The world is divided into two very distinct types of person: those who love cracking their knuckles, and those that truly, utterly despise it with every fiber of their being. So what is actually happening to your fingers when you pull them back a little too far? A team of researchers decided to attempt to answer this question once and for all, using ultrasound techniques to image fingers during a session of thorough knuckle-cracking. At a meeting of the Radiological Society of North America in Chicago, they announced that it is probably the formation of bubbles within the joints that produces the noise.
Scientists know that the sound isn’t caused by bones or muscles snapping against each other – unless you’ve been accidentally a bit too aggressive – but it is something to do with the lubricating fluid between many of your joints. This “synovial” fluid stops our bones and cartilage grating too much on each other, and allows us a greater degree of flexibility.
When we stretch our fingers, we release gas into the fluid, which forms small bubbles – this process is known as tribonucleation. These bubbles form rapidly, and they burst almost as quickly, either when they expand to a certain size within the joint and pop or when the joint collapses back in on them, violently compressing them. There has been some debate as to what part of the process causes the noise, however: is it the sudden emergence of the bubbles, or their quick collapse?
A magnetic resonance imaging (MRI) study last April looked extremely carefully at the formation of these bubbles within the metacarpophalangeal joint (MPJ), the base of the finger. The noise seemed to come from the initial formation of the bubbles, and the mystery was declared solved.
However, ultrasound imaging can see processes happening within the body up to 100 times faster and detect things up to 10 times smaller than MRI scanners. A new team of scientists, led by radiologist Robert D. Boutin from the University of California, Davis, decided to conduct the same study using this technology instead.
Participants were asked to crack each knuckle while being observed through an ultrasound machine. Thirty of the participants were regular knuckle crackers, and 10 were not. After imaging over 400 MPJ cracks, the researchers announced that they saw something far more spectacular than they were expecting: A bright “explosion” accompanied the bubble formation and collapse.
“What we saw was a bright flash on ultrasound, like a firework exploding in the joint,” Boutin told The Washington Post. “It was quite an unexpected finding.” These flashes occurred so frequently that the radiologists were able to ascertain which videos, when muted, showed a cracking joint with up to 94 percent accuracy.
Although this study definitively shows that it is indeed the action of bubbles that cause the cracking noise, the researchers still cannot be certain whether it is the formation or the collapse that ultimate causes the noise.
“I will tell you that we consistently saw the bright flash in the joint only after we heard the audible crack. Never the other way around,” Boutin continued. “Perhaps that supports the bubble formation theory, not the bubble popping theory.”
An additional finding of the results is that the joints that are cracked do become significantly more flexible – albeit only on a temporary basis. As a previous, famous 60-year-long study also showed, this study found no negative health effects associated with habitual cracking. So perhaps it doesn’t hurt to be this sort of crack-addict.