"Pull my finger," a phrase embraced by school-aged kids and embarrassing uncles the world over, is now being used to settle a decades-long debate about what happens when you crack your knuckles. In a new study published April 15 in PLOS ONE, an international team of researchers led by the University of Alberta used MRI video to determine what happens inside finger joints to cause the distinctive popping sounds heard when cracking knuckles. For the first time, they observed that the cause is a cavity forming rapidly inside the joint.

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

Scientists have debated the cause of joint cracking for decades, dating back to 1947 when U.K. researchers first theorized vapour bubble formation as the cause. That was put in doubt in the 1970s when another team of scientists instead fingered collapsing bubbles as the cause.

The idea for the project was born when Nanaimo chiropractor Jerome Fryer approached Kawchuk about a new knuckle-cracking theory. They decided to skip the theories and, with U of A colleagues Jacob Jaremko, Hongbo Zeng, Richard Thompson and Australian Lindsay Rowe, decided to actually look inside the joint.

But to find an answer, the team needed someone capable of cracking knuckles on demand--a job that fell to Fryer himself. Kawchuk said most people have the ability to crack their knuckles, but unlike most, Fryer can do it in every finger, and after the standard recuperation time, he can do it again.

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

Fryer's fingers were inserted one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked. MRI video captured each crack in real time--occurring in less than 310 milliseconds.

In every instance, the cracking and joint separation was associated with the rapid creation of a gas-filled cavity within the synovial fluid, a super-slippery substance that lubricates the joints.

"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."