Medicine: Why Knuckles Crack

As schoolboys and a good many annoyed mothers and teachers are aware, knuckle joints can be made to crack. What no one has fully understood is why. Some have speculated that the noise is caused by the snapping of bone against bone, or by the movement of tendons over bony projections within the joint. A trio of British researchers has now solved this minor medical mystery. According to Anthony Unsworth, Duncan Dowson and Verna Wright of the University of Leeds, knuckle noise results from the explosion of gas bubbles in the synovial fluid that fills the joint.

The three base their finding...

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