The tarsal tunnel is the region beneath the flexor retinaculum on the medial side of the ankle. The contents of the tarsal tunnel are the posterior tibial nerve and the posterior tibialis artery and two accompanying veins. The posterior tibial nerve most commonly divides into the medial and lateral plantar nerves within this tunnel.

Tarsal Tunnel Syndrome is traditionally known as a single region of compression on the posterior tibial nerve by the flexor retinaculum. However, there are actually three other sites of compression, in addition to the tarsal tunnel, that need to be decompressed in order to correct the patient’s problems. The other sites are:

The calcaneal tunnel (for the medial calcaneal n., d)
The medial plantar tunnel (for the medial plantar n.,b)
The lateral plantar tunnel (for the lateral plantar n., c)

Variations predisposing a person to nerve compression at the ankle:
The posterior tibial nerve most commonly divides into the medial and lateral plantar nerves within this tunnel. In about 5% of people, this division will occur proximal to the entrance of the tunnel, so that at the time these nerves enter the tunnel they already occupy about twice the volume of the posterior tibial nerve.