Stenosing Tenosynovitis (Trigger Finger): A1 Pulley Thickness on Ultrasound
Anatomy and Study Objective
The A1 pulley is located at the level of the metacarpophalangeal joint and surrounds the flexor tendons of the finger. On a transverse ultrasound section, it appears as a thin hypoechoic structure around the tendons. The A2 and A4 pulleys are critical for preventing flexor tendon bowstringing; A2 may also thicken in trigger finger.
Normal Thickness and Thresholds
According to Diagnostic Ultrasound: Musculoskeletal (Griffith, 2025), the normal thickness of the A1 pulley is < 0.5 mm. In stenosing tenosynovitis (trigger finger), the A1 pulley diffusely thickens to 1–2 mm.
| Parameter | Normal | Trigger Finger |
|---|---|---|
| A1 Pulley Thickness | < 0.5 mm | 1–2 mm |
| Echogenicity of Thickening | — | hypoechoic |
Measurement Technique
The thickness of the annular pulley should be measured in the transverse plane at 10 or 2 o'clock positions (or 11/1 o'clock), as the base of the pulley expands and measurements there give overestimated values. On a longitudinal ultrasound scan, the thickening of the pulley is less recognizable than on a transverse scan.
Additional Ultrasound Features
In addition to A1 pulley thickening, trigger finger may reveal:
- hyperemia of the pulley on color/power Doppler imaging;
- tendinosis or tenosynovitis of the flexor tendons;
- nodular thickening or ganglion cyst formation;
- deviation of the flexor tendons as they pass under the A1 pulley.
Dynamic Study
Dynamic imaging allows assessment of the restriction of flexor tendon movement. Movement may be clearly restricted, passive, or relatively preserved with pulley elevation. The superficial flexor tendon is usually more restricted than the deep one, although distinguishing them on ultrasound is generally not possible.
Differential Diagnosis
In gouty tophus, crystalline aggregates produce a comet tail artifact; deposits typically form a large heterogeneous hypoechoic mass with calcified foci and hyperemia. GCTTS (giant cell tumor of the tendon sheath) does not calcify. In stenosing tenosynovitis, the tendon sheath may be distended with anechoic fluid without hyperemia.
Frequently asked questions
What is the normal thickness of the A1 pulley?
The normal thickness of the A1 annular pulley is less than 0.5 mm (Griffith, 2025).
To what thickness does the A1 thicken in trigger finger?
In trigger finger, the A1 pulley diffusely thickens to 1–2 mm, and the thickening is hypoechoic.
In which plane and position should the A1 pulley be measured?
It should be measured in the transverse plane at 10/2 o'clock positions (or 11/1 o'clock), as the base of the pulley expands. Thickening is less visible on a longitudinal scan.
What other changes can be seen in trigger finger?
Hyperemia of the pulley on color Doppler, tendinosis or tenosynovitis of the flexor tendons, nodular thickening or ganglion cyst, deviation of tendons under the A1 pulley. Dynamic study is useful for assessing movement restriction.
Can the A2 pulley also thicken?
Yes, in trigger finger, the A2 pulley can also thicken. A2 and A4 are critical pulleys that prevent flexor tendon bowstringing.