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 on observations and X-ray photographs of 17 patients who volunteered to have their finger joints stretched on a specially designed machine. The tests showed that stretching increases the space between the finger bones, thus reducing pressure on the clear, viscous synovial fluid that lubricates the joints. This causes tiny gas bubbles to form within the fluid. As the pressure continues to decrease, these bubbles burst and release their energy as noise. The gas does not escape. Instead, as the joint returns to normal position, the gas is reabsorbed into the synovial fluid over a period of 15 minutes. This explains why most knuckle crackers must wait a while for the satisfaction of performing an encore.

This file is automatically generated by a robot program, so reader's discretion is required.