This formation and collapse of bubbles is called “cavitation.” Synovial fluid is fluid within joints. And when a person pulls a finger to crack their knuckle, for example, the sudden, dramatic increase in surrounding pressure pops the gas bubbles, partially or totally, making a sound. This gas is released as a bubble.Ĭracking joints may create a vacuum that the gases then fill. When a person extends a joint, this lowers the pressure inside it, releasing any gases, such as carbon dioxide.