Solving the Mystery of Knuckle Cracking

The prevalence and availability of multi-million dollar MRI machines has done amazing things for healthcare in developed countries, allowing physicians to see inside the human body in real-time, no invasive surgery necessary. So perhaps it’s no surprise that even the most mundane of human activities has just been magnetically imaged in the name of science: knuckle cracking.

Gregory N. Kawchuk, of the Department of Physical Therapy at the University of Alberta, Edmonton, and his colleagues aren’t the first to publish research on our often noisy joints. As they report in their new paper in PLOS ONE, the first scientific study came in 1947, attributing the mysterious knuckle noises to the space in between joints expanding rapidly, creating a crack. In 1971, the theory flipped, when a different group used the same methods to draw an opposite conclusion—that it was the collapse of an air bubble created by the rapid expansion that caused the noise.

That’s probably the explanation that you heard growing up, as it’s the idea that’s stood for forty-plus years since. (You may have also heard of the dangers of cracking knuckles, but one extremely diligent scientist did all he could to prove it’s not the case.) “Unfortunately, no direct evidence exists to resolve these differing perspectives regarding the mechanism of joint cracking,” write Kawchuk and his co-authors, leading them to investigate with technology unavailable to the previous researchers.

The real-time nature of MRIs produced a decidedly not high-definition video of 3.2 frames per second, enough for Kawchuk to capture the exact moment of knuckle crack. They even created a mechanical knuckle-cracker, allowing the willing human subject to remain motionless while his pointer finger was pulled.

This time, the evidence was clear: “tribonucleation is the process which governs joint cracking.” It’s the rapid separation creating air bubbles (that’s the tribonucleation part), not their subsequent collapse, that enables our nervous habits to continue. Who knew MRIs could be so useful? You have to wonder what such technology can tell us about nail biting, hair twirling, and chewing on your pencil!

Images: PLoS ONE, doi:10.1371/journal.pone.0119470