This is a patient with tennis elbow
I can see there is a hyper echoic spot at Radial collateral ligament.
How to describe this pathology? Avulsion fracture or enthesopathy or any other differential diagnosis?
You raise a great point as we need to attempt to classify these echogenic areas correctly as with enthesophytes, avulsions, loose bodies, osteophytes, exostosis, crystal deposition etc.
By definition an avulsion is where a piece of bone at an enthesis is pulled off by a traumatic overloading of the tissue attached at the enthesis.
Depending on the location it may be caused by the joint capsule attachment (ligament) , tendon attachment, retinaculum attachment or plantar / volar plate attachment. In all of these cases you should see a relationship between the disrupted collagen and the fragment of bone avulsion.
A loose body on the other hand is usually associated with chronic OA and it is a piece of bone / cartilage that has chipped off as part of the OA process and is not related to a collagen enthesis attachment point but rather is in an intra-articular location.
What is it in your case?
It is difficult for me to be absolutely sure and a plain radiograph will certainly help.
It could be a small loose body jammed under the radial collateral ligament.
It could also be a small traction enthesophyte,
There is certainly some vascularity in the joint capsule suggesting a degree of synovitis of the joint.