A 55-year-old man has a displaced trimalleolar ankle fracture. Which component, when fractured and exceeding 25% of the tibial articular surface on CT, mandates open reduction and fixation of the posterior malleolus?

• A Medial malleolus involvement alone
• B Fibula fracture above the level of the syndesmosis
• C Posterior malleolus (posterior tibial fragment) > 25% of the tibial plafond surface ✓
• D Talar shift > 2 mm on stress radiograph

Explanation

The posterior malleolus contributes to ankle stability; when its fragment involves more than 25% of the tibial articular surface, it must be fixed to restore articular congruence, prevent posterior talar subluxation, and reduce early post-traumatic arthritis. Below 25%, the talar dome contacts primarily the intact anterior surface and the fragment can be left unfixed. Fibular fixation and medial malleolus fixation are addressed separately; talar shift guides overall mortise restoration, not specifically posterior malleolus fixation threshold.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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