The ankle is often thought of as a single hinge, but it is actually a complex, weight-bearing joint system that connects the lower leg to the foot. Formally known as the talocrural joint, it is engineered for stability and controlled movement to manage the forces of walking, running, and jumping. The structure is formed by the interaction of three distinct bones that must fit together precisely to maintain proper function.
The Three Primary Ankle Bones
The three bones that create the ankle joint are 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 the majority of the body’s weight. At its lower end, the tibia features a bony projection on the inside of the ankle known as the medial malleolus, which helps stabilize the joint.
Positioned parallel to the tibia is the fibula, a much thinner bone located on the outer side of the leg. The fibula plays a smaller role in weight bearing compared to the tibia, but its presence is important for ankle stability. The lower tip of the fibula forms the lateral malleolus, the prominent bony bump visible on the outside of the ankle.
Fitting snugly between the ends of the tibia and fibula is the talus, one of the seven tarsal bones of the foot. The talus acts as a bridge, transferring weight from the leg bones down to the heel bone, or calcaneus. The superior surface of the talus is dome-shaped and covered in articular cartilage to allow smooth movement within the joint socket.
How the Ankle Bones Articulate
The unique way the three bones fit together creates the highly stable talocrural joint. This articulation is frequently compared to a carpentry design called a mortise and tenon joint. In this analogy, the ends of the tibia and fibula are tightly bound by strong ligaments to form a bracket-shaped socket, which is the mortise.
The talus acts as the tenon, a wedge-shaped block that fits securely into the mortise. This tight, hinged arrangement primarily allows movement in only one plane. The movements permitted at the ankle joint are dorsiflexion (lifting the foot toward the shin) and plantarflexion (pointing the foot downward).
The upper part of the talus is wider toward the front (anteriorly) than the back (posteriorly). When the foot is in dorsiflexion, the wider anterior portion of the talus is driven into the mortise, creating maximum bone-on-bone contact. This action locks the joint tightly, making the ankle more stable. Conversely, during plantarflexion, the narrower part of the talus is engaged, making the joint less stable and more susceptible to injury.
Bone Fractures
The bony prominences and the joint’s load-bearing surfaces are common sites for ankle fractures, typically caused by rotational forces or high-impact compression. A fracture involving only one of the malleoli is called a unimalleolar fracture, most often affecting the lateral malleolus of the fibula. When both the medial malleolus of the tibia and the lateral malleolus of the fibula are broken, the injury is classified as a bimalleolar fracture.
A more complex injury is the trimalleolar fracture, which includes breaks in the medial malleolus, the lateral malleolus, and the posterior portion of the tibia (posterior malleolus). These multiple breaks significantly disrupt the stability of the ankle mortise and often require surgical repair. A high-energy injury known as a Pilon fracture involves a break in the distal, weight-bearing surface of the tibia. Pilon fractures usually result from vertical forces, such as a fall from height, which drives the talus upward into the bottom of the tibia, leading to a severe fracture involving the joint’s articular cartilage.