Science and Tech


Knuckle-Pop MRI Helps Explain That Awful Sound

University of Alberta researchers used MRI video to capture what happens when a person pops a knuckle and where that popping sound comes from.
Posted at 6:10 PM, Apr 15, 2015

Call it habit or arthritis-inducing or just plain annoying. Chances are you've heard or seen someone popping or cracking their knuckles. (Video via University of Alberta

But now, scientists are giving us a better look at a popping joint (than many of us probably would've asked for) and possibly settling the scientific debate on how that noise actually happens.

In a study nicknamed the "pull my finger study," a team of researchers led by the University of Alberta put a subject's finger in tube that slowly yanked on the digit. The researchers used magnetic resonance imaging — or MRI — video to capture the formation of a cavity inside the synovial fluid, which is located between joints. 

Greg Kawchuk, the lead author of the study, explains what we see in the video, saying: "It's a little bit like forming a vacuum. 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 images from the "pull my finger study" go against the so-called bubble collapse theory of knuckle popping, in which bubbles were believed to form in the synovial fluid. When a person pops their knuckles, those bubbles were believed to collapse, causing the sound. (Video via Vox)

As for how unhealthy or healthy cracking a joint might be, other research outside of this University of Alberta study has found the force of a pop or crack can do damage, but there's no long-term link between popping and, say, arthritis.

So, naturally, that's one thing the University of Alberta researchers hope to explore next. The study can be found in the journal PLOS ONE

This video includes images from Jaysin Trevino / CC BY 2.0 and Greg Kawchuk et al. / CC BY 4.0.