Common Peroneal Nerve

For Medical Students

What It Shows

The common peroneal nerve passes behind the fibular head and around its neck, where it is superficial and vulnerable. 2.10.6 covers the common peroneal nerve.

It appears on axial images as a discrete structure with internal fascicles. 5.5.13 covers the neurovascular structures.

It matters because a nerve injury changes the consent, the rehabilitation and the prognosis more than most ligament findings do. 2.10.7 covers the surgical anatomy.

How It Is Acquired

Assess it on axial images from the popliteal fossa around the fibular neck. 5.5.13 covers the neurovascular structures.

Compare its size and signal with the tibial nerve in the same study. 2.10.6 covers the common peroneal nerve.

Look for discontinuity, enlargement or loss of the fascicular pattern. 5.5.13 covers the neurovascular structures.

Look at the muscles it supplies for denervation change. 5.5.10 covers the tendons.

Record the finding explicitly in any lateral injury. 5.20.1 covers structured reporting.

What Normal Looks Like

A discrete nerve of even calibre with a visible fascicular pattern. 5.5.13 covers the neurovascular structures.

A course behind the fibular head and around the neck with no displacement. 2.10.6 covers the common peroneal nerve.

No oedema in the muscles of the anterior and lateral compartments. 5.5.10 covers the tendons.

Symmetry with the tibial nerve. 5.5.13 covers the neurovascular structures.

What Abnormal Looks Like

Enlargement with loss of the fascicular pattern. 5.5.13 covers the neurovascular structures.

Discontinuity, which is the finding that changes the prognosis. 2.10.6 covers the common peroneal nerve.

Displacement or stretching around a fibular fracture. 2.3.2 covers the attachments.

Denervation oedema in the muscles supplied. 5.5.10 covers the tendons.

A ganglion along its course, which is a separate and treatable diagnosis. 5.7.7 covers the ganglion cysts.

MRCS Level

How It Is Acquired

In a viva the expected answer includes the nerve in the lateral assessment.

The expected points are the axial images, the comparison with the tibial nerve, the calibre and fascicles, the muscles, and the explicit record.

Say that a corner injury carries a recognised risk to this nerve. 2.10.7 covers the surgical anatomy.

Say that clinical examination establishes the deficit. 4.13.8 covers the combined PLC injuries.

Say that denervation change in muscle is an indirect sign worth looking for. 5.5.10 covers the tendons.

What Abnormal Looks Like

The reasoning expected is that the nerve is tethered as it crosses the fibular neck, so a varus or rotational injury that displaces the fibula stretches it, which is why the lateral injuries and the nerve travel together. 2.10.6 covers the common peroneal nerve.

Continuity on imaging with a clinical deficit suggests a stretch injury rather than a division, which carries a different prognosis. 5.5.13 covers the neurovascular structures.

Denervation change appears in muscle before the nerve's own appearance settles, so the muscles are the earlier sign. 5.5.10 covers the tendons.

A knee dislocation puts this nerve and the vessels at risk together, and the vessels take priority. 5.5.13 covers the neurovascular structures.

How Good It Is

Magnetic resonance imaging shows this nerve well and does not grade the deficit.

Its best contribution is continuity, calibre and the denervation change in muscle. 5.5.13 covers the neurovascular structures.

It is limited because a stretch injury can look normal, because the deficit is clinical, and because recovery is followed by examination rather than by imaging. 4.13.8 covers the combined PLC injuries.

No figures appear for accuracy on this page, under decision 028, and no nerve calibre values or recovery intervals appear either: those are published quantities from particular series and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the nerve changes what the patient is told before any operation. 2.10.7 covers the surgical anatomy.

Continuity with a deficit is watched, and discontinuity is a different conversation. 5.5.13 covers the neurovascular structures.

A corner reconstruction passes close to the nerve, so its pre-operative state has to be documented. 5.10.6 covers the post-operative PLC imaging.

A fibular osteotomy or hardware near the neck carries the same risk. 5.17.4 covers the osteotomy.

Denervation change gives an objective record where the examination is equivocal. 5.5.10 covers the tendons.

Where It Misleads

The commonest error is omitting the nerve from a lateral injury report. 5.20.8 covers common reporting errors.

A normal looking nerve is read as excluding a lesion. 5.5.13 covers the neurovascular structures.

Denervation oedema is read as a muscle strain. 5.5.10 covers the tendons.

A ganglion along the nerve is reported as a cyst with no comment on the nerve. 5.7.7 covers the ganglion cysts.

Post-operative change is read as a new lesion. 5.10.6 covers the post-operative PLC imaging.

Fellowship Level

Where It Misleads

At fellowship level the deficit is already present and the question is whether it will recover. 4.13.8 covers the combined PLC injuries.

Imaging contributes continuity and the muscles' state, while recovery is a clinical and electrophysiological question. 5.5.13 covers the neurovascular structures.

A documented pre-operative deficit protects the patient and the surgeon, and the imaging record is part of that. 5.20.1 covers structured reporting.

Serial muscle appearance tracks denervation and reinnervation, and is rarely requested for that purpose. 5.5.10 covers the tendons.

A ganglion within the nerve is a separate and treatable cause that is missed whenever the nerve is not looked at. 5.7.7 covers the ganglion cysts.

The useful report states the nerve's continuity, calibre, fascicular pattern and the muscles supplied, in every lateral injury. 5.20.1 covers structured reporting.