The posterior tibial nerve travels down the back of the leg and around to the medial side of the ankle(big toe side), running behind the big bony prominence on the medial side of the ankle noted as the medial malleolus. Just behind the medial malleolus is the posterior tibial tendon, and then the flexor digitorum longus tendon and finally the posterior tibial artery, nerve and several veins. Covering over the tendons, artery and nerve is a sheet of tissue called the flexor retinaculum. Underneath the flexor retinaculum at the level of the ankle, this space is known as the tarsal tunnel. the posterior tibial nerve splits at the level of the tarsal tunnel into the medial plantar nerve and the lateral plantar nerve. The tendons, arteries and nerves and travel around the side of the ankle into the bottom or plantar aspect of the foot. The medial plantar nerve supplies sensation to the medial side of the sole of the foot as well as sensation to the first, second and third toe. The lateral plantar nerve supplies sensation to the lateral or outer part of the sole and as well as the fourth and fifth toe. A small branch called the medial calcaneal nerve comes off the posterior tibial nerve inside the tarsal tunnel, running to the medial side of the heel and supplying sensation to the medial or inner side of the heel.
Any condition that produces increased pressure or inflammation in the tarsal tunnel may cause irritation or compression neuropathy of the posterior tibial nerve. This is known as Tarsal Tunnel Syndrome. Symptoms include numbness on the sole or plantar aspect of the foot, as well as tingling, burning or pain. There may also be weakness or atrophy of the intrinsic muscles. In some cases a claw toe deformity of the toes may develop secondary to this weakness. The diagnosis is made by obtaining a history of the above symptoms, numbness or tingling on the plantar side of the foot, a positive Tinel's test and possible atrophy of the intrinsic muscles. Usually electrodiagnostic tests such as nerve conduction velocity (NCV) or electromyography(EMG) studies are ordered.
Several causes of tarsal tunnel syndrome exist. inflammation and swelling of the tendons may result from overuse or inflammatory conditions such as rheumatoid arthritis or lupus. Repetitive stretching or tension on the nerve may occur in patients with flat feet with increased pronation. Synovitis or inflammation of the underlying ankle joint or subtalar joint may produce pressure upward on the tarsal tunnel. A ganglion cyst may also grow into the tarsal tunnel, causing symptoms. Sometimes the collection of veins or venous plexus around the nerve swell, producing increased pressure on the nerve with standing and weight-bearing.
Treatment of tarsal tunnel syndrome maybe conservative with anti-inflammatories to reduce inflammation and arch supports and orthotics to reduce pronation in patients with flat feet. In severe cases or one conservative treatment fails surgery may be required. During surgery a curved incision is made on the medial side of the ankle over top of the tarsal tunnel. The flexor retinaculum over top of the tarsal tunnel is split, opening up the tarsal tunnel and relieving pressure. Any mass in the tarsal tunnel is removed. A swollen venous plexus is split and cauterized, reducing pressure on the nerve. The nerve is freed, down to the foot. The skin is closed and a dressing and boot applied. There is no weight-bearing for 6 weeks. During therapy, the boot is removed and early range of motion of the ankle and foot begun. This allows the nerve to mobilize and reduce scarring. Results are approximately 75% successful in major studies. risks with this condition and surgery are anesthesia related risks, wound healing problems infection, continued irritation of the nerve and a condition called reflex sympathetic dystrophy, resulting in stiffness and pain of the foot.