• Manuela Callari

Why it is not a failure to leave academia...

After many hypotheses and various theories, today a mathematical model may finally be able to explain the sounds produced by knuckle cracking.

MRI scans from the 2015 study by Kawchuk and co-workers

Some love it. Some loathe it. Some think that it helps lubricate your finger joints. Some will tell you that it causes arthritis. Wherever you stand, have you ever asked yourself what causes the sound of knuckle cracking? It is a debate that has attracted the curiosity of scientists for over 100 years. After many hypotheses and various theories, today a mathematical model may finally be able to explain the sounds produced by knuckle cracking.

Bones don’t have hinges, they are held together by ligaments. Inside of each joint, where two bones meet, there is a lube called synovial fluid which keeps the two bones from grinding on each other. The synovial fluid is held tight by the ligaments and there is gas dissolved into it. You can imagine it like a fizzy drink trapped in a sealed bottle. When the bones are pulled apart, there is a sudden pressure drop in the joint. Just like when you open the lid of your fizzy drink, lower pressure in the joint allows the gas to aggregate and form tiny bubbles which will

consequently pop.

In early studies, researchers hypothesized that the cracking sound was caused by vibrations in the tissues during the rapid motion of pulling the fingers apart. Other attributed the sound to a sudden tightening of the ligaments around the joint. However, it was in 1971 that Unsworth and co-workers gave the most convincing theory. Through extensive experiments they concluded that the sounds are caused by the popping of the gas bubbles formed in the synovial fluid during knuckle cracking.

This theory was widely accepted in the science world for more than 40 years until Kawchuk and his collaborators used Magnetic Resonance Imaging (MRI) to observe what was happening to a volunteer’s finger during knuckle cracking. Contrary to the previous theory, they hypothesized that sounds are the result of the formation of the bubbles rather than their collapse. In fact, the MRI scans showed the presence of gas bubbles in the synovial fluid even after the cracking.

Today, Abdul Barakat, professor at the École Polytechnique in France and Vineeth Suja, researcher at Stanford University in California, have presented a mathematical model that may explain the sounds produced by knuckle cracking once and for all.

The two researchers have simulated the knuckle cracking through a series of mathematical equations and a geometrical representation of the joint. Their findings agree with the Unsworth theory, confirming that the sound is generated when the tiny bubble formed in the synovial fluid collapse. Comparing the mathematical prediction with recordings of actual knuckle cracking from three volunteers, they found that the sound predicted by their mathematical model well matches the volume and the frequency of the recordings.

Moreover, Baraket and Suja’s model shows that the sound is generate even if only partial collapse of the bubble occurs. This may explain why Kawchuk observed the presence of residual bubbles in the MRI scans and finally resolve the contradictions between the older theories.