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About half of cases of tarsal tunnel syndrome are idiopathic. Certain anatomic variations such as pes planus and hindfoot valgus deformity increase the risk of the syndrome, as these configurations increase the tensile load upon the posterior tibial nerve. Trauma with subsequent scarring and fibrosis within the tarsal canal has been implicated, and the syndrome is found with greater frequency in patients with rheumatoid arthritis and diabetes. MRI is particularly helpful in detecting masses that may cause tarsal tunnel syndrome. Mass-like etiologies include ganglion cysts, neurogenic tumors (Fig 4), varicosities (Fig 5), lipomas, severe tenosynovitis, and accessory muscles.