The fibula is the smaller of the two long bones in the lower leg, running parallel to the tibia, or shin bone. The term “distal” refers to the part of a limb situated away from the center of the body. The distal fibula is the bottom end of this slender bone, located at the ankle joint. This area forms a component of the ankle’s structure, a joint subjected to significant daily forces.
Defining the Distal Fibula
The distal end of the fibula is the prominent, bony bump on the outer side of the ankle, known as the lateral malleolus. It extends further downward than the inner ankle bone, providing inherent stability. The slender fibula is significantly thinner than the robust tibia and bears only a small fraction of the body’s weight.
The lateral malleolus is flattened, meaning it is wider from front to back. Its inner surface is smooth, featuring an articular facet that contacts the side of the talus, one of the main bones of the foot. Behind this smooth surface is a depression called the malleolar fossa, which serves as an attachment site for stabilizing ligaments.
Role in Ankle Stability
The primary function of the distal fibula is providing mechanical stability to the ankle joint complex, not weight support. Together with the lower end of the tibia, the lateral malleolus forms a protective socket, often described as a mortise, that securely cups the talus bone. This configuration prevents excessive side-to-side shifting of the talus, allowing the ankle to function primarily as a hinge joint for up and down movement of the foot.
The distal fibula serves as the anchor point for the lateral collateral ligaments of the ankle. These structures include the anterior talofibular, posterior talofibular, and calcaneofibular ligaments. Originating on the lateral malleolus and attaching to bones in the foot, they collectively resist the foot’s tendency to roll inward (inversion). The fibula’s position also helps limit the external rotation of the ankle.
Common Injuries and Causes
Given its exposed position, the distal fibula is frequently involved in ankle trauma. The most common injury is a lateral malleolus fracture, a break often caused by twisting or rolling the ankle. Fractures range from a small avulsion fracture, where a ligament pulls a bone fragment away, to a more complete break. Inversion injuries, where the foot turns sharply inward, are a frequent mechanism for this damage.
A high ankle sprain, or syndesmotic injury, is another pathology in this region. This occurs when the ligaments connecting the distal fibula and tibia (the syndesmosis) are torn. The mechanism is typically a forceful external rotation of the foot relative to the leg, which pries the bones apart. This type of sprain is often more debilitating than a standard lateral ankle sprain because it compromises the integrity of the ankle mortise.
Repetitive stress from activities like long-distance running can lead to a distal fibula stress fracture. This is a small crack that develops gradually over time due to continuous microtrauma. Any injury to the distal fibula or its surrounding ligaments can lead to ankle joint instability, requiring timely diagnosis and appropriate treatment to prevent long-term functional issues.