How do ankle and first metatarsophalangeal joint positions affect flexor hallucis longus tendon excursion at the retro-talar pulley?

BACKGROUND: The Flexor Hallucis Longus (FHL) tendon plays a vital role in foot biomechanics, particularly in first metatarsophalangeal joint (MTP1) flexion and gait stability. Excessive strain on the tendon, common in athletes, can lead to inflammation and functional hallux rigidus. However, the contributions of ankle and MTP1 joint movements to FHL tendon dynamics remain unclear. This study aims to analyze the relationship between ankle and MTP1 joint positions and their effect on the FHL tendon excursion at the retro-talar pulley. We hypothesize that ankle and MTP1 movements significantly influence tendon displacement, with plantarflexion producing the greatest variation. MATERIALS AND METHODS: Nine cadaveric specimens (18 feet) were used. Two suture markers were placed on the FHL tendon: one proximal and one distal to the pulley, both positioned with the ankle and MTP1 joint in a neutral reference position (defined as the reference starting point, with the ankle at 90° and the metatarsophalangeal joint at 0°). The ankle (neutral, dorsiflexion, plantarflexion) and MTP1 joint (neutral, dorsiflexion, plantarflexion) positions were combined, resulting in nine total positions. For each configuration, the displacement of both markers relative to the pulley edges was measured, and the mean displacement was calculated. Tendon excursion was defined as the difference between this mean displacement and the displacement measured in the neutral ankle-neutral MTP1 reference position. An increase in tendon distance was defined as a proximal sliding excursion of the tendon within the retro-talar pulley, whereas a decrease reflected distal tendon sliding. RESULTS: The study involved 7 males (77.7%), with an average age of 82.7 ± 3.4 years. The greatest tendon distance variation is proximal sliding during ankle plantarflexion. With the ankle maintained in plantarflexion, tendon excursion varied according to MTP1 position, reaching 1.3 ± 0.7 cm with MTP1 plantarflexion, 1.2 ± 0.4 cm with MTP1 in a neutral position, and decreasing to 0.44 ± 0.5 cm with MTP1 dorsiflexion. CONCLUSION: FHL distance variation at the retro-talar pulley was influenced by the positions of the ankle and MTP1 joint positions, with plantarflexion producing the greatest proximal sliding, whereas dorsiflexion resulted in a distal sliding. Although based on a static, load-free cadaveric model with probably partial activation of the windlass mechanism influencing these results, this relationship may help to better understand the mechanical interaction between ankle and first metatarsophalangeal joint positions and their influence on FHL tendon excursion. LEVEL OF EVIDENCE: IV; retrospective case series.