Common Peroneal Nerve

For Medical Students

Structure and Location

The common peroneal nerve is the smaller of the two divisions of the sciatic nerve. It separates from the tibial division in the lower thigh and runs down and laterally towards the fibula.

At the knee it runs along the inner border of the biceps femoris tendon. The two descend together, nerve behind and medial, tendon in front and lateral.

It then crosses the back of the lateral head of gastrocnemius, passes behind the fibular head, and winds forwards around the neck of the fibula.

At the neck it lies directly on bone with skin, fascia and almost nothing else over it. Conventional teaching places that point about 4 cm below the tip of the fibular styloid.

Having rounded the neck it passes under the edge of peroneus longus and divides into a superficial branch, which supplies the peroneal muscles and the skin of the lower leg, and a deep branch, which supplies the muscles that lift the foot.

Relations

In front of it at the knee: the biceps femoris tendon, then the lateral collateral ligament, then the joint. The order from back to front is nerve, tendon, ligament.

Conventional teaching places the nerve about 2 cm posteromedial to the biceps tendon at the level of the fibular head.

Deep to it: the lateral head of gastrocnemius proximally, then the fibula itself at the neck.

Superficial to it at the neck: skin and fascia. There is no muscle belly over it there, which is why the nerve can be rolled under a finger.

It gives branches before it divides: the lateral sural cutaneous nerve to the skin of the calf, and a recurrent articular branch that runs back up to the knee.

Function

The deep branch supplies the muscles that dorsiflex the foot and extend the toes. Losing it produces a foot that cannot be lifted.

The superficial branch supplies the muscles that evert the foot and the skin over most of the lower leg and foot.

The nerve carries no motor supply above the knee except to the short head of biceps femoris, which 2.10.5 covers.

Clinical Relevance

A foot drop after a knee injury is this nerve. The anatomy makes it the only structure at the knee whose injury produces that sign.

It is injured because of where it sits. A nerve lying on bone with no muscle over it has nothing to absorb a blow or a stretch.

It should be tested and recorded at the first assessment of any lateral knee injury. A deficit found later cannot otherwise be dated.

Injury to it is covered by 6.26 and management by 7.49. This page covers where it is and what lies beside it.

MRCS Level

Structure and Location

Divide the course into three parts for description: the thigh segment along biceps, the crossing behind the fibular head, and the fibular neck segment under peroneus longus.

Each part has its own hazard. The thigh segment can be stretched, the fibular head segment can be caught in a fixation, and the neck segment can be compressed.

At the neck the nerve enters a fibro-osseous tunnel formed by the attachment of peroneus longus to the fibula. The tunnel tethers it, so the nerve cannot move away from a force applied there.

Conventional teaching gives the nerve a diameter of the order of 5 mm at the neck, which makes it palpable in most people.

The recurrent articular branch leaves before the division and runs proximally to the anterolateral knee. It is small and it is often sacrificed without being seen.

Relations

The relationship to biceps femoris is the one to know, because it does not change with injury or with scarring. The nerve is posteromedial to the tendon in the same fascial plane.

The popliteofibular ligament attaches to the fibular styloid within about 2 cm of the nerve, so every fibular fixation is a fixation near it. 2.10.3 covers that.

The lateral head of gastrocnemius lies deep to the nerve proximally and gives it a cushion. That cushion ends at the fibular head.

The fibular neck has no soft tissue envelope worth the name. Skin, fascia, nerve, bone.

Blood Supply and Innervation

The nerve is supplied by a longitudinal vasa nervorum running in its epineurium, fed segmentally from the popliteal and inferior lateral genicular arteries.

Stripping the nerve from its bed over a long segment divides those segmental feeders. A mobilised nerve is a devascularised nerve in proportion to how far it has been mobilised.

The nerve's own branches at the knee are the lateral sural cutaneous nerve and the recurrent articular branch, both arising before the division.

Function

Deep branch: tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius, and the first web space.

Superficial branch: peroneus longus and brevis, and the skin of the lateral leg and dorsum of the foot.

A lesion at the neck takes both branches. A lesion of the deep branch alone spares eversion, which is how the level is worked out clinically.

The recurrent articular branch contributes to anterolateral knee sensation, which is why numbness there can accompany a nerve injury without a foot drop.

Clinical Relevance

Record dorsiflexion, eversion and first web space sensation separately. The three together localise the level, and a single note saying foot drop does not.

Examine the nerve before any lateral operation and again afterwards. The anatomy makes a deficit likely enough that an undocumented baseline is a problem.

Expect the nerve to be palpable at the neck in most people. Rolling it under a finger confirms its position before an incision is planned.

Avoid pressure at the fibular neck in any dressing, cast or positioning, because the nerve lies on bone there with nothing over it.

FRCS Level

Relations

Treat the nerve as the fixed point of the lateral side and the ligaments as what is arranged around it. Every other lateral structure can be approached from more than one direction; the nerve cannot be moved.

Its tether at the fibular neck means a force applied anywhere along the segment below the knee is transmitted to the nerve rather than absorbed by its mobility.

The distance from the nerve to the three lateral fixation points is small and roughly equal: the fibular styloid, the fibular head and the posterolateral capsule all lie within a couple of centimetres of it.

Function

The nerve's tolerance is the limiting factor in lateral surgery, not the strength of any fixation. A construct that holds and a nerve that does not work is a failed operation.

Traction, compression and division are three different injuries with three different outcomes, and only the third is obvious at the time.

Flexing the knee shortens the path the nerve takes and reduces tension on it. That is a property of the anatomy and it is available throughout an operation.

Clinical Relevance

Identify the nerve rather than avoid its territory in any posterolateral approach. The territory is the operative field and avoidance is not possible.

Document dorsiflexion, eversion and first web space sensation before and after. A baseline that does not exist cannot be returned to.

Flex the knee while working near the fibula, since the position reduces the tension the nerve is carrying.

Consider the nerve when planning any correction that lengthens the lateral side. The nerve is tethered and it does not accommodate.

Avoid long mobilisation when a short one will do, because each centimetre takes segmental supply with it.

Fellowship Level

Clinical Relevance

This section is the nerve as it is met in lateral knee surgery. It is approach anatomy. Injury belongs to 6.26, management of an injured nerve to 7.49, and every technique named below to its own chapter in Section 7.

Which operations put it in the field. Posterolateral corner and collateral ligament reconstruction, fibular head and neck fixation, osteotomy and deformity correction, lateral meniscal repair passed inside-out or outside-in, posterolateral portals, biceps tenodesis, and removal of lateral hardware. The common thread is not the operation. It is a point of fixation or passage within two centimetres of a nerve that cannot move.

Why it is injured more than its size suggests. Three anatomical facts compound: it lies on bone at the neck with no muscle cover, it is tethered in a fibro-osseous tunnel so it cannot slide away from a force, and its blood supply is segmental along its length rather than entering at a hilum. Each one is tolerable alone and together they make a nerve that is easy to reach and hard to treat well.

Identify rather than avoid. The nerve is found by following the posteromedial border of the biceps femoris tendon distally, in the fascial plane the two share. That relationship survives scar, swelling and chronicity, which is why it is the landmark worth relying on when it is most needed. Where the biceps insertion is avulsed the landmark has gone with it, and the exposure starts proximally instead.

The three injuries. Division is obvious at the time and is the least common. Compression comes from a retractor held against the neck, from a dressing, a cast or a positioning support, and from swelling in a closed lateral compartment; it is recoverable in proportion to its duration. Traction is the one that is missed, because nothing happens at the time: a tethered nerve carried over a correction, a lengthening, or a limb held in extension while the lateral side is opened, is stretched by an amount nobody measured. Of the three, traction is the commonest and the quietest.

Position is a tool. Flexing the knee shortens the path the nerve takes around the fibula and reduces its tension, and the position is available at any moment without changing the plan. A nerve relaxed by 60 or 90 degrees of flexion tolerates retraction that the same nerve in extension does not.

Mobilisation costs supply. The vasa nervorum enter segmentally along the nerve's bed, so a nerve lifted over several centimetres has lost the feeders along that length. Where a nerve has to be mobilised to be protected, the shortest mobilisation that achieves it is the right one, and the trade is explicit rather than free.

Releasing the tunnel is an anatomical decision, not a reflex. The fibro-osseous tunnel at the neck is where a swollen nerve has nowhere to go, so a procedure expected to produce swelling beside it, or a correction that increases tension through it, raises the question. 7.49 carries when it is done; the anatomy here says why the question arises at that point and not elsewhere along the course.

The recurrent articular branch is usually sacrificed and rarely mentioned. It crosses the field of a lateral approach and costs anterolateral sensation and nothing motor, which is why it is accepted. A patient told about a numb patch beforehand is in a different position from one who discovers it.

Record the examination as three findings and not one: dorsiflexion, eversion and first web space sensation, before and after. The three separate a deep branch lesion from a complete one and a new deficit from an old one. A note saying foot drop does neither.