Does the sound of cracking knuckles make you flinch? What about a film of the joints in action? MRI scans have been used to capture the moment your joints go pop.
Gregory Kawchuk at the University of Alberta, Canada, and colleagues used a cable to slowly pull a man's fingers in an MRI scanner until the joints cracked. The sound was thought to come from the collapse of an air bubble, but in the scans the air cavity that formed in the fluid around the separating joints persisted after the noise.
A mysterious flash also appeared just before the crack. Kawchuk thinks it may be caused by cartilage releasing fluid as the tension on the joints rises.
"The scans don't allow us to explore the mechanism of sound production but like all sound, it has to come from a production of energy that causes vibrations to travel through the air," says Kawchuk. Higher resolution MRI scans might help solve the mystery, he says.
Knuckle cracking is thought to be harmless but the team want to image the parts of the joint not captured by their existing scans. This should enable them to find out where the air bubble ends up and investigate how the rest of the joint is affected by the cracking.
Journal reference: PLoS One, DOI: 10.1371/journal.pone.0119470
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