Instead, Gregory Kawchuk from the University of Alberta, Canada, and his team found that the cracking and joint separation were associated with rapid creation of a gas-filled cavity within the synovial fluid.
"It's a little bit like forming a vacuum," Kawchuk said in a press release. "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 cavity doesn't look like multiple bubbles at all, but rather more like a single crescent moon shape next to the joint.
The researchers could see this via MRI video that captured each knuckle crack -- lasting just 310 milliseconds -- in real time.
To capture the video, Kawchuk and his team had participants insert a finger one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked. The participants were completely unharmed.
The researchers surprisingly noticed that a white flash appears just before the knuckle cracks.
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"No one has observed it before," said Kawchuk.
He and his colleagues suspect that the flash is caused by water suddenly being drawn together just before the joint cracks.
Occasional joint cracking is harmless and even happens on its own, but the jury is still out on whether or not frequent knuckle-cracking causes health problems.
Kawchuck next plans to use even more advanced MRI tech to uncover what happens to the joint after it pops, and how this could affect the individual over time.
Photo: Man cracking the knuckles on his right hand. Credit: Jaysin Trevino, Wikimedia Commons