The fibula is the thin, long bone that runs along the outer side of your lower leg, parallel to the larger shinbone (tibia). While it’s often overshadowed by the tibia, the fibula plays a critical role in stabilizing your ankle and serves as an anchor point for muscles and ligaments throughout the lower leg. It carries only about 6.4% of your body weight, but removing or damaging it can seriously compromise your ability to walk.
Where the Fibula Sits in Your Leg
The fibula runs the full length of your lower leg on the outside, from just below the knee to the ankle. At the top, it sits slightly behind and to the side of the tibia but does not form part of the knee joint itself. At the bottom, the fibula widens into a bony bump you can feel on the outer side of your ankle. That bump is called the lateral malleolus, and it forms the outer wall of the ankle joint by connecting with the talus bone of the foot.
A tough sheet of connective tissue called the interosseous membrane stretches between the tibia and fibula along nearly their entire length, binding them together while still allowing slight movement between the two bones. At the lower end, four distinct ligaments form a structure called the syndesmosis, which locks the fibula tightly to the tibia right above the ankle. These ligaments run in different directions, creating a stable frame that keeps the ankle joint secure during movement.
What the Fibula Actually Does
People often hear the fibula described as a non-weight-bearing bone, and that’s mostly true. In a neutral standing position, the fibula handles roughly 6.4% of the load passing through your leg. But that number shifts depending on how your foot is positioned. When your foot flexes upward (as in walking uphill), the fibula takes on more weight. When your foot points downward, the load decreases. Rolling the ankle outward also increases the force on the fibula.
Its real job is lateral stability. The lateral malleolus at the bottom of the fibula acts as a buttress that prevents your ankle from rolling outward. Without it, the ankle joint becomes unstable and prone to collapse in that direction. The fibula also serves as an attachment site for several important muscles in the lower leg, including those that help you point your toes, flex your foot, and stabilize your ankle during side-to-side movements. Ligaments running from grooves on the fibula’s lower end provide additional leverage during ankle motion.
How It Compares Across Species
The human fibula is remarkably slender compared to the tibia, which has led some anatomists to describe it as a vestigial bone. In most non-human mammals, the fibula is a much more substantial structure. Burrowing and swimming animals like moles and armadillos have a large, robust fibula fused to the tibia at both ends, providing a powerful base for the muscles that drive their paws through soil or water. Fast-running animals like horses have gone the opposite direction: their fibula has fused almost entirely with the tibia, leaving only a small free piece at the bottom.
Among primates and carnivores, the fibula remains a separate, mobile bone. This mobility allows for the rotation and varied positioning of the foot that climbing and grasping demand. In humans, the fibula retains that mobile design even though we no longer climb habitually, and its slenderness reflects the fact that bipedal walking shifted nearly all weight-bearing responsibility to the tibia.
Common Fibula Fractures
Fibula fractures most often happen at or near the ankle. The classic scenario is a twisted ankle where the foot rolls or rotates with enough force to snap the bone. Doctors classify these fractures partly by where the break occurs relative to the syndesmosis (the ligament complex connecting the fibula to the tibia just above the ankle joint).
A break below the syndesmosis is generally the least complicated. The ligaments holding the tibia and fibula together remain intact, so the ankle stays relatively stable. A fracture at the level of the syndesmosis may or may not involve ligament damage, and treatment depends on whether the ankle joint has shifted. A break above the syndesmosis is the most concerning, because the ligaments between the two bones are almost always torn as well, leaving the ankle unstable.
The mechanism matters too. A foot that’s turned inward and then forced to rotate outward tends to produce a spiral fracture of the lower fibula. A foot that’s turned outward and then twisted can fracture the fibula higher up the shaft. These patterns help surgeons decide whether the break needs surgical repair or can heal in a cast or boot.
Healing and Recovery
Most fibula fractures heal completely in six to eight weeks. Because the fibula carries so little weight, many people with an isolated fibula fracture can put some weight on the injured leg during recovery, though your doctor will tell you when it’s safe to do so based on the specific break. More complex fractures involving the ankle joint or torn ligaments take longer and may require surgery to restore alignment.
The fibula receives its blood supply primarily from the peroneal artery (also called the fibular artery), which runs along the back of the interosseous membrane between the two leg bones. Good blood flow to the fracture site is one reason the fibula tends to heal reliably, though smoking or vascular disease can slow the process.
The Fibula as a Surgical Building Material
One of the fibula’s most remarkable roles in modern medicine is as a donor bone for reconstructive surgery. Because it’s long, straight, and not essential for weight-bearing, surgeons can harvest a section of fibula along with its blood supply and transplant it to rebuild bone elsewhere in the body. This technique is called a fibula free flap.
The procedure is most commonly used to reconstruct the jawbone after cancer surgery. Surgeons make precise cuts in the harvested fibula segment and reshape it to match the curved contour of the jaw. Because the transplanted bone arrives with its own blood vessels, it can remodel and even grow stronger over time in response to use, and dental implants can later be placed directly into it. The vascular pedicle (the stalk of blood vessels keeping the graft alive) can extend up to 15 cm when the bone is taken from the lower portion of the fibula. Surgeons can even include a patch of skin and underlying tissue up to 10 by 20 cm, and if a sensory nerve is reconnected at the new site, that skin can regain feeling.
Beyond jaw reconstruction, fibula grafts are used to replace damaged sections of the tibia, femur, or arm bones after trauma, infection, or tumor removal. The graft tolerates early weight-bearing better than non-living bone grafts and carries a lower risk of gradually breaking down over time.