Why is the ankle joint least stable and most prone to dislocation when the foot is fully plantarflexed?
- A The deltoid ligament relaxes completely in plantarflexion
- B The calcaneofibular ligament becomes slack and detaches from the malleolus
- C The narrow posterior end of the trochlear surface of the talus occupies the mortise, allowing side to side play ✓
- D The tibia rotates internally, opening the syndesmosis widely
Correct answer: C. The narrow posterior end of the trochlear surface of the talus occupies the mortise, allowing side to side play
Explanation
The trochlear (superior articular) surface of the talus is wider anteriorly than posteriorly. In dorsiflexion the broad anterior part wedges firmly between the medial and lateral malleoli, giving maximum bony stability. In full plantarflexion the narrow posterior part sits in the mortise, leaving side to side laxity, which is why landing in plantarflexion risks dislocation. Ligament tension changes alone do not explain this bony mechanism.
Reference: Gray's Anatomy for Students, 4th ed.
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