A1 Pulley Sonographic Appearance SAX vs LAX

[David Smith]

Hi Stephen,

I was assessing a patient for bilateral trigger fingers in 4th digits. On SAX exam, I was surprised to see a fairly thin pulley. As expected, the pulley appeared as a anechoic thin shadow arching over the FDS and FDP tendons.

I have seen sonographic images of thickened pulleys in LAX and they often are not anechoic anymore but have that particle board appearance.

Do you ever see a difference in appearance in the same pulley, with SAX showing the shadow and LAX showing a more echogenic appearance? Are the pitfalls in missing a thickened pulley such as using too much transducer pressure?

As I said, I was surprised to see the patient easily show me the triggering on exam, and then not see any obvious pathology on the US exam.

Thanks,
David

[Stephen Bird]

Hi Dave,

This makes perfect sense and it is all about ultrasound physics and specifically anisotropy.
When you scan short axis only the bit of the pulley at the top of the arch is specular to the ultrasound beam and hence this part will look more echogenic and “collagen” like. The components of the A1 pulley that are running down the side of the tendons has a collagen direction which will ensure anisotropy and hence they look black. Any part of the pulley will look echogenic and “collagen” if you can hit the fibre direction at close to 90 degrees.
This also explains why it looks more echogenic in the long axis as you are only seeing the “apex” of the pulley and you are perfectly specular to the collagen lines.

It is also interesting that the finger triggered when the pulley looked thin.
I have seen this many times where a thin but tight pulley causes triggering. It is often in the setting of a degree of tenosynovitis which makes the tendons larger and no longer able to fit beneath the pulley apparatus.

Steve

[David Smith]

Great answer Steve. I will be more careful with my SAX angle as up to know, on SAX, even the top arch of the pulley looks anechoic. I will play around with it.

One question: if the pulley is nice and thin, any particular reason someone would get tenosynovitis enough to make the tendon thicker and have trouble passing through the pulley? Just genetics and/or bad luck?

Thanks,
David

[Stephen Bird]

When I see a triggering finger or thumb with a normal looking A1 pulley it is usually due to a degree of tenosynovitis. When this occurs the tendon(s) becomes larger and then friction occurs and the friction exacerbates it making the tendon swell more etc etc and bingo you have a trigger.

Anything that leads to tenosynovitis will be the culprit. May be overuse, may be underlying rheumatoid. Diabetes is also a risk factor and no doubt sometimes it is just bad luck.

We generally treat these patients with a catabolic injection into the flexor sheath as a first up intervention.

Steve

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