FDS tendons !

The flexor digitorum superficialis tendons run through the wrist into the fingers; they bend the fingers at the middle joints.

Official name: Flexor digitorum superficialis tendons

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What it is

Four flexor tendons through the carpal tunnel to the middle bones of the fingers; they bend the middle finger joints and can be cut in palm or finger lacerations.

What it does

Bend the fingers at the middle joints.

Key facts

Attaches: Four tendons through the carpal tunnel (middle and ring in front of index and little), each splitting at the proximal phalanx to the sides of the middle phalanx
Function: Flex the proximal interphalangeal and MCP joints; tested with the other fingers held straight; the FDP tendon passes through its split
Tight in: Finger and wrist extension

Relations

Course: From the forearm the four tendons pass through the carpal tunnel within the ulnar bursa, fan out in the palm and enter the fibrous sheaths at the A1 pulleys, where they split around the FDP
Deep: FDP tendons, behind them in the carpal tunnel and palm until they come through the FDS split over the proximal phalanx
Superficial: Median nerve just under the flexor retinaculum in the carpal tunnel; in the palm the superficial palmar arch and the palmar aponeurosis
Lateral: FPL tendon, in its own sheath, the most radial tendon in the carpal tunnel
Medial: Ulnar nerve and artery, passing over the flexor retinaculum in Guyon's canal

How to find it

With the other fingers held flat on the table, bending one finger at the middle joint shows its FDS working. In the distal forearm the tendons are felt ulnar to the palmaris longus tendon when the fingers flex against resistance.

Examination

Isolated FDS test: the examiner holds the other fingers fully straight and asks for flexion of the free finger; PIP flexion with a floppy DIP means the FDS is intact, none means it is cut
Index finger: its FDP often has its own belly, so the standard test is unreliable; a pinch with the PIP bent and the DIP straight shows the FDS instead
Tenodesis effect: wrist extension normally curls the fingers; a finger that stays straight suggests divided flexor tendons

On imaging

Ultrasound shows tendon continuity during active and passive movement, finds retracted ends before repair and shows tenosynovitis within the sheath.

Clinical notes

Flexor zones: I distal to the FDS insertion, II from the A1 pulley to the FDS insertion, III palm, IV carpal tunnel, V forearm
In zone II both tendons lie in the sheath; repair aims to keep them gliding without adhesions, usually with early protected movement
The ring finger FDS is a common donor in tendon transfers, for example to restore thumb opposition
'Spaghetti wrist': deep cuts at the front of the wrist (zone V) often divide several FDS tendons together with the median and ulnar nerves

Anatomical variants

The little finger FDS is often weak, linked to the ring finger tendon or absent, so a weak isolated test there may be normal
Accessory slips or bellies can appear in the palm

Memory aid

'Superficialis splits, profundus perforates': the FDS divides so the FDP can pass through it to the fingertip.

Test yourself

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

Where do the FDS tendons insert, and what is Camper's chiasm?

Each splits over the proximal phalanx to let the FDP through; the halves cross again behind it (Camper's chiasm) and insert on the sides of the middle phalanx.

How is an FDS tendon tested in isolation?

The other fingers are held straight, locking their shared FDP; PIP flexion of the free finger with a floppy DIP shows an intact FDS.

What is flexor zone II, and why is it difficult?

From the A1 pulley to the FDS insertion; both flexor tendons share a tight sheath, so repairs tend to form adhesions.

Which FDS tendon is a common donor in tendon transfers?

The ring finger FDS, for example to restore thumb opposition.

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

Its names

Official Latin (TA2): tendines musculi flexoris superficialis digitorum TA2 2486
Official Spanish (DeCS): Tendones del músculo flexor superficial de los dedos
English (TA2): Flexor digitorum superficialis tendons

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

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Representative model, for educational purposes only.

FDS tendons highlighted on the Durgical 3D model