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The fibula is the thinner of the two bones in the lower leg. It sits parallel to the tibia, or shin bone, and despite its size, plays many important roles that facilitate leg movement. Though the tibia and the fibula are about the same length, the fibula is significantly thinner. This shows the difference between the bones’ functions. While the tibia bears the body’s weight and absorbs stress, the fibula acts as a supporting structure for the tibia.

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Proximal End

The upper end of the fibula sits just behind the head of the tibia, underneath the knee. Anatomically, this is the proximal end, though some physicians call it the head. Proximal is an anatomical term that describes an object that is closer to the center of the body. The head of the fibula expands outward in all directions and has an irregular quadrate form; in other words, it is square-like but has irregular sides. At the top of the head, a thick, pointed shape emerges. This shape is the apex or styloid process of the fibula. Immediately under the head, the fibula constricts to form the thin neck of the bone.

proximal head fibula PALMIHELP / Getty Images
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Shaft

Between both ends of the fibula is a long, thin stretch of bone -- the shaft. In cross sections, the shaft has a triangular shape, though this shape becomes more irregular towards the ankle. Additionally, as the shaft stretches toward the ankle, it begins to twist slightly. Because the shaft has many attachment sites for muscles, its shape varies slightly from person to person to better fit their muscles. There are three key surfaces and borders of the fibul: the medial, lateral, and posterior. Likewise, there are three borders: anterior, posterior, and interosseous.

shaft fibula xray R&A Studio / Getty Images
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Borders of the Fibula

The borders of the fibula are sharp edges that stretch along the bone’s long axis.

  • The anterior border begins at the head of the fibula and stretches to the ankle, where it eventually splits into two ridges.
  • The interosseous border acts as the attachment point for the fibrous interosseous membrane that helps connect the tibia and the fibula, forming the knee joint.
  • The posterior border runs along the back of the fibula and initially has a round shape. However, its edge becomes sharper closer to the ankle.
fibula shaft borders Jan-Otto / Getty Images
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Surfaces of the Fibula

The surfaces of the fibula lie between each of the three borders. Between the interosseous and anterior borders is the medial surface. This section acts as the attachment site for the muscles that allow the foot to extend and the toes to point upward. The lateral surface sits between the posterior and anterior borders. Between the posterior and interosseous borders is the posterior surface. At the proximal end, the posterior surface is very narrow, though it begins to widen as it reaches the ankle. This section has attachment sites for the muscles that point the toes downward.

fibula xray surfaces eyenigelen / Getty Images

Distal End

Proximal is the anatomical term for referencing things that are closer to the center of the body, and distal is the opposite, referring to sections that are further away from the center. In the case of the fibula, this is the end closest to the ankle. The most prominent feature of the distal end is the lateral malleolus, a bony growth that sits on the outer edge of the ankle and helps form the ankle joint.

doctor distal fibula ericsphotography / Getty Images

Functions

The tibia is the load-bearing bone in the lower leg, and the majority of the fibula’s functions revolve around providing support for the larger bone. A number of ligaments attach on the distal end of the fibula to stabilize the ankle and assist with its movements. Additionally, the lateral fibular collateral ligament connects to the fibular head and helps stabilize the knee. In general, the fibula provides very little support for the knee.

support stabilize leg spyderskidoo / Getty Images

Articulations

In anatomy, articulations are areas that act as places of movement and are generally interchangeable with the term "joint." The fibula forms three joints, two with the tibia and one with the talus in the foot. The fibular head forms the proximal tibiofibular joint with lateral tibial condyle. The condyle is one of two round projections on the head of the tibia. The lateral malleolus articulates with the fibular notch of the tibia, forming the distal tibiofibular joint. At the ankle, the lateral malleolus articulates with the talus in the foot.

ankle joint articulation PeopleImages / Getty Images

Muscles

One way the fibula provides stability for the tibia is by serving as an attachment site for many important leg muscles.

  • The biceps femoris tendon attaches to the fibular head.
  • The shaft has attachment points for the fibularis longus, fibularis brevis, extensor digitorum longus, and extensor hallucis longus tendons.
  • The fibularis tertius attaches to the the distal end of the fibula.
calf muscles fibula milan2099 / Getty Images

Bone Development

When the body first starts developing, the bones are not the firm, hard structures they eventually become. As a person grows, their bones begin to harden and reshape through the process of ossification. The fibula begins ossification around the eighth week of pregnancy. The ossification begins in the shaft and moves outward to the proximal and distal ends. The process is usually complete by the time a person is in their 20s.

ultrasound pregnancy development FatCamera / Getty Images

Surgical Considerations

The fibula has unique surgical uses. Because it does not support much weight, it is not completely necessary, and surgeons can use the bone in mandibular reconstructive procedures. The fibula is thin, long, and has great vascularity, so the body can easily adjust to bone grafts and subsequent dental implants. Usually, when doctors use the fibula as a graft, they take only the shaft and leave the two ends of the bone in the leg.

doctor mandibular fibula Steve Debenport / Getty Images

Disclaimer

This site offers information designed for educational purposes only. You should not rely on any information on this site as a substitute for professional medical advice, diagnosis, treatment, or as a substitute for, professional counseling care, advice, diagnosis, or treatment. If you have any concerns or questions about your health, you should always consult with a physician or other healthcare professional.