Cavity Forming Behind Knuckle-Cracking Sound: Study

Finding a reason behind the centuries-old common practice of knuckle cracking and the sound it give out, researchers said it was due to quick cavity formation between the joints that gives out the sound.

Scientists have debated the cause of joint cracking for decades, dating back to 1947 when British researchers first theorised vapour bubble formation as the cause. but it was not convincing for many until now. In 1970s another team of scientists said collapsing bubbles as the cause.

“We call it the ‘pull my finger study’ and actually pulled on someone’s finger and filmed what happens in the MRI. When you do that, you can actually see very clearly what is happening inside the joints,” explained lead author Greg Kawchuk, professor in the faculty of rehabilitation medicine, the University of Alberta.

To find an answer, the team asked acclaimed chiropractic physician Jerome Fryer to volunteer for the test. Fryer practices chiropractic care for sports injuries. He was asked to join the research.

Fryer’s fingers were inserted one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked. In an MRI video, each crack in real time was captured — occurring in less than 310 milliseconds.

In every instance, the cracking and joint separation was associated with the rapid creation of a gas-filled cavity within the synovial fluid — a super-slippery substance that lubricates the joints.

“It is 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 is associated with the sound,” Kawchuk noted.

The ability to crack your knuckles could be related to joint health and researchers are planning to see its use in learning ahead the possibility of joint problems long before they do take place in human body.

The study has been published in PLOS ONE.

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