A1 pulleys !

The A1 pulleys are bands at the base of each finger that hold the flexor tendons to the bone; a thickened A1 pulley causes trigger finger.

Official name: A1 pulleys of the flexor tendon sheaths

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What it is

Short rings of tough tissue over the flexor tendons at the base of each finger and the thumb, in the palm.

What it does

Hold the flexor tendons close to the bone at the knuckle; a thickened pulley causes trigger finger.

Key facts

Attaches: Palmar plate over each MCP joint
Function: Keeps the flexor tendons close to bone; site of trigger finger
Tight in: Finger flexion

Relations

Contents: FDS tendons and FDP tendons (FPL tendon in the thumb) inside the flexor tendon sheath
Behind: Volar plate of the MCP joint, from which it arises, and the metacarpal head
Below: The A2 pulley on the shaft of the proximal phalanx (finger pulleys)
Lateral: Digital nerves and digital arteries on each side; in the thumb the radial digital nerve crosses in front of it

How to find it

The examiner presses over the metacarpal head in the palm while the person slowly bends and straightens the finger; in trigger finger a tender nodule glides under the examiner's finger and catches at the edge of the pulley.

Examination

Palpation during active flexion: a tender nodule that catches or clicks at the A1 pulley confirms triggering; a finger that locks in flexion and needs the other hand to straighten it is a more advanced grade

On imaging

Ultrasound shows a thickened, hypoechoic A1 pulley and a swollen tendon beneath it; dynamic scanning shows the tendon catching. Ultrasound can also guide a percutaneous release or an injection.

Common problems

Trigger finger: the tendon or the A1 pulley thickens, so the finger catches or locks when bending, then snaps straight.
More common with diabetes, in the thumb, ring and middle fingers.

How it is diagnosed

A tender nodule in the palm at the base of the finger and catching felt on bending.

Treatment

Splint, activity changes and a steroid injection, which often works.
Release of the A1 pulley through a small cut or with a needle when it keeps coming back.

Clinical notes

Paediatric trigger thumb: a fixed flexed thumb IP joint with a palpable nodule (Notta node) in a young child, sometimes on both sides
Release can be open or percutaneous; incomplete release and accidental cutting of A2 are the main technical errors
Several trigger digits at once are more common with rheumatoid arthritis and amyloidosis

Trigger finger

Test yourself

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

Which pulley is released in trigger finger surgery, and which must be preserved?

Only the A1 pulley is divided; keeping A2 intact prevents bowstringing.

Which nerve is at risk in thumb A1 pulley release?

The radial digital nerve, which crosses obliquely in front of the pulley close to the skin.

Where is the proximal edge of the A1 pulley of the ring and little fingers?

Near the distal palmar crease (the proximal palmar crease for the index; the MCP flexion crease for the thumb).

Why is the A1 pulley usually preserved in rheumatoid arthritis?

Releasing it can worsen ulnar drift; tendon synovectomy is preferred.

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

Its names

Official Latin (TA2): pars anularis vaginae fibrosae digiti manus TA2 2585
Official Spanish (DeCS): Porción anular I de la vaina fibrosa de los dedos de la mano
English (TA2): A1 pulleys of the flexor tendon sheaths
Also called: A1 pulley · annular pulley · Annular part of the fibrous digital sheath (A1)

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Metacarpophalangeal Joints; Anatomy, Shoulder and Upper Limb, Hand Metacarpal Phalangeal Joint.

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Representative model, for educational purposes only.

A1 pulleys highlighted on the Durgical 3D model