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Understand
The anatomy of the retro-talar tunnel and the flexor hallucis longus
FHL arises from the blockage of the flexor hallucis longus tendon in an osteofibrous tunnel located beneath the ankle. Understanding this anatomy means understanding where and how the conflict occurs, and why five anatomical variations make each patient a particular case.
Understand
Where the conflict occurs
Functional Hallux Limitus is caused by a blockage in the gliding of the flexor hallucis longus (FHL) tendon, also called the long flexor of the big toe. This blockage occurs in a precise and anatomically well-defined location, the retro-talar tunnel. It is here that the tendon becomes wedged during walking and can no longer glide freely. The conflict is seen at the end of the stance phase, just before propulsion, when the ankle moves into dorsiflexion.
The flexor hallucis longus muscle
The FHL muscle originates on the deep posterior surface of the leg and, at its other end, inserts distally on the distal phalanx of the big toe. Its function is plantarflexion of the big toe and, secondarily, plantarflexion of the ankle. The junction between the muscle and its tendon, called the musculotendinous junction, lies just upstream of the retro-talar tunnel.
Flexor Hallucis Longus
Anatomical path from muscle to big toe
Origin deep posterior surface of the leg. Path descends, crosses the retro-talar tunnel, runs beneath the foot. Insertion distal phalanx of the big toe. Function plantarflexion of the big toe and the ankle. Junction located just upstream of the retro-talar tunnel.
The retro-talar tunnel, site of the conflict
The retro-talar tunnel is an osteofibrous channel located on the posteromedial aspect of the ankle, behind the astragalus (the talus). It is made up of two structures, one bony and one fibrous, which together form an obligatory passage for the tendon. It is in this passage that the tendon changes direction abruptly, almost at a right angle, to leave the leg and join the big toe parallel to the sole of the foot.
The trochlear bony groove — At the front of the tunnel, the tendon rests on a bony groove carved into the talus. This groove acts as a point of reflection for the tendon, which at this spot changes direction by almost 90 degrees. This abrupt reflection is what makes its gliding vulnerable to any disturbance.
The fibrous pulley, or retinaculum — At the back, the tunnel is closed off by a fibrous pulley called the retinaculum. Its structure is analogous to the reflection pulleys that hold the deep flexors in the hand. But its very rigidity can, in the case of an anatomical variation, turn the passage into a trap.
Retro-talar tunnel
Inferior view, identifiable structures
1. Retro-talar pulley. 2. Groove of the talus. 3. Subtalar joint. 4. Groove of the calcaneus.
Key point
The retro-talar tunnel is the foot’s equivalent of the carpal tunnel in the hand, but its bony rigidity is more pronounced. It is this rigidity that makes the conflict possible, and that explains why release maneuvers must be performed in this precise area.
Five major anatomical variations
The anatomy of the tunnel is not the same in everyone. Dissections and more than 1,500 endoscopic tenolyses performed at the Centre du pied have made it possible to identify five broad categories of variation, which may explain why some patients develop FHL while others seem protected.
Muscular variation
Position and volume of the muscle
- A low-lying musculotendinous junction predisposes to the conflict
- Hypertrophied muscle (dancers, skaters on demi-pointe)
- Excessive muscle volume reduces the space within the tunnel
Bony variation
Posterior talar tubercle or os trigonum
- A bulky posterior talar tubercle
- Os trigonum (a small accessory bone behind the talus)
- Visible on X-ray or CT scan
Fibrous variation
Texture and extensibility of the pulley
- A fibrous pulley that is more or less extensible depending on age
- Variable collagen content
- Poor tolerance of forced dorsiflexion
Synovial variation
Inflammatory state and accordion effect
- Chronic inflammatory state or synovitis
- Thickening of the synovial sheath
- An accordion effect that hinders gliding
Tendinous variation
Fraying or tendon nodule
- Fraying of the tendon fibers
- A tendon nodule indicating micro-tears
- The signature of a long-standing FHL
Clinical consequence
These variations add up and explain why FHL has no single cause but rather a combination of predisposing factors. Identifying the variation or variations at play in a given patient guides the treatment choice — conservative in moderate cases, surgical by endoscopic tenolysis in severe or resistant cases.
Key scientific article
The dissection work carried out at the Centre du pied was the subject of a publication in the Journal of Anatomy, which constitutes the scientific reference on the identification of the retro-talar pulley.
Scientific publication
Identification of the retrotalar pulley of the Flexor Hallucis Longus tendon
Journal of Anatomy, anatomical study conducted at the Centre Orthopédique d’Ouchy
« After more than 1,500 operations, the finding is always the same: the tendon jams at exactly the same spot in the tunnel. What the clinical examination suggested, the surgery confirmed directly — this is indeed where the condition plays out. »
Dr Jacques Vallotton, Centre Orthopédique d’Ouchy
Understand through video
Four video sequences make it possible to visualize concretely the anatomy of the flexor hallucis longus and the retro-talar tunnel, from dissection to the lectures of the Medicol Congress.
Anatomy of the flexor hallucis longus (FHL). What do we really know?
Anatomy of the Flexor Hallucis Longus
Anatomy Workshop
Anatomy
Next step
Understanding how the tunnel jams in motion
Anatomy lays the foundations. Biomechanics explains how and why, with each step, this tunnel becomes a tendon trap for some patients.
In brief
Tendon involved
FHL
Site of the conflict
retro-talar tunnel
Bony structure
groove of the talus
Fibrous structure
retinaculum
Variations
5 categories
At-risk population
dancers, skaters
Go further
Related pages
Biomechanics
How anatomy becomes mechanism, the role of the tenodesis effect and the breakdown of synchronism.
Definition of FHL
The concept of FHL and how it differs from osteoarthritic hallux rigidus.