Posted by Dr. Ed on 11/13/07 13:51
Theresa:
The tarsal tunnel is a grouping of compartments within a ligament on the medial side of the ankle. The tendons, nerves, arteries and veins to the top of the foot course over the front of the ankle and top of the foot. It is not possible for that to be the case on the bottom of the foot as one would step on those structures. As such those structures to the bottom of the foot were designed to 'loop around' the inside of the ankle (medial) and enter the bottom of the foot via a 'portal' known as the porta pedis.
The tarsal tunnel contains compartments. The first compartment contains the tibialis posterior tendon (the tendons which pulls up on the arch); the second compartment contains the flexor digitorum longus tendon (the long flexor tendons to the lessor toes that enable us to curl our toes); the third compartment the posterior tibial nerve, posterior tibial artery and its accompanying veins; the fourth compartment compartment contains the flexor digitorum longus tendon (the tendon that flexes the big toe and allows us to push off).
The posterior tibial nerve is the nerve that is affected in tarsal tunnel syndrome. The posterior tibial nerve give off medial calcaneal branches which go toe the bottom of the heel to provide sensation then divides into its two terminal branches, the medial plantar nerve and the lateral plantar nerve. That branching can occur within the tarsal tunnel or below it. The medial and lateral plantar nerve then take a turn to go into the back portion of the arch in an area known as the porta pedis. The porta pedis is partially 'surrounded' by the abductor hallucis muscle (the muscle that pulls the big toe away from the second toe). If that muscle is enlarged it can decrease the size of the portal pedis making it more likely that the nerve branches become entrapped there. This problem has been termed 'distal tarsal tunnel syndrome' although that usage is not universal.
The lateral plantar nerve, soon after entering the foot gives off a branch known as the inferior calcaneal nerve or Baxter's nerve which runs along the bottom of the heel bone, basically between the bone and the origin of the plantar fascia. It is postulated that that nerve can become entrapped in that location.
Here is a diagram of the nerve distribution to the skin on the bottom of the foot: http://en.wikipedia.org/wiki/Image:Gray834.svg
Dr. Ed
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