Lower limb

Fibula

Before you say anything

A slender bone that carries almost no weight, which is exactly why surgeons harvest it for grafts. Students trip over it twice: they mistake the head for the malleolus, and they say it takes part in the knee. It does not. It takes part in the ankle, and the common peroneal nerve winding round its neck is the applied point that always follows.

Which side? Three checks

Hold it like this first: Hold it vertically with the expanded, flattened lower end downwards, the malleolar fossa on that end facing medially and backwards, and the apex of the head at the upper end pointing upwards and backwards.

Check 1 — Compare the ends
One end is flattened from side to side and descends to a point; the other carries a small knob with an upward apex
The flattened end is the lower end — the lateral malleolus
Check 2 — Find the malleolar fossa
A deep pit on one surface of the lower end, behind the articular facet
It faces medially and backwards, towards the talus and tibia, which fixes the side
Check 3 — Look at the articular facet
A smooth triangular facet on the inner aspect of the lower end
It faces the talus and so is medial, confirming the answer

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Say these first

Slender and non-weight-bearing
A small head with an upward apex at one end
A flattened lateral malleolus with a malleolar fossa at the other

Do not confuse it with

The tibia
The fibula is slender with a small knob-like head; the tibia is stout with two broad condyles and a subcutaneous medial surface.
The ulna
Both are slender, but the ulna's upper end is a large hook with a trochlear notch, while the fibula's upper end is a small rounded head.

Examiner trap

Saying the fibula forms part of the knee joint because its head reaches the tibial condyle. That articulation is the separate superior tibiofibular joint, outside the knee.

Memory hook

Fibula: fossa faces the tibia.

Why it matters

The common peroneal nerve winds round the neck of the fibula, where a fracture or a tight plaster can injure it and cause foot drop with loss of sensation over the dorsum of the foot.
The shaft carries little weight, so a long segment can be removed as a vascularised bone graft to reconstruct the mandible or a limb.
In a Pott's fracture the lateral malleolus is fractured as the foot is forcibly everted.

Senior’s 30-second takeaway

Fibula — the whole bench in four lines. Read this on the way in.

If you see this: A slender leg bone · A small head with an apex at one end · A flattened malleolus with a deep pit at the other
If asked to identify it: This is a fibula; the malleolar fossa and articular facet on the lower end face medially, which gives the side.
If asked one more: It takes no part in the knee joint but does form the lateral wall of the ankle.
If asked why it matters: The common peroneal nerve winds round its neck, so injury there causes foot drop.

Then stop. Let the examiner ask the next question.

🩺 Before you say anything

Which side? Three checks

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Senior’s 30-second takeaway

Before you say anything