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Tarsal tunnel syndrome (TTS) refers to an entrapment of the posterior tibial nerve as it descends from the leg to the foot. This condition was first described by Keck and Lam in 1962. The terminal aspect of the posterior tibial nerve (L4-S1 nerve distribution) supplies the motor function to the muscles of the foot and the sensory innervation to the bottom of the foot. Varying degrees of entrapment of this nerve may effect either motor function, sensory function or both.

There are any number of reasons that tarsal tunnel occurs. Contributing factors include trauma, varicose veins, bone spurs and soft tissue tumors such as ganglionic cysts. Other contributing factors include biomechanical instability of the foot and ankle. Each of these contributing factors places pressure on the posterior tibial nerve creating the symptoms of tarsal tunnel syndrome. Most cases are ideopathic, meaning that the entrapment appears to be due to direct pressure from the lacinate ligament with no other visible cause.

Treatment of tarsal tunnel syndrome

Conservative care for tarsal tunnel syndrome includes injectable cortisone, and most importantly, support of the arch. Many studies have shown that the pronated or flat foot is much more prone to tarsal tunnel syndrome. Rigid arch support has been shown to decrease strain on many of the structures (nerve and tendon) that pass from the leg to the foot through the tarsal canal.

Tarsal tunnel syndrome may be treated surgically with a release of the lacinate ligament and exploration of the tarsal canal with decompression of the posterior tibial nerve. Most peripheral nerves are slow to respond to surgical procedures. The recovery period for patients undergoing tarsal tunnel surgery may vary from 3 months to 18 months. The outcome of the procedure varies and seems to depend upon the nature of the entrapment, the damage that the posterior tibial nerve had sustained prior to surgery and a host of other factors.

Other Nerve Entrapments of the Foot

Tarsal tunnel syndrome and Morton's Neuroma are by far and away the two most common peripheral nerve entrapment found in the foot. There are other areas of the foot where peripheral nerves may be entrapped by endogenous bone or soft tissue structures, or by exogenous factors such as shoes. Each of these additional locations of peripheral nerve entrapment require specialized care with either shoe modifications, injections or surgical decompression of the nerve at the level of the entrapment.

Anterior tarsal tunnel syndrome is another peripheral nerve entrapment found in the foot. Anterior tarsal tunnel syndrome is common to skiers. It's simply an entrapment of the nerves on the top of the foot (intermediate dorsal cutaneous and deep peroneal nerves) from tight ski boots. Symptoms include numbness and achiness of the toes.

The sural nerve, on the outer portion of the foot may also become entrapped. Symptoms of a sural nerve entrapment include numbness of the outside of the foot and small toe.

The calcaneal branch of the posterior tibial nerve may also be entrapped as it descends from the tarsal canal to supply sensory innervation to the bottom of the heel. This condition is an important differential diagnosis in treating heel pain and should always be considered a possibility when treating plantar fasciitis.

Lacinate Ligament - A broad thin band running from the tibia to the calcaneus that keeps each of the tendon and neurovascular structures of the ankle in place as they descend from the leg to the foot. Without the lacinate ligament, these structures would have a tendency to bowstring or displace as the ankle moved.

Surgical decompression of a peripheral nerve - a surgical procedure used to release any adhesion, scar tissue or soft tissue structure that may inhibit the normal function of the peripheral nerve.

Tarsal tunnel syndrome occurs on the inside of the ankle, just behind the ankle bone. Many of the structures that govern the function of the foot pass through a tunnel in this area referred to as the porta pedis or tarsal canal. These structures include arteries, veins, nerves and multiple tendons. As these structures round the inside of the ankle, they are held in place by a broad ligament known as the lacinate ligament. The purpose of the lacinate ligament is to prevent these vital structures from bowstringing or popping out of position with motion such as walking or running.

The posterior tibial nerve is the primary nerve that passes through the porta pedis. The posterior tibial nerve is susceptible to problems as it passes under the lacinate ligament. If for any reason pressure is applied to the posterior tibial nerve, symptoms of tarsal tunnel will occur.

The flexible flatfoot has been discussed in many papers as a contributing biomechanical factor in cases of TTS. The flexible flatfoot is considered by many to be a poorly functioning foot due to its' inability to bear load effectively. As the arch of the foot decreases, pressure within the tarsal canal increases. Also, as the height of the medial arch decreases, the structures on the inside of the arch are required to traverse a longer distance subsequently placing tension on many of the structures of the medial foot, including the posterior tibial nerve.

Diagnosis of tarsal tunnel can be difficult due to the vague symptoms described by most patients. Many patients describe a dull achy sensation in the bottom of the foot. What confuses the diagnosis in many instances is the fact that the posterior tibial nerve bifurcates, or splits at the level of the lacinate ligament. The two branches of the posterior tibial nerve continue on to supply sensation to the medial (inside) and lateral (outside) aspects of the bottom the foot. Therefore, depending on the level of the bifurcation of the nerve, the symptoms can vary and include only a part or all of the bottom of the foot. The picture at left shows the course of the posterior tibial nerve as it descends into the foot (red). The black dotted circle describes an area of the medial ankle that is often painful for those with TTS. The blue shaded area details a focal entrapment of the calcaneal branch of the posterior tibial nerve.

A common test used to diagnose TTS is to tap on the inside of the ankle along the course of the posterior tibial nerve. A peripheral nerve that is irritated by entrapment will tingle or give an electric shock sensation at the level of the entrapment referred to as a Tinel's Sign.

Other diagnostic tools that can be used include a nerve conduction study. This study, often called an EMG (short for electromyography), evaluates the electrical conductivity of the posterior tibial nerve as it passes beneath the lacinate ligament. A decrease in conductivity indicates a possible entrapment of the nerve. The use of an EMG in testing for TTS is not conclusive. Many cases of TTS may have a normal EMG.

Arthritis

Gout

Peripheral nerve entrapment
At the level of L3 to S1
Entrapment beneath the piriformis muscle
Entrapment behind the knee
Entrapment at the head of the fibula

Posterior tibial tendonitis

This article was written by Jeffrey A. Oster, DPM, C.Ped and last updated 3/24/05.

Additional references include;

Keck, C., The tarsal tunnel syndrome. JBJS. 44-A:180-182, 1962

Lam, S., A tarsal tunnel syndrome. Lancet 2:1354-1355, 1962

G. Archar, D., Lewis, J., DiDomenico, L., Hypertrophic sustentaculum tali causing tarsal tunnel syndrome: a case report. J. Foot Surg 40:2 110-113, 2001