This is an interesting one,
Yes indeed you can see the proximal tibia-fibular joint with ultrasound,
It is fairly simple as you can easily palpate the head of the fibula and then simply line this up with the tibia.
The joint can get OA and synovitis and the joint capsule ligaments can be damaged by trauma or degenerative change.
This is what I suspect has happened in your patient and like other joints this leads to a ganglion arising from the joint through the capsule.
It is possible for a ganglion to become “intra-muscular” and this occurs where there is a small defect in the muscle fascia and this might be anatomical where a vein / artery or nerve is entering or exiting the fascia or it may be a congenital deficiency in the fascia. Either way it is a possible entry site for a ganglion looking for a path of least resistance to grow along.
You can see the same thing with a bakers cyst which ends up intra-muscular inside medial gastrocnemius, and I saw one recently that became intramuscular inside soleus.
So I think this is an intra-muscular extension of a proximal tibia-fibular joint ganglion and it looks like it is inside peroneus longus.
Remember also the proximal tibia-fibular joint ganglions have another trick up their sleeve and they can find a path of least resistance along the intra-articular branch of the common peroneal nerve. They can track along this and become an intramural ganglion of the common peroneal nerve.
Sneaky eh!!
Steve