GOOD NEWS. Scientists think they know what makes knuckles crack. I know, right? How did we not know that already? It turns out the subject has actually been a topic of debate for more than half a century. In a study published Wednesday in PLOS, researchers report that they finally just threw those ivory ticklers into an MRI to see what was going on. They still have to confirm their findings, but they believe the sound that we hear when cracking knuckle joints is caused by the formation of a bubble in joint fluid.

Maybe you've heard that one before — it was proposed as an explanation for the popping back in 1947. But in 1971, another group of researchers claimed it was actually the collapse of that bubble that made the noise, not the formation. In the new study, University of Alberta researchers claim that modern imaging technology actually supported the older theory.

"There have always been these warring camps: The formation of the bubble makes the sound, or the collapse of the bubble makes the sound," lead author Greg Kawchuk said. The study came about when chiropractor Jerome Fryer approached Kawchuk about the prevalent theories and the men realized that no one had ever examined the phenomenon closely in an MRI.

In addition to serving as a co-author for the research, Fryer also served as its sole subject: He's apparently great at cracking on command.

To control the cracks and catch them with the MRI, the researchers connected Fryer's fingers to tubes that slowly pulled just enough to induce a joint pop.

With just one subject (even though he offered up all 10 fingers) and no actual sound recording in the MRI, the study is far from a conclusive end to the debate. But in every single test crack, the formation of a cavity was seen at the exact moment the sound was heard. Furthermore, Kawchuk and his colleagues didn't observe any collapse of the cavity at all — at least not in the immediate aftermath of the knuckle cracking — so they don't see how it could cause the noise.

"We still have to go the extra step to prove it," Kawchuck said, explaining that they plan on repeating the experiment with specialized sound equipment, "but we saw the cavity form when the sound was made, and then saw it lingering for a long time after."

Why should we care? Kawchuck points out that not everyone can crack their knuckles, and he's curious about what that might say about joint health. Studies have suggested that cracking knuckles doesn't cause early arthritis (no matter what your parents told you), so he wonders why some joints can do it and others can't.

"Is it something some people just have to learn, like whistling, or are their joints without the ability to crack? We'd like to use the method we explored in this experiment to learn more about knuckle cracking and how it relates to overall joint health," Kawchuck said.

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