What really happens when you crack your knuckles

Nicky Phillips, April 16, 2015

VIDEO: http://www.smh.com.au/technology/sci-tech/what-really-happens-when-you-crack-your-knuckles-20150416-1mlqk9.html

In what might be one of the most obscure scientific discoveries of 2015, researchers have identified once and for all what happens when you crack your knuckles.

In what might be one of the most obscure scientific discoveries of 2015, researchers have identified once and for all what happens when you crack your knuckles.

It had been assumed for almost 50 years that joint cracking – which gives great relief to some yet makes others shiver with disgust – occurred when a change of pressure inside the joint caused gas bubbles to pop as the knuckle was bent or pulled.

Now a team of international researchers, including an Australian, who used magnetic resonance imaging (MRI) to watch this event in real time, have overturned that view.

Researchers have used MRI to see what happens when a knuckle cracks.

They found that rather than gas bubbles collapsing, when a man's fingers were gently pulled to force a joint crack the images show a "cavity" or small pocket of gas forming and remaining inside the joint.

"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 the lead researcher Gregory Kawchuk, of the University of Alberta.

"It's a little bit like forming a vacuum," said Dr Kawchuk, a chiropractor and biomedical engineer. The whole thing took less than 310 milliseconds.

"Following these events, the resulting cavity was never seen to collapse," said the researchers, which included University of Newcastle radiographer Lindsay Rowe.

Their findings are published in the peer-reviewed science journal PLoS One.

The group said whether cracking knuckle damaged joints was a contentious issue, with previous research suggesting "habitual knuckle cracking" did not increase joint degeneration. They hope to do further work to confirm this. The imaging costs of the present study were funded by the Canadian Chiropractic Research Foundation, however the funders had no role in the study design or analysis.

"By defining the process underlying joint cracking, its therapeutic benefits, or possible harms, may be better understood," they said.

Researcher and physiotherapist Suzanne Snodgrass, who was not involved in the research, said for years it had been suggested that the joint-cracking sound was caused by an increase in space around the joint as it was pulled or bent, which subsequently decreased the pressure around the knuckle.

Because of the sudden change in pressure, dissolved gases inside the synovial fluid rush into the joint creating a gas bubble that collapses to make a "crack", she said.

"This is what we teach students before this paper," said Dr Snodgrass, an associate professor at the University of Newcastle.

"We might change that now," she said.