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The Posterior Tibial Nerve is a larger nerve that courses around the inside aspect of the ankle. At the level of the ankle, it courses under the flexor retinaculum (also called the Laciniate ligament or medial annulare ligament). The flexor retinaculum can cause excessive pressure to be exerted along the nerve which in turn leads to pain. There are several reasons that this occurs.
Cavus foot is a fancy way of saying a high arched foot. The high arched foot is opposite a foot with excessive pronation. The problem with this foot structure is that it is extremely rigid. This leads to very stiff joints and ligaments. The flexor retinaculum is usually very tight, and any motion of the hindfoot will press the nerve against the retinaculum. This can be treated with a good orthotic. This works, because theoretically, this will control even small movement in the hindfoot. If this fails, then surgical release of the retinaculum is performed.
Pronation is a three dimensional motion that is initiated in the hindfoot, but affects the entire foot and ankle. A more specific explanation is given in the technical side note. Excessive pronation leads to extended and abnormal stress applied to the inside aspect of the ankle. This “pushes” the nerve and adjacent structures against the flexor retinaculum. Controlling excessive pronation, usually through an orthotic, can alleviate this abnormal pressure and in turn, alleviate the pain. If this persists, then the flexor retinaculum is surgically released (similar to carpal tunnel release).
Soft Tissue Mass (space occupying lesion)
This is considerably more rare than the above causes. This may be an actual tumor that has developed in the tarsal tunnel. In 99% of the cases, this is a benign growth. Imaging studies such as diagnostic ultrasound or MRI can help determine the presence of a space occupying lesion. Occasionally, a muscle is too large or long, and pushes into the tarsal tunnel space. In almost all of these cases (aside from a malignancy), surgical excision of the lesion (including space occupying muscle) cures the problem.
Control of blood glucose is extremely important in diabetes, and this is only one of a myriad of reasons. When blood glucose is persistently abnormal, sorbitol is produced as a byproduct. This is can concentrate in nerves and lead to swelling due to osmosis. Swelling of the nerve exerts abnormal pressure against the flexor retinaculum and leads to tarsal tunnel syndrome. Tight control of blood glucose helps to control this. If it persists and nerve tests show the nerve to be viable, then surgical release leads to high cure rates.
Direct and indirect injury to the ankle can lead to soft tissue damage which subsequently leads to scarring. Excessive scarring can lead to nerve entrapment. Although this is not entrapment against the flexor retinaculum, it can still mimic tarsal tunnel syndrome.