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Knuckle cracks unraveled
December 6, 2015, 1:59 pm

Radiologists believe they have figured out why knuckles crack, and the good news is that it does no immediate harm to your hand. Ultrasound readings of people cracking their knuckles reveals a bright flash, "like a firework exploding in the joint," said lead researcher Dr. Robert Boutin, a professor of radiology at University of California.

The flash comes from a gas bubble forming in the joint, but until now researchers could not agree whether the sound was caused by the bubble popping or the bubble forming. By syncing the ultrasound with audio, radiologists think they now have the answer. They found that there was a 10 millisecond gap between sound of the crack and the visible flash on the ultrasound. "The sound is not coming from the bubble popping. It's actually the bubble forming," said Boutin.

Between 25 percent and 50 percent of people regularly crack their knuckles. To study the phenomenon, radiologists asked 40 people, aged 18 to 63, to stretch their fingers a total 400 times. Participants included 30 individuals with a history of habitual knuckle cracking and 10 who did not usually crack their knuckles.

There have been several theories over the years and a fair amount of controversy about what is happening in the joint when it cracks. Boutin clarified, "We're confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint."

The bubble is created from dissolved gas that is suspended in fluid that lubricates a person's joints.  When a person stretches their finger, the act creates negative pressure that draws out the gas. The tiny micro-bubbles of gas then suddenly coalesce into one large bubble, and the crack appears to come from the tiny bubbles crashing together.

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