Any doubt over what causes knuckles to crack appears to have been dispelled by scientists who used an MRI scanner to visualise the process.
Their scans showed that when the bones of the joint are pulled apart, the surrounding connective tissue capsule is stretched. As the volume of the capsule is stretched, its internal pressure drops allowing gases dissolved in the synovial fluid to come out of solution and form a space, or ‘bubble’, through a process known as cavitation. It is this process that accounts for the cracking noise, according to recent research published in PLOS ONE, and not, as previously thought, the subsequent collapse of the bubble.
These observations are consistent with tribonucleation, a process where opposing surfaces resist separation until a critical point where they separate rapidly, resulting in vapour cavities that do not collapse instantaneously. Observed previously in vitro, this work provides the first in-vivo demonstration of tribonucleation on a macroscopic scale and, as such, provides a new theoretical framework to investigate health outcomes associated with joint cracking.
It takes up to 30 minutes for the gas to redissolve in the synovial fluid. This is why it is not possible to crack a knuckle immediately after a previous cracking.
The good news for the between 25–54% of people that crack their knuckles is that there is little evidence for any ill effects, based on studies that have varied considerably in their approach. Winner of the 2009 Ig Nobel Prize Donald Unger, for example, cracked the knuckles on his left hand only at least twice a day for 60 years but found no difference between his hands after more than half a century of cracking. A number of larger scale studies have also found no link to osteoarthritis.
Knuckle cracking may simply be a nervous, if rather annoying, habit. One study linked the practice to manual labour, fingernail biting, smoking and drinking alcohol. It remains unclear how any link with arthritis first found credence. People who already have arthritis sometimes find their joints crack because the cartilage has been damaged, but this seems more likely to be a consequence of damage rather than a cause.
But the habit should still not be guilt-free, as the authors of a study into a possible link with osteoarthritis concluded. They said: “The chief morbid consequence of knuckle cracking would appear to be its annoying effect on the observer.”