There is no compelling scientific evidence that cracking your knuckles causes arthritis. But a long-standing habit could affect joint function down the road.
A joint is the point at which two (or more) bones meet. Ligaments connect the bones to one another, and a joint capsule surrounds the whole thing. It is filled with a natural lubricant called synovial fluid that helps the joint move smoothly. When you make the motion to crack your knuckles, the joint is pulled apart, expanding the capsule. This decreases the pressure inside the capsule, forcing gasses dissolved in the synovial fluid to release into the space to equalize it. That rush of gas causes the pop! sound -- so satisfying to you, so annoying to everyone else in the room. It's like popping the top on a can of soda.
It takes about 30 minutes for the carbon dioxide, nitrogen and oxygen to dissolve back into the synovial fluid. That's why you have to wait a while before you can get a second pop.
Cracking your knuckles feels good because it stretches the joint and stimulates the nerve endings found there. Can it be dangerous? Well, while it's unlikely that knuckle cracking can do the cartilage damage that leads to arthritis, it can lead to instability in the joint and loss of grip strength and hand function.
