足首関節

The ankle, or talocrural joint, is a uniaxial hinge joint that allows two kinds of movements: dorsiflexion and plantarflexion. This functional classification limits the ankle to forward and backward foot motion. The joint's structure prevents sideways movement, making it ideal for supporting body weight during walking and standing.

The ankle joint is formed by three bones: the talus from the foot articulates with the distal end of the tibia from the leg. The medial malleolus of the tibia and lateral malleolus of the fibula also articulate with the talus, firmly holding it in position and preventing side-to-side motion.

Hyaline cartilage covers the articular surfaces of the ankle joint, protecting bones from friction and facilitating smooth joint movement. This cartilage layer reduces wear on bone surfaces during dorsiflexion and plantarflexion, enabling pain-free motion and supporting the joint's weight-bearing function.

Multiple strong ligaments support the ankle joint and prevent abnormal movements. The medial deltoid ligament resists excessive eversion, while the lateral ligaments—anterior and posterior talofibular ligaments and calcaneofibular ligament—block excess inversion. Together, they stabilize the talus and prevent twisting motions.

The fibrous capsule surrounds the ankle joint and attaches to the articular surfaces of the tibia, malleoli, and talus. This connective tissue layer encloses the joint, providing structural support and helping to contain synovial fluid that lubricates the articulating surfaces during movement.

The superior aspect of the talus bone is square-shaped with three areas of articulation. The top articulates with the tibia to bear body weight, while the sides articulate with the medial and lateral malleoli. This geometry ensures stable weight distribution and prevents lateral displacement of the foot.

The medial side features the broad deltoid ligament, which resists excessive eversion. The lateral side has three smaller ligaments: anterior and posterior talofibular ligaments connecting the talus to the fibula, and the calcaneofibular ligament connecting the calcaneus to the fibula. Both systems prevent abnormal side-to-side motion.