Researchers crack the secret to what makes our knuckles crack

16 Apr 2015

A man cracking his knuckles

After holding our breath for decades, the world can now rest easy knowing that the reason why our knuckles crack has been discovered, and it has nothing to do with air bubbles forming.

As part of a test by the University of Alberta Faculty of Rehabilitation Medicine called the Pull my Finger study, a test subject’s fingers were placed under an MRI machine scanner that looked at what changes when a joint is cracked by someone.

Aside from being unnerving for some to hear, the previous theories that have been around since 1947 that suggested it was the result of air bubble formation in the synovial fluid that lubricates our joints were almost correct and it is, in fact, the result of a vacuum cavity, according to CNet.

Suggesting the study, and participating in it, chiropractor Jerome Fryer is known as the ‘knuckle cracker’ for his particular ability and, as part of the test, placed each of his fingers in tubes that would tighten and pull each finger until it cracked, which was then picked up by the MRI machine.

Posting their findings in a research paper, the team, led by Prof Greg Kawchuk, discovered with absolute certainty that 310 milliseconds after the joint was pulled, a vacuum cavity would form, creating the distinctive pop.

“Fryer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team,” said Prof Kawchuck of Fryer’s participation in the study.


The MRI scans of Fryer cracking his knuckles before (left) and after (right). Image via PLOS One/Kawchuk et al

Important discoveries for joint health

However, the experiment wasn’t undertaken just for the fun of it, as the team was also hoping to better understand joint health, which, as is well-known, decreases considerably as human beings get older.

Aside from discovering the vacuum cavity, the team also discovered for the first time that right before the joint cracked a flash of white would appear on the MRI, which, they believe, indicates that water is suddenly drawn to the joint prior to the pop.

“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,” Prof Kawchuck said.

On the topic of joint health, the study also establishes that the age-old story that knuckle cracking has a seriously detrimental effect on your joints is a myth, no doubt to the displeasure of people who hate the sound.

“The data fails to support evidence that knuckle cracking leads to degenerative changes in the metacarpal phalangeal joints in old age,” the study concludes. “The chief morbid consequence of knuckle cracking would appear to be its annoying effect on the observer.”

And just in case you actually wanted to hear Fryer crack his knuckles…

Man cracking his knuckles image via Jaysin Trevino/Flickr

Colm Gorey was a senior journalist with Silicon Republic