Knuckle-Cracking Riddle Finally Solved

"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 online on April 15, 2015 in an open-access article in PLOS ONE, an international team of researchers led by the University of Alberta (U of A) used MRI video to determine what happens inside finger joints to cause the distinctive popping sounds heard when cracking knuckles. The article is titled “Real-Time Visualization of Joint Cavitation.” For the first time, they observed that the cause is a cavity forming rapidly inside the joint, not the collapse of that cavity. In the work, the scientists present direct evidence from real-time MRI that the mechanism of joint cracking is related to cavity formation rather than cavity collapse. "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 Dr. 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 vapor 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, British Columbia chiropractor Jerome Fryer approached Dr. Kawchuk about a new knuckle-cracking theory. They decided to skip the theories and, with University of Alberta colleagues Jacob Jaremko, Hongbo Zeng, Richard Thompson, and Australian Lindsay Rowe, decided to actually look inside the joint.
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