There is an effusion of the PIP joint.
The synovium appears thickened and there is increased vascularity with your Doppler assessment.
Your text says middle finger, the images are labelled index. I presume index is correct.
No trauma I presume.
On some of the images it looks like there might be some crystal deposition on the cartilage.
Is there a history of gout?
I would include gout in my differentials.
I would also include rheumatoid disease and OA.
Rheumatoid and OA like this joint, where as gout and CPPD prefer the MCP joint.
Get a plain radiograph to see if there is any OA changes or evidence of rheumatoid or gout erosions.
The radiograph wil also tell us if there is any hydroxyapatite or CPPD crystals present.
So it is an inflamed joint with a large effusion, which is likely why the range of movement is restricted.
If you follow the typical distribution patterns rheumatoid and OA are the conditions that like this joint, not so much gout or CPPD
But given your pictures, I would not exclude gout as an option even though it is not a typical location.
Are any other joints affected with similar symptoms?
If you have a second joint involved a rheumatological consult is worth while.
Whatever the cause I am surprised it was not a painful joint!
What did the palmar aspect of the joint and the volar plate look like?
Later today I will try and record a webinar for the website which outlines the typical distribution patterns of these diseases in the hand.
I think you will like it,
Steve.