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Why Knuckles Crack

Adapted from National Public Radio.

Scientists think they may have solved an old question about the cracking of knuckles: Why does it do that?

The crack apparently comes from a bubble forming in the fluid within the joint when the bones separate. It’s like a tiny air bag inflating. This theory about knuckle-cracking was first proposed in 1947 but challenged in the 1970s.

One guy, who is really good at cracking his knuckles volunteered to put his hand inside a special MRI scanner, and made a movie of the inside of his knuckles as they pulled on the end of each finger to make it crack. What they saw was clear: The cracking sound comes when a bubble forms between the bones of the knuckle joint — not when it collapses.

The discovery challenges the common misconception that knuckle-cracking causes arthritis.

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