So what causes the “popping” sound?
Your knuckles are joints covered by a capsule (the joint capsule). Within this capsule there is fluid, which acts as a lubricant and also contains nutrients for the adjacent bone surfaces. A variety of gases are continuously dissolved in this fluid. When one cracks a knuckle, the stretching of the capsule lowers the pressure inside the joint and creates a vacuum, which is filled by the gas previously dissolved in the fluid. This creates a “bubble” which then bursts producing the “popping” or “cracking” sound. It takes a while until these gases are re-dissolved in the fluid, which explains why knuckles cannot be “re-cracked” immediately.
There is no evidence that cracking knuckles causes arthritis. However, there are studies associating knuckle cracking with injury of the ligaments surrounding the joint or dislocation of the tendons.
What causes arthritis?
There are different kinds of arthritis with the major categories being two: The inflammatory type such as rheumatoid arthritis and the degenerative type best known as osteoarthritis (wear and tear). The causes for either are not well known and research focuses on explaining the mechanisms leading to these diseases. For the inflammatory arthritis an unknown exposure to environmental stimuli is considered possible. For the “wear and tear arthritis” instead, aging and excessive mechanical stress may play a role in accelerating the damage in the joints.