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# Fibularis longus tendon

The fibularis longus tendon runs behind the outer ankle bone and under the sole to the [big toe](https://durgical.com/en/anatomy/great-toe/) 's metatarsal; it supports the arch.

Official name: Tendon of fibularis longus

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## What it is

The tendon of the [fibularis longus](https://durgical.com/en/anatomy/fibularis-longus-muscle/). It runs behind the outer ankle bone, around the cuboid and across the sole to the [first metatarsal](https://durgical.com/en/anatomy/first/).

## What it does

Turns the foot outward, steadies the ankle and helps support the arches.

## Key points

- Turns sharply around the cuboid, where a sesamoid (os peroneum) may lie; a painful, fractured or split os peroneum is a recognised cause of lateral foot pain
- A torn superior fibular retinaculum lets both fibular tendons slip over the lateral malleolus, usually after forced dorsiflexion; a small flake off the malleolus on X-ray (fleck sign) suggests it
- A high-arched, inward-tilted (cavovarus) foot and lateral ankle instability overload the fibular tendons

## Key facts

**Attaches**
Behind the lateral malleolus (superficial to the brevis), below the fibular trochlea, through the groove of the cuboid and across the sole to the [medial cuneiform](https://durgical.com/en/anatomy/medial-cuneiform-bone/) and base of the first metatarsal; an os peroneum may lie in it

**Function**
Eversion and plantarflexion of the first ray; supports the transverse and lateral longitudinal arches

**Tight in**
Inversion with dorsiflexion

## Relations

**Course**
Down the lateral leg, behind the lateral malleolus, along the side of the [calcaneus](https://durgical.com/en/anatomy/calcaneus/), round the groove of the [cuboid](https://durgical.com/en/anatomy/cuboid-bone/) and obliquely across the deep sole to the inner midfoot

**Deep**
[Fibularis brevis tendon](https://durgical.com/en/anatomy/tendon-of-fibularis-brevis/) behind the malleolus; the [calcaneofibular ligament](https://durgical.com/en/anatomy/calcaneofibular-ligament/) (CFL) below it

**Superficial**
[Fibular retinacula](https://durgical.com/en/anatomy/superior-fibular-retinaculum/); the [sural nerve](https://durgical.com/en/anatomy/sural-nerve/) and [small saphenous vein](https://durgical.com/en/anatomy/small-saphenous-vein/) run just behind and below

**Below**
In the sole, a tunnel under the cuboid roofed by the [long plantar ligament](https://durgical.com/en/anatomy/long-plantar-ligament/)

## How to find it

With the foot turned out against resistance, both tendons are felt behind and below the lateral malleolus; the longus is followed under the outer foot toward the cuboid, while the brevis heads to the base of the [fifth metatarsal](https://durgical.com/en/anatomy/fifth/).

## Examination

- Resisted eversion: pain or weakness along the tendon suggests tendinopathy or a tear
- Resisted eversion and dorsiflexion while the ankle circles: a tendon felt or seen slipping over the malleolus indicates subluxation

## On imaging

On radiographs, an os peroneum that has moved upward or split suggests a longus rupture; dynamic ultrasound shows subluxation; on MRI the curve round the malleolus can show falsely high signal (magic angle effect).

## Common problems

- Tendinitis, tears, or slipping out of its groove behind the outer ankle bone, often after ankle sprains.

## Clinical notes

- Recurrent dislocation is treated by repairing the retinaculum, sometimes with deepening of the fibular groove
- When one fibular tendon is badly damaged it can be sutured to the other (tenodesis)
- Some centres harvest all or the front half of this tendon as a graft for knee ligament reconstruction; its effect on the arch and the first ray is still debated

## Anatomical variants

- Extra slips to the bases of other metatarsals are common; the medial cuneiform attachment is usual but not constant
- An os peroneum is found in a variable share of feet and may be cartilage rather than bone

## Memory aid

Behind the lateral malleolus, 'brevis by the bone': the brevis lies against the [fibula](https://durgical.com/en/anatomy/fibula/), the longus outside it.

## Test yourself

Think of the answer, then tap the question to see it.

## Where does the fibularis longus tendon insert?

On the medial cuneiform and the base of the first metatarsal, after crossing the sole from the groove of the cuboid.

## Which sesamoid may lie in the fibularis longus tendon, and why does it matter?

The os peroneum, at the cuboid; a painful, fractured or split os peroneum causes lateral foot pain.

## What lets the fibular tendons dislocate over the lateral malleolus, and what is the X-ray sign?

A torn superior fibular retinaculum, usually after forced dorsiflexion; a small flake off the malleolus (fleck sign).

## Which fibular tendon lies against the bone behind the lateral malleolus?

The fibularis brevis ('brevis by the bone'); the fibularis longus lies superficial to it.

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
musculus fibularis longus TA2 2652

**Official Spanish (DeCS)**
Tendón del peroneo largo DeCS D013710

**English (TA2)**
Tendon of fibularis longus

**Also called**
musculus peroneus longus · peroneus longus muscle

## Related

- [CFL](https://durgical.com/en/anatomy/calcaneofibular-ligament/)
- [Long plantar ligament](https://durgical.com/en/anatomy/long-plantar-ligament/)
- [Fibular retinacula](https://durgical.com/en/anatomy/superior-fibular-retinaculum/)
- [Ankle tendon sheaths](https://durgical.com/en/anatomy/anterior-tarsal-tendon-sheaths/)
- [Fibularis longus](https://durgical.com/en/anatomy/fibularis-longus-muscle/)
- [Cuboid](https://durgical.com/en/anatomy/cuboid-bone/)
- [Medial cuneiform](https://durgical.com/en/anatomy/medial-cuneiform-bone/)
- [Fibula](https://durgical.com/en/anatomy/fibula/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=14088); [Anatomy, Bony Pelvis and Lower Limb: Calf Peroneus Longus Muscle](https://www.ncbi.nlm.nih.gov/books/NBK546650/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/tendon-of-fibularis-longus/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/tendon-of-fibularis-longus/