In a study published Wednesday by the scientific journal PLOS ONE, the researchers used MRI video to determine what triggers the joints in the finger to cause the distinct sound. They observed that the cracking sound is caused by the rapid formation of a gas-filled cavity within the synovial fluid, a substance that lubricates the space between joints.
“We call it the ‘pull my finger study’— and actually pulled on someone’s finger and filmed what happens in the MRI,”explained by lead author Greg Kawchuk, a professor of rehabilitation medicine at the University of Alberta, in a press release. “When you do that, you can actually see very clearly what is happening inside the joints.”
One of the study’s co-authors, chiropractor Jerome Fryer, volunteered his knuckles as he initially approached Kawchuk with the hypothesis. His fingers were inserted individually into a flexible tube linked to a cable that was pulled on until his joint cracked — which occurred in less than 310 milliseconds.
The researchers watched what happened as the joints separated, and found that the force pulling the bones apart overwhelms the ability for them to stick together. A momentary drop in pressure allowed gas in the synovial fluid to rapidly fill the cavity between bones when the stress was applied. This results in the formation of bubbles, and that infamous “pop.”
“It’s a little bit like forming a vacuum,” said Kawchuk. “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 researchers also detected a white flash that appears just before the popping sound, which the researchers suspect occurs from water rushing together before the joint cracks. Kawchuk hopes to analyze this further stating that, “No one has observed it before.”
While scientists have determined that habitual knuckle cracking does not appear to cause long-term harm, Kawchuk believes these new findings have paved the way for new research into the benefit or harm of joint cracking.
“It may be that we can use this new discovery to see when joint problems begin long before symptoms start, which would give patients and clinicians the possibility of addressing joint problems before they begin,” he said.
