The Knuckle-Cracking Debate Gets Twisty
There’s a new twist in the debate about why knuckles crack.
Yes, it turns out that “why knuckles crack” is actually a debate — despite the existence of the phenomenon for approximately all of human history, researchers still don’t know why it happens. And that’s not for a lack of trying to understand: According to a new paper in the journal Scientific Reports, researchers have been looking into the question since the early 1900s.
The new paper posits that knuckle cracking happens because of the collapse of tiny air bubbles in the fluid that surrounds the joints. But a 2015 study on the subject essentially concluded the opposite, saying that the cracking sound occurs when these bubbles form, not when they pop.
Snap crackle pop
Over the years, scientists have theorized that the distinctive sound of a cracking knuckle could come from everything from vibrations in the tissue to the tightening of joint capsules to the collapse or formation of bubbles. This bubble formation is known as cavitation, and it occurs when the liquid synovial fluid that lubricates joints gets pulled apart, creating a sudden decrease in pressure and a resulting gas-filled space. In the case of joints, the gas is about 80 percent carbon dioxide, said Vineeth Chandran Suja, a doctoral candidate in chemical engineering at Stanford University who studies bubble dynamics.