Deep Peroneal Nerve Anatomy

Origin and root value

• Terminal branch of the common peroneal nerve, itself the smaller terminal division of the sciatic nerve (L4, L5, S1, S2).
• Deep peroneal nerve root value: L4, L5 and S1, predominantly L5.
• The common peroneal nerve gives, before it divides, the recurrent articular branch to the knee and contributes the lateral sural cutaneous nerve to the calf. Both leave above the fibular neck, which is why they can be spared in a distal lesion.

The fibular neck

• The common peroneal nerve passes from the popliteal fossa along the medial border of the biceps femoris tendon, crosses the lateral head of gastrocnemius, and winds around the neck of the fibula approximately 2 to 4 cm distal to the tip of the fibular head.
• Here it lies subcutaneously, directly on periosteum, covered only by skin, fat and deep fascia. It is palpable and rollable in most people.
• It then passes deep to the arcade of the peroneus longus - the fibrous proximal edge of the muscle - and divides within the substance of the muscle into the deep and superficial peroneal nerves.

Anterior compartment course

• The deep peroneal nerve pierces the anterior intermuscular septum to enter the anterior compartment.
• It runs distally on the anterior surface of the interosseous membrane, initially between tibialis anterior and extensor digitorum longus, and in the distal leg between tibialis anterior and extensor hallucis longus.
• It is joined by the anterior tibial artery shortly after entering the compartment. The nerve and artery travel together for the whole length of the compartment, the nerve crossing from lateral, to anterior, and back to the lateral side of the artery.

At the ankle

• The nerve passes beneath the inferior extensor retinaculum with the artery, in the anterior tarsal tunnel.
• At the level of the ankle joint it lies between the tendons of extensor hallucis longus and extensor digitorum longus, immediately lateral to the artery, which becomes the dorsalis pedis.

Terminal division

• Lateral terminal branch: passes laterally deep to extensor digitorum brevis, expands into a small ganglion-like swelling, and supplies extensor digitorum brevis and extensor hallucis brevis, with articular twigs to the tarsal and tarsometatarsal joints.
• Medial terminal branch: continues with the dorsalis pedis artery onto the dorsum of the foot, passes over the first intermetatarsal space, and divides into the dorsal digital nerves of the first web space - supplying the adjacent sides of the hallux and second toe. It frequently gives a small motor twig to the first dorsal interosseous.

The parent nerve is developed in common peroneal nerve anatomy, its sibling in superficial peroneal nerve anatomy, and the emergency it detects in compartment syndrome.

The anterior compartment of the leg

In the foot

• Extensor digitorum brevis and extensor hallucis brevis, through the lateral terminal branch. These are the only intrinsic foot muscles supplied by the peroneal nerve; every other intrinsic muscle of the foot is tibial.
• A twig to the first dorsal interosseous is frequently present.

The proximal branch order matters clinically

• The branch to tibialis anterior leaves early, close to the point at which the nerve enters the anterior compartment. A lesion at the fibular neck therefore takes out tibialis anterior along with everything else, which is why fibular neck lesions present with a complete foot drop.
• The branch to extensor hallucis longus leaves further distally, which is why an evolving anterior compartment syndrome or a partial lesion often shows weak hallux dorsiflexion before weak ankle dorsiflexion. Test the great toe specifically.
• Extensor digitorum brevis is the most distal muscle, and its wasting is the sole motor sign in anterior tarsal tunnel syndrome.

The first dorsal web space

• The entire cutaneous territory of the deep peroneal nerve is the web space between the hallux and the second toe, and the adjacent sides of those two toes - an area of a few square centimetres.
• This makes it the most anatomically specific sensory territory in the lower limb: numbness here, with normal sensation elsewhere on the dorsum of the foot, is a deep peroneal nerve lesion and essentially nothing else.

Articular sensory supply

• Branches to the ankle joint as the nerve passes the anterior capsule.
• Branches from the lateral terminal branch to the tarsal, tarsometatarsal and intertarsal joints.
• These articular branches carry deep midfoot and ankle pain and are the anatomical basis for a deep peroneal block in dorsal foot surgery.

Boundaries with the neighbours

Practical testing points

• Test with light touch and pinprick, comparing directly with the same point on the other foot and with the adjacent dorsum of the foot.
• Test before analgesia and before a regional block, and record the finding. A femoral or popliteal block abolishes this sign entirely, which is a recognised reason for a delayed diagnosis of compartment syndrome.
• Preserved first web sensation does not exclude an evolving compartment syndrome. It is an early sign, not a perfect one, and objective pressure measurement remains the arbiter.

Sites of compression, proximal to distal

1. The fibular neck The dominant site. The nerve is subcutaneous against bone with essentially no protective padding. Causes include a plaster or tight bandage, prolonged positioning in lithotomy or lateral decubitus, habitual leg crossing, prolonged squatting or kneeling, rapid weight loss with loss of the fat pad, a fibular neck fracture, and direct trauma.

2. The peroneus longus arcade (the fibular tunnel) The fibrous proximal edge of the peroneus longus origin forms an arch through which the nerve passes just before it divides. A tight arcade produces a compressive neuropathy with the same clinical picture as a fibular neck lesion.

3. Superior tibiofibular joint ganglion An intraneural ganglion tracking along the articular branch from the superior tibiofibular joint is a classic and frequently missed cause of a spontaneous peroneal palsy. Recurrence after simple decompression is high unless the articular branch is divided and the joint addressed. An extraneural ganglion compressing the nerve is also described.

4. The anterior intermuscular septum Where the deep peroneal nerve pierces the septum to enter the anterior compartment - a fixed point that tethers the nerve and is relevant in traction injuries.

5. The anterior compartment itself Not an entrapment but a closed space. Rising pressure compresses the nerve along its whole intracompartmental length.

6. The inferior extensor retinaculum - the anterior tarsal tunnel The distal entrapment site, discussed in detail in the clinical section.

7. Distal to the retinaculum The nerve can be entrapped where the tendon of extensor hallucis brevis crosses over it on the dorsum of the foot, a distinct and reported site producing first web symptoms without proximal signs.

Relations to remember for the operating theatre

• At the fibular neck: the nerve is between skin and bone. Anything placed on the lateral proximal leg - a retractor, a clamp, a pin, a positioning post - is on the nerve.
• In the compartment: the nerve lies on the interosseous membrane, deep to the muscle bellies, immediately adjacent to the anterior tibial artery. A drill or saw exiting the posterolateral tibial cortex reaches them both.
• At the ankle: the neurovascular bundle lies between extensor hallucis longus and extensor digitorum longus. The standard anterior approach to the ankle works medial to extensor hallucis longus, retracting the tendon and the bundle laterally together.
• In the foot: the dorsalis pedis artery and the medial terminal branch cross the first intermetatarsal space - directly in the field of a first tarsometatarsal or Lisfranc procedure.

Why this nerve dominates the diagnosis

The anterior compartment of the leg is the compartment most commonly affected by acute compartment syndrome, and the deep peroneal nerve runs its entire length. Nerve tissue is more sensitive to ischaemia than muscle, so the nerve declares the problem first.

Clinical sequence

1. Pain out of proportion to the injury, poorly relieved by opioid analgesia.
2. Pain on passive stretch - passive plantarflexion of the toes and ankle.
3. Paraesthesia in the first dorsal web space - the first objective sign, and the one that is specific to this compartment.
4. Weakness of hallux and then ankle dorsiflexion.
5. Pallor, paralysis and pulselessness are late and unreliable - a normal dorsalis pedis pulse never excludes the diagnosis.

The objective threshold

• McQueen and Court-Brown monitored anterior compartment pressure continuously for 24 hours in 116 tibial diaphyseal fractures. Using an absolute pressure of 30 mmHg as a threshold would have committed 43 per cent of patients to fasciotomy, and 40 mmHg would still have led to 23 per cent unnecessary fasciotomies.
• Using a differential pressure - diastolic blood pressure minus compartment pressure - of less than 30 mmHg identified all three genuine compartment syndromes with no missed cases.
• Practical rule: measure the differential pressure, not the absolute pressure, and decompress when it is below 30 mmHg for more than two hours. The duration is part of the threshold and is routinely dropped: McQueen and Duckworth's later review states the criterion as "the differential pressure being less than 30 mmHg for more than 2 h", and it is that sustained reading which carries a sensitivity of 94 per cent and a specificity of 98 per cent, against a sensitivity of only 13 to 64 per cent for clinical findings. A single transient dip below 30 is not the trigger; a single reading above it is not reassurance either. Measure within 5 cm of the fracture, where pressures are highest.

Fasciotomy and the nerve

• The standard two-incision, four-compartment fasciotomy uses an anterolateral incision, roughly 2 cm anterior to the fibular shaft, to open the anterior and lateral compartments.
• The superficial peroneal nerve lies in the lateral compartment and is the nerve at risk from this incision; the deep peroneal nerve lies deeper, on the interosseous membrane, and is at risk from over-enthusiastic deep dissection.
• Release the anterior compartment fascia along its whole length with long scissors, staying just anterior to the intermuscular septum, and identify the septum before releasing the lateral compartment.
• Document first web sensation and dorsiflexion power after fasciotomy as the new baseline.

Chronic exertional compartment syndrome

• Exercise-induced anterior compartment pain with transient first web paraesthesia and foot drop that resolves with rest is the classic history in a runner.
• Diagnosis is by post-exercise compartment pressure measurement; treatment is fasciotomy or fasciectomy of the affected compartments.
• Do not confuse it with a medial tibial stress syndrome or a stress fracture, neither of which produces a neurological symptom.

Localising the lesion

The two discriminators worth memorising

• Eversion separates the deep peroneal nerve from the common peroneal nerve, because peroneus longus and brevis are supplied by the superficial peroneal nerve.
• Inversion and hip abduction separate a peroneal nerve lesion from an L5 root lesion, because tibialis posterior and gluteus medius are L5 muscles that do not travel in the peroneal nerve. This is the single most commonly missed distinction in a foot drop viva.

Common causes at the fibular neck

• Positioning and pressure: lithotomy, lateral decubitus, prolonged bed rest, a tight cast, habitual leg crossing, prolonged squatting.
• Trauma: fibular neck fracture, knee dislocation - where the common peroneal nerve is injured in a substantial proportion, particularly with posterolateral corner disruption - and direct blows.
• Iatrogenic: total knee arthroplasty, particularly after correction of a severe fixed valgus deformity with a flexion contracture; proximal tibial osteotomy; proximal fibular surgery; lateral meniscal and posterolateral corner repair.
• Mass lesions: intraneural ganglion from the superior tibiofibular joint, schwannoma, osteochondroma of the fibular neck.
• Systemic: diabetes, vasculitis, leprosy, hereditary neuropathy with liability to pressure palsies, and rapid weight loss.

Management principles

• Compressive and positional palsies are usually neuropraxias and recover; remove the cause, splint the foot in neutral with an ankle-foot orthosis to prevent equinus contracture, and re-examine.
• Electrodiagnostic baseline at 3 to 4 weeks, repeated at 3 months.
• A new palsy after total knee arthroplasty is managed by removing all dressings, flexing the knee to relieve tension on the nerve, and re-examining; explore if there is a mechanical cause.
• A palsy with a mass lesion is explored - a superior tibiofibular joint ganglion needs the articular branch divided as well as the ganglion decompressed, or it recurs.
• Established, unrecoverable palsy: an ankle-foot orthosis, or reconstruction by posterior tibial tendon transfer through the interosseous membrane to the dorsum of the foot, with or without a tendo-Achilles lengthening.

The anatomy of the tunnel

• Roof: the inferior extensor retinaculum.
• Floor: the fascia overlying the talus and navicular.
• Contents: four tendons (tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius), the anterior tibial artery and vein, and the deep peroneal nerve.

Presentation

• Dorsal midfoot and ankle pain with paraesthesia or dysaesthesia in the first web space.
• Worse with tight footwear, tight laces, ski or skate boots, prolonged plantarflexion (which tightens the retinaculum over the nerve), and at night.
• Wasting of extensor digitorum brevis is the only motor sign, is subtle, and is routinely missed.
• If the compression is distal to the point where the lateral branch leaves, the presentation is purely sensory - first web numbness with no wasting at all.

Causes

• Extrinsic: footwear pressure, laces, straps, direct trauma.
• Intrinsic: dorsal osteophytes from tibiotalar or talonavicular arthritis narrowing the floor, ganglion, a prominent os intermetatarseum, oedema, previous fracture or fracture callus, and recurrent inversion injury with a tethered nerve.
• Distal to the tunnel: entrapment where the extensor hallucis brevis tendon crosses the nerve has been described as a distinct entity, relieved by decompression at that level.

Diagnosis

• Tinel sign between the extensor hallucis longus and extensor digitorum longus tendons.
• Reproduction of symptoms on forced plantarflexion or direct pressure over the retinaculum.
• Electrodiagnosis: prolonged peroneal distal motor latency with reduced amplitude, and abnormalities confined to extensor digitorum brevis. Remember the accessory peroneal nerve and the temperature pitfall.
• Imaging: weight-bearing radiographs for dorsal osteophytes; magnetic resonance imaging or ultrasound for a ganglion or a mass.
• A diagnostic local anaesthetic block into the tunnel that abolishes symptoms is strongly supportive.

Management

• Non-operative first, and it frequently works: change the footwear, re-lace to avoid the dorsal midfoot, use a tongue pad, modify activity, consider a corticosteroid injection, and treat any associated tendinopathy.
• Surgical decompression of the inferior extensor retinaculum, with removal of any dorsal osteophyte or ganglion, when non-operative measures fail. Decompression can be performed at the retinaculum or more distally at the tarsometatarsal level, depending on where the compression is localised.
• Counsel that the diagnosis is uncommon and frequently over- or under-diagnosed, and that the results of decompression depend heavily on correct localisation.

Proximal tibial osteotomy

• Peroneal nerve dysfunction after high tibial osteotomy is more common than the clinical picture suggests. In one pre- and post-operative electrophysiological study of 11 patients undergoing a barrel-vault osteotomy, three (27 per cent) had electrophysiologically detectable peroneal lesions, of whom only one was clinically symptomatic. The lesions persisted for a prolonged period and some were permanent.
• Mechanisms: the fibular osteotomy or fibular head excision performed to allow closure; traction on the nerve during correction; retractors on the posterolateral tibia; a saw or drill exiting the posterolateral cortex; and post-operative compartment syndrome or haematoma. Do not assume the medial opening wedge avoids this. Miller's series of 46 medial opening wedge osteotomies recorded complications in 17 patients (36.9 per cent) - but the reported breakdown is loss of correction in 7, lateral cortex fracture in 4, venous thromboembolism in 2, delayed union in 2 and symptomatic hardware in 2, which accounts for all 17 with no peroneal palsy and no compartment syndrome among them. The honest reading is that the medial approach moves the work away from the nerve without abolishing the risk, and that a series of 46 is far too small to measure a complication of this frequency at all.
• Prevention: if a fibular osteotomy is required, perform it in the middle third of the fibular shaft - at least 10 cm distal to the fibular head and comfortably proximal to the syndesmosis - or excise the fibular head, or release the proximal tibiofibular joint instead. Identify and protect the nerve if working near the neck. Avoid a circumferential dissection at the neck.

Proximal fibular surgery

• Fibular head and neck resection, proximal fibular osteotomy and fixation of a proximal fibular fracture all demand that the nerve be identified first and traced through the peroneus longus arcade before any bone work.
• The nerve is not a structure to be avoided by memory here - it must be seen.

Total knee arthroplasty

• Peroneal palsy is an uncommon but well recognised complication, most often after correction of a severe fixed valgus deformity with a flexion contracture, where the correction lengthens the nerve.
• Epidural analgesia can mask an evolving palsy. Remove the dressing, flex the knee to 20 to 30 degrees, and re-examine as the first response to a new post-operative foot drop.

Ankle arthroscopy and the anterior approach

• The anterocentral portal is avoided precisely because the deep peroneal nerve and the anterior tibial artery lie directly beneath it.
• The anteromedial portal is placed medial to the tibialis anterior tendon; the anterolateral portal is placed lateral to peroneus tertius, where the superficial peroneal nerve is the structure at risk.
• In the anterior approach to the ankle, used for pilon fixation, arthrodesis and total ankle arthroplasty, the interval is medial to extensor hallucis longus, and the neurovascular bundle is retracted laterally with the tendon. Persistent retraction on the bundle is the mechanism of post-operative first web numbness.

Foot surgery

• The medial terminal branch and the dorsalis pedis artery cross the first intermetatarsal space, directly in the field of a first tarsometatarsal fusion, a Lapidus procedure and Lisfranc fixation.
• A dorsal approach between the first and second metatarsals must protect the branch, and a dorsomedial cutaneous branch of the superficial peroneal nerve is at risk in the same field.
• Tight postoperative dressings and casts over the dorsum of the foot compress the nerve at the retinaculum and produce a temporary first web numbness that patients find alarming.

Exposing the nerve at the fibular neck

• Position: lateral decubitus or supine with a bump, knee flexed to relax the nerve.
• Incision: curvilinear, centred over the fibular head, extending posterior to the biceps femoris tendon proximally and distally over the neck.
• Landmark: identify the biceps femoris tendon and follow its medial border distally. The common peroneal nerve is always on the medial side of this tendon - this is the single most reliable way to find it.
• Trace the nerve distally over the neck, divide the arcade of peroneus longus, and follow the division into the deep and superficial branches.
• Do not skeletonise the nerve or strip it circumferentially over a long segment; it depends on its longitudinal epineurial vessels.

Decompression for compressive neuropathy

• Release the deep fascia over the nerve, the arcade of the peroneus longus, and any fibrous band at the entry into the anterior compartment.
• Look for an intraneural ganglion if the palsy was spontaneous - a fusiform swelling of the nerve, sometimes with visible cystic fluid. Decompress the cyst, divide the articular branch to the superior tibiofibular joint, and address the joint, or the ganglion recurs.
• Postoperative: early mobilisation, an ankle-foot orthosis until dorsiflexion returns, and serial examination.

Prevention checklist for the whole team

1. Pad the fibular neck of the dependent leg in every lateral case and every lithotomy case, and re-check after position changes.
2. Document the pre-operative neurological status of both legs before any knee or proximal tibial procedure.
3. Do not allow a circumferential dressing or cast to sit tightly over the fibular neck or the dorsum of the foot.
4. Examine and document first web sensation and hallux dorsiflexion in recovery, before regional analgesia wears off, and again the next morning.
5. Treat a new deficit as a mechanical problem until proven otherwise - remove dressings, flex the knee, and re-examine, rather than ordering an investigation first.

Nerve reconstruction

• Sharp transection: immediate epineurial repair.
• Gap: interposition sural nerve grafting, though results for the peroneal nerve are historically less good than for upper limb nerves because of the long regeneration distance to tibialis anterior.
• Traction lesions after knee dislocation: often long-segment, and outcomes after grafting are modest. The decision to graft is made on the length of the lesion at exploration, typically at 3 to 6 months.
• Nerve transfer: transfer of a branch of the tibial nerve - typically to the soleus or the lateral head of gastrocnemius - to the deep peroneal nerve or its motor branch to tibialis anterior is an option that shortens the regeneration distance. Outcomes are variable and it remains a technique for specialist units.
• Timing: the same 1 mm per day regeneration principle applies, and the distance from the fibular neck to tibialis anterior is short, which is why an early recovery is common in neuropraxia and why failure by 3 to 6 months should prompt a decision.

Tendon transfer for established palsy

• Posterior tibial tendon transfer through the interosseous membrane to the dorsum of the midfoot is the standard reconstruction for an unrecoverable foot drop.
• Prerequisites: a plantigrade, supple foot with no fixed equinus, adequate posterior tibial power (grade 5), and a compliant patient.
• A tendo-Achilles lengthening or gastrocnemius recession is frequently needed at the same sitting.
• The transfer restores a functional dorsiflexion "brake" rather than true active dorsiflexion; realistic counselling matters.
• Alternative: a well-fitted ankle-foot orthosis remains an entirely legitimate definitive treatment, particularly in an older or lower-demand patient.

Preventing the secondary deformity

• The single most important early intervention in any peroneal palsy is a splint holding the ankle at neutral. An equinus contracture develops quickly, is far harder to treat than the palsy itself, and can render an otherwise reconstructable foot unreconstructable.