"Knuckle cracking over the years will cause repetitive trauma to the joints and cartilage," Seattle neurosurgeon Rod Oskouian told The Washington Post.
Oskouian and his three colleagues pored over 26 sometimes-contradictory papers regarding the mechanisms and effects of knuckle cracking, beginning with a 1911 German treatise titled "On the Dispute About Joint Pressure." He did so, he said, after becoming fascinated by the universal inability of his students through the years to explain what makes that cracking noise.
Modern scholars now agree that bones themselves aren't cracking, but rather that the movement creates a bubble of gas in the synovial fluid lubricating the joints. Researchers still don't know if it is the bubble's formation or subsequent pop that makes the noise, but Oskouian said the mechanics are similar to a chiropractor's "adjustment" of the spine, which also elicits a cracking sound.