The Effects of Foot Position and Load on Tibial Nerve Tension

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Abstract
Patients with tarsal tunnel syndrome of unknown etiology do poorly after surgical decompression. Although surgical decompression addresses the soft tissue constraints, it ignores the role of osseous support. Some authors have suggested that a pes planus deformity (i.e., valgus hindfoot and abducted forefoot) is an unrecognized cause of tarsal tunnel syndrome due to increased tibial nerve tension. An in vitro study was performed on nine cadaveric feet to determine the effects of foot position and load on tibial nerve tension. Tensile forces placed through the tibial nerve were measured when the foot was placed in dorsiflexion, eversion, combined dorsiflexion-eversion, and then under cyclical load and increasing internal rotation at 5° increments from 0° to 20°. The nerve tension was reassessed after the creation of a pes planus deformity under the previous conditions. Tibial nerve tension in the stable and unstable foot was significantly increased by eversion, dorsiflexion, and combined dorsiflexion-eversion. Tibial nerve tension was significantly greater in an unstable foot when compared with a stable foot during eversion, dorsiflexion, and combined dorsiflexion-eversion. In the stable foot, tibial nerve tension was significantly increased during axial loading with increasing internal rotation when compared with 0° rotation. The increased tibial nerve tension in the stable foot was significant with increasing internal rotation when 0° was compared with 10°, 15°, and 20°. In the unstable foot, the tibial nerve tension was significantly increased with increasing internal rotation compared with the nerve tension at 0° of rotation. The increased tibial nerve tension in the unstable foot was significant with increasing internal rotation when 0° was compared with 5°, 10°, 15°, and 20°. When stability of the foot and internal rotation were compared independently, each factor increased tibial nerve tension. However, these factors acting together did not significantly compound the increase in nerve tension. This study demonstrates that tibial nerve tension is increased in an unstable foot compared with a stable foot during eversion, dorsiflexion, combined dorsiflexion-eversion, and cyclical load with increasing internal rotation.

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