Cystic lesions in distal semimembranosus

[Samuel Katumba]

Hi Steve,
I scanned a 40year old female who complained of vague pain around the posterior left knee joint-worse on squatting-this had lasted about 4months.
She reported no fever, no other systemic disease.
Attached are images of posterior thigh (a little suboptimal).
There are multiple cystic foci noted in semimembranosus, normal muscle echotexture is distorted. No hyperemia on color doppler.
I thought this could be myositis or Pyomyositis-problem is no fever reported and the symptoms had lasted 4months, besides no increased color signals on color doppler.
maybe it was muscle infarction but no know history of diabetes
maybe it was a muscle/tendon a tear, but patient is not an athlete and no reported history of trauma.
after some days, I thought of tendinosis, but all these suggestions do not seem to accurately account for the findings in this muscle. What do you think??

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[Stephen Bird]

Hi Samuel,

Nice images,

In the poster-medial corner of the knee cystic lesions are very common.

Mostly they are classic Bakers cysts but there are also other ganglion type cysts that commonly occur.

There is also a bursa sitting deep to the semimemb tendon just proximal to its insertion to the tibia that can be distended with fluid.

Bakers cysts and importantly the other ganglion type cysts of the poster-medial corner can become intramuscular and work their way inside medial gastric, soles or semimemb.

I suspect this is what you are seeing and this will account for the chronic history and the feeling of tightness in the poster-medial corner of the knee.

As you are looking at an intramuscular abnormality it would seem mandatory to perform an MRI just in case you are looking at a sarcoma, however this seems less likely given the appearance. Ant intramuscular lesion that is identified without trauma (i’e’e a muscle tear) should go through this check. This article looks at some of the variations

https://journals.sagepub.com/doi/full/10.1177/8756479317733750

I don’t think it is tendinosis with myxoid degeneration as it extends beyond the tendon collagen and into the muscular part of the muscle.

This is also a nice article:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4094974/

The images also show low levels echoes within the cystic areas which is not typical of a ganglion or cyst extension.

A venous thrombosis event is also a possibility, however I think this is also unlikely.

These are my thoughts,

I do not have a specific diagnosis for you, but an MRI sill settle the argument,

Please let us know if you can get some follow up,

Steve

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