A new answer to the knuckle cracking puzzle

The pop of a cracking knuckle is not from a bubble popping but from one being born. For decades scientists argued over the source. Some said it was a bubble collapsing inside the joint. Others said it came from the bones separating. Gregory Kawchuk from the University of Alberta settled the debate with real-time MRI. He pulled a volunteer's finger inside a tube while a scanner filmed the joint. The joint space widened from 0.93 to 1.89 millimeters too fast for fluid to fill it. That pressure drop opened a gas cavity in the synovial fluid. The crack came from the cavity's birth not its collapse. The whole event took under 310 milliseconds. This process called tribonucleation resolved a dispute from 1947 and 1971. The work appeared in PLOS ONE in April 2015.

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