Why do knuckles crack?

Each knuckle joint is surrounded by a capsule filled with synovial fluid, which contains dissolved gases such as carbon dioxide. When you stretch or bend the joint, pressure inside the capsule drops suddenly and a gas-filled cavity forms, a process called tribonucleation. A 2015 study using real-time MRI showed that the cracking sound coincides with this cavity forming. It usually takes around 20 minutes for the gas to dissolve back into the fluid, which is why you cannot crack the same knuckle again straight away. Despite the old warning, studies have found no clear link between habitual knuckle cracking and arthritis.