A 28-year-old man presents to the ED with acute ankle pain after an inversion injury during a football match. An AP mortise view X-ray is obtained. The structure marked **D** (talar dome in mortise) appears to have uniform joint space of 3 mm around it with no widening or asymmetry. Which of the following injuries is MOST likely ruled out by this normal appearance of the structure marked **D**?

Explanation

## Why "Syndesmotic injury with talar shift" is right The talar dome in mortise view normally maintains a uniform joint space of 3–5 mm around it, forming a symmetric "bracket" with the distal tibia and both malleoli. Widening of the clear space (>4 mm) or asymmetric positioning of the talar dome indicates talar shift, which is the hallmark radiographic sign of syndesmotic injury or disruption of the ankle mortise. A normal appearance of the structure marked **D** with uniform joint space directly excludes syndesmotic injury with talar displacement. Per Maheshwari Orthopedics 10e, the mortise view is specifically designed to detect such disruptions by bringing both malleoli into the same plane. ## Why each distractor is wrong - **Anterior talofibular ligament (ATFL) rupture**: This is the most commonly injured ligament in inversion ankle sprains. ATFL rupture does not necessarily cause widening of the mortise or talar shift on AP mortise view; the radiograph may appear normal. Clinical examination (anterior drawer test) is more sensitive for ATFL injury than imaging. - **Isolated medial malleolus fracture**: A fracture of the medial malleolus alone does not necessarily displace the talar dome or widen the mortise if the deltoid ligament remains intact and syndesmotic structures are uninjured. The talar dome position can remain normal. - **Deltoid ligament disruption without talar displacement**: Deltoid ligament injury (part of structure **B**) can occur without causing talar shift. If the syndesmotic ligaments and lateral structures remain intact, the talar dome may maintain its normal mortise position and uniform joint space. **High-Yield:** A normal, symmetric talar dome in mortise view with uniform 3–5 mm joint space excludes syndesmotic injury and talar shift; widening >4 mm indicates syndesmotic disruption requiring urgent orthopedic evaluation. [cite: Maheshwari Orthopedics 10e — AP Ankle Mortise View, Ankle Mortise Anatomy, Syndesmotic Injury]