Cracking Knuckles

A cracking finger knuckle at right. A resting knuckle at left. 

In fifth grade I was in line to go to gym class, and someone in line cracked his knuckles which I thought was strange. Naturally I had to crack my knuckles too. It took me years to quit.

It is annoying when other people crack knuckles, but it was satisfying when I did it myself.

Cavitation causes the knuckle cracking sound. Cavitation is a fluid dynamics term that means bubbles form in a fluid. Research is pretty clear that the bubble formation and popping is the cause of the sound. It does not sound like bubbles breaking -- it seems like it should have to do with the bones or the tendons -- but it doesn't.

The joint fluid is approximately at atmospheric pressure of course, but pulling the joint causes instantaneous low pressure and bubbles form. Measurements show resting joint fluid is at about 5% lower pressure than atmospheric pressure, that is -4 torr (500 pascal) depending on the joint.  This is a little surprising, but if the pressure were higher than atmospheric it would try to leak out. Clearly the joint fluid is well-contained since it stays there.

Physiologists say the joint fluid is "viscous", but measurements show the viscosity is only 100-300 centipoise (0.1-0.3 Pa-s), which is like pancake syrup,  way thinner than road-tar or Vaseline. Normal fluid is a saline solution with about 3% protein. It can have a little 0.04% glucose, and should not have  blood or lactic acid, which are signs of disease.

For a second or so, the joint is 15%  to 25% larger than before, and this means the joint fluid is foamy. The bubbles are almost all carbon dioxide, which means it comes from respiration of the adjacent cells. Once the cracking occurs the joint becomes more extensible, and the joint stretches. Usually it stretches to the limits of the surrounding muscle, and this can cause muscle damage -- and muscle damage lowers strength. People, who crack their hands, have weaker hands.  The joint cannot crack again until the gas is reabsorbed in to the joint fluid.

I was leaving work Friday, and my jaw joint made a cracking noise -- something that never happens to me. I thought, this joint cracking is a really odd phenomena, and knowing how it works does not make it  less odd.