Why is there a “popping” sound when you crack your knuckles, and is it dangerous to crack them?

A number of reasons have been given for the characteristic “popping” sound associated with someone cracking their knuckles. One reason is that when a joint is contracted, small ligaments or muscles may pull tight and snap across the bony protuberances of the joint. Another possibility is that when the joint is pulled apart, air can pop out from between the bones, creating a vacuum that produces a popping sound. A third reason, discovered by British scientists in 1971, is that when the pressure of the synovial fluid is reduced by the slow articulation of a joint tiny gas bubbles in the fluid may burst, producing the popping sound. Research has not shown any connection between knuckle cracking and arthritis. One study found that knuckle cracking may be the cause of soft tissue damage to the joint capsule and a decrease in grip strength. The rapid, repeated stretching of the ligaments surrounding the joint is most likely the cause of damage to the soft tissue. Some researchers believe that since the bones of the hand are not fully ossified until approximately age 18, children and teenager who crack their knuckles may deform and enlarge the knuckle bones. However, most researchers believe knuckle cracking does not cause serious joint damage.