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, from behind the lateral malleolus, around the cuboid and across the entire sole to the base of the first metatarsal.

Two functions. That course gives it eversion at the hindfoot and plantarflexion of the first ray at the forefoot, and the second 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, it drives the first metatarsal into plantarflexion and sets off the chain that ends in a cavovarus foot, which is why examiners return to this muscle so often.

Eversion. Peroneus longus everts (pronates) the foot at the subtalar and transverse tarsal joints, the primary action it shares with peroneus brevis. Because the tendon passes posterior to the ankle axis with only a small moment arm, it is also a weak ankle plantarflexor.

Plantarflexion of the first ray. This action is unique to peroneus longus, and it is the one that matters clinically. By pulling the first metatarsal base plantarwards and laterally, the muscle depresses the first metatarsal head.

Arch support. It maintains the transverse and lateral longitudinal arches. Pulling the first metatarsal base toward the cuboid approximates the medial and lateral columns and stabilises the transverse arch at push-off. Acting as a plantar sling with tibialis posterior beneath the midfoot, it also stabilises the longitudinal arch.

The windlass. At terminal stance the great toe dorsiflexes at the metatarsophalangeal joint, tightening the plantar fascia and raising the arch. For the windlass mechanism to work the first metatarsal head must be firmly loaded against the ground, and peroneus longus supplies that load: it plantarflexes the first ray at exactly the moment the windlass engages, stabilising the medial column so that push-off occurs through a rigid tripod.

Too little. Without peroneus longus, after a rupture or a peroneus longus to brevis tenodesis, the first ray dorsiflexes under load. The resulting hypermobile first ray transfers load to the second metatarsal, causing transfer metatarsalgia and occasionally a second metatarsal stress fracture. This is a genuine and underappreciated complication of tenodesis, and one candidates should be able to name.

Too much. Unopposed peroneus longus in cavovarus drives the first ray plantarwards and sets off the chain below.

Palpation. The muscle belly is palpable along the lateral leg over the upper two-thirds of the fibula. The tendon can be felt behind the lateral malleolus and 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, deep in the sole, cannot be palpated.

Isolating peroneus longus. Stabilise the hindfoot, push up under the first metatarsal head and ask the patient to press down. Resisted first ray plantarflexion is the only clinical test that specifically loads peroneus longus rather than the evertors as a group.

Assessing a cavovarus foot

Never correct a cavovarus foot without answering three questions. Look first for the peek-a-boo heel from the front and at hindfoot alignment from behind.

Why is it cavovarus? A cavovarus foot in a young patient is a neurological diagnosis until proven otherwise. Around two-thirds of cases have an identifiable neurological cause, most commonly Charcot-Marie-Tooth disease. Examine the hands, the reflexes, the family and the spine: look for intrinsic wasting, clawing, areflexia and hand involvement, examine the parents, and consider nerve conduction studies and genetic testing. If the deformity is unilateral, progressive or of recent onset, obtain a spinal MRI to exclude a tethered cord, syrinx, diastematomyelia or an intraspinal tumour.

Is the hindfoot flexible? The Coleman block test decides whether the calcaneus needs an osteotomy: a flexible hindfoot needs forefoot surgery, a fixed one needs a lateralising calcaneal osteotomy added, and a rigid, arthritic hindfoot needs arthrodesis.

Is the Achilles tight? Perform the Silfverskiold test in every case. Gastrocnemius contracture is present in most cavovarus feet and must be corrected, because failing to correct it guarantees recurrence and a failed transfer.

Radiographs. Weightbearing anteroposterior and lateral foot views show:

• An increased Meary angle (talus-first metatarsal, apex dorsal)
• An increased calcaneal pitch (greater than about 30 degrees)
• A positive Hibbs angle
• A bell-shaped or hourglass sinus tarsi
• On the lateral view, overlap of the metatarsals with the first metatarsal appearing more plantar, which 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 and how it presents. Tears occur at the two watershed zones, which are also the zones of high friction. The patient has lateral or plantar-lateral foot pain, weak eversion, and specifically weakness or pain on resisted first ray plantarflexion. A chronic rupture presents with a progressive dorsiflexed first ray and can precipitate a cavus-to-planus change or transfer metatarsalgia.

The radiograph. On an oblique foot radiograph, proximal migration of an ossified os peroneum is pathognomonic of a peroneus longus rupture distal to the sesamoid. Diastasis of a bipartite or multipartite os peroneum suggests a fracture or an interposed tear.

MRI and associations. Longitudinal splits appear as a C-shaped or bisected tendon on axial images, and fluid in the sheath out of proportion to the other tendon suggests tenosynovitis. 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)

Sobel and colleagues described a spectrum 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

It presents as plantar-lateral midfoot pain, worse on resisted first ray plantarflexion and on inversion stretch. Immobilisation and orthoses come 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. The superior peroneal retinaculum is avulsed or stripped from the posterolateral fibula, often lifting a periosteal sleeve with a thin rim of cortical bone. That rim is the fleck sign on an anteroposterior ankle radiograph, and it is diagnostic.

Eckert and Davis classification grades the plane of failure:

• 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).
• Grade IV, added in some descriptions: 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 is 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

It is rare compared with anterior compartment syndrome, and usually seen as part of a four-compartment leg injury. The signs are 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 release is described under Surgical Relevance.

Superficial peroneal nerve entrapment

The nerve is entrapped where it pierces the deep fascia, causing exercise-related lateral leg pain with dorsal foot paraesthesia and no motor loss. There is a recognised association with chronic exertional lateral compartment syndrome, and with a fascial defect and muscle herniation at the exit point. The 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, with the sural nerve behind and below the fibular tip.

Posterolateral approach to the distal tibia. The interval lies between the peroneals (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. Because the common peroneal nerve divides within peroneus longus, 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, so transilluminate and mark the nerve first.

Lateral compartment fasciotomy. Through an anterolateral incision 2 cm lateral to the tibial crest, the same incision used for the anterior compartment, identify the anterior intermuscular septum and open the lateral compartment posterior to it. The superficial peroneal nerve lies within the compartment, so 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 matters more than the technique. It is indicated for:

• Cavovarus reconstruction, to remove the plantarflexion force on the first ray and augment weak eversion
• An 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 its proximal stump side-to-side to peroneus brevis, 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. Correcting cavovarus means weakening the strong side and augmenting the weak, and this is why the tenodesis is a standard component of cavovarus reconstruction: it addresses both halves of the problem with one manoeuvre.

Prerequisite. The peroneus brevis must be intact and functional. If both tendons are diseased, tenodesis is not an option. A flexor digitorum longus or flexor hallucis longus transfer to the base of the fifth metatarsal restores eversion using a functioning muscle from the deep posterior compartment. Where both tendons are irreparable with substantial scarring, a staged reconstruction places a silicone rod in the peroneal sheath to create a pseudosheath, followed at three months by an allograft or hamstring autograft reconstruction.

Counselling. Loss of first ray plantarflexion leaves the hypermobile or dorsiflexed first ray and transfer metatarsalgia described under the windlass. In a foot that is not cavus this is a genuine downside; in a cavovarus foot it is the therapeutic aim.

Peroneus longus as a graft. The longus tendon is occasionally harvested as an autograft, for example in ligament reconstruction elsewhere, precisely because tenodesing the residual stump to brevis preserves eversion and losing first ray plantarflexion is generally tolerated. Counsel about the hypermobile first ray.

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.