Watch: See knuckle crack on MRI for first time

Posted at 2:40 PM, Apr 15, 2015
and last updated 2015-04-16 10:22:53-04

When you pull my finger — what causes the sound?

A new study shows for the first time a knuckle cracking on MRI video. The real-time images challenge the long-standing belief that the cracking sound is caused by the bursting of a bubble in the joint.

For the study, published Wednesday in the journal PLOS ONE, University of Alberta researchers used a cable to pull a man’s finger while watching on a MRI scanner.

In the video, the knuckle joint separates, and a bubble of gas forms inside the slippery synovial fluid between the bones. Each crack lasts about one-third of a second.

But the bubble doesn’t burst. It’s re-absorbed during a 20-minute rest period where cracking is no longer possible. The researchers concluded that the cracking sound is caused by the rapid separation of the joint, not a breaking of the bubble as was previously believed.

"It's a little bit like forming a vacuum," said study authorGreg Kawchuk in a media release. "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."

Cracking knuckles does not cause arthritis or other damage, even though the force inside the joint should have been high enough to damage it, the study said.

In a 1998 study, physician Donald Unger cracked the knuckles on only on his left hand every day for 60 years. Neither hand developed arthritis.

Gavin Stern is a national digital producer for the Scripps National Desk.