Source: Harvard Health

Victor Liaw

Many people crack their knuckles to release tension or feel like they’re giving their fingers a stretch. The cracking results in a loud pop or snapping sound, which can be unsettling to other people. Although the science behind knuckle cracking is not definitive, the most popular theory involves a process known as bubble collapse. Bending and distracting the joints creates a space and a relative vacuum, which causes the formation of gas bubbles in the joint fluid which then pop, resulting in the cracking sound. A second and more recent theory suggests that the sound is not produced by the popping bubbles but rather by the formation of the cavity itself. There have been studies that support both theories, so it’s difficult to say which one is correct.

For some time, the practice was thought to increase the risk of arthritis. However, multiple scientific studies have shown that knuckle cracking does not increase the chance of osteoarthritis. Most of these experiments included large numbers of patients, but perhaps the most peculiar experiment was done by Dr. Donald Unger. For 50 years, he would crack the knuckles on his left hand at least twice a day while doing nothing for his right hand, and he did not develop arthritis at all, with “no apparent differences between the two hands.”  

https://www.medicalnewstoday.com/articles/259603.php

http://www.jabfm.org/content/24/2/169

http://www.scientificamerican.com/article/crack-research/