FYFD

Cracking one’s knuckles produces an unmistakable popping noise that satisfies some and disconcerts others. The question of what exactly causes the popping noise has persisted for more than fifty years. It’s generally agreed that separating the two sides of a joint causes low enough pressures to form a cavitation bubble in the sinovial fluid of the joint. But researchers have been divided on whether it’s the formation or the collapse of this bubble that’s responsible for the sound. Studying the phenomenon firsthand is difficult with today’s imaging technologies -- none of them are fast enough to capture a behavior that takes only 300 milliseconds. As a result, scientists are turning to mathematical modeling and numerical simulation. 

A recent study tackled the problem by modeling a joint that already contains a bubble and examining the bubble’s response to changes in pressure inside the joint. The pressure changes alter the bubble’s size and cause it to generate sound. When compared to experiments of people cracking their knuckles, the simulated sounds are remarkably similar in both amplitude and frequency. It’s not even necessary for the bubble to collapse completely to make the noise. Just a partial collapse is enough to sound just like that old, familiar pop. (Image credit: G. Kawchuk et al.; research credit: V. Chandran Suja and A. Barakat; via Gizmodo)

Joints like our knuckles are lubricated with liquid called the synovial fluid. When manipulated, these joints can pop or crack audibly. For half a century, researchers have thought the cracking sound joints under tension make was the result of bubbles in the synovial fluid collapsing. But a new cine magnetic resonance imaging (MRI) study shows that the sound is generated during bubble inception and that the cavity persists after the sound. When the bones of the joint are pulled, viscous forces resist their separation. With enough force, the joints separate suddenly, causing a pressure drop in the synovial fluid that forms a vapor-filled cavity in the joint. According to the real-time MRI observations, this is when the sound is generated. The cavity does eventually dissipate, they found, but only well after the pop. The whole joint-cracking process is consistent with the tribonucleation mechanism seen in machinery.  (Image credit: G. Kawchuk et al.; GIF via skunkbear, source video)