'Pull my finger' study cracks mystery of knuckle noise

Scientists at the University of Alberta did a lot of finger pulling -- all in the name of serious study into joint health. In a study published in the journal Plos One, researchers may have finally figured out what is going on when we crack our knuckles.

It started with Nanaimo B.C. chiropractor Jerome Fryer. He tells As It Happens host Carol Off that he's long been curious about knuckle cracking, and volunteered himself as a test subject. 

"Throughout my career I always wanted to answer that question," he says.

It helps that Dr. Fryer can spontaneously crack his knuckles on command. It earned him the moniker the "Wayne Gretzky of knuckle cracking."

The prevailing wisdom is that a bubble of air collapses in the joint as it is pulled. According to the new research, that's not the case.

When Dr. Fryer's fingers were examined with MRI imaging, he says, "We saw a void -- a dark spot -- that occurs at the same time as the finger cracks."

Researchers concluded that, in fact, a cavity rapidly forms inside the joint as the finger is pulled. 

According to one of the study's authors Greg Kawchuk, "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 next step is to investigate the health implications of knuckle cracking. It may be that cracking knuckles -- while annoying others -- could also be good for us.