Flexor digitorum profundus muscle

Origin and insertion

Flexor digitorum profundus originates from four sites; the superior three-quarters of the anterior surface of the ulna, the adjacent part of the interosseous membrane, the coronoid process of ulna and the aponeurosis of the flexor carpi ulnaris muscle. From this broad origin, the muscle takes an inferior course towards the hand.

At the level of the distal third of the forearm, it gives off a broad tendon. This crosses the superficial surface of the pronator quadratus and enters the hand by passing beneath the flexor retinaculum. Upon entering the hand, the tendon splits into four slips that attach to the palmar surfaces of the bases of the distal phalanges of digits 2-5. The muscle fibers are arranged in a way that the medial part of the muscle inserts into digits 4 and 5, while the lateral part inserts to digits 2 and 3.

Relations

Flexor digitorum profundus lies in the medial part of the forearm flexor compartment. It is located deep to the flexor digitorum superficialis muscle along its entire course. In the forearm, the median nerve crosses the lateral part of the anterior surface of the muscle.

The tendons of flexor digitorum profundus also lie deep to those of flexor digitorum superficialis. On their way towards the distal phalanges of the digits, each of the four tendons pass through the interval bounded by the terminal slips of the flexor digitorum superficialis. The lumbrical muscles of the hand attach to the deep surfaces of the flexor digitorum profundus tendons.

Innervation

Flexor digitorum profundus has a dual innervation;

• The medial part of the muscle, that inserts to the fourth and fifth digits, is innervated by the ulnar nerve (C8-T1)
• The lateral part, that inserts to the second and third digits, is innervated by the median nerve, via anterior interosseous branch (C8-T1)

Blood supply

Flexor digitorum profundus is supplied by the inferior ulnar collateral and ulnar recurrent arteries at its origin. The superior part of the muscle belly is supplied either by the ulnar artery or by its branch; the common interosseous artery. The rest of the muscle is vascularized by branches of the ulnar, anterior interosseous and median arteries.

Function

Flexor digitorum profundus muscle is a powerful flexor of the fingers. As it pulls the distal phalanges towards the hand, it causes flexion of the digits 2-5 at the metacarpophalangeal and interphalangeal joints. The muscle can act on its own but it usually works in synergy with the flexor digitorum superficialis, lumbricals and flexor digiti minimi brevis muscles to perform this action.

This muscle is also an accessory wrist flexor, aiding the flexor carpi ulnaris, flexor carpi radialis, palmaris longus, flexor digitorum superficialis and flexor pollicis longus muscles to flex the hand at the wrist joint.

The actions of the flexor digitorum profundus are important for establishing and maintaining a strong grip of the hand.

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