To some, it may be an extremely grisly bad habit, but many others enjoy the addiction of cracking their joints from time to time. It is extremely therapeutic to hear and feel the pop and relief of pressure, but where does the sound come from and is it something to worry about? To find out, researchers put a hand under MRI and asked the person to crack his knuckles.
“Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. Unfortunately, bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon. Here we present direct evidence from real-time magnetic resonance imaging that the mechanism of joint cracking is related to cavity formation rather than bubble collapse.”
“Our results offer direct experimental evidence that joint cracking is associated with cavity inception rather than collapse of a pre-existing bubble. These observations are consistent with tribonucleation, a known process where opposing surfaces resist separation until a critical point where they then separate rapidly creating sustained gas cavities. Observed previously in vitro, this is the first in-vivomacroscopic demonstration of tribonucleation and as such, provides a new theoretical framework to investigate health outcomes associated with joint cracking.”
So basically, the crack sound comes from an air cushion that rapidly forms between the joints that are momentarily being separated, so that the joints can snap back together again without harm. Whether or not joint cracking is bad for your bones still remain to be seen.
Watch the MRI video below of the knuckle cracking action.