Peroneus Longus Anatomy

Peroneus longus (fibularis longus) is the longer, more superficial and more proximally arising of the two muscles of the lateral (peroneal) compartment. Its tendon has the longest and most convoluted course of any tendon in the lower limb: it descends behind the lateral malleolus, turns forward beneath the peroneal tubercle of the calcaneus, then turns sharply plantarwards and medially around the lateral border of the cuboid, crosses the entire sole obliquely, and inserts on the plantar-lateral aspect of the base of the first metatarsal and the medial cuneiform.

That course gives it two entirely different functions - eversion at the hindfoot and plantarflexion of the first ray at the forefoot - and it is the second of these that dominates its clinical importance. Peroneus longus is the only muscle that plantarflexes the first ray. When it acts unopposed, because tibialis anterior (the first ray dorsiflexor) has weakened, the first metatarsal is driven into plantarflexion, the forefoot pronates relative to the hindfoot, and the hindfoot compensates into varus. That is the pathomechanics of the cavovarus foot in a single sentence, and it is the reason examiners return to this muscle so often.

Actions

• Eversion (pronation) of the foot at the subtalar and transverse tarsal joints - the primary action shared with peroneus brevis.
• Plantarflexion of the first ray - unique to peroneus longus, and the action that matters clinically. By pulling the first metatarsal base plantarwards and laterally it depresses the first metatarsal head.
• Weak ankle plantarflexion - the tendon passes posterior to the ankle axis but with a small moment arm.
• Transverse arch support - by pulling the first metatarsal base toward the cuboid it approximates the medial and lateral columns and stabilises the transverse arch at push-off.
• Longitudinal arch stabilisation - it acts as a plantar sling with tibialis posterior beneath the midfoot.

Peroneus longus versus peroneus brevis

First ray mechanics and the windlass

• At terminal stance the great toe dorsiflexes at the metatarsophalangeal joint, tightening the plantar fascia and raising the arch - the windlass mechanism. For the windlass to work, the first metatarsal head must be firmly loaded against the ground.
• Peroneus longus supplies that load. It plantarflexes the first ray at exactly the moment when the windlass engages, stabilising the medial column so that push-off occurs through a rigid tripod.
• In its absence (peroneus longus rupture, or a peroneus longus to brevis tenodesis) the first ray dorsiflexes under load - a hypermobile first ray - and the load transfers to the second metatarsal, causing transfer metatarsalgia and second metatarsal stress fracture. This is a genuine and underappreciated complication of tenodesis, and one candidates should be able to name.
• In excess (unopposed peroneus longus in cavovarus) the first ray is driven plantarwards, the forefoot pronates, and the hindfoot compensates into varus.

Palpation

• The muscle belly is palpable along the lateral leg over the upper two-thirds of the fibula.
• The tendon is palpable behind the lateral malleolus and can be traced forward below the peroneal tubercle, a small palpable prominence on the lateral wall of the calcaneus roughly 1 to 2 cm distal and anterior to the fibular tip.
• The insertion cannot be palpated - it is deep in the sole.
• Isolate peroneus longus by asking for plantarflexion of the first ray against resistance while the examiner stabilises the hindfoot: push up under the first metatarsal head and ask the patient to press down. This is the only clinical test that specifically loads peroneus longus rather than the evertors as a group.

Named tests and signs

Assessing a cavovarus foot properly

• Peek-a-boo heel from the front and hindfoot alignment from behind.
• Coleman block test for hindfoot flexibility.
• Silfverskiold test - gastrocnemius contracture is present in most cavovarus feet and must be corrected.
• Full neurological examination, including the hands. A cavovarus foot in a young patient is a neurological diagnosis until proven otherwise, and around two-thirds of cases have an identifiable neurological cause, most commonly Charcot-Marie-Tooth disease. Examine for intrinsic wasting, clawing, areflexia, and hand involvement, examine the parents, and consider nerve conduction studies and genetic testing.
• Spinal examination and MRI if the deformity is unilateral, progressive or of recent onset, to exclude a tethered cord, syrinx, diastematomyelia or an intraspinal tumour.
• Radiographs: weightbearing anteroposterior and lateral foot views. Look for an increased Meary angle (talus-first metatarsal, apex dorsal), an increased calcaneal pitch (greater than about 30 degrees), a positive Hibbs angle, and the bell-shaped or hourglass sinus tarsi. On the lateral view the overlap of the metatarsals with the first metatarsal appearing more plantar confirms the plantarflexed first ray.

Related pages. The tendon it runs with, and what it does to it: Peroneus Brevis Anatomy — the longus is the compressing wedge that splits the brevis in the fibular groove — plus Peroneus Tertius Anatomy and Superficial Peroneal Nerve Anatomy, whose 3-to-18 cm exit range makes the lateral incision unsafe by rule. The deformity it drives: Cavovarus Foot — plantarflexion of the first ray is the forefoot-driven cause the Coleman block test interrogates. Its own pathology: Peroneal Tendon Tears, Peroneal Tendon Subluxation and Peroneal Tendon Disorders. Where it turns the corner: Calcaneal Fractures and Subtalar Arthritis for the peroneal tubercle and cuboid tunnel, and Lateral Ankle Instability and Lateral Ankle Sprains for the retinacular injury that starts peroneal pathology.

Peroneus longus tears and rupture

• Where: at the two watershed and high-friction zones - the retromalleolar groove and the cuboid tunnel.
• Presentation: lateral or plantar-lateral foot pain, weakness of eversion, and specifically weakness or pain on resisted first ray plantarflexion. Chronic rupture presents with a progressive dorsiflexed first ray and can precipitate a cavus-to-planus change or transfer metatarsalgia.
• Radiographic clue: proximal migration of an ossified os peroneum on an oblique foot radiograph is pathognomonic of a peroneus longus rupture distal to the sesamoid. Diastasis of a bipartite os peroneum suggests a fracture or an interposed tear.
• MRI: longitudinal splits appear as a C-shaped or bisected tendon on axial images; fluid in the sheath out of proportion to the other tendon suggests tenosynovitis.
• Association: a hypertrophied peroneal tubercle (present in a minority of calcanei, and more prominent in cavovarus) causes attritional wear as the tendon rides over it.

Painful os peroneum syndrome (POPS)

• A spectrum described by Sobel and colleagues encompassing:
  • Acute os peroneum fracture or diastasis of a multipartite ossicle.
  • Chronic os peroneum fracture with a callus and stenosing tenosynovitis.
  • Attrition or partial rupture of peroneus longus proximal or distal to the ossicle.
  • Frank rupture with proximal ossicle migration.
  • A hypertrophied peroneal tubercle entrapping the tendon.
• Presents as plantar-lateral midfoot pain, worse on resisted first ray plantarflexion and on inversion stretch.
• Management: immobilisation and orthoses first; surgery involves excision of the ossicle with tendon repair, or, if the tendon is irreparable, tenodesis of peroneus longus to peroneus brevis proximally.

Peroneal tendon subluxation and dislocation

• Mechanism: a forceful dorsiflexion with sudden reflex contraction of the peroneals, classically a skiing injury with the ski tip catching, or a football or basketball injury. It is frequently misdiagnosed as a lateral ankle sprain.
• Pathoanatomy: avulsion or stripping of the superior peroneal retinaculum from the posterolateral fibula, often lifting a periosteal sleeve with a thin rim of cortical bone - the fleck sign on an anteroposterior ankle radiograph, which is diagnostic.
• Eckert and Davis classification:
  • Grade I: the retinaculum is elevated from the lateral malleolus with the tendons lying between bone and periosteum.
  • Grade II: the fibrocartilaginous ridge is elevated with the retinaculum.
  • Grade III: a cortical avulsion fragment is displaced with the retinaculum (the fleck sign).
  • A grade IV is added in some descriptions where the retinaculum is avulsed from its posterior attachment.
• Predisposing factors: a shallow, flat or convex retromalleolar groove, a low-lying peroneus brevis muscle belly, an accessory peroneus quartus muscle (present in roughly 10 to 20 per cent of feet), retinacular laxity, and hindfoot varus.
• Treatment: acute injuries in low-demand patients may be treated in a cast in slight plantarflexion and inversion for six weeks, but recurrence is common and most athletes are treated surgically. Surgery comprises superior peroneal retinaculum repair or reconstruction, combined with fibular groove deepening where the groove is shallow, and correction of hindfoot varus where present.

Peroneal tendinopathy and tenosynovitis

• Overuse pain along the tendons behind and below the lateral malleolus, worse with activity on uneven ground.
• Always look for the underlying cause: hindfoot varus, a lateral ligament-deficient ankle, a hypertrophied peroneal tubercle, a low-lying brevis belly or a peroneus quartus. Treating the tendinopathy without correcting hindfoot varus guarantees recurrence.

Lateral compartment syndrome

• Rare compared with the anterior compartment, and usually seen in the context of a four-compartment leg injury.
• Signs: pain on passive inversion, tenderness over the lateral compartment, and numbness on the dorsum of the foot sparing the first web space (superficial peroneal territory).
• The lateral compartment is released through the same anterolateral incision as the anterior compartment, working posterior to the anterior intermuscular septum, with the superficial peroneal nerve at risk.

Superficial peroneal nerve entrapment

• Entrapment where the nerve pierces the deep fascia 10 to 12 cm above the lateral malleolus, causing exercise-related lateral leg pain with dorsal foot paraesthesia and no motor loss.
• A recognised association with chronic exertional lateral compartment syndrome and with a fascial defect and muscle herniation at the exit point.
• Diagnosis is clinical, with a Tinel sign at the exit point and reproduction on resisted dorsiflexion-eversion or passive plantarflexion-inversion; a diagnostic local anaesthetic block is confirmatory.

Approaches and intervals

• Lateral approach to the fibula and ankle: the peroneal tendons lie posterior to the fibula, and the sural nerve roughly 1.5 cm posterior and inferior to the fibular tip. A posterolateral approach to the distal tibia uses the interval between the peroneals and flexor hallucis longus.
• Posterolateral approach to the tibia: between peroneus longus and brevis (lateral compartment) and soleus and flexor hallucis longus (posterior), reaching the posterior tibial surface. The peroneal artery lies deep in this interval.
• Proximal fibular exposure and fibular head resection: the common peroneal nerve divides within peroneus longus at the fibular neck; the nerve must be identified proximal to the muscle and traced through it before any osteotomy or resection. This is the highest-risk nerve exposure in the leg.
• Lateral ankle arthroscopy: the anterolateral portal risks the intermediate dorsal cutaneous branch of the superficial peroneal nerve; transilluminate and mark the nerve first.
• Lateral compartment fasciotomy: through an anterolateral incision 2 cm lateral to the tibial crest, the anterior intermuscular septum is identified and the lateral compartment opened posterior to it. The superficial peroneal nerve lies within the compartment; direct the fasciotomy posterior and distal with care.

Peroneus longus to peroneus brevis tenodesis

This is the operation examiners ask about, and the reasoning behind it is more important than the technique.

• Indications:
  • Cavovarus reconstruction - to remove the plantarflexion force on the first ray and augment weak eversion.
  • Irreparable peroneus longus tear with an intact brevis.
  • Painful os peroneum syndrome where the tendon cannot be repaired.
• Technique: through a lateral incision behind and below the fibula, expose both tendons in the common sheath. Divide peroneus longus proximal to the diseased or deforming segment and suture the proximal longus stump side-to-side to the peroneus brevis tendon, with the foot in neutral to slight eversion and appropriate tension, using two or three non-absorbable mattress sutures over a 2 to 3 cm overlap. Excise the distal longus segment or leave it if it is not symptomatic.
• Effect: the power of peroneus longus is redirected into eversion through the brevis insertion at the fifth metatarsal base, and its plantarflexing effect on the first ray is abolished.
• Prerequisite: the peroneus brevis must be intact and functional. If both tendons are diseased, tenodesis is not an option - the choices are then a flexor digitorum longus or flexor hallucis longus transfer to the fifth metatarsal base, or a staged reconstruction with a silicone rod and later hamstring allograft.
• Complication to counsel: loss of first ray plantarflexion creates a hypermobile or dorsiflexed first ray with transfer load to the second metatarsal, producing transfer metatarsalgia and occasionally a second metatarsal stress fracture. In a foot that is not cavus this is a genuine downside; in a cavovarus foot it is the therapeutic aim.

Cavovarus reconstruction - where peroneus longus fits

Anatomical variation across populations

• Os peroneum ossification is reported in roughly 5 to 25 per cent of feet, with the wide range reflecting differences in imaging modality and population. It is bilateral in the majority when present, and multipartite in a minority - a fact that matters when interpreting a suspected fracture.
• Peroneus quartus is an accessory muscle present in approximately 10 to 20 per cent of feet in cadaveric series, usually arising from peroneus brevis and inserting on the retrotrochlear eminence of the calcaneus. It crowds the retromalleolar groove and is associated with brevis tears and subluxation.
• Retromalleolar groove morphology varies widely: concave in the majority, but flat or even convex in a significant minority - the anatomical substrate for peroneal tendon instability.
• Superficial peroneal nerve course is highly variable, including bifurcation proximal to the fascial exit and an anterior compartment course in a minority.
• Peroneal tubercle size varies substantially and hypertrophy is more common in cavovarus feet.

Side-by-side guidance

Resource-dependent practice

• Well-resourced settings: MRI to characterise tendon tears and groove morphology, nerve conduction studies and genetic testing for CMT, weightbearing CT for three-dimensional deformity assessment.
• Limited-resource settings: the Coleman block test, the peek-a-boo heel sign, the Silfverskiold test and a careful neurological examination cost nothing and answer almost every question needed to plan cavovarus surgery. Weightbearing radiographs suffice for surgical planning. Plantar fascia release, peroneus longus to brevis tenodesis, first metatarsal dorsiflexion osteotomy and a Dwyer calcaneal osteotomy all require only basic instruments and are fully deliverable at district hospital level.
• Peroneal tendon surgery requires no implants beyond suture and anchors, and groove deepening can be performed with an osteotome and impactor.

Registry and outcome signals

• There is no registry for peroneal tendon or cavovarus surgery. Cohort series consistently report that failure of cavovarus reconstruction is due to under-correction, unaddressed equinus, and progression of the underlying neuropathy, rather than to failure of the tendon work itself.
• Series of peroneal tendon dislocation consistently report a high recurrence rate with non-operative treatment in athletes and reliable results with retinacular repair combined with groove deepening.