The ankle is a hinge joint that lets the foot point up and down. It carries the whole body weight with every step.
The mortise
The lower ends of the tibia and fibula form a slot, the mortise, that grips the top of the talus. The bony bumps on each side are the medial malleolus (tibia) and the lateral malleolus (fibula).
Lateral ligaments
Three ligaments support the outer ankle: the anterior talofibular, calcaneofibular and posterior talofibular ligaments. Rolling the foot inward (inversion) usually sprains the anterior talofibular ligament first.
Medial ligament and syndesmosis
The strong deltoid ligament supports the inner side, so medial injuries often come with a fracture. The syndesmosis binds the tibia and fibula together above the joint, and its injury is called a high ankle sprain.
Achilles tendon
The calf muscles attach to the calcaneus through the Achilles tendon, the main plantar flexor of the foot. A sudden snap during push-off suggests a rupture.
At the bedside
The Ottawa ankle rules call for an X-ray when there is bone tenderness at the back edge or tip of either malleolus, or the patient cannot take four steps. Squeezing the calf (Thompson test) checks whether the Achilles tendon is intact.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).