SadieB replied: "The popping sound is gas escaping and does no harm in itself. Arthritis is not caused by this. Think about what chiropractors do. Often this is used as a way of reducing tension. However, if you pull or push on your joints to pop them it can do damage to the tendons in your hands. Habitual knuckle crackers are more likely to have hand swelling and lower grip strength attributed to stretched tendons. So it is really more about how one does this and proper technique is needed to prevent damage."