The ankle is a complex junction connecting the lower leg to the foot, bearing the body’s weight and providing mobility and stability. This structure is not formed by a single bone, but by the articulation of several distinct skeletal components. To understand what is commonly called “the ankle bone,” it is necessary to examine the individual bones that constitute this joint.
The Talus: The Central Ankle Bone
The bone most often referred to as the ankle bone is formally known as the Talus, or sometimes the astragalus. This small but robust bone sits directly above the heel bone, serving as the link between the foot and the lower leg. The Talus is classified as one of the seven tarsal bones that form the rear and mid-foot, and it is the second largest of this group.
Its unique shape features a dome-like superior surface, known as the trochlea, which is specifically designed to articulate with the leg bones. This surface acts like a hinge, allowing for the primary movements of the ankle: dorsiflexion, which is pulling the toes upward, and plantarflexion, which is pointing the toes downward. Due to its position, the Talus is almost entirely covered in cartilage and has no muscle attachments, meaning its blood supply can be delicate.
The Ankle Joint: Where Three Bones Meet
The true ankle joint, or talocrural joint, is a highly stable hinge joint formed by the Talus articulating with two bones from the lower leg. These two bones are the Tibia, commonly known as the shin bone, and the Fibula, the thinner bone that runs parallel to the Tibia. The inferior ends of the Tibia and Fibula wrap around the Talus to create a highly congruent socket.
This specific arrangement is often described as a mortise and tenon structure, borrowing terms from carpentry. The Tibia and Fibula form the three-sided mortise, or socket, which tightly holds the Talus, the tenon, in place. This design provides significant side-to-side stability, especially when the foot is pulled upward, or dorsiflexed.
Clarifying Common Ankle Terminology
The prominent bony points felt on either side of the ankle are the Malleoli, and these are often the structures people mistake for the single ankle bone. These protrusions are not separate bones but are actually the rounded ends of the Tibia and the Fibula. The inner bump, which is closer to the body’s midline, is called the medial malleolus and is the distal end of the Tibia.
The outer bump, which is more noticeable, is the lateral malleolus and forms the end of the Fibula. These malleoli function as the bony walls of the ankle joint’s mortise, preventing the Talus from shifting sideways. They stabilize the joint and act as attachment points for the strong ligaments that secure the ankle complex.
The Tarsals: Neighboring Foot Bones
The Talus is anatomically part of a larger group of seven bones collectively known as the Tarsals, which make up the hindfoot and midfoot. While the Talus forms the ankle joint with the leg bones, it also sits upon the largest of the tarsal bones, the Calcaneus, commonly known as the heel bone. This articulation forms the subtalar joint, which is responsible for the foot’s side-to-side rotation.
The remaining tarsal bones include the Navicular, the Cuboid, and three Cuneiform bones. These bones are tightly bound together by ligaments to create the arches of the foot, which are necessary for shock absorption and weight distribution.