Dimensional Changes of the Tibial Nerve and Tarsal Tunnel in Different Ankle Joint Positions in Asymptomatic Subjects.

The tarsal tunnel is a clinically important fibrous osseous conduit for the tibial nerve and associated tendons. It is mechanically dynamic, and normal ankle movements appear to change the tunnel shape, potentially having an impact on the tibial nerve. The objective of this study was to measure changes in the tibial nerve and tarsal tunnel dimensions in plantarflexion and dorsiflexion of the ankle joint in healthy subjects. A cross-sectional study with 13 volunteer subjects and a total of 18 records was designed. The cross-sectional area, anterior-posterior distance, transverse distance, and flattening ratio of the tibial nerve were measured by using ultrasound in plantarflexion and dorsiflexion of the ankle joint. The anterior-posterior distance of the tarsal tunnel was also measured. The tunnel anterior-posterior distance significantly increased during plantarflexion (p < .001) and decreased during dorsiflexion (p = .027) of the ankle. From plantarflexion to dorsiflexion of the ankle, the tibial nerve cross-sectional area significantly decreased (p = .035). The anterior-posterior distance also decreased significantly (p < .001), whereas the transverse distance increased (p < .001), thus decreasing the flattening ratio of the tibial nerve (p < .001). Ankle joint position determined significant changes in the shape and dimensions of the tibial nerve at the tarsal tunnel.