Whether brought on by nervousness or done unknowingly, knuckle cracking is a common occurrence. But why do joints make a cracking sound at all? Researchers from the University of California, Davis have helped solve this mystery once and for all, by using an ultrasound imagining technique.
Their findings were presented at the annual meeting of the Radiological Society of North America (RSNA), and explained how an ultrasound showed a bright flash when a knuckle is cracked. That led them to discover that the sound came from the formation of a gas bubble inside the joint.
In the study, Dr. Robert D. Boutin and colleagues examined 40 healthy adults, including 17 women and 23 men (age range 18-63), utilizing an ultrasound to record every attempted crack of the knuckle at the base of each finger, known in medical parlance as the metacarpophalangeal joint.
Ten of the participants stated that they do not generally engage in knuckle cracking, while 30 claimed they did so approximately 20 times per day for 40 years. In this process, all of the participants were evaluated for grip strength, range of motion, and the laxity of the joint. The cracks occurred in 62 of the 400 joints that were examined.
Dr. Boutin stated that the bright flash seen on the ultrasound was like a firework exploding in the joint, something he has never seen before. He also mentioned that over the years, there has been several theories and fair amount of controversy regarding what happens when the joint cracks.
When a knuckle is cracked either by finger stretching or bending a finger backwards the joint expands. This decreases the pressure between the joint and the ligaments that connect the bones and the joint capsule. The depressurization causes gases, such as carbon dioxide, nitrogen, and oxygen that are dissolved in the synovial fluid, to form little bubbles that quickly fill in the empty space.
As the joints settle back into place, the fluid also returns to its rightful place and pops those little bubbles which causes the recognizable cracking sound.
As to whether cracking knuckles is good for you, harmful or neither, the team said this is still unknown. Physical examinations of participants showed no signs of pain, swelling or disability, nor were any differences observed in grip strength or joint laxity between the knuckle crackers and non-crackers. However, Dr. Boutin stated that further research will be needed in the future to measure any long-term hazard or benefit.