Flexor Retinaculum: Tarsal Tunnel Syndrome

Flexor Retinaculum is present in the medial aspect of the ankle. It attaches to the medial malleolus on the lower end of the tibia. The Medial Calcaneus bone is also known as the heel bone.  The posterior tibial nerve passes under the flexor retinaculum. It gives the medial and lateral plantar nerves.

• There is only one tibialis posterior tendon on the medial side of the ankle, coming from the calf region and passing under the cover of the flexor retinaculum.
• Flexor digitorum longus tendon — it flexes the toes– under cover of the flexor retinaculum. The posterior tibial artery and nerve are present under the flexor retinaculum.
• Harry – Flexor hallucis longus —anterior to posterior

The flexor retinaculum forms the roof of a tunnel, and the tarsal bone forms the floor. This is known as the Tarsal tunnel.

Tarsal Tunnel Syndrome

Tarsal tunnel syndrome is a posterior tibial nerve compression under the flexor retinaculum, similar to carpal tunnel syndrome. The medial and lateral plantar nerves are compromised.

When these two branches (MPN and LPN) are compromised, the sole experiences sensory loss and muscle paralysis. When lumbrical interossei are paralyzed, the hand shows a Claw-Hand deformity similar to that of the foot.

Clawfoot Deformity

The two functions of lumbricals and interossei are MCP joint flexion and IP joint extension. Lumbrical interossei paralysis can happen due to ulnar and median nerve compromise at the level of the wrist. This causes the muscles in the Anterior forearm to become more powerful. This results in flexion. The Anterior and posterior muscles are balanced by lumbrical interossei. 

In claw foot. The metatarsophalangeal (MTP) joint is in hyperextension. There is Interphalangeal flexion, and the lumbrical interossei are paralyzed. There will be claw foot deformity where leg muscles are more powerful.

Causes of the MTP Extension

As we have discussed earlier in the blog,

• Anterior TDH— dorsiflexor or extensor 
• Extensors are more powerful
• Posterior – flexor TDH– tibialis posterior, flexor digitorum and flexor hallucis
• If lumbricals and interossei are not working, anterior leg muscles become more powerful. Therefore, anterior leg muscles cause MTP hyperextension.
• Posterior calf leg muscles cause IP extension.

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Injury to the posterior tibial nerve causes paralysis of sole muscles - lumbrical interossei.

The long saphenous vein passes anterior to the medial malleolus. The tibialis anterior passes under the extensor retinaculum. Anterior to tibia. Peroneus tertius is a small muscle passing anterior to the tibia. 

Frequently Asked Questions

Q: What is passing under the cover of the flexor retinaculum?

Answer: The posterior tibial nerve passes under the flexor retinaculum. It gives the medial and lateral plantar nerves.

Q: What happens when these two branches (MPN and LPN) are compromised?

 Answer: There will be sensory loss on the sole and paralysis of the muscles in the sole.

Q: What is the structure that passes deep to the flexor retinaculum?

1. Posterior tibial artery
2. Long saphenous vein
3. Tibialis anterior
4. Peroneus Tertius

Answer: (a) Posterior tibial artery

Q: What happens when there is an injury to the posterior tibial nerve?

Answer: Paralysis of sole muscles

Q: What happens in clawfoot?

Answer: In claw foot, the metatarsophalangeal (MTP) joint is in hyperextension.

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