Effect of foot and ankle position on tarsal tunnel compartment volume

Background. Tarsal tunnel pressure is increased when the foot and ankle are positioned in eversion or inversion from neutral, aggravating symptoms of tarsal tunnel syndrome in some patients. Space-occupying lesions may cause tarsal tunnel syndrome. We hypothesized that positional change of the foot and ankle from neutral to eversion or inversion causes decreased tarsal tunnel compartment volume that may aggravate symptoms of posterior tibial nerve entrapment. Methods: MRI of 13 ankles in nine healthy subjects in three positions (neutral, eversion, inversion) were obtained with respect to the malleolar-calcaneal plane; this plane was defined by the distal tip of the anterior colliculus of the medial malleolus, the medial tubercle of the posterior calcaneal tuberosity, and the lateral tubercle of the posterior calcaneal tuberosity. The borders of the tarsal tunnel noted on the MRI were traced with a computer digitizing apparatus to determine the cross-sectional area of the tarsal tunnel on each image, and the slice thickness and interspace distance for the seven central images were used to calculate tarsal tunnel volume. Results: The mean tarsal tunnel volume was significantly greater when the foot and ankle were in neutral position (21.5 +/- 0.9 cm(3)) than in either full eversion (18.0 +/- 0.9 cm(3); p<0.001) or inversion (20.3 +/- 1.0 cm(3); p<0.001). Conclusions: The results support the hypothesis that eversion and inversion of the foot and ankle cause decreased compartment volume of the tarsal tunnel and increased tarsal tunnel pressure that may contribute to symptoms of posterior tibial nerve entrapment in tarsal tunnel syndrome. Clinical relevance: Neutral immobilization of the foot and ankle may relieve symptoms of posterior tibial nerve entrapment in tarsal tunnel syndrome by minimizing pressure on the nerve and maximizing tarsal tunnel compartment volume available for the nerve.