Anatomical Structure and Arrangement of the Muscles in the Posterior Compartment (Part 2)

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At the musculotendinous junction of these three deep muscles, each tendon courses through the medial aspect of the foot, ultimately reaching the plantar surface. Their distinctive pathways are particularly relevant for the tibialis posterior, enabling it to exert a strong supination (inversion) effect.

A key anatomical feature is that the tibialis posterior, flexor digitorum longus, and the tibial neurovascular bundle pass through the tarsal tunnel, located deep beneath the flexor retinaculum. This structure is analogous to the carpal tunnel in the wrist and is clinically significant due to tarsal tunnel syndrome—a condition caused by compression of the tibial nerve beneath the flexor retinaculum, leading to sensory disturbances in the plantar aspect of the foot.

Tendinous Pathways of the Deep Posterior Muscles

Flexor Hallucis Longus (FHL)

The tendon of the flexor hallucis longus (FHL) runs through a specialized groove formed between:
  • The posterior talar tubercle
  • The inferior border of the sustentaculum tali
This groove is covered by fibrous bands and lined with synovial membrane, forming a tendinous canal that stabilizes the tendon’s position.

Notably, the FHL tendon lies deeper (more lateral) compared to the tibialis posterior and flexor digitorum longus tendons.

Flexor Digitorum Longus (FDL)

The FDL tendon passes posterior to the medial malleolus, crosses the ankle joint, and continues distally. Near the bases of the metatarsal bones, it splits into four individual tendons, which attach to the plantar surfaces of the distal phalanges of toes 2–5, excluding the hallux.

The FHL tendon, after entering the plantar aspect of the foot, runs between the sesamoid bones of the first metatarsophalangeal joint, finally inserting onto the plantar surface of the distal phalanx of the hallux.

Tibialis Posterior (TP)

The tendon of the tibialis posterior is positioned anterior to the FDL tendon as they both pass through the groove behind the medial malleolus.

It continues distally, coursing between the deep layer of the flexor retinaculum and the superficial layer of the deltoid ligament.

At its distal attachment, the tendon divides into superficial and deep branches, inserting onto almost all tarsal bones (excluding the talus) and the bases of the proximal metatarsals.

The most prominent distal insertion of the tibialis posterior is the navicular tuberosity, which can be palpated a few centimeters proximally along the medial foot arch during adduction and inversion movements.

Biomechanical Significance of the Tibialis Posterior

  • The primary tendon provides mechanical support to the spring ligament, which is essential for stabilizing the medial longitudinal arch.
  • Tibialis posterior dysfunction (such as tendon rupture or overstretching) often leads to:
    • Collapse of the medial longitudinal arch
    • Lowering of the talus height, resulting in progressive flatfoot deformity

Pulley Mechanisms and Functional Implications

Both the tibialis posterior and flexor digitorum longus utilize the medial malleolus as a fixed pulley, redirecting their force posteriorly to the ankle’s axis of rotation. These tendons are securely anchored by the flexor retinaculum as they pass behind the medial malleolus.

The FHL, in contrast, relies on a unique pulley system formed by:

  • The medial and lateral talar tubercles
  • The sustentaculum tali of the calcaneus

This specialized subtalar pulley system ensures the efficient transmission of plantarflexion forces, optimizing propulsion during the push-off phase of gait.