I’ve played guitar for ten years and believe the injury was caused
from overplaying and incorrect
playing (pressing down too hard on the fretboard, not warming up, and playing for hours straight) I had the same injury 4 years ago in the same knuckles but it was less severe and it healed itself in four months with rest, diet, ice, anti inflammatories, topical CBD oil, and lots of water.
I tried all of the things I had done in the past and they have helped but not fixed it.