There's some disagreement among physiologists about knuckle-cracking and how it produces its characteristic popping sound. Now, Canadian scientists have used MRI scans to watch what happens inside a cracking finger joint in real time – and their observations may have settled this longstanding debate for good.

Above: Images of the hand in the resting phase before cracking (left). The same hand following cracking with the addition of a post-cracking distraction force (right). Note the dark, interarticular void (yellow arrow). Caption and image credit: Kawchuk et al/PLOS

For decades, researchers have assumed that the sound of a popping knuckle comes from either the formation or collapse of a gas cavity inside a joint. What both camps have lacked is hard observational evidence.

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Scientists led by Gregory Kawchuk from the University of Alberta decided to take a crack at this problem by using an MRI to examine knuckle-popping in action. The team recruited a volunteer (yes, just one) who inserted his fingers, one at a time, into a tube connected to a cable that slowly pulled on the digit until the knuckle joint cracked.

Looks like fun. (Kawchuk et al/Plos)

By this method, the researchers were able to capture cracks — which happen in less than 310 milliseconds – in real-time.

Analysis of the video showed that, when our articular surfaces are separated, a cavity quickly forms inside our finger joints. This formation, the researchers speculate, is what's causing the popping sound. Each time it happened, the cracking and joint separation could be linked to the rapid creation of a gas-filled cavity within the synovial fluid, a slippery substance that lubricates the joints.

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The researchers likened it to a vacuum. As noted by Kawchuk in a statement: "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."

Fascinatingly, the researchers also observed a strange white flash appear just before the popping happens. They think it has something to do with water suddenly being drawn together just before the joint cracks, but further research is required. Consequently, they're not quite ready to say if knuckle-cracking is helpful or harmful.

Read the entire study at PLOS One: “Real-Time Visualization of Joint Cavitation”.