PLOS One/Kawchuk et al.

What is it that makes that popping sound when you crack your knuckles? If you think it's vacuum cavities forming in the synovial fluid of the joint, give yourself a gold star: a team of researchers led by the University of Alberta Faculty of Rehabilitation Medicine have confirmed that that is precisely what it is.

How? By pulling the fingers of a test subject inside an MRI machine.

"We call it the 'pull my finger study' -- and actually pulled on someone's finger and filmed what happens in the MRI," said lead author of the study published in PLOS One, Professor Greg Kawchuk of the Faculty of Rehabilitation Medicine. "When you do that, you can actually see very clearly what is happening inside the joints."

The theory of bubbles in the joint was first floated in 1947: UK researchers JB Roston and R Wheeler Haines hypothesised that cracking the knuckles caused bubbles to form in the synovial fluid; this, they believed, caused the sound. In 1971, however, another study came along that proposed that it was not the formation, but the collapse of the bubble that produced the audible effect -- in other words, that it was the bursting of the bubble that made a noise.

Other hypothetical sources of the knuckle-cracking noise included stretching ligaments, or the adhesions in the joints snapping -- but the bubble idea has always been the strongest, since X-rays taken directly after cracking a joint show a gas bubble inside that joint. But whether or not it was the formation or collapse of the bubble had still been something of a mystery.

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The idea for the study came from Nanaimo chiropractor Jerome Fryer, who approached Professor Kawchuk with a theory. Rather than beat around the bush, they decided to take a direct look using magnetic resonance imaging -- with champion knuckle-cracker Fryer as the guinea pig.

"Fryer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team," Professor Kawchuk said.

Fryer's fingers were inserted, one at a time, into a tube attached to a cable; this tube slowly pulled on each finger until the knuckle cracked. And, in each instance, it was absolutely the formation of the bubble in the synovial fluid that was associated with the popping sound, occurring within 310 milliseconds.

University of Alberta, Kawchuk et al.

"It's a little bit like forming a vacuum," Professor Kawchuk explained. "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."

Solving a decades-old mystery was far from the team's only focus, though -- as fun as that was. The team believes studying joint cracking could help them better understand joint health -- such as the contradiction between the amount of force required to crack a joint (enough to cause damage to hard surfaces) and the fact that it doesn't appear to do long-term harm.

One thing they found, for instance, was a flash of white in the MRI just before the joint popped -- something no one had ever documented before. Professor Kawchuk believes it was water suddenly being drawn into the joint, and plans to use more advanced MRI to study what happens in the joint just before and after the pop.

"It may be that we can use this new discovery to see when joint problems begin long before symptoms start, which would give patients and clinicians the possibility of addressing joint problems before they begin," he said.

The 1971 team may have missed the mark on the cause of the sound, but they did get at least one thing correct.

"The data fail to support evidence that knuckle cracking leads to degenerative changes in the metacarpal phalangeal joints in old age," the study concludes. "The chief morbid consequence of knuckle cracking would appear to be its annoying effect on the observer."