The ankle is a complex joint system connecting the lower leg to the foot. While often referred to as a single structure, its stability and function depend on the precise alignment of multiple bones. This joint allows for the dynamic motion required for walking, running, and standing, playing a central role in movement and weight transfer.
The Primary Bony Components
The core ankle joint, known as the talocrural joint, is formed by the articulation of three bones: the tibia, the fibula, and a bone of the foot. The tibia (the largest lower leg bone) and the fibula (the smaller bone) form a socket. Their distal ends widen to create bony prominences on either side of the ankle called malleoli, which stabilize the joint. The medial malleolus is the bump on the inside (lower part of the tibia), and the lateral malleolus is the bump on the outside (lower end of the fibula). Together, the tibia and fibula create a fork-like structure, called the ankle mortise.
The Talus: The Ankle Bone
The talus is often considered the true “ankle bone,” acting as the bridge between the leg and the foot. This small, irregularly shaped bone sits directly beneath the tibia and fibula, transferring the body’s entire weight to the foot. The talus is unique because it lacks direct muscle or tendon attachments; its position is stabilized entirely by surrounding ligaments and the bony mortise. It has one of the highest percentages of its surface covered by articular cartilage, the smooth tissue allowing joints to glide. This extensive cartilage coverage limits the bone’s blood supply, making it vulnerable to disruption. The talus also articulates with the heel bone (calcaneus) beneath it, forming the subtalar joint.
Function and Range of Motion
The ankle joint is classified as a hinge joint, designed to handle weight-bearing and forward motion. Its structure allows movement mainly in one plane: the up-and-down motion of the foot. The two primary movements are dorsiflexion (lifting the front of the foot upward toward the shin) and plantarflexion (pointing the toes downward). The interlocking structure of the talus within the bony mortise provides increased stability during dorsiflexion. The total range of motion is typically 65 to 75 degrees, with plantarflexion usually accounting for the greater share of movement.
Common Ankle Bone Injuries
Injuries involving the ankle bones are fractures, which differ from common ankle sprains that involve ligaments. The most frequent type of bone injury involves the malleoli, resulting in fractures of the distal tibia or fibula. These are commonly referred to as lateral malleolus, medial malleolus, or bimalleolar fractures (if both are broken). A fracture to the talus, though less common, is often a more serious concern. Since the talus has a limited blood supply, a fracture can easily disrupt blood flow, potentially leading to osteonecrosis, where bone tissue dies. Fractures that displace the talus from its socket are unstable and frequently require surgical intervention to restore proper alignment and joint stability.