Abstract
BACKGROUND: Isolated rupture of the flexor hallucis longus (FHL) tendon is a scarce condition, with limited reported cases documented since 1980. Most reported cases have been managed surgically, using approaches such as direct repair, tendon transfers, or grafting, with variable outcomes. Meanwhile large chronic flexor hallucis longus (FHL) tendon defects exceeding 10 cm are rare and lack a standardized surgical treatment approach, particularly in physically active individuals. We present a distinctive case of a delayed, complete traumatic FHL tendon rupture in an active-demand individual, successfully treated using a combination of ultrasound-guided tendon localization and arthroscopy-assisted tendon allografting to address a tendon gap exceeding 10 cm. Given the rarity of this technique and the favorable postoperative recovery, we aim to contribute this case to the existing literature and review previously reported cases of isolated FHL tendon rupture. CASE PRESENTATION: A 48-year-old male recreational basketball player without past medical history presented two months after a traumatic complete FHL rupture, confirmed on MRI, with a tendon defect greater than 10 cm, pronounced push-off weakness, loss of hallux interphalangeal (IP) joint flexion, and significant psychological distress after a landing injury. Although acceptable functional outcomes have been reported following flexor hallucis longus (FHL) transfer in Achilles tendon reconstruction, these findings reflect a different clinical context. In this case of isolated FHL rupture, preservation of native hallux flexion was prioritized based on the patient’s activity demands. With limited guidance in the existing literature for managing such a large chronic defect, we proceeded with a minimally invasive approach combining ultrasound-guided identification of the tendon stump and arthroscopic debridement of the fibro-osseous tunnel of the FHL. The large gap was reconstructed using a flexor digitorum longus (FDL) tendon allograft. Postoperatively, the patient demonstrated marked improvement in push-off stability, recovery of hallux flexion strength, and renewed confidence during athletic activities. CONCLUSION: The current literature provides limited descriptions of the surgical management of flexor hallucis longus (FHL) ruptures with tendon defects greater than 10 cm, and these cases therefore remain challenging. In this case, reconstruction using an ultrasound-guided and arthroscopy-assisted approach with a flexor digitorum longus (FDL) graft was feasible and associated with a favorable clinical outcome. Although this technique has been infrequently reported, our findings suggest that it may be considered as one of several possible options when alternative methods are limited or unsuitable, particularly in active individuals for whom restoration of hallux function is important. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12891-026-09612-4.