The talocalcaneonavicular joint (TCN-j) is supported by the spring ligament, which has recently been revealed to be part of the joint capsule complex, along with the tendinous sheath of the tibialis posterior and flexor digitorum longus (FDL). Nonetheless, the FDL’s role in TCN-j stability has received limited attention. This study aimed to elucidate the positional relationships between the FDL and sustentaculum tali, which comprises the TCN-j. We hypothesized that the FDL runs medial to the sustentaculum tali, and its course significantly changes from the sitting to the standing position. Six ankles from six body donors were investigated, and seven ankles from seven volunteers were assessed using ultrasonography. The FDL was three-dimensionally located inferomedial to the sustentaculum tali. The FDL tendinous sheath was attached to the sustentaculum tali or connected by the tibialis posterior via the tendinous sheath. Based on the in vivo ultrasound image, the FDL location relative to the sustentaculum tali was maintained; however, the curvature of the FDL course was significantly more prominent in standing than in sitting. The FDL force against the bending moment may prevent the excessive eversion of the foot and aid the conventional spring ligament’s contribution to TCN-j stability for maintaining the longitudinal arch.

距跟舟关节 (TCN-j) 由弹簧韧带支撑，弹簧韧带最近被发现是关节囊复合体的一部分，还有胫骨后肌和趾长屈肌 (FDL) 的腱鞘。尽管如此，FDL 在 TCN-j 稳定性中的作用受到的关注有限。本研究旨在阐明 FDL 和包含 TCN-j 的 sustentaculum tali 之间的位置关系。我们假设 FDL 在大载突内侧运行，并且其路线从坐姿到站立姿势有显着变化。对来自六名身体捐献者的六个脚踝进行了调查，并使用超声检查对来自七名志愿者的七个脚踝进行了评估。FDL 在三维上位于大载托下内侧。FDL 腱鞘附着在大腿托上，或通过腱鞘与胫骨后肌相连。根据活体超声图像，FDL 相对于大载距的位置得以保持；然而，FDL 课程的曲率在站立时明显比坐着时更突出。抵抗弯曲力矩的 FDL 力可以防止足部过度外翻，并有助于传统弹簧韧带对 TCN-j 稳定性的贡献，以维持纵向足弓。