Peroneus Brevis Anatomy

Peroneus brevis (fibularis brevis) is the shorter, deeper and more distal of the two lateral compartment muscles. It arises from the distal two-thirds of the lateral fibula, so its muscle belly extends further distally than that of peroneus longus, and its tendon runs anterior and deep to the longus in the retromalleolar groove, directly against the bone. It inserts on the styloid process at the base of the fifth metatarsal.

Three facts follow from that anatomy and account for almost everything clinically important about the muscle:

1. Because it lies against the fibula with the longus tendon pressed over it, peroneus brevis is the tendon that develops longitudinal split tears.
2. Because it inserts on the fifth metatarsal base, it is implicated in the fracture patterns of that bone and is a fixed point that must be respected in every lateral foot procedure.
3. Because it is the strongest and most direct evertor, it is the muscle whose loss matters most - and yet it was historically harvested wholesale for lateral ligament reconstruction, which is exactly why modern practice avoids that.

Actions

• Eversion (pronation) of the foot at the subtalar and transverse tarsal joints - it is the strongest and most efficient evertor, with a larger and more direct eversion moment arm than peroneus longus.
• Weak ankle plantarflexion - the tendon passes behind the ankle axis with a small moment arm.
• Abduction of the forefoot, pulling the lateral column laterally.
• Dynamic lateral ankle stabiliser - by resisting inversion it protects the anterior talofibular and calcaneofibular ligaments. Reflex peroneal contraction is a key component of the dynamic defence against an inversion sprain, and slowed peroneal reaction time is a recognised feature of chronic ankle instability.

The antagonist pairings

Why the strength mismatch matters

• In a normal foot the balance between tibialis posterior and peroneus brevis is maintained by neural control, not by equal strength. Tibialis posterior is stronger, but it is not asked to work unopposed.
• In a length-dependent neuropathy such as Charcot-Marie-Tooth disease, the more distal and smaller muscles fail first: peroneus brevis weakens while tibialis posterior remains strong, and the inversion moment goes unopposed. Combined with a strong peroneus longus plantarflexing the first ray, this generates the classic cavovarus foot.
• Restoring eversion is therefore a therapeutic aim in cavovarus, and it is one of the two things a peroneus longus to peroneus brevis tenodesis achieves - the other being abolition of first ray plantarflexion.

The fifth metatarsal base as a mechanical node

• The tuberosity is loaded by three structures: the peroneus brevis tendon (dorsolateral), the lateral band of the plantar aponeurosis (plantar), and the lateral collateral ligament of the calcaneocuboid joint.
• In a sudden inversion with the foot plantarflexed, tension in these structures avulses the tuberosity - the zone 1 avulsion fracture. Cadaveric and clinical work implicates the lateral band of the plantar fascia at least as much as the peroneus brevis, and many authorities now regard the plantar fascia as the dominant deforming structure.
• Because it is a metaphyseal, well-vascularised, minimally displaced avulsion in cancellous bone, it heals reliably with symptomatic treatment.
• Contrast with the Jones fracture, which sits distal to this at the metaphyseal-diaphyseal junction, in a vascular watershed, in cortical bone, in a region loaded by the adduction-and-plantarflexion moment across the lateral column rather than by a simple avulsion. That is why it behaves entirely differently.

Palpation

• The tendon is palpable behind the lateral malleolus, deep to peroneus longus, and can be traced forward above the peroneal tubercle to the base of the fifth metatarsal, which is subcutaneous and easily felt as a prominence at the mid-lateral border of the foot.
• Ask for resisted eversion with the foot in slight plantarflexion to isolate the peroneals from the extensor digitorum longus and peroneus tertius, which also evert but from a dorsiflexed position.
• Isolating brevis from longus: peroneus brevis is best assessed by resisted eversion (which it does more efficiently than longus); peroneus longus is assessed by resisted plantarflexion of the first ray. Pain or weakness in one but not the other narrows the diagnosis.

Named tests and signs

Imaging

• Radiographs: anteroposterior, lateral and oblique foot views for the fifth metatarsal base - the oblique view is essential and the fracture is often invisible on the other two. Look also for a fleck sign at the distal fibula on an ankle series, indicating retinacular avulsion.
• The two mimics of a fifth metatarsal base fracture in a child or adolescent:
  • Unfused apophysis - the physeal line runs parallel to the shaft (longitudinal), whereas a fracture line runs transverse to the shaft. It appears around age 9 to 11 and fuses by around 12 to 15.
  • Os vesalianum - a rounded, corticated accessory ossicle proximal to the tuberosity within the peroneus brevis tendon, present in a small percentage of feet.
• MRI: longitudinal splits appear as a C-shaped, chevron-shaped or bisected brevis tendon on axial images, sometimes with the longus tendon nestling in the split. Also assesses retinacular integrity, groove morphology, a low-lying muscle belly and a peroneus quartus.
• Ultrasound: dynamic assessment is the best modality for demonstrating subluxation, which is a movement phenomenon and can be entirely normal on a static MRI.
• CT or weightbearing CT: best for groove morphology, the peroneal tubercle, and hindfoot alignment.

Related pages. The tendon it is crowded by, and the rest of the group: Peroneus Longus Anatomy — the wedge that splits the brevis against the sharp posterior fibular edge — plus Peroneus Tertius Anatomy and Superficial Peroneal Nerve Anatomy, its motor supply. What goes wrong with it: Peroneal Tendon Tears, Peroneal Tendon Subluxation — the retinacular failure that starts the cascade — and Peroneal Tendon Disorders. What its insertion does: Jones Fractures, Fifth Metatarsal Stress Fractures and Metatarsal Fractures — the tuberosity avulsion is the brevis pulling off its own footprint. Why the tendon is loaded in the first place: Lateral Ankle Instability and Lateral Ankle Sprains for the instability that drives attrition, and Cavovarus Foot, the hindfoot alignment that must be corrected or the repair fails. Neighbouring hindfoot pathology: Subtalar Arthritis and Calcaneal Fractures, which displace the peroneal tendons laterally.

Fifth metatarsal base fractures

• The Torg classification of zone 2 and 3 injuries grades them radiographically as acute (a sharp fracture line, no intramedullary sclerosis), delayed union (a widened fracture line with some sclerosis) and nonunion (complete medullary sclerosis). Sclerosis indicates that non-operative management is unlikely to succeed.
• Hindfoot varus is the single most important predisposing factor for zone 2 and 3 fractures and for their recurrence. A fifth metatarsal stress fracture or a refracture after screw fixation should always trigger an assessment of hindfoot alignment and a Coleman block test. Failing to correct varus means the screw will simply break or the bone will refracture.
• Screw technique: a solid or cannulated intramedullary screw of the largest diameter the canal will accept, inserted from the tip of the tuberosity in line with the medullary canal ("high and inside"), engaging the diaphysis. A screw that is too short, too narrow, or that straightens the natural curvature of the metatarsal and causes distal cortical impingement, predisposes to failure. The sural nerve is at risk at the entry point.

Peroneus brevis longitudinal split tears

• Prevalence: described in a substantial proportion of asymptomatic cadaveric specimens as well as in symptomatic patients, so a split on MRI must be correlated with symptoms and examination.
• Presentation: chronic posterolateral ankle pain and swelling, worse on uneven ground, often with a history of a "sprain that never got better". Weakness of eversion is variable.
• The Krackow and Bohne classification and the description by Sobel and colleagues stratify splits by extent, from partial-thickness fraying to a complete bisection with the longus lying between the two limbs.
• Almost always associated with at least one of: superior peroneal retinacular laxity, a low-lying muscle belly, a peroneus quartus, a flat or convex fibular groove, a hypertrophic peroneal tubercle, lateral ankle instability, or hindfoot varus.
• Treatment:
  • Less than 50 per cent of the tendon involved: debride and tubularise with a running non-absorbable suture.
  • Greater than 50 per cent involved, or the tendon is unusable: tenodesis to peroneus longus proximally and distally, excising the diseased segment - provided the longus is intact.
  • Both tendons unusable: flexor digitorum longus or flexor hallucis longus transfer to the fifth metatarsal base, or a staged reconstruction with a silicone rod followed by an allograft or hamstring autograft.
  • In every case: decrowd the groove (debulk a low-lying belly, excise a peroneus quartus), repair the superior peroneal retinaculum, deepen a flat groove, and correct hindfoot varus.

Peroneal tendon subluxation and dislocation

• Mechanism: sudden forced dorsiflexion with reflex peroneal contraction, classically a skiing injury, frequently misdiagnosed as a lateral ankle sprain.
• Pathoanatomy: the superior peroneal retinaculum strips from the posterolateral fibula with a periosteal sleeve, sometimes taking a rim of cortex - the fleck sign, diagnostic on an anteroposterior ankle radiograph.
• Eckert and Davis grades I to III describe increasing depth of that failure, from elevation of the retinaculum alone (I), through elevation with the fibrocartilaginous ridge (II), to a displaced cortical avulsion (III); a grade IV in some descriptions is avulsion from the posterior attachment.
• Treatment: cast immobilisation in slight plantarflexion and inversion for six weeks is an option acutely in a low-demand patient, but recurrence is common; surgical repair or reconstruction of the retinaculum, with fibular groove deepening if the groove is flat or convex, is standard in athletes.
• Intrasheath subluxation is a distinct entity in which the tendons swap position within an intact retinaculum, producing a painful snap without visible dislocation. It is diagnosed on dynamic ultrasound and is easily missed on static MRI.

Peroneal tendinopathy and tenosynovitis

• Overuse pain along the tendons, worse with lateral loading. Always look for the underlying cause.
• A hypertrophied peroneal tubercle (more common in cavovarus feet) causes attritional wear as the tendons ride over it and can be resected.
• Subfibular impingement in a valgus hindfoot compresses the tendons between the calcaneus and the fibula - here the cause is the opposite alignment, and correcting the flatfoot is the treatment.

Denervation and neuromuscular disease

• Peroneus brevis weakens early in Charcot-Marie-Tooth disease, and its loss against a strong tibialis posterior is one of the two engines of cavovarus.
• Common peroneal nerve palsy at the fibular neck causes complete loss of eversion together with drop foot.
• Isolated superficial peroneal nerve lesion causes loss of eversion with preserved dorsiflexion and sensory loss on the dorsum of the foot sparing the first web space.

Peroneus brevis harvest for lateral ankle ligament reconstruction - and why it fell out of favour

Fibular groove deepening

• Indication: a flat or convex retromalleolar groove in a patient with peroneal tendon subluxation or dislocation, or recurrent instability after a previous retinacular repair.
• Techniques:
  • Indirect (impaction) deepening: a drill or burr is introduced through a small cortical window at the fibular tip and the cancellous bone hollowed out; the posterior cortex is then impacted into the void with a tamp, deepening the groove while preserving the smooth cortical and fibrocartilaginous gliding surface. This is the preferred technique because it retains the natural bearing surface.
  • Direct (osteoperiosteal flap) deepening: an osteoperiosteal flap is raised from the posterior fibula, cancellous bone removed beneath it, and the flap replaced.
  • Simple burring of the groove is discouraged because it removes the fibrocartilage and leaves a raw cancellous surface against the tendon.
• Always combined with repair or reconstruction of the superior peroneal retinaculum, and with correction of hindfoot varus where present.

Fifth metatarsal base fixation

• Approach: a short longitudinal incision just proximal and dorsal to the tuberosity, or a percutaneous entry, in both cases with the sural nerve identified or protected.
• Entry point: at the very tip of the tuberosity, aiming "high and inside" - dorsal and medial - so the screw follows the medullary canal rather than perforating the lateral cortex.
• Screw: the largest diameter the canal will accept (commonly 4.5 to 5.5 mm), and long enough to engage good diaphyseal bone but not so long that it straightens the natural lateral bow of the metatarsal, which levers the distal fragment and causes cortical impingement or a distal fracture.
• Adjuncts: bone graft or bone marrow aspirate for a sclerotic nonunion; plantar plating is an alternative in revision.
• Do not forget the alignment: a Coleman block test and an assessment of hindfoot varus is mandatory in a recurrent or stress fracture, and a lateralising calcaneal osteotomy may be needed.

The muscle as a donor

• Tendon donor: historically a major graft source (Evans, Watson-Jones, Chrisman-Snook), now largely superseded. It remains a reasonable revision graft as a split tendon where no alternative exists.
• Recipient of a transfer: peroneus brevis is a common recipient - the peroneus longus is tenodesed to it in cavovarus and in irreparable longus tears, and a flexor digitorum longus or flexor hallucis longus transfer is anchored to the fifth metatarsal base when the brevis itself is unusable.
• Muscle flap: never used. It is small, its supply is by unnamed perforators, and its loss would remove the strongest evertor.

Anatomical variation across populations

• Peroneus quartus is present in roughly 10 to 20 per cent of feet in cadaveric series, with reported prevalence varying widely by population and by whether the definition includes small slips. Its most common form arises from the peroneus brevis and inserts on the retrotrochlear eminence of the calcaneus.
• Retromalleolar groove morphology: concave in the majority (commonly quoted around 80 per cent), flat in roughly 10 to 20 per cent, and convex in a small minority. Groove morphology is the single most important anatomical predictor of peroneal tendon instability.
• Low-lying peroneus brevis muscle belly extending distal to the tip of the fibula is common and is a recognised contributor to a crowded groove.
• Os vesalianum is an uncommon accessory ossicle within the peroneus brevis tendon proximal to the fifth metatarsal tuberosity.
• Peroneus digiti minimi slip from the brevis to the fifth toe extensor apparatus is present in the majority of feet.

Side-by-side guidance

Resource-dependent practice

• Well-resourced settings: MRI to characterise the tendons and groove, dynamic ultrasound for subluxation, weightbearing CT for alignment and groove morphology, suture tape augmentation of anatomic repairs, early screw fixation for elite athletes.
• Limited-resource settings: the diagnosis of a peroneal tendon problem is clinical - tenderness and swelling behind the fibula, pain and weakness on resisted eversion, a provocable snap. The peek-a-boo heel and the Coleman block test cost nothing and answer the alignment question. Zone 1 fractures require nothing more than a hard-soled shoe. Zone 2 fractures can be managed successfully with six to eight weeks of non-weightbearing immobilisation in a non-athlete. A Brostrom-Gould repair requires only suture. Groove deepening by impaction needs only an osteotome and a tamp.
• The single most transferable message across settings is that hindfoot varus must be sought and corrected, because it is free to diagnose and it determines whether everything else will hold.

Registry and outcome signals

• There is no registry for peroneal tendon or lateral ligament surgery. Comparative cohort data consistently favour anatomic repair over tenodesis reconstruction for function and for preservation of hindfoot motion.
• Athletic cohorts consistently report that fifth metatarsal fracture refracture correlates with undersized screws, premature return to play and uncorrected hindfoot varus, rather than with the fixation principle itself.
• Series of peroneal tendon dislocation consistently report high recurrence with non-operative management in athletes and reliable results with retinacular repair combined with groove deepening where the groove is unfavourable.