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. Therefore, 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.
Anterior tarsal tunnel syndrome is another common
peripheral nerve entrapment of the foot. Anterior tarsal tunnel
syndrome is found on the top of the foot (anterior or dorsal foot).
Anterior tarsal tunnel 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.
Anterior tarsal tunnel syndrome is an entrapment of the deep peroneal
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 anterior tarsal tunnel syndrome 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 anterior tarsal tunnel syndrome 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 to correct anterior tarsal tunnel
syndrome. 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 Metzembaum 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.
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 tird 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
