Knuckle cracking may be annoying, but researchers did not understand how the sharp cracking sound was produced by the practice until now. This new study shows what most medical professionals thought about the practice is wrong.

Magnetic resonance imaging (MRI) scans were used to observe bones and other structures in the hand as a volunteer's knuckles were cracked. Researchers found a cavity, filled with gas, is formed as stress is applied to knuckles. This deposit is formed within the synovial fluid, which serves to cushion joints.

"It's 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's associated with the sound," Gregory Kawchuk from University of Alberta said.

The study involved the participant placing individual fingers inside a tube, connected to a cable. As the wire was pulled, the knuckles cracked, while the MRI recorded actions occurring within the digit. Strange white flashes were also seen in the MRI images just prior to the familiar crack. This was the first time ever that this effect was seen by medical researchers. Investigators believe this energy may result from water being drawn close together in the joint just prior to the sound.

"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," Kawchuk said.

Previous research suggested that the cracking sound associated with the practice was caused by the popping of gas bubbles in finger joints. This new study contradicts that idea, revealing it is the creation of gas pockets that drives the sound. Gas was once thought to be present in multiple bubbles just before release, but this new research shows just a single crescent-shaped bubble within joints. Each crack took just 310 milliseconds (less than a third of a second) to complete.

Future research on the phenomenon could utilize MRI equipment with more advanced capabilities that will allow investigators to record what happens in a joint after cracking. This study could help determine if the practice of knuckle-cracking is healthy for hands or if such activity could be dangerous to the well-being of hands and fingers.

Many people believe that knuckle-cracking can lead to a host of health problems, including arthritis. However, most health professionals believe that occasional cracking of joints is harmless. Debate still continues over the long-term effects of the practice.

Investigation of the underlying causes behind the familiar sound of cracking knuckles was detailed in the journal PLOS One.