Researchers share MRI video of knuckle cracking

EDMONTON, Alberta, April 16 (UPI) -- University of Alberta researchers shared an MRI video showing what goes on inside the hand when someone cracks a knuckle.

The video, released as part of a study published Wednesday in journal PLOS ONE, shows the knuckle joint separating and creating a bubble of gas in the synovial fluid between the bones when the researchers used a cable to pull the man's finger and crack his knuckle.

The University of Alberta researchers, led by Professor Greg Kawchuk, said the gas bubble does not burst, as previously believed, but instead is reabsorbed during the 20-minute period following the knuckle crack. The team said the cracking sound is caused by the rapid separation of the joint, rather than the previously-believed breaking of the bubble.

"It's a little bit like forming a vacuum," Kawchuk said. "As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created and that event is what's associated with the sound."

The study backed up previous research indicating joint cracking does not damage the body or lead to arthritis.