FDP !

The flexor digitorum profundus is the deepest finger flexor of the forearm; it bends the fingertips and then the whole finger.

Official name: Flexor digitorum profundus

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What it is

The deep forearm muscle with four tendons to the fingertips.

What it does

Bends the fingertips and the whole finger.

Key facts

Origin: Anterior and medial ulna, interosseous membrane
Insertion: Bases of the distal phalanges of digits 2–5
Action: Flexes the DIP joints (the only muscle that does)
Nerve: Lateral half (index, middle): anterior interosseous nerve; medial half (ring, little): ulnar nerve (C8–T1)
Blood supply: Ulnar artery, anterior interosseous artery

Relations

Superficial: Flexor digitorum superficialis (FDS), with the median nerve between them; the ulnar nerve and ulnar artery lie on its medial part under flexor carpi ulnaris
Deep: Front and medial surfaces of the ulna and the interosseous membrane; pronator quadratus lies behind its tendons above the wrist
Lateral: Flexor pollicis longus (FPL), with the anterior interosseous nerve and artery running between them on the interosseous membrane
Course: Its tendons pass through the carpal tunnel deep to FDS, give origin to the lumbricals in the palm, pierce the FDS split over the proximal phalanx and reach the distal phalanx

How to find it

Deep on the front of the ulna; during a strong grip its upper belly is felt contracting just in front of the subcutaneous posterior border of the ulna. In surgery it is reached by retracting flexor carpi ulnaris and FDS.

Examination

FDP test: with the middle phalanx held straight, the person bends the fingertip; no DIP flexion means a cut or avulsed tendon or a nerve lesion
OK sign: a squared-off 'pinch' instead of a round O between thumb and index (index DIP and thumb IP stay straight) indicates anterior interosseous nerve palsy
Weak DIP flexion of the ring and little fingers with a normal index places an ulnar nerve lesion at or above the elbow

On imaging

Ultrasound tracks the tendons deep to FDS and locates a cut or avulsed tendon as a gap with a retracted end, which helps plan surgery. In jersey finger (avulsion from the distal phalanx, most often the ring finger), a lateral finger radiograph looks for a bony fragment from the distal phalanx base, at the DIP joint or higher. MRI if ultrasound is unclear.

Clinical notes

Jersey finger needs early recognition: the retracted tendon loses its blood supply from the vincula, and the level of retraction guides repair
Zone II flexor injuries cut FDS and FDP together inside the same sheath
The ulnar paradox explains why clawing may get worse as a high ulnar nerve recovers and FDP regains power before the hand muscles

Anatomical variants

The index usually has its own belly and moves independently, while the other three tendons arise from a common belly
Martin–Gruber anastomosis: motor fibres cross from the median (often the anterior interosseous) to the ulnar nerve in the forearm in a minority of limbs (figures vary widely), confusing lesion localisation

Test yourself

Think of the answer, then tap the question to see it.

What is the nerve supply of the FDP (flexor digitorum profundus)?

Lateral half (index, middle): anterior interosseous nerve; medial half (ring, little): ulnar nerve.

What is a jersey finger, and which finger is most often affected?

Avulsion of the FDP tendon from the distal phalanx when a gripping finger is forced straight; most often the ring finger.

What is the ulnar paradox involving the FDP?

A high ulnar nerve lesion also weakens the ring and little finger FDP, so those fingers claw less than after a lesion at the wrist.

What is the quadriga effect of the FDP?

The middle, ring and little tendons share a belly, so tethering or shortening one limits flexion of its neighbours.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): flexor profundus digitorum TA2 2491
Official Spanish (DeCS): Músculo flexor profundo de los dedos
English (TA2): Flexor digitorum profundus

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Flexor Digitorum Profundus Muscle; Anatomy, Shoulder and Upper Limb, Hand Long Flexor Tendons and Sheaths.

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Representative model, for educational purposes only.

FDP highlighted on the Durgical 3D model