Superficial Peroneal Nerve Anatomy

Origin

• Terminal branch of the common peroneal nerve, arising at the fibular neck within the substance of peroneus longus, beneath the arcade of that muscle.
• Root value L4, L5 and S1, predominantly L5 and S1.

In the lateral compartment

• Descends between the peroneus longus and brevis posteriorly and the anterior intermuscular septum anteriorly.
• Gives motor branches to peroneus longus proximally and to peroneus brevis more distally.
• Compartment variation is the examinable point. In 85 cadaveric legs, the nerve:
  • ran within the lateral compartment from origin to fascial exit in 73 per cent,
  • took a variable course in the lateral compartment then crossed into the anterior compartment in 14 per cent,
  • divided in two, with branches in both the anterior and lateral compartments, in 12 per cent,
  • and in one leg never lay deep to peroneus longus at all, running along the deep surface of the fascia throughout.
• Practically: more than a quarter of legs have superficial peroneal nerve tissue in the anterior compartment. That is why a surgeon releasing the anterior compartment must not assume the nerve is safely elsewhere.

The fascial exit

• The nerve pierces the deep crural fascia in the distal third of the leg to become subcutaneous.
• The classic teaching figure is 10 to 12 cm proximal to the tip of the lateral malleolus.
• The measured cadaveric range is 3 to 18 cm - a range that makes any single number unsafe to rely on.
• The exit point is a fixed tethering point and is the site of superficial peroneal nerve entrapment.

Terminal branches

• Medial dorsal cutaneous nerve: the more medial branch. Crosses the anterior ankle and extensor retinaculum, and supplies the medial side of the hallux and the second and third web spaces - the adjacent sides of the second, third and fourth toes.
• Intermediate dorsal cutaneous nerve: the more lateral branch. Crosses the anterolateral ankle and supplies the dorsolateral foot, and the third and fourth web spaces - the adjacent sides of the third, fourth and fifth toes.
• The division frequently occurs before the nerve pierces the fascia, which is why two separate subcutaneous branches are often found at the ankle rather than a single trunk.
• Articular branches supply the ankle and subtalar joints.

The parent nerve is developed in common peroneal nerve anatomy and its motor twin in deep peroneal nerve anatomy. The procedures that threaten it most are covered in ankle impingement syndromes and compartment syndrome.

The lateral compartment

The functional pair to understand

• Peroneus longus versus tibialis anterior is the first ray couple. Peroneus longus plantarflexes the first ray; tibialis anterior dorsiflexes it.
• Peroneus brevis versus tibialis posterior is the hindfoot couple. Brevis everts; tibialis posterior inverts.
• In Charcot-Marie-Tooth disease the classic pattern is a weak tibialis anterior and a weak peroneus brevis with a relatively strong peroneus longus and strong tibialis posterior - producing a plantarflexed first ray, forefoot-driven hindfoot varus, and the cavovarus foot. This is why the transfer of peroneus longus to peroneus brevis is part of cavovarus reconstruction: it removes the deforming first ray plantarflexor and augments the deficient evertor.

The diagnostic value of eversion

• Eversion is the single most useful motor test when localising a peroneal problem.
• Foot drop with preserved eversion localises to the deep peroneal nerve.
• Foot drop with lost eversion localises to the common peroneal nerve at or above the fibular neck.
• Weak eversion with weak inversion points to an L5 root lesion, because tibialis posterior is L5 but travels in the tibial nerve.

The territory

• The distal anterolateral leg, and then almost the entire dorsum of the foot and toes.
• Medial dorsal cutaneous nerve: medial border of the hallux and the second and third web spaces.
• Intermediate dorsal cutaneous nerve: dorsolateral foot and the third and fourth web spaces.

The three exclusions - and they are all examinable

1. The first dorsal web space is deep peroneal, not superficial peroneal.
2. The lateral border of the foot and the lateral side of the little toe are sural.
3. The medial border of the foot, as far as the first metatarsophalangeal joint, is saphenous - a femoral nerve branch, and the only sensory territory below the knee that is not sciatic.

Sural versus superficial peroneal - the border that examiners test

Practical mapping

• Test the fourth web space for the intermediate dorsal cutaneous nerve, the dorsum of the second and third toes for the medial dorsal cutaneous nerve, the first web space for the deep peroneal nerve, and the lateral border of the foot for the sural nerve. Four points, four nerves, ten seconds.
• A patient with numbness confined to the dorsum of the foot that spares the first web and the lateral border has a superficial peroneal nerve lesion, and the level of the lesion is determined by whether eversion is also weak.
• Overlap between adjacent cutaneous territories is common, so a small area of preserved sensation within a territory does not exclude a complete lesion.

The fascial exit - the entrapment site

• The nerve is tethered where it pierces the crural fascia in the distal third of the leg. This is the superficial peroneal tunnel, and it is the site of entrapment.
• A fascial defect over the lateral compartment, often with a muscle hernia through it, is the most consistently reported associated finding: in one operative series, fascial defects were present in 11 of 24 legs treated for entrapment, and an anomalous nerve course in 5.
• Chronic exertional lateral compartment syndrome raises pressure behind the exit and is an occasional cause of the same syndrome.

Traction and stretch

• Inversion ankle sprain stretches the nerve as it crosses the anterolateral ankle, and a traction neuropraxia is a recognised cause of persistent lateral ankle pain and dorsal foot dysaesthesia after a sprain that otherwise appears to have healed.
• Displaced distal fibular fractures stretch the nerve over the fracture, and a pre-operative deficit should be documented before fixation.

Iatrogenic compression and division

• Anterolateral ankle arthroscopy portal - the single commonest site.
• Lateral compartment fasciotomy with blind distal scissors.
• The proximal end of a distal fibular plate incision, where the nerve crosses the fibula anteriorly.
• Lateral ankle ligament reconstruction and the sinus tarsi approach.
• Tight dressings, casts and boot straps over the anterolateral ankle.

Relations that matter in theatre

• The anterior intermuscular septum is the key intra-operative landmark. The nerve lies immediately posterior to it, in the lateral compartment. Identify the septum, and you know where the nerve is.
• The short saphenous vein and its tributaries mark the sural nerve posterolaterally; the superficial veins over the anterolateral ankle mark the superficial peroneal branches. A vein in your incision is a warning.
• The extensor retinaculum at the ankle lies deep to the nerve branches, not superficial to them - the branches cross on top of it.
• The peroneal tendons lie posterior to the lateral malleolus, well behind the nerve at that level.

The commonest iatrogenic injury in the operation

Injury to the superficial peroneal nerve at the anterolateral portal is the most frequent complication of anterior ankle arthroscopy, and the branch usually involved is the intermediate dorsal cutaneous nerve.

The evidence that it is preventable

• In a controlled cadaveric course study of 60 novice surgeons, one group received standard teaching on portal placement and the other received an additional lecture specifically on portal-related complications and the structures at risk.
• The rate of superficial peroneal nerve injury fell from 25 per cent in the standard group to 3.6 per cent in the group taught about complications.
• The same study found hindfoot endoscopy significantly safer than anterior arthroscopy overall - 5 per cent of structures injured versus 13.9 per cent.

Portal technique that protects the nerve

1. Mark the nerve before anything else - plantarflexion, inversion, fourth toe flexion, skin pen.
2. Establish the anteromedial portal first, medial to the tibialis anterior tendon, where the saphenous nerve and vein are the structures at risk.
3. Localise the anterolateral portal with a needle under direct arthroscopic vision, and use transillumination through the skin to identify the nerve as a shadow.
4. Place the portal lateral to the peroneus tertius tendon, at or just above the joint line.
5. Incise skin only with the blade, then spread bluntly with a mosquito haemostat down to capsule, and introduce a blunt trocar.
6. Never use an anterocentral portal - the deep peroneal nerve and anterior tibial artery lie directly beneath it.
7. Use non-invasive distraction rather than an invasive distractor where possible, and avoid excessive traction time.
8. Examine and document superficial peroneal territory sensation postoperatively.

If it is injured

• A new area of numbness on the dorsum of the foot after arthroscopy is common and often a neuropraxia from portal stretch; it usually recovers over weeks to a few months.
• A painful, Tinel-positive spot at the portal that persists beyond three to six months indicates a divided branch or a branch tethered in scar, and warrants a diagnostic block and consideration of exploration.
• Excision of a symptomatic neuroma with proximal transposition into muscle is the salvage - excision alone recreates the problem in the same subcutaneous plane.

The nerve in the compartment you are opening

The superficial peroneal nerve lies within the lateral compartment, immediately posterior to the anterior intermuscular septum. Any fasciotomy of the leg therefore has to deal with it deliberately.

Two-incision four-compartment fasciotomy

• Lateral incision: longitudinal, approximately 2 cm anterior to the fibular shaft, centred over the intermuscular septum.
• Find the septum first. Once the fascia is exposed, identify the anterior intermuscular septum - the perforating vessels emerging along it are a helpful marker.
• Anterior compartment: release the fascia anterior to the septum, running the scissors proximally toward the patella and distally toward the ankle, staying anterior.
• Lateral compartment: release the fascia posterior to the septum, running the scissors proximally toward the fibular head and distally toward the lateral malleolus.
• The distal lateral release is where the nerve is divided, because the nerve has become subcutaneous by that level and lies directly in the path of scissors aimed at the malleolus. Either extend the skin incision distally and visualise the nerve, or make a separate small distal counter-incision and release the last few centimetres under direct vision.
• Remember the anterior compartment variation. More than a quarter of legs have superficial peroneal nerve tissue in the anterior compartment - so the nerve can be injured during the anterior release as well.

Chronic exertional compartment syndrome

• Lateral compartment release for chronic exertional compartment syndrome carries the same risk in a lower-stakes setting, with the added problem that the operation is elective and the patient's expectations are high.
• Use a small open incision centred on the fascial defect if there is one, identify the nerve directly, and release under vision rather than through a minimal incision with a fasciotome.

Documentation

• Record superficial peroneal territory sensation and eversion power before and after every fasciotomy. A post-fasciotomy deficit that was not documented pre-operatively is indefensible.

Superficial peroneal nerve entrapment (superficial peroneal tunnel syndrome)

• Presentation: pain over the distal anterolateral leg with paraesthesia or dysaesthesia over the dorsum of the foot, characteristically brought on by exercise and relieved by rest. Often mistaken for chronic exertional compartment syndrome or a stress fracture.
• Site: at the point where the nerve pierces the crural fascia, 10 to 12 cm above the lateral malleolus.
• Associated findings: a fascial defect with a muscle hernia over the lateral compartment, present in nearly half of operated legs in one series; an anomalous nerve course in a further group; and occasionally a genuine chronic lateral compartment syndrome behind the exit.
• Diagnosis: entirely clinical. Local tenderness with a Tinel sign at the exit point, the three provocative manoeuvres, a palpable fascial defect, and abolition of symptoms by a diagnostic local anaesthetic block. Nerve conduction studies, radiographs and resting compartment pressures are typically normal.

Management

• Non-operative first: activity modification, relative rest, correction of training errors, orthoses where a cavovarus or overpronating foot is loading the peronei, and a corticosteroid injection at the exit point.
• Operative decompression: fasciotomy of the superficial peroneal tunnel with neurolysis, releasing the fascia both proximal and distal to the exit and enlarging any fascial defect rather than repairing it. Local fasciectomy is an alternative.
• Outcomes are honest rather than spectacular. In one series of 24 legs, 9 patients were satisfied, 6 improved but limited in sport, 3 unchanged and 1 worse - roughly three-quarters cured or improved, with results less good in athletes. A later prospective series using strict clinical diagnostic criteria reported 80 per cent free of symptoms or satisfied.
• Counsel accordingly: the operation reliably helps daily activity, less reliably restores high-level sport.

Trauma

• Inversion ankle sprain: a traction neuropraxia of the nerve at the anterolateral ankle. Suspect it when lateral ankle pain persists beyond the expected recovery, particularly with dorsal foot dysaesthesia and a positive Tinel sign over the nerve. Most settle with time, desensitisation and proprioceptive rehabilitation.
• Distal fibular fracture: the nerve is stretched over a displaced fibula, and the deficit is present before treatment. Document it pre-operatively - a deficit first recorded after fixation is assumed to be iatrogenic.
• Fibular neck fracture and knee dislocation: injure the common peroneal nerve, so both superficial and deep territories are involved.
• Crush and degloving injuries of the anterolateral leg divide the nerve along with the skin.

Elective foot and ankle surgery

• Lateral ligament reconstruction (Brostrom and its modifications): the intermediate dorsal cutaneous branch crosses the incision. Elevate the flap carefully and retract the branch with it.
• Sinus tarsi and extended lateral approaches for calcaneal fracture: the branch crosses the proximal limb of the incision.
• Distal fibular plating: the nerve crosses the fibula anteriorly several centimetres above the tip of the malleolus, so it lies at the proximal end of a standard lateral incision.
• Hallux valgus and first metatarsophalangeal surgery: the medial dorsal cutaneous branch runs directly over the dorsomedial aspect of the first metatarsophalangeal joint and is the nerve responsible for post-operative dorsomedial numbness and neuroma after a medial approach.
• Lesser toe and midfoot surgery: dorsal incisions in the second, third and fourth intermetatarsal spaces cross the terminal branches.
• Peroneal tendon surgery: the branches lie anterior to the tendons at the ankle but cross the field in a distal peroneal tendon transfer.

Nerve blocks and grafts

• Ankle block: a superficial peroneal block is placed as a subcutaneous ring across the anterior ankle from the tibialis anterior tendon to the lateral malleolus, because the nerve has already divided into multiple branches at that level.
• Graft donor: the superficial peroneal nerve can be harvested as a sensory graft, but it sits behind the sural nerve as first choice because the resulting numb dorsum is more troublesome than a numb lateral border, and because a neuroma on the dorsum of the foot sits under the shoe.
• Nerve biopsy: the superficial peroneal nerve, often taken together with an adjacent peroneus brevis muscle biopsy, is a standard site for combined nerve and muscle biopsy in suspected vasculitis.

Universal principles for anterolateral leg, ankle and foot incisions

1. Mark the nerve before prep, exsanguination or distension. Plantarflex, invert, flex the fourth toe, look tangentially, mark with a pen.
2. Incise skin only. Then spread bluntly, parallel to the expected course of the branches.
3. Treat superficial veins as markers. Where there is a vein, there is usually a nerve branch.
4. Use blunt trocars and blunt haemostats, never a sharp obturator or a blind scissor pass into subcutaneous fat.
5. Cross branches at right angles with the branch visible and mobilised, rather than dissecting along them.
6. Release retractors periodically. Sustained retraction on a subcutaneous sensory nerve produces a traction neuropraxia even when the nerve is anatomically intact.
7. Avoid a circumferential tight dressing over the anterolateral ankle.
8. Document sensory examination before and after every case in this region.

Positioning and adjuncts

• Prone or lateral positioning for hindfoot work changes the apparent line of the nerve; re-mark it in the operative position, not in the anaesthetic room position.
• Transillumination through the arthroscope is a genuinely useful adjunct at the anterolateral portal and costs nothing.
• Regional block after marking, not before. A block placed before the nerve is marked removes the ability to elicit paraesthesia during localisation.

Recognised intra-operatively

• Direct epineurial repair under magnification if the ends are clean and can be apposed without tension. Even where sensory recovery is imperfect, an intact repair is much less likely to produce a painful neuroma than a free stump.
• Small gap: a conduit or a short interpositional graft, most conveniently from the sural nerve or a segment of the same nerve.
• If repair is not possible: resect the stump proximally and bury it in the peroneal muscle belly, well proximal to the ankle and away from any shoe pressure point. Never leave a divided stump in subcutaneous fat over the dorsum of the foot.

Recognised postoperatively

• A new area of dorsal foot numbness alone: observe, reassure and desensitise. Most portal and retraction lesions recover.
• A painful, pinpoint, Tinel-positive spot at the incision or portal persisting at three to six months: this is a neuroma or a tethered branch.
  • Confirm with a diagnostic local anaesthetic block placed proximal to the point.
  • Exclude and treat complex regional pain syndrome first.
  • Then explore: neurolysis for a tethered branch in continuity, or excision with proximal transposition into muscle for a true neuroma.
• Shoe modification and padding are legitimate definitive treatments in a patient who does not want further surgery, because the pain here is largely pressure-driven.

Tendon transfers involving its muscles

• Peroneus longus to peroneus brevis transfer is a standard component of cavovarus reconstruction, most often in Charcot-Marie-Tooth disease. It removes the deforming plantarflexion force on the first ray while augmenting the weak evertor. The superficial peroneal nerve branches to both muscles must be preserved during the transfer.
• Peroneus brevis is a donor for lateral ligament reconstruction in some historical techniques (Evans, Chrisman-Snook) - now largely superseded by anatomical repair, partly because sacrificing the prime evertor is a real functional cost.
• Peroneus longus is used as a graft source in some ligament reconstructions; the same caution applies, and the nerve branches must be protected during harvest.

After a common peroneal nerve palsy

• When the superficial peroneal nerve is lost as part of a common peroneal palsy, the loss of eversion compounds the foot drop and produces an equinovarus foot, which is far harder to brace than a pure drop foot.
• Posterior tibial tendon transfer through the interosseous membrane addresses both the dorsiflexion and the varus, and this is one reason it is preferred to a simple orthosis in an active patient with a complete palsy.
• Splint at neutral from day one regardless of the reconstruction plan, because an equinovarus contracture develops quickly and closes off the reconstructive options.