Anatomy of Flexor Digitorum Longus

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Anatomy of Flexor Digitorum Longus

Origin – It arises from the posterior surface of mid tibia immediately below the soleal line and medial to the tibial origin of the Tibialis posterior. It is medial to an imaginary vertical line that separates the tibialis posterior muscle from the flexor digitorum longus.
The tendon of the flexor digitorum longus passes behind the medial malleolus In a groove.
The groove also contains the tendon of the Tibialis posterior.

TOM DICK AND HARRY is a mnemonic for the neurovascular arrangement in the Flexor Retinaculum of Ankle

Tibialis posterior
Flexor Digitorum longus
Posterior tibial Artery
Tibial Nerve
Hallucis longus.

Cross sectional arrangement.

It is in the deep posterior compartment of the leg.
Knot of Henry: point of crossing over of 2 tendons flexor digitorum longus and Flexor Hallucis longus.
After the tendons curve under the medial malleolus and talus ,they begin to converge and cross each other.
The Flexor digitorum longus crosses the Flexor Hallucis Longus in the medial compartment of the foot and become more plantar [superficial].
The 2 tendons are connected by a strong tendinous slip.
Tibialis posterior insufficiency or rupture is not uncommon and may be missed. When insufficiency or rupture is diagnosed one of the treatment option is to borrow the tendon next to tibialis posterior tendon inorder to reinforce the function of tibialis posterior.

The logical choice of tendons is the one next to the tibialis posterior tendon-flexor digitorum longus tendon FDL.uses as a tendon transfer not alone but in conjunction with other procedure , usually bony procedures.

Because of the intersection between the flexor hallucis longus and Flexor digitorum longus tendons, if transection and transfer of the flexor digitorum longus tendon is done proximal to the Knot of Henry to correct the tibialis posterior dysfunction ,this will result in retention of the function of the big toe and the lesser toes .

Insertion :

flexor digitorum longus – the tendon expands and us joined by the quadratus plantar muscle.
It then divides into 4 tendons that are inserted into the base of the Distal phalanges of the 2nd to 5th toes.

Nerve Supply- tibial Nerve