Gas. Actually, the cracking of joints, especially in a younger person is due to a buildup or gas in the joints from normal metabolism .As the gas is forefully pushed out of the joint a cracking or popping noise is heard. It is not harmful to the joint. Cracking noises in older joints can be a sign of true arthritis.
Movement. The soft tissues of a joint can be tight after injury and burns. When these tissues are moved by the muscle, there is a change of pressure in the joint and a snap can occur. Clicking is usually a soft tissue interacting with a bone (such as "snapping hip" painful snaps can be helped by stretching and avoiding unnatural joint movements. Ask your dr more about joint protection & bone health.