Flexor digitorum profundus

Musculus flexor digitorum profundus

Also known as: FDP

A deep forearm flexor whose four tendons pierce the FDS tendons to reach the distal phalanges. It is the only muscle that flexes the DIP joints and has dual (median and ulnar) innervation.

Overview

FDP arises from the upper three-quarters of the anterior and medial surfaces of the ulna and the interosseous membrane. Its tendons pass through the carpal tunnel deep to FDS. In the palm they give origin to the lumbricals, then pass through the split FDS tendons to insert on the bases of the distal phalanges of digits 2–5. Only the tendon to the index finger has an independent belly. The other three share a common belly, so they tend to flex together.

FDP flexes the distal interphalangeal (DIP) joints and then helps flex the PIP and MCP joints and the wrist. It is the main flexor in a sustained grip. Dual innervation: the lateral part (index and middle fingers) is supplied by the anterior interosseous nerve (median) and the medial part (ring and little fingers) by the ulnar nerve.

Anatomy

Origin

Upper three-quarters of the anterior and medial surfaces of the ulna and the interosseous membrane.

Insertion

Bases of the distal phalanges of digits 2–5.

Action

Flexion of the DIP joints, then the PIP and MCP joints and the wrist.

Innervation

Lateral half (digits 2–3): anterior interosseous nerve (median). Medial half (digits 4–5): ulnar nerve.

Nerve roots

C8–T1

Arterial supply

Anterior interosseous artery and ulnar artery (muscular branches).

Relations

Lies on the ulna deep to FDS and FCU. The ulnar nerve and artery lie on its anterior surface medially. The anterior interosseous nerve and artery lie between it and FPL on the interosseous membrane.

Clinical significance

Jersey finger

Avulsion of the FDP tendon from the distal phalanx, most often the ring finger, when a player grabs an opponent's jersey. The patient cannot flex the DIP joint. It requires surgical repair.

Ulnar paradox

A high (elbow) ulnar lesion paralyses the medial half of FDP, so the ring and little fingers claw less than with a low (wrist) lesion. In the low lesion, intact FDP pulls the fingers into flexion, worsening the clawing caused by the paralysed lumbricals.

Anterior interosseous nerve palsy

Weakness of FPL and the index finger FDP prevents the patient from making an "OK" sign; they form a flat pinch instead. There is no sensory loss.

Test yourself

Which muscle flexes the distal interphalangeal joints of the fingers?

Flexor digitorum superficialis
Flexor digitorum profundus
Lumbricals
Palmar interossei

Answer: Flexor digitorum profundus

FDP inserts onto the bases of the distal phalanges. FDS inserts on the middle phalanges and flexes the PIP joints. The lumbricals flex the MCP joints and extend the interphalangeal joints.