Tarsal Tunnel Syndrome

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's no universal agreement between clinicians and surgeons as to why tarsal tunnel syndrome occurs. Contributing factors that may cause tarsal tunnel syndrome include trauma to the tarsal canal, varicose veins, bone spurs and soft tissue tumors such as ganglionic cysts. Other contributing factors include biomechanical instability of the foot caused by pronation (flattening of the arch) and supination (high arch). Each of these contributing factors ultimately results in compression of the posterior tibial nerve. The site where the nerve is compressed is also called an entrapment.

Testing for tarsal tunnel syndrome with EMG (electromyography) is often imprecise and misleading. The most reliable method of assessing tarsal tunnel syndrome is percussion of the nerve. This test is called a Tinel's sign. Two fingers are used to briskly tap the medial ankle just behind the ankle bone (medial malleolus). An electrical shock sensation is called a positive Tinel's sign and indicates the location of the entrapment. The pictures to the left show the location of the posterior tibial nerve (in green) as it descends the leg behind the inside ankle bone (medial malleolus). Two common areas of entrapment are found as the posterior tibial nerve passes beneath the lacinate ligament (in pink) and/or the upper margin of the abductor hallucis muscle (dotted line). The posterior tibial nerve passes deep to the muscle at the dotted line. This is the most common location for entrapment of the posterior tibial nerve.

The posterior tibial nerve splints into three branches at the level of the medial ankle. A positive Tinel's sign may be positive for all three branches or just one isolated branch. This variation in the Tinel's sign depends upon the level of the entrapment and whether the entrapment is proximal or distal to the triforcation (split) of the nerve into its' three branches.

An alternative method of nerve conduction testing proposed by Lee Dellon, MD, of Baltimore MD, uses a device called a pressure specified sensory device or PSSD. This method incorporates two point discrimination and pressure testing to quantify sensory loss due to nerve entrapments. My personal experience with PSSD is that it is difficult to duplicate findings between providers, and may therefore lead to misrepresentation in testing data.

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. 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 and the damage that the posterior tibial nerve had sustained prior to surgery.

The pictures below show the steps involved in a tarsal tunnel release. Image 1 shows the planned incision line running behind the medial ankle to a level just distal to the upper margin of the abductor hallucis muscle. Dissection is carried down through the subcutaneous space where in Image 2 we see the lacinate ligament identified. Image 3 shows the lacinate ligament released and direct visualization of the posterior tibial nerve and associated vein and artery. Image 4 shows the posterior tibial nerve isolated. The nerve and surrounding soft tissue is inspected for any visual defect including tumors or evidence of trauma. Image 5 shows the dissection Baxter's nerve. And Image 6 shows the fibrous tissue that invests the abductor hallucis muscle. The fibrous tissue surrounding the abductor hallucis extends into the porta pedis. A tarsal tunnel release requires dissection of this fibrous tissue into the porta pedis to insure a complete release.

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

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 bone or soft tissue structures, or by external 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.

Entrapment of the deep peroneal nerve is another common peripheral nerve entrapment of the foot. The deep peroneal nerve is found on the top of the foot (anterior or dorsal foot). A deep peroneal nerve entrapment is often found in conjunction with a small bump of bone at the metatarsal cuneiform joint. This bump of bone is called a saddle bone deformity or met-cuneiform exostosis. A deep peroneal nerve entrapment is an entrapment of the nerve as it descends over the top of the saddle bone deformity. Compression of the deep peroneal nerve occurs when we wear shoes that apply pressure to the saddle bone deformity. The result is that the deep peroneal nerve is compressed between the shoe and bone. We'll often see a deep peroneal nerve entrapment in skiers, those who wear clogs or folks who lace their shoe very tightly. In each case, the direct pressure from the shoe compresses the nerve over the bump of bone. A Tinel's sign can be elicited at the top of the foot with a positive Tinel's sign resulting in electrical shock sensation to the first and second toes.

Treatment of a deep peroneal nerve entapment can be accomplished by avoiding the reasons for direct compression of the deep peroneal nerve. If the compression is due to shoe laces that are laced too tight, loosen the shoe or skip and eyelet of the shoe. An easy way to eliminate pressure to the deep peroneal nerve is to use a modified tongue pad. Split the tongue pad longitudinally (from heel to toe) and place the adhesive backed pad on the under side of the tongue of the shoe. Use the pad to create a gap so that direct pressure to the deep peroneal nerve is decreased. Tongue pads also work well in clogs and ski boots.

The following images show the steps used to perform a surgical decompression of the deep peroneal nerve. Image 1 shows the planned incision overlying the 1st metatarsal cuneiform joint. Image 2 shows dissection through the subcutaneous space. The extensor retinaculum is underscored by a pair of Metzenbaum scissors. Image 3 shows the complete release and the deep peroneal nerve. Image 4 shows final closure of the incision. This procedure is performed in a surgery center of hospital using a local anesthetic. The procedure takes approximately 45 minutes to complete. Patients are able to walk on the foot for short distanced immediately following surgery.

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 most common location of sural nerve entrapment is at the lateral heel, just below the lateral ankle bone (fibula or lateral malleolus) at the peroneal tubercle. Percussion to this site may elicit a positive Tinel's sign with distribution of a shock sensation to the little toe.

It is not uncommon for the sural nerve to be injured during surgery. The sural nerve is often in the center of an incision for flatfoot procedures or ankle fracture repairs. Exposure to the air and retraction of the nerve during surgery may cause temporary loss of sensation in the lateral (outside) foot to the 5th toe. Return of normal sensation may take up to a year following surgery.

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 deep to the lacinate ligament and abductor hallucis muscle. 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, or pronated foot, 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. 

Diagnosing tarsal tunnel syndrome can be difficult due to the vague symptoms described by patients. Many patients describe a dull, achy sensation in the medial ankle and the plantar foot. The pain is described as vise like and is not relieved by short periods of rest. Pain does not occur with the first step on the foot, but rather increases with the duration of time spent on the foot. Tarsal tunnel syndrome occurs most commonly in one foot but can be found in both feet at the same time.

The Tinel's sign, as previously described, is the most specific test used to determine the extent and location of the entrapment.

What often makes the diagnosis tarsal tunnel syndrome more challenging is the fact that the posterior tibial nerve trifurcates, or splits into three parts at the level of the lacinate ligament. The two main branches of the posterior tibial nerve continue on to supply sensation to the medial (inside) and lateral (outside) aspects of the bottom the foot, while the third branch travels to the bottom of the heel. Therefore, depending on the level of the triforcation 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. This isolated entrapment of the calcaneal branch is called Baxter's entrapment.

Arthritis

Entrapment, lumbar spine, at the level of L3 to S1

Entrapment, sciatic nerve, beneath the piriformis muscle

Entrapment, posterior tibial nerve, behind the knee

Entrapment, common peroneal nerve, at the head of the fibula

Gout

Navicular fracture

Nerve tumor

Posterior tibial tendonitis (PTTD)

• Flatfeet

• Posterior Tibial Tendon Dysfunction

• Sinus Tarsi Syndrome

• Arch Pain

• Ankle Pain

This article was written by Jeffrey A. Oster, DPM and last updated 2/8/13.

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