The next time you crack your knuckle, know that you’re actually creating little gas-filled bubbles in the fluid that lubricates your knuckle’s joints. The cavities appear because the bones at the joints separate rapidly, creating a low-pressure volume that’s filled by gas ‘pumped’ out of the higher-pressure fluid.
This is what Greg Kawchuk, a professor at the University of Alberta, Canada, and his colleagues discovered, by observing a participant crack knuckles under the gaze of an MRI scanner. Their findings are reported in a paper in PLOS ONE, published April 15. He attributed the noise specifically to the sudden formation of the cavity in the synovial fluid, “a little bit like forming a vacuum”.
For its apparent simplicity, the study actually seeks to lay to rest the question of what causes the sound when knuckles are cracked. Since the early 1900s, multiple explanations have been advanced. All of them agreed that a cavity was involved in the cracking, but the primary contention was if the cracking sound was caused by a cavity forming or a cavity collapsing.
Kawchuk’s use of the MRI rules in favor of cavity-formation. In fact, it also shows that the cavity persists well after the cracking is done, or as the paper puts it, “past the point of sound production”. So the cracking couldn’t have arisen from a collapsing cavity. Here’s a film – the first one of knuckle-cracking – showing what the MRI revealed.
By way of an application in diagnosis, the PLOS ONE paper also notes,
… cine MRI revealed a new phenomenon preceding joint cracking; a transient bright signal in the intra-articular space. While not likely visualized gas given the imaging parameters employed, we do not have direct evidence to explain this observation. We speculate this phenomenon may be related to changes in fluid organization between cartilaginous joint surfaces and specifically may result from evacuation of fluid out of the joint cartilage with increasing tension. If so, this sign may be indicative of cartilage health and therefore provide a non-invasive means of characterizing joint status.
The bit about “non-invasive means” is enticing, although these are still early days and Kawchuk & co.’s words are purely speculative. Another diagnostic avenue pursued in the past has sought to understand the link between knuckle-cracking and osteoarthritis. On this, the last-word remains elusive.
A 1989 study had found that the energy released during knuckle-cracking was more than enough to damage cartilage, while a 2011 study found that the habit didn’t actually affect the risk of acquiring osteoarthritis.