I don't think my case has anything to do with arthritis. I think it is the demand I put on the little fingers (by holding a big bat handle) that made parts of them (tendons?) elongate. (It seems those parts are unable to return to their original state.)
My cousin is a chiroprator. He visited me some two months ago and looked at the little fingers. He said there is no big concern.
Unfortunately, the condition is getting worse. I can bend the little fingers with no problem. However, if my hands rest for a while (say over 30 minutes), and I release the fingers from a curled position to a flat position, there is great resistance and some pain.