A cavity forming rapidly inside our finger joints may cause the popping sound heard when cracking knuckles, according to a real-time, MRI based study published in the open-access journal PLOS ONE by Gregory Kawchuk from University of Alberta, Canada and colleagues.

Scientists have debated the cause of joint cracking for decades and to get to the root of what happens, the authors of this study used MRI video to observe for the first time what happens inside a joint when it cracks. The authors visualized ten finger joints from one participant by inserting them one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked. MRI video captured each crack in real time--occurring in less than 310 milliseconds.

In every instance, the cracking and joint separation were associated with rapid creation of a gas-filled cavity within the synovial fluid, a slippery substance that lubricates the joints. "It's a little bit like forming a vacuum," Kawchuk said. "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 team also observed the presence of a white flash that appears just before cracking. "No one has observed it before," says Kawchuk, an occurrence he believes is water suddenly being drawn together just before the joint cracks. Kawchuk said he'd like to use even more advanced MRI technology to understand what happens in the joint after the pop, and what it all could mean for health. The authors suggest the findings may pave the way for new research into the potential therapeutic benefits or harms of joint cracking.