The ankle is a complex structure connecting the leg to the foot, crucial for stability and movement. While often considered a single entity, it is a joint formed by multiple bones and supporting structures. This intricate design allows for a wide range of motion, enabling essential daily activities like walking, running, and maintaining balance.
More Than One Bone: The Ankle’s Bony Components
The primary ankle joint, known as the talocrural joint, involves the articulation of three main bones: the tibia, the fibula, and the talus. The tibia, commonly called the shin bone, is the larger of the two lower leg bones and bears most of the body’s weight. Its lower end forms the medial malleolus, the prominence on the inside of the ankle.
Adjacent to the tibia is the fibula, a thinner bone that runs parallel to the tibia in the lower leg. The distal end of the fibula forms the lateral malleolus, the protrusion on the outside of the ankle. Together, the tibia and fibula create a bracket-shaped socket, often described as a mortise, into which the talus fits. This mortise-and-tenon arrangement provides stability to the ankle joint.
The talus is a tarsal bone positioned directly beneath the tibia and fibula. Its superior surface, called the trochlear surface, is shaped like a pulley and articulates with the inferior surface of the tibia and the malleoli of both the tibia and fibula. The talus is wider at its front portion and narrower at the back, meaning the ankle joint is most stable when the foot is flexed upwards (dorsiflexed) as the wider part of the talus fits snugly into the mortise.
The Ankle’s Supporting Structures and Range of Motion
Beyond its bony framework, the ankle joint relies on a network of ligaments and tendons for stability and movement. Ligaments are strong fibrous bands that connect bones to other bones, keeping the joint aligned and preventing excessive motion. On the outer side of the ankle, three lateral ligaments originate from the fibula and connect to the talus and calcaneus. These include the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL), and the posterior talofibular ligament (PTFL). The ATFL is often the most frequently injured ligament during ankle sprains, especially with inversion injuries.
On the inner side of the ankle, the deltoid ligament complex provides stability. This strong, triangular band connects the tibia to bones in the foot, including the talus, calcaneus, and navicular. The deltoid ligament helps prevent the foot from rolling excessively outward (eversion). Due to its size and strength, isolated deltoid ligament injuries are less common than lateral ankle sprains, requiring significant force.
Tendons, which connect muscles to bones, cross the ankle joint and facilitate movement. The Achilles tendon, a major tendon, connects the calf muscles to the heel bone. It is essential for pointing the foot downward, an action known as plantarflexion. Peroneal tendons run along the outside of the ankle, stabilizing the foot and assisting with outward movements.
The ankle joint primarily allows for two main movements: dorsiflexion and plantarflexion. Dorsiflexion involves lifting the front of the foot upwards. Conversely, plantarflexion is the action of pointing the foot and toes downward, like pressing a car pedal. While the talocrural joint is responsible for these primary movements, other movements like inversion (turning the sole of the foot inward) and eversion (turning the sole of the foot outward) occur mainly at the subtalar joint, located just below the ankle joint between the talus and calcaneus. These accessory movements are important for adapting to uneven surfaces.