Last week we discussed creaky knees, this week on to cracking knuckles.
People have asked whether cracking their knuckles is bad for them, or whether it causes arthritis. It doesn't.
There have been several decent studies on the topic, and none found increased rates of joint damage or cartilage wearin people who crack their knuckles, even among habitual crackers. Given that about 20 per cent of the population regularly crack their knuckles, this is good news.
One knuckle cracker, Dr Donald Unger, used himself as a study subject. For 50 years he ran a personal experiment, cracking the knuckles of his left hand at least twice a day, but never those on his right hand. An estimated 36,000 cracks later he had no arthritis in either hand. Not scientific, but neat nonetheless and well-supported by later, more rigorous studies.
Also neat is what actually makes the sound: collapsing bubbles in the joint fluid. By pulling our finger bones apart, we're creating negative pressure between the bones. The lowered pressure causes microscopic bubbles dissolved in the joint fluid to expand into big bubbles.
A similar principle applies to uncorking a bottle of champagne. With a sudden lowering of the pressure in the bottle, dissolved gases expand, becoming visible and audible.
In the joint a similar thing happens, but the bubbles don't just pop individually and fizz like champagne in a glass. Instead the bubbles merge and form a mega-bubble in the space between the separated knuckle bones.
Eventually such an intensely low pressure is reached that joint fluid, which is normally as thick as honey, suddenly gets sucked from the edges of the joint into the low-pressure bubble space between the bones. The bubble collapses forcefully in a miniature implosion, creating the typical sound of a cracked knuckle. For the next 15 minutes or so, the knuckle is not "re-crackable". It takes that long for gases to dissolve back into the joint fluid, ready for the next go.
