographs:

  • Technique: 10-15 pound weights attached to each wrist during AP view
  • Controversy: Pain may limit patient cooperation; questionable clinical utility
  • Evidence: Recent studies suggest limited value in acute setting
  • Alternative: Manual stress applied by examiner (less standardized)

 

Radiographic Measurements and Interpretation

Coracoclavicular Distance Assessment:

  • Measurement technique: Shortest distance between superior coracoid and inferior clavicle
  • Normal variations: Can vary 15-20% between individuals
  • Pathological significance:
    • Type II: <25% increase
    • Type III: 25-100% increase
    • Type V: >100% increase (typically >25mm absolute)

 

AC Joint Space Evaluation:

  • Normal width: 1-3mm (narrows with age)
  • Pathological widening: >7mm suggests ligamentous disruption
  • Pitfalls: Joint space can appear normal in acute Type I injuries
  • Age considerations: Degenerative changes common after age 40

 

 

Diagnostic Ultrasound: The Emerging Modality

Ultrasound Advantages in AC Joint Assessment:

  • Real-time evaluation of joint stability
  • Dynamic assessment during shoulder movement
  • No radiation exposure
  • Cost-effective compared to MRI
  • Point-of-care capability in clinic setting

 

Ultrasound Technique for AC Joint Evaluation:

  • Patient positioning: Seated, arm relaxed at side
  • Probe placement: Linear high-frequency transducer (12-15 MHz)
  • Scanning planes: Coronal and sagittal over AC joint
  • Key structures: AC joint capsule, CC ligaments, surrounding musculature

 

Ultrasound Findings by Injury Grade:

Type I Injuries:

  • AC joint: Minimal fluid, intact joint capsule
  • CC ligaments: Normal echogenicity and thickness
  • Surrounding tissues: Minimal soft tissue swelling

Type II Injuries:

  • AC joint: Joint space widening, capsular disruption
  • CC ligaments: Hypoechoic areas suggesting partial tear
  • Dynamic assessment: Increased mobility with stress

Type III-V Injuries:

  • AC joint: Complete capsular disruption
  • CC ligaments: Complete discontinuity or absence
  • Clavicular position: Abnormal relationship to acromion
  • Soft tissue: Extensive hematoma and edema

 

Ultrasound Limitations:

  • Operator dependent technique
  • Limited penetration in large patients
  • Incomplete visualization of deeper structures
  • Learning curve for accurate interpretation

 

 

Advanced Cross-Sectional Imaging

MRI Indications and Findings:

  • Sequences: T1, T2, STIR, proton density with fat suppression
  • Planes: Coronal, sagittal, axial oblique
  • AC ligament assessment: Best seen on coronal T2 sequences
  • CC ligament evaluation: Sagittal planes optimal for conoid and trapezoid ligaments
  • Associated injuries: Rotator cuff tears, labral pathology, bone marrow edema

 

CT Evaluation:

  • Indications: Complex fracture patterns, surgical planning
  • 3D reconstruction: Valuable for understanding spatial relationships
  • Bone detail: Superior to MRI for cortical bone assessment
  • Limitations: Poor soft tissue contrast compared to MRI

Technical Errors:

  • Inadequate penetration: Obscures bony detail
  • Patient positioning: Rotation can simulate pathology
  • Bilateral comparison: Essential for accurate assessment
  • Magnification differences: Can affect measurements

Interpretation Errors:

  • Normal variants: Os acromiale, congenital AC joint variations
  • Degenerative changes: Can mimic acute injury
  • Previous injury: Chronic changes may confuse acute assessment

Concomitant pathology: Missed associated injuries.

Red Flags: When to Refer Immediately

As a chiropractor, recognizing when AC joint injuries require immediate orthopedic consultation is critical:

  • Type IV, V, VI injuries (surgical candidates)
  • Neurovascular compromise (rare but serious)
  • Associated fractures (coracoid, acromion, clavicle)
  • Open injuries
  • Failed conservative management after 3-6 months

Radiographic Interpretation Mastery

  1. Bilateral comparison is non-negotiable – AC joint anatomy varies significantly between individuals; unilateral assessment is unreliable
  2. CC distance measurements – Use shortest distance between superior coracoid and inferior clavicle; normal is 11-13mm in men, 9-11mm in women
  3. Zanca view superiority – 15° cephalic tilt with 50% reduced penetration eliminates superimposition and reveals subtle pathology
  4. Axillary lateral necessity – Only view that reliably detects Type IV posterior displacement; if patient can’t position, consider CT
  5. Weight-bearing views controversy – Limited evidence for clinical utility; patient discomfort often limits diagnostic value
  6. Age-related changes – AC joint degeneration begins in 30s; distinguish acute injury from chronic degenerative changes

 

Ultrasound Integration Strategies

  1. Real-time dynamic assessment – Evaluate joint stability during shoulder movement; particularly valuable for borderline cases
  2. Point-of-care efficiency – Immediate evaluation can guide initial treatment decisions and patient counseling
  3. CC ligament visualization – High-frequency transducers can identify partial vs. complete CC ligament tears
  4. Comparative technique – Always scan contralateral side; normal variants are common
  5. Learning curve consideration – Requires dedicated training for reliable interpretation; consider teleradiology consultation for complex cases

 

Advanced Diagnostic Considerations

  1. Associated injury screening – 20-30% of AC joint injuries have concomitant rotator cuff pathology
  2. Chronic vs. acute differentiation – Bone marrow edema on MRI helps distinguish acute from chronic injuries
  3. Functional imaging – Kinematic MRI or ultrasound can assess dynamic instability patterns
  4. 3D CT reconstruction – Valuable for complex cases requiring surgical planning
  5. Nerve conduction studies – Consider in Type IV-VI injuries with neurological symptoms

After diving into this detailed guide on acromioclavicular (AC) joint dislocations, you’re now equipped with practical, clinic-ready insights to improve how you assess and manage shoulder separations.

Here’s what you should walk away with:

Confident Diagnosis
You now know how to distinguish between a mild sprain and a high-grade separation using the Rockwood classification. Recognize key clinical signs—like the cross-body adduction test and visible step-off deformities—and correlate them with imaging findings.

Smarter Imaging Protocols
Standardized imaging is a game changer. Always get bilateral AP views, a proper Zanca view, and an axillary lateral to avoid missing posterior or subtle injuries. Don’t forget—ultrasound can offer real-time soft tissue assessment, especially in borderline cases.

When to Refer
Not every injury needs surgery, but Types IV, V, and VI do. If there’s neurovascular compromise, open injury, or failure of conservative management, refer fast.

Teleradiology = Clinical Edge
Partnering with a teleradiology service ensures rapid, expert interpretation—especially when the imaging is unclear or time-sensitive.

Bottom line: Mastering AC joint dislocations isn’t just about knowing anatomy—it’s about integrating clinical judgment, imaging, and referral timing to make the right call when it matters most.

Frequently asked questions

What are the specific radiographic measurements I should know for AC joint evaluation?

Normal CC distance is 11-13mm in men, 9-11mm in women.

AC joint space should be 1-3mm (widens to >7mm in Type II injuries).

Type III injuries show 25-100% increase in CC distance compared to contralateral side.

Type V demonstrates >100% increase (typically >25mm absolute).

Always measure the shortest distance between superior coracoid and inferior clavicle for CC distance.

Ultrasound is excellent for real-time dynamic assessment and soft tissue evaluation but should complement, not replace, radiographic evaluation. It’s superior for visualizing CC ligament tears, joint capsule disruption, and associated soft tissue hematomas. Sensitivity approaches 90% for complete AC separations when performed by experienced operators. However, it’s operator-dependent and has limitations in deeper structure visualization.

Learn diagnostic ultrasound.

Type III-VI injuries have 20-30% incidence of concomitant rotator cuff tears, especially in patients >40 years. Type IV injuries carry risk of brachial plexus injury due to posterior displacement. High-energy mechanisms (MVA, significant falls) increase risk of rib fractures, pneumothorax, or clavicle fractures. Consider cervical spine imaging in high-energy trauma. Chronic Type III injuries may develop secondary impingement syndrome.

Acute injuries show: bone marrow edema on MRI, soft tissue swelling on ultrasound, sharp cortical margins on radiographs, and clinical correlation with recent trauma.

Chronic degenerative changes demonstrate: joint space narrowing, osteophyte formation, subchondral sclerosis, and gradual onset symptoms.

Key differentiator: Acute CC ligament injury causes increased CC distance; degenerative disease typically maintains normal CC relationships.

Working with a DACBR (Doctor of Chiropractic, Board Certified in Radiology) or specialized musculoskeletal radiologist ensures precise interpretation of subtle imaging findings and accurate classification of AC joint injuries and other musculoskeletal conditions. These radiology experts utilize standardized measurements to assess injury severity, detect associated pathologies that are often missed in routine evaluations, and provide evidence-based treatment recommendations that enhance patient outcomes.

This level of expertise is especially critical in:

  • Borderline Type III AC joint injuries, where conservative vs. surgical management is debated

  • Complex shoulder injuries that require surgical planning

  • Medico-legal cases that demand clear, defensible radiological interpretation

For chiropractors seeking to improve diagnostic accuracy, streamline referrals, and boost practice credibility, partnering with a DACBR-led teleradiology service is a powerful step toward elevated patient care.

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