The structure indicated is the calcaneofibular ligament of the foot.
The calcaneofibular ligament is one of three ligaments found on the lateral aspect of the ankle. The lateral ligaments of the ankle joint consists of:
- Anterior talofibular ligament
- Posterior talofibular ligament
- Calcaneofibular ligament
The calcaneofibular ligament has its superior attachment on the malleolar fossa on the lateral malleolus, and its inferior attachment on the lateral surface of the calcaneus. Running across the calcaneofibular ligament are the tendons of the peroneus longus and peroneus brevis.
Learn more about the anatomy of the ankle joint in this tutorial.