Introduction:
The shoulder joint is the most frequently dislocated joint in the body due to its wide range of motion and relatively shallow glenoid fossa. Dislocations may be anterior (most common), posterior, inferior (luxatio erecta), or rarely superior. Imaging is essential for accurate diagnosis, identifying associated injuries, and planning management.
Types of Shoulder Dislocation:
| Type | Frequency | Direction of Humeral Head Displacement |
|---|---|---|
| Anterior | ~95% | Anteroinferior to glenoid |
| Posterior | ~2–4% | Posterior to glenoid |
| Inferior | <1% | Inferoinferior displacement |
| Superior | Rare | Superior displacement due to trauma |
Imaging Modalities and Approach:
1. Plain Radiography (Initial Modality of Choice):
Standard Views:
- AP view
- Scapular Y view
- Axillary view (essential to avoid missing posterior dislocation)
- Modified trauma axial view if patient cannot abduct
Radiographic Signs:
A. Anterior Dislocation:
- Humeral head displaced inferomedially to glenoid
- “Subcoracoid” position of head (under coracoid)
- Associated injuries:
- Hill-Sachs lesion (posterolateral humeral head compression fracture)
- Bankart lesion (anteroinferior glenoid labrum fracture)
- Greater tuberosity fracture
B. Posterior Dislocation:
- Humeral head appears fixed in internal rotation (“light bulb sign”)
- Rim sign: widened glenohumeral joint space
- Trough sign: impaction fracture of anteromedial humeral head
C. Inferior Dislocation:
- Humeral head lies inferior to glenoid
- Arm held overhead in abduction (“Luxatio erecta”)
2. Ultrasound (USG):
- Useful in emergency settings to confirm dislocation and evaluate rotator cuff
- Not a replacement for radiographs/MRI
3. CT Scan:
- Best for bony detail
- Used when:
- Complex fracture-dislocation
- Surgical planning
- Evaluation of glenoid bone loss, Hill-Sachs, or Bankart lesions
- 3D reconstruction helps in preoperative assessment
4. MRI:
- Ideal for soft tissue evaluation
- Best to assess:
- Labral tears (Bankart, ALPSA, Perthes lesions)
- Capsuloligamentous injuries
- Rotator cuff pathology
- Chronic instability signs
- MR arthrography improves sensitivity for labral and capsular injuries
Key Imaging Findings by Dislocation Type:
| Type | Key Signs |
|---|---|
| Anterior | Subcoracoid head, Hill-Sachs, Bankart |
| Posterior | Light bulb sign, rim sign, trough sign |
| Inferior | Humeral head below glenoid, abducted arm |
| Chronic | Glenoid bone loss, rotator cuff tear, capsule laxity |
Associated Lesions to Evaluate:
- Bony Bankart (glenoid rim fracture)
- Hill-Sachs lesion
- Labral tears (anterior, posterior)
- HAGL lesion (humeral avulsion of glenohumeral ligament)
- Rotator cuff tears
- Axillary nerve injury (clinical correlation)
Conclusion:
A systematic imaging approach is crucial in shoulder dislocation to confirm the type, identify associated soft tissue and bony injuries, and guide treatment. X-rays remain the first-line tool, while MRI and CT provide valuable additional information for surgical planning and assessment of complications or instability.
