Flexor digitorum longus muscle

Structure

The flexor digitorum longus muscle arises from the posterior surface of the body of the tibia, from immediately below the soleal line to within 7.5 cm of its lower extremity, medial to the tibial origin of the tibialis posterior muscle. It also arises from the fascia covering the tibialis posterior muscle.

The fibers end in a tendon, which runs nearly the whole length of the posterior surface of the muscle. This tendon passes behind the medial malleolus, in a groove, common to it and the tibialis posterior, but separated from the latter by a fibrous septum, each tendon being contained in a special compartment lined by a separate mucous sheath. The tendon of the tibialis posterior and the tendon of the flexor digitorum longus cross each other ("dig over tib") at a spot above the medial malleolus, the so-called chiasma crurale. It passes through the tarsal tunnel.

It passes obliquely forward and lateralward, superficial to the deltoid ligament of the ankle-joint, into the sole of the foot, where it crosses over the tendon of the flexor hallucis longus at the level of the navicular bone at a location known as the "master knot of Henry" (also referred to as chiasma plantare), and receives from it a strong tendinous slip.

It then expands and is joined by the quadratus plantæ muscle, and finally divides into four tendons, which are inserted into the bases of the last phalanges of the second, third, fourth, and fifth toes, each tendon passing through an opening in the corresponding tendon of the flexor digitorum brevis muscle opposite the base of the first interphalangeal joint.

Variation

Flexor accessorius longus digitorum, not infrequent, origin from fibula, or tibia, or the deep fascia and ending in a tendon which, after passing beneath the laciniate ligament, joins the tendon of the long flexor or the quadratus plantæ.