Thumb is stiff & tight in the knuckle, pain in knuckle when bent back but xray & ultrasound are clear. on going for 4 months, caused by punching?

Early arthritis?: Joint stiffness & pain sounds like early arthritis. If you have been an avid boxer, then osteoarthritis (from overuse of the thumb) seems to be in the works. Often, X-rays & ultrasound cannot detect the changes of early arthritis, especially if it is located in a small joint like those in the hand. Best to see a rheumatologist or find another way to make diagnosis. Ice the thumb, avoid over-use.