The ankle is often thought of as a single joint, but it is a complex structure connecting the lower leg to the foot. This region involves multiple bones working together to facilitate movement and bear the body’s weight. The common name “ankle bone” is a simplification, as several bony elements form this major articulation. This article clarifies the specific bones that form the true ankle joint and those that provide necessary support to the rear foot.
Defining the True Ankle Joint
The articulation responsible for the primary up-and-down motion of the foot is known formally as the talocrural joint. This true ankle joint is a synovial hinge joint formed by three distinct bones: the Tibia, the Fibula, and the Talus. The Tibia, or shin bone, is the larger of the two lower leg bones and bears the majority of the body’s weight.
The distal, or lower, end of the Tibia forms the inner bump of the ankle, which is called the medial malleolus. The Fibula, the smaller leg bone, runs parallel to the Tibia and forms the outer ankle bump, known as the lateral malleolus. Together, the bottom surfaces of the Tibia and Fibula create a secure, bracket-shaped socket. This socket is anatomically referred to as the ankle mortise.
The Talus, the topmost bone of the foot, fits snugly into this mortise. The Talus transmits the forces from the leg to the rest of the foot bones, making it a central component of the lower limb’s weight distribution system.
The Supporting Tarsal Bones
Beyond the talocrural joint, the ankle complex includes a group of seven short, irregularly shaped bones known as the Tarsals. These bones comprise the rear section of the foot, providing a stable foundation for the ankle joint above them. The Talus is technically one of these seven Tarsals, but its function in the main joint sets it apart from the others.
The largest Tarsal bone is the Calcaneus, commonly known as the heel bone, which sits directly beneath the Talus. The Calcaneus is the first bone to make contact with the ground during walking and plays a significant role in supporting body weight. It articulates with the Talus at the subtalar joint, but not with the Tibia or Fibula.
The remaining five Tarsals form the midfoot. These smaller bones are tightly bound by ligaments, forming the arches of the foot.
The five midfoot Tarsals include:
- The Navicular
- The Cuboid
- The three Cuneiforms (medial, intermediate, and lateral)
Movement and Stability
The bony architecture of the talocrural joint is specialized to allow two main movements: dorsiflexion and plantarflexion. Dorsiflexion pulls the foot upward toward the shin, while plantarflexion points the foot downward, like pressing an accelerator pedal. This hinge-like motion occurs as the Talus rotates within the mortise.
The shape of the Talus contributes significantly to stability, as it is wider across its anterior portion than its posterior portion. When the ankle is in dorsiflexion, the wider part of the Talus is forced into the mortise, creating a tighter, more stable joint. Conversely, the joint is less stable when the foot is fully pointed in plantarflexion.
Ligaments surrounding the joint maintain stability by resisting excessive movement and preventing dislocation. On the inside of the ankle, the fan-shaped deltoid ligament resists the foot turning too far outward, or over-eversion. The outside of the ankle is reinforced by a group of three lateral ligaments, including the anterior and posterior talofibular ligaments and the calcaneofibular ligament, which primarily guard against over-inversion.