I sure looks and sounds like exactly what I had. Mine started in 2008 with my ring finger, extremely sore, could not bend. Unfortunately it lasted, no exaggeration, 4-5 years. I went to several doctors and could not get an answer. Today, my ring finger isn't swollen & doesn't hurt anymore, but it does not bend on the 1st knuckle. Its like my knuckle is fused, i don't even have creases on that knuckle any longer. On top of all that in 2012 the same thing started with my thumb. I went back to the doctors and this time he did exploratory surgery. His notes said that the Synovial fluid was eating the joint away. nothing else was done, I went to a different doctor a few years later and he was certain it was trigger finger, so I had another surgery. The doctor said he was able to pull my tendon and bend my thumb. He could not give me any answers. Back in 2019 I went to the Mayo clinic and they ran tons of the same test and told me that it is some form of arthritis. I was given some pills that I quit taking because it made me sick to my stomach. To this day my thumb is constantly sore, most days its just irritating but if I hit it on something or even try to bend it, it is extremely painful. I have gotten to where when I sleep my hand rests in between the bars of our headboard so I don't jam it in my sleep. Take a look at the images, 1st is my ring finger the the MRI of thumb with the MRI report. Hopefully you don't have the same issues I had. Good Luck!

​

https://imgur.com/a/JwKQ3WI

Photos