Stephen Bird

Hi Lisa.

When I scan the tarsal tunnel, I begin in the distal calf behind the medial malleolus, I identify the tibia’s posterior tendon and then look posterior to it to find the tibial nerve.
I follow the tibial nerve distally and the first branch you see is a single fascicle which arises and heads posteriorly in the subcutaneous space towards the apex of the calcaneum. This is the medial calcanea nerve. I look at this when the patient has medial heel pain. it is damaged through pressure from shoes that are poorly fitted or from direct trauma as it is in the subcutaneous space and fairly unprotected.

Then I keep moving distally watching the tibial nerve in short axis and you see it divide into the medial and lateral plantar nerves. The medial one is closer to the medial malleolus. I follow these distally in the short axis. They are now under the flexor retinaculum and hence inside the tarsal tunnel.
I think of the tarsal tunnel as having two components. The first is the area underneath the flexor retinaculum. In this area any space occupying lesion can cause nerve compression.
Potential space occupying lesions are:
Subtalar joint arthritis and synovitis
Subtalar joint ganglion
Ankle joint arthritis or synovitis
Deltoid ligament complex pathology
Spring ligament pathology
Nerve sheath tumour
Tib post, FDL or FHL pathology like tenosynovitis
Any other miscellaneous space occupying lesion.

If you look at the bones as you pass through the tarsal tunnel you have the tibia most medial, then the talus adjacent to the tibia and then the calcaneum including the sustentaculum tali of the calcaneum adjacent to the talus. Hence you have an ankle joint between tibia and talus and a subtler joint between the talus and the calcaneum. So both of these joints can cause trouble.

There is a branch that comes off of the lateral plantar nerve. It is Baxters nerve, which is now called the first branch of the lateral plantar nerve. It is a motor only nerve and it passes under the plantar fascia and motor drives the abductor digiti minimi muscle. I don’t look directly for this nerve, but what I do is look at the muscle of abductor digiti minimi assessing for isolated atrophy change where the other foot intrinsic muscles look normal but the abductor digiti minimi has atrophy, indicating neural dysfunction. The nerve is usually trapped under the plantar fascia, rather than in the tarsal tunnel, hence I look altho he muscle rather than the nerve.

Next I follow the medial plantar nerve into the longitudinal medial arch of the foot. It follows the FDL and FHL tendons into the arch to become the knot of Henry. To pass into the arch it has to pass through what I refer to as the second part of the tarsal tunnel. This is where is passes between the pronator quadratus muscle and the abductor hallucius muscle. In some patients these muscles are hypertrophic for biomechanical reasons and can cause compression of the nerve as it passes between the muscle bellies.
At the know of Henry, any tenosynovitis of the FHL or FDL can cause compression of the adjacent medial plantar nerve. Remember a bit of fluid in the FHL sheath is normal as it communicates with the ankle joint.

The lateral plantar nerve can now be traced more distally and it goes into the second part of the tarsal tunnel as described and may suffer the same compression. Then it passes between the flexor digitorum breves muscle and the quadratus plantae muscles where is is fairly protected and hence I don’t see much pathology here. One exception is a plantar vein thrombosis that can irritate the lateral plantar nerve.

And that is all there is to it !


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