How do you differentiate between an anterior and posterior shoulder dislocation?

Differentiating Anterior from Posterior Shoulder Dislocation

Obtain a standard trauma radiograph series including anteroposterior (AP) views in internal and external rotation PLUS an axillary or scapula-Y view—the axillary/scapula-Y view is essential because AP views alone frequently miss posterior dislocations, which are misdiagnosed in over 60% of cases initially. 1, 2, 3

Clinical Examination Findings

Anterior Dislocation (95% of cases)

• Arm position: Abducted and externally rotated 4
• Shoulder contour: Loss of normal rounded contour with anterior fullness 4
• Axilla: Increased vertical diameter anteriorly 4
• Range of motion: Restricted internal rotation and adduction 5

Posterior Dislocation (Often Missed)

• Arm position: Adducted and internally rotated, locked in internal rotation 3, 6
• Shoulder contour: Subtle posterior fullness with anterior flatness of the shoulder 3
• Range of motion: Marked loss of external rotation and abduction—this is the key clinical finding 3
• Mechanism clues: History of seizures, electroshock, or fall onto a flexed, adducted arm should immediately raise suspicion 3, 6

Critical pitfall: Always compare the affected shoulder to the unaffected side, as posterior dislocations present with subtle findings that are easily missed without careful bilateral comparison. 3

Radiographic Evaluation Algorithm

Step 1: Initial Imaging

• Standard trauma series required: AP views in internal and external rotation PLUS axillary or scapula-Y view 1, 2
• Why the third view is mandatory: Glenohumeral dislocations can be misclassified on AP views alone—posterior dislocations are frequently missed without proper orthogonal views 1, 2

Step 2: Radiographic Signs

Anterior dislocation findings:

• Humeral head displaced anteriorly and inferiorly to the glenoid on axillary/Y views 1
• Look for associated Hill-Sachs deformity (posterolateral humeral head compression fracture) 1
• Look for bony Bankart lesion (anterior glenoid rim fracture) 1

Posterior dislocation findings:

• Humeral head displaced posteriorly relative to glenoid on axillary/Y views 1, 7
• "Light bulb sign" on AP view (humeral head appears symmetric/internally rotated) 3, 7
• Look for reverse Hill-Sachs deformity (anteromedial humeral head compression fracture) 6
• Look for posterior glenoid rim fractures 6

Step 3: Advanced Imaging if Diagnosis Unclear

• CT scan: Should be performed if symptoms persist or radiographs are equivocal, as CT better characterizes fracture patterns and can definitively identify posterior dislocations 1, 7

Critical Management Considerations

Before attempting reduction:

• Radiography must confirm the dislocation type and identify associated fractures 2, 5
• Assess for neurovascular compromise, particularly in older patients and those with proximal humeral fractures 2
• Never attempt reduction without radiographic confirmation—attempting reduction of an unconfirmed dislocation could worsen fracture-dislocations 2

Post-reduction:

• Obtain post-reduction radiographs to confirm successful reduction and evaluate for fractures that may have been obscured 2
• Reassess neurovascular status 2

Major pitfall: Posterior dislocations are missed in over 60% of cases initially because clinicians fail to obtain proper orthogonal views (axillary or scapula-Y) and miss the subtle clinical findings of posterior fullness and loss of external rotation. 3, 7 The correct diagnosis is often delayed for months or years, leading to significant morbidity. 3