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Fibularis longus tendon

The fibularis longus tendon is the distal extension of the fibularis longus muscle, one of the key tendons in the lateral compartment of the leg. It plays a crucial role in ankle eversion, plantarflexion, and stabilization of the transverse arch of the foot. The tendon follows a long oblique course, curving around the lateral malleolus and under the foot to insert on the medial cuneiform and first metatarsal base.

Due to its long trajectory, narrow path, and passage beneath fibro-osseous tunnels, the fibularis longus tendon is prone to tendinopathy, tears, and subluxation, especially in athletes and patients with chronic ankle instability.

Synonyms

  • Peroneus longus tendon

  • Lateral peroneal tendon

Origin, Course, and Insertion

  • Origin: Continuation of the fibularis longus muscle, arising from the head and proximal two-thirds of the fibula and intermuscular septa

  • Course:

    • Tendon passes posterior to the lateral malleolus, within a common synovial sheath with fibularis brevis

    • Curves around the lateral calcaneal wall, passing through the fibular (peroneal) groove of the cuboid bone

    • Crosses obliquely beneath the foot in a fibro-osseous tunnel

  • Insertion: Plantar surfaces of the medial cuneiform and the base of the first metatarsal bone

Tendon Attachments

  • Retained within the fibular retinacula (superior and inferior) at the ankle

  • Stabilized in the cuboid groove, sometimes reinforced by an os peroneum (sesamoid bone) within the tendon

  • Inserts firmly at the first ray of the foot, reinforcing the medial arch

Relations

  • Superiorly: Fibularis brevis tendon (shares sheath at lateral malleolus)

  • Inferiorly: Cuboid groove and plantar structures of the foot

  • Laterally: Lateral malleolus and calcaneus

  • Medially: Crosses under midfoot bones toward medial cuneiform and first metatarsal

Function

  • Eversion: Major evertor of the foot, balancing inversion forces of tibialis posterior

  • Plantarflexion: Assists gastrocnemius–soleus complex in pushing off

  • Arch support: Maintains the transverse arch of the foot by pulling the first metatarsal medially and downward

  • Dynamic stabilizer: Prevents medial collapse of the foot during gait

Clinical Significance

  • Tendinopathy: Overuse injuries common in runners, dancers, and skiers

  • Tears: Longitudinal splits and degenerative ruptures occur in chronic ankle instability

  • Subluxation/dislocation: From retinacular tears or laxity around lateral malleolus

  • Accessory ossicles: Os peroneum pathology may cause tendon entrapment or fracture

  • Symptoms: Lateral ankle pain, snapping tendon, weakness in eversion and push-off

MRI Appearance

  • T1-weighted images:

    • Normal tendon: uniform low signal (dark band)

    • Muscle belly: intermediate signal intensity

    • Surrounding fat planes: bright, helping tendon delineation

    • Tears: focal discontinuity or hyperintense clefts

  • T2-weighted images:

    • Normal tendon: dark, low signal (darker than on T1)

    • Tendinopathy: intermediate-to-bright signal thickening within tendon

    • Tears: linear or irregular bright hyperintense signals extending through tendon fibers

    • Fluid in sheath: bright hyperintense, suggesting tenosynovitis

  • STIR:

    • Normal tendon: dark band

    • Tendinopathy/edema: hyperintense signal along tendon or sheath

    • Excellent for detecting inflammatory changes

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: dark, sharply outlined

    • Tears: bright linear signal indicating fiber disruption

    • Superior for detecting longitudinal splits and subtle partial tears

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal enhancement

    • Synovitis or peritendinitis: peripheral or sheath enhancement

    • Tears: enhancing granulation tissue at site of injury

CT Appearance

Non-Contrast CT:

  • Tendon seen as linear soft tissue density along lateral ankle and foot

  • Bony anatomy: fibular groove, cuboid tunnel, os peroneum easily visualized

  • Indirect signs of pathology: tendon thickening, displacement, or os peroneum fracture

Post-Contrast CT (standard):

  • Tendon itself does not enhance

  • Peritendinous enhancement may suggest synovitis, inflammation, or adjacent scar tissue

MRI image

Fibularis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Fibularis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Fibularis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI image

Fibularis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00003

CT image

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