FIBULA

 

The fibula, or calf bone, is a slender, cylindrical bone located on the posterior part of the leg. It is found lateral to the tibia, connected above and below. The tibia bears more weight than the fibula. The fibula extends past the lower end of the tibia distally and forms part of the ankle joint and provides stability to the joint. The fibula has a proximal head, narrow neck, long shaft, and distal lateral malleolus.

PROXIMAL END OR HEAD OF FIBULA

The head of the fibula is irregular in shape and projects anteriorly, posteriorly, and laterally. There is a round, flattened area on the medial part of the fibular head known as a facet. The fibular facet articulates with the inferolateral part of the lateral tibial condyle, forming the proximal tibiofibular joint. There is a short neck of the fibula below the head of the fibula. The common peroneal nerve travels posterolaterally to the neck of the fibula.

The apex of the head, also known as the styloid process, projects upwards from its posterolateral aspect.

BODY OR SHAFT

The shaft of the fibula is triangular in cross section. There are 3 borders: anterior, interosseous, and posterior; and 3 surfaces: lateral, medial, and posterior.

BORDERS

Anterior Border

The border starts from below the anterior aspect of the fibular head and continues distally toward the lateral malleolus, where it diverges into 2 ridges that surround the triangular subcutaneous surface.

Interosseous or Medial Border

The medial border lies medial to the anterior border, where it might be indistinguishable, but more on the posterior aspect. It ends below at the upper end of a roughened area above the talar facet of the lateral malleolus. It has attachment to the interosseous membrane, which also forms the tibiofibular joint. The anterior and interosseous border has attachment to the dorsiflexors.

Posterior Border

The border runs at the back of the fibula; proximally, it is rounded. The border becomes prominent at the distal end with the medial margin of the groove of the lateral malleolus. Proximally it lies with the styloid process.

SURFACES

Lateral Surface

The lateral surface is between the anterior and posterior borders laterally, which is broad and deeply grooved. The proximal part of the surface faces laterally, and it spirals down distally and then faces posterolaterally. The surface then posteriorly continues in the posterior groove of the lateral malleolus. The lateral surface provides attachment to the peroneal muscles.

Medial Surface

The medial surface is between the anteromedial and posteromedial borders. It is grooved distally by the origin of tibialis anterior. In the upper 2/3rd, it is quite narrow, almost 1mm or less.

Posterior Surface

The posterior surface is between the interosseous and posterior borders and is the largest. The proximal 2/3 is rough and narrow and is divided by a longitudinal medial crest and separated from the interosseous border by a grooved surface medially for the nutrient foramen. The remaining 1/3rd surface distally is facing posteriorly in the proximal half, and in the distal half it faces medially. This surface provides attachment to plantar flexors.

LOWER END OR LATERAL MALLEOLUS

The lower end of the fibula forms the lateral malleolus of the leg; it projects distally & posteriorly. It is a bony projection on the lateral part of the ankle. The lateral malleolus extends posteroinferiorly, which is round and rough anteriorly and a broad groove posteriorly. The medial surface has a triangular articular facet, vertically convex, with its apex distal, which articulates with the lateral talar surface. The lateral surface is subcutaneous. The distal end provides attachment to several ligaments of the ankle joint for support and stability.

VASCULAR SUPPLY

At the posterior surface in the middle, a distally directed nutrient foramen is there where the fibular artery enters it. It is perfused from the periosteum, which receives small branches from the fibular artery. The proximal head and epiphysis are supplied by a branch of the anterior tibial artery.

NERVOUS SUPPLY

The nerves that supply knee and ankle joints, i.e., the genicular branch of the common fibular nerve and the deep fibular nerve, respectively, innervate the proximal and distal ends of the fibula. The superficial and deep fibular nerves innervate the muscles attached to the fibula and fibular periosteum.

MUSCLE & LIGAMENT ATTACHMENTS

·       Biceps femoris inserts on the head of the fibula on the anterolateral slope of the apex, and the insertion is C-shaped.

·       Extensor digitorum longus originates at the anterior surface of the fibula on the proximal half of the medial surface.

·       Extensor hallucis longus originates from the medial side of the fibula from the posterior half of the middle 2/4th.

·       The fibularis tertius originates from the distal anteromedial surface of the fibula.

·       Peroneus longus originates from the whole width of the lateral surface in its proximal third, i.e., the head of the fibula.

·       Peroneus brevis originates from the inferior 2/3rd of the inferior and anterolateral surface of the fibula.

·       Soleus originates from the head of the fibula, the posterior border of the fibula, and the soleal line.

·       Tibialis posterior arises from the posterior surface of the fibula between the medial crest and the interosseous border.

·       Flexor hallucis longus originates distal to soleus on the posterior side of the fibula near the interosseous membrane.

OSSIFICATION

The fibula is part of the appendicular skeleton and develops through endochondral ossification. The fibula ossifies from three centers: one from the shaft and two from the extremities. The ossification center at the shaft appears around the 8th gestational week; at the distal end, it appears around the 1st year, and at the proximal end, it appears around the 3rd year in females and the 4th year in males.

The lower end of the fibula fuses with the shaft around the 15th year in females and the 17th year in males. The proximal epiphysis unites with the shaft around the 17th year in females and the 19th year in males.

The fibula violates the law of ossification as secondary appears first at the lower end rather than the proximal end. The lower-end epiphysis appears early due to its pressure epiphysis. The proximal end epiphysis fuses last as it is the growing end of the bone, which continues to grow afterwards with the upper end of the tibia.