Australasian Science: Australia's authority on science since 1938

What Makes a Knuckle Crack?

An international research group has identified what happens when you crack your knuckles.

It had been assumed for almost 50 years that joint cracking occurs when a change of pressure inside the joint causes gas bubbles to pop as the knuckle is bent or pulled. Now researchers have overturned that view after using magnetic resonance imaging to watch the process in real time.

Rather than gas bubbles collapsing, the images show a “cavity” or small pocket of gas forming and remaining inside the joint when a finger is gently pulled to force a joint to crack. The entire process takes less than 310 ms.

“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,” said lead researcher Gregory Kawchuk, of the University of Alberta. “It’s a little bit like forming a vacuum.”

“Following these events, the resulting cavity was never seen to collapse,” said the researchers, which included radiographer Lindsay Rowe of the University of Newcastle. The findings are published in PLOS ONE.

It’s not known whether knuckle-cracking damaged joints, with previous research suggesting that habitual knuckle-cracking does not increase joint degeneration. The researchers hope to do further work to confirm this.