An Ultrasonography Study of Anatomical Variability and Dimensional Changes of the Tibial and Plantar Nerves with Change in Foot and Ankle Position

This anatomical study used high resolution ultrasonography (US) to compare the plantar nerves to the tibial nerve after mapping branching patterns in relation to the malleolar-calcaneal axis in the tarsal tunnel (TT).  Additionally, dimensional changes in the vertical diameter (VD), longitudinal diameter (LD), and cross-sectional area (CSA) of the tibial and plantar nerves were examined in response to static changes of maximal dorsiflexion (DF), plantarflexion (PF), and neutral weight bearing positions (WB) from the neutral non-weight bearing (NWB) position.  Finally, modifying effects due to age, sex, footedness, body mass index, foot pronation and nerve dimensions were investigated.

            An US examination was conducted bilaterally to examine the nerves in the TT for both feet of men (n=40) and women (n=38) in NWB, DF, PF, and WB.  Analyses combined the plantar nerve dimensions into composite values and aggregated these data with corresponding tibial nerve values; tibial, lateral plantar, and medial plantar nerve sets were analyzed separately. 

            Analysis demonstrated that US could be used to study plantar nerve branching variability within the TT, with meaningful comparisons between the plantar nerves and the TN.  Effects due to sex were observed when transitioning from non-weight bearing PF to a WB state for the TN and LPN.  Pronation had no effect on the WB condition for this young normative population, but LD lengthened from NWB to WB.  Compressive forces changed both the VD and CSA nerve dimensions from the PF condition in all nerves; producing a rounding of the nerves in PF and a flattening in the other conditions.