A 28-year-old male presents to the emergency department following a motor vehicle accident with severe right ankle pain and swelling. Clinical examination reveals tenderness over the lateral malleolus and inability to bear weight. An anteroposterior (AP) radiograph of the ankle appears normal. However, the radiologist notes that the medial clear space (distance between the medial border of the talus and the medial malleolus) measures 5 mm on the AP view, compared to 3 mm on the contralateral side. What is the most appropriate next imaging step to confirm the suspected diagnosis?

Explanation

## Clinical Scenario Analysis This patient has imaging findings suggestive of a **syndesmotic injury** (high ankle sprain) with widening of the medial clear space, indicating disruption of the deltoid ligament and/or syndesmotic ligaments. The normal AP radiograph initially masks the injury because static views do not stress the ligamentous structures. ## Role of Stress Radiographs **Key Point:** Stress radiographs (external rotation or inversion stress views) are the gold standard for detecting occult syndesmotic and ligamentous injuries when plain radiographs are equivocal or normal. **High-Yield:** The medial clear space >5 mm (or >2 mm difference from the contralateral side) on AP view, or >6 mm on mortise view, indicates syndesmotic disruption and is an indication for stress imaging or advanced imaging. ## Imaging Hierarchy for Ankle Injuries | Imaging Modality | Indication | Sensitivity for Ligament Injury | |---|---|---| | **Plain radiographs (AP, lateral, mortise)** | Initial screening; detect fractures | Low for pure ligamentous injury | | **Stress radiographs** | Equivocal plain films; assess syndesmotic integrity | High; cost-effective | | **MRI** | Detailed soft-tissue assessment; chronic instability; surgical planning | Very high but expensive | | **CT** | Fracture characterization; 3D anatomy for complex fractures | Not first-line for ligament assessment | | **Ultrasound** | Dynamic assessment; operator-dependent; useful in experienced hands | Moderate; limited availability | ## Clinical Pearl **Syndesmotic injuries are frequently missed on initial radiographs.** The external rotation stress test (Kleiger test) combined with external rotation stress radiographs is the most practical and cost-effective next step in this clinical scenario. If the medial clear space widens further under stress, syndesmotic disruption is confirmed, and the patient requires immobilization or surgical fixation depending on severity. ## Why Stress Radiographs Are Preferred Here 1. **Cost-effective:** Significantly cheaper than MRI or CT. 2. **Readily available:** Can be performed in the ED or radiology department. 3. **Diagnostic accuracy:** Excellent sensitivity and specificity for syndesmotic injury when properly performed. 4. **Clinical utility:** Results directly guide management (conservative vs. operative). [cite:Rockwood and Green's Fractures in Adults Ch 53]