The fibula is a long bone, one of the two bones in your lower leg alongside the tibia (shinbone). It is the third longest bone in the human body, and like all long bones, it has a cylindrical shaft connecting two wider ends. Despite its length, the fibula carries surprisingly little of your body weight, serving instead as a critical anchor for muscles and a stabilizer for your ankle joint.
What Makes the Fibula a Long Bone
Bones are classified into five types based on their shape: long, short, flat, irregular, and sesamoid. Long bones are defined by having a shaft (called the diaphysis) that is significantly longer than it is wide, capped at each end by rounded portions (the epiphyses). The fibula fits this definition clearly. Its shaft is shaped like a three-sided prism and runs the length of your outer calf, connecting a knobby head at the top to the bony bump you can feel on the outside of your ankle at the bottom.
Like other long bones such as the femur, humerus, and tibia, the fibula contains a medullary cavity filled with bone marrow. The outer layer is dense cortical bone, which gives it strength and rigidity. This internal structure is one reason the fibula works so well as a donor bone for surgical grafts, with material properties including strength and elasticity that hold up even when transplanted to other parts of the body.
How the Fibula Connects to Other Bones
The fibula articulates with the tibia at two points. At the top, the head of the fibula meets the outer edge of the tibia just below the knee. At the bottom, the two bones are bound together by a tough set of ligaments called the syndesmosis, which resists rotational and translational forces during movement. Between these two joints, a sheet of connective tissue called the interosseous membrane runs the full length of both bones, linking them together and serving as a barrier against upward displacement of the ankle bone (the talus).
The lower tip of the fibula forms the lateral malleolus, the prominent bump on the outside of your ankle. This structure is essential to ankle stability. Together with the medial malleolus on the inner side (part of the tibia), it creates a mortise, a bracket-shaped socket that grips the talus and keeps the ankle joint aligned. If either malleolus fractures, the ankle ring is disrupted and often becomes unstable.
Weight Bearing and Primary Function
The fibula bears only about 6.4% of your body weight when your ankle is in a neutral position. The tibia handles the rest. This minimal load-bearing role is why surgeons can remove a section of fibula for use as a bone graft without significantly compromising leg function.
Where the fibula truly earns its importance is as a muscle attachment site. Nine major muscles either originate from or insert onto the fibula, controlling movements of the knee, ankle, foot, and toes. The peroneal muscles (fibularis longus and brevis) run along the outer surface and are responsible for pointing your foot downward and turning it outward. The soleus, one of the main calf muscles, originates partly from the back of the fibula’s head and upper shaft. Muscles that extend and flex your toes, including those controlling the big toe independently, also anchor to the fibula’s surface. Even the biceps femoris, a hamstring muscle, inserts onto the fibula’s head, linking it to knee flexion and hip extension.
Common Fibula Injuries
Fibula fractures most often occur near the ankle. Doctors classify these fractures using the Danis-Weber system, which is based on where the break sits relative to the syndesmosis. A Type A fracture occurs below the syndesmosis and generally does not make the ankle unstable. Type B fractures happen at the level of the syndesmosis, and Type C fractures occur above it, often causing significant instability that requires surgical repair.
A particular pattern called a Maisonneuve fracture involves a break high up on the fibula near the knee, combined with a fracture or ligament tear at the inner ankle. Because the visible swelling is near the ankle, the upper fibula fracture can be missed if the full leg isn’t examined. High ankle sprains, which involve damage to the syndesmosis ligaments rather than the bone itself, are another common injury tied to the fibula’s lower end.
The Fibula as a Surgical Donor Bone
The fibula is considered the best donor source for vascularized bone grafts used to reconstruct large defects in other bones. Surgeons harvest a segment of fibula along with its blood supply intact, which allows the transplanted bone to heal faster and even grow in length in children. These grafts have two to four times the strength, toughness, and elasticity of non-vascularized grafts, and they can survive in challenging environments like tissue that has been scarred by radiation or affected by infection.
This technique is used to rebuild sections of the femur, tibia, upper arm bone, and pelvis after tumor removal, with defects of 6 to 10 centimeters or larger being the typical threshold for choosing a vascularized fibula graft. The fibula’s dual blood supply, from both its outer surface vessels and an internal nutrient artery, is what makes this possible. The nutrient vessels enter along the middle portion of the bone, with no nutrient vessels found within 5 centimeters of either end.