Fibula

The fibula (plural: fibulae) is the smaller of the two bones of the leg. It is not directly involved in the transmission of weight but is important for ankle stability and acts as a source for numerous muscle attachments. It is commonly raised as a flap for reconstructive surgery.

Gross anatomy

Osteology

The fibula runs posterolateral to the tibia and consists of a head, neck, shaft and distal malleolus.

Head

The proximal head is irregular with a small superiorly pointed tubercle called the styloid process (formed from the posterior and lateral aspects). The proximomedial head articulates with the tibia to form the superior tibiofibular joint via a rounded facet. 

The head faces in a proximal and anteromedial direction. Inclination of this surface can vary from horizontal to 45º . The neck sits inferior to the head.

Shaft

The shaft can vary in shape. It has three borders and three surfaces.

The borders are:

• anterior border: runs from the anterior aspect of the head to the triangular shaft just proximal to the lateral malleolus. Distally it turns laterally to form the posterior wall of the triangle. 
• posterior border: runs from the styloid process to the medial aspect of the posterior groove of the lateral malleolus. 
• medial border (also known as the interosseous border): sits medial to the anterior border and descends to the rough area proximal to the talar facet. 

The surfaces are:

• lateral surface: between the anterior and posterior borders. Its lower third surface faces posteriorly. Distally it is continuous with the lateral malleolus’s posterior groove.    
• medial surface (sometimes called the anterior surface): between the anterior and medial borders. It faces forward and medial and is narrowed proximally. 
• posterior surface: between the medial and posterior borders. Superiorly until its lower third, it is divided by the medial crest, a prominent vertical ridge. The medial posterior surface is concave. The lateral posterior surface initially faces posteriorly but ends facing medially at the lower end just above the talar facet, the rough area for attachment of the transverse tibiofibular ligament. 

Lateral malleolus

Laterally, it is continuous with the triangular area of the shaft and has a convex surface. Medially, its triangular facet articulates with the talus, and it has a rough area just above this facet. The malleolar fossa sits on the posterior surface. The lateral malleolus forms the lateral part of the mortise and extends more distally and posteriorly than the medial malleolus.

Articulations

The fibula contributes to the ankle joint but is not involved in the knee joint directly:

• proximal
  • superior tibiofibular joint
• distal
  • talocrural joint
  • inferior tibiofibular joint (tibiofibular syndesmosis joint)

The malleolar mortise is formed by the arch of the distal tibia and fibula. 

Attachments

Musculotendinous

Proximal/head

• biceps femoris
• extensor digitorum longus
• soleus

Medial surface

• extensor digitorum longus
• peroneus tertius
• extensor hallucis longus

Lateral surface

• peroneus longus
• peroneus brevis

Posterior surface

• tibialis posterior
• soleus
• flexor hallucis longus 

Ligamentous

• capsule of superior tibiofibular joint and the anterior and posterior proximal tibiofibular ligaments
• fibular collateral ligament of the knee joint
• interosseous membrane
• interosseous ligament of inferior tibiofibular joint
• distal tibiofibular syndesmosis
  • Anterior inferior and posterior inferior tibiofibular ligaments
• talocrural joint
  • anterior and posterior talofibular ligaments
  • calcaneofibular ligament
•  superior extensor retinaculum and superior peroneal retinaculum of the ankle

Relations

The lateral part of the fibular neck accommodates the common peroneal nerve. The peroneus longus and brevis tendons pass behind the lateral malleolus with the peroneus tertius passing in front.

In relation to the fascial compartments of the leg

• the anterior compartment sits anterior to the fibula
• the lateral compartment sits lateral to the fibula
• the deep posterior compartment sits medial to the fibula
• the superficial posterior compartment sits posterior to the fibula     

See muscles of the lower limb.

At the lateral malleolus, the peroneus brevis tendon passes behind the malleolus closest to the bone, anterior to the peroneus longus tendon (memory aid: brevis closest to bone).

Blood supply

Branches of the fibular artery enter the shaft of the fibula at the nutrient foramen on the posterior surface, just proximal to the midpoint. As function can remain normal post removal of the fibular shaft, the middle third of the fibula is often the site of bone grafting. These fibular branches are important for the harvest of the fibular shaft for plastic surgery reconstruction (free fibular graft, usually for mandibular reconstruction). Arterial anastomoses at the knee and ankle also provide metaphysial vessels to the fibula.

Variant anatomy

• fibular hemimelia
• shaft size and shape
• transverse convex lateral malleolus
• superior peroneal retinaculum sesamoid bone

Radiographic features

Plain radiograph

It is important to understand the 3-dimensional orientation of the lateral malleolus within the ankle. The lateral malleolus is longer, larger and more posterior than the medial malleolus. Hence on a true lateral radiograph of the ankle, it is posterior to the medial malleolus.

Development

Ossification

There are three ossification centers of the fibula.

The 8 fetal week is when the primary ossification center appears for the fibular shaft.

Two further secondary ossification centers are for the upper and lower fibula end. The fibula is unlike most long bones where the initially appearing secondary ossification center often fuses last. In the fibula, the upper center appears in the third to fourth year whilst the lower center appears earlier in the first year. The upper ossification center fuses to the shaft around the seventeenth to nineteenth year whilst the distal epiphysis unites with the shaft around the fifteenth to seventeenth year. Ossification tends to occur later in males. 

Further ossification centers may occur. See ossicles of the foot.

History and etymology

Due to its resemblance to a clasp (modern day safety pin), fibula in latin means brooch/pin.

Related investigations

Fractures of the lateral and medial malleolus are common in soccer and basketball players. Fibular fractures often occur 2-6 cm proximal to the distal lateral malleolus . Fracture dislocations of the ankle joint and tibial fractures may be associated. Fractures often involve disrupted muscle attachments and may compromise gait due to the distal fibula role in the stability of the ankle.

Spiral fracture of the proximal fibula and associated disruption of the distal tibiofibular syndesmosis and deltoid ligament can occur. See Maisonneuve fracture.

Avulsion fractures of the proximal fibula may occur from a blow to the anteromedial tibia in knee extension or knee hyperextension with an internally rotated tibia. See: arcuate sign.

Plain radiograph

• knee series
• tibia/fibula series
• ankle series

Cross-sectional imaging

• leg CT
• leg CTA (for fibular free graft workup)
• leg MRI

Related pathology

• ankle fractures
  • Weber classification
  • Lauge-Hansen classification
• Maisonneuve fracture