Final answer:
The articular capsule encases the ankle joint, surrounding the joint cavity and providing stability, particularly through the anterior talofibular ligament on the lateral side.
Step-by-step explanation:
The capsule that encases the ankle joint is known as the articular capsule. This structure is a connective tissue that surrounds the joint cavity of a synovial joint, such as the ankle or talocrural joint. The articular capsule provides stability to the joint by encasing the articulations between the talus bone and the malleoli of the tibia and fibula. On the lateral side of the ankle, the anterior talofibular ligament, an intrinsic ligament, is located between the talus bone and the lateral malleolus of the fibula, supporting the talus at the talocrural joint and resisting excess inversion of the foot. This ligament is part of the articular capsule system that functions to stabilize the ankle joint.