Fibula

The fibula is a long slender bone being expanded both as its upper and lower ends.

The upper end, or head, is expanded in all directions having on its superomedial side a facet for articulation with the lateral condyle of the tibia. Lateral to this articular facet is the apex of the head which projects upwards. The rest of the upper end is roughened for the attachment of biceps femoris. Just below the head is the neck, around which runs the common peroneal nerve.

The shaft varies considerably in individuals and its features are often difficult to recognize. The fibula actually has three borders and three surfaces. The anterior border is more prominent inferiorly where it widens into a smooth, triangular, subcutaneous area continuous with the lateral surface of the malleolus. It runs from below the anterior aspect of the head passing vertically down to the triangular area described above. Medial to the anterior border is the interosseus border, again often poorly marked. Extending from the neck, it lies close to the anterior border in its upper third, but then passes backwards and medially to join the apex of the roughened triangular area superior to the malleolar articular surface. The posterior border begins below the lateral aspect of the head and neck, and passes down to the medial margin of the posterior surface of the lateral malleolus. This border is rounded and more difficult to trace. The lateral surface of the fibula is concave and posterolateral to the anterior border. It becomes convex as it winds round posterior to the triangular subcutaneous area to the posterior surface of the malleolus. It is roughened for the attachment of the peroneal muscles. The anterior surface is a very narrow strip between the anterior and interosseus borders at its upper end, expanding as it continues downwards. The posterior surface is more expanded than the anterior and lateral surfaces, being divided by a vertical ridge, the crest, into a medial and lateral part similar to the tibia. The region between the crest and the interosseus border is concave and usually divided by an oblique line, whereas the region between the crest and posterior border is flat and roughened in its upper part by the attachment of soleus.

The lower end can be recognized readily, being flattened on its medial and lateral sides and having posteriorly a deep malleolar fossa. On its medial side, just above this fossa, is a triangular area which is smooth for articulation with the lateral surface of the body of the talus. Just above the articular area on the medial side of the lower end is an elongated roughened area for attachment of the interosseus ligament of the inferior tibiofibular joint, and just below this is the malleolar fossa. The fibula varies in shape according to the muscles that are attached to it, and it can be seen that it carries no weight, but contributes to the lateral stability of the ankle joint.

Ossification

The primary centre appears in the shaft during the seventh week in utero, again spreading so that at birth only the ends are cartilaginous. The secondary centre for the distal end appears during the second year and fuses with the body between 17 and 19 years. The secondary centre for the proximal end appears slightly later, during the third or fourth years, and also fuses with the body later, between 19 and 21 years.

At birth, the fibula is relatively thick, being about half as thick as the tibia in the third prenatal month. As development and growth continue, the disparity between the thickness of the fibula and tibia increases to give the relative adult proportions. The distal end of the fibula does not reach below the medial malleolus until after its ossification has begun, that is after the second year. It is only after this time that the adult relations of the malleoli can be seen.

Palpation

The head of the fibula can be readily palpated on the posterolateral side below the knee joint. If the hands are placed on the lateral sides of the calves and moved up towards the knees, the bony head can be felt projecting laterally. The head can also be easily palpated if the fingers are placed in hollow on the lateral side of the knee when it is flexed to 90°. Little of the shaft can be palpated as it is surrounded by muscles; however in its lower third an elongated triangular area can be palpated on the lateral aspect which can be traed down to the lateral malleolus. The lateral malleolus is easily palpated on the lateral side of the ankle projecting down to a point 2.5cm below the level of the ankle joint.