Cracking joints is manipulating one's joints to produce a distinct cracking or popping sound. It is sometimes performed by physical therapists, chiropractors, osteopaths, and masseurs in Turkish baths. The cracking of joints, especially knuckles, was long believed to lead to arthritis and other joint problems. However, medical research has not demonstrated such a connection. The cracking mechanism and the resulting sound is caused by carbon dioxidecavitation bubbles suddenly partially collapsing inside the joints. To be able to crack the same knuckle again requires waiting about 15 minutes for the bubbles to reform.
Causes
For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was uncertain. Suggested causes included:
There were several hypotheses to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it. When a spinal manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low-pressure environment, some of the gases that are dissolved in the synovial fluid leave the solution, making a bubble, or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of the resultant gas bubble are thought to be mainly carbon dioxide, oxygen and nitrogen. The effects of this process will remain for a period of time known as the "refractory period," during which the joint cannot be "re-cracked," which lasts about twenty minutes, while the gases are slowly reabsorbed into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate. In 2015, research showed that bubbles remained in the fluid after cracking, suggesting that the cracking sound was produced when the bubble within the joint was formed, not when it collapsed. In 2018, a team in France created a mathematical simulation of what happens in a joint just before it cracks. The team concluded that the sound is caused by bubbles' collapse, and bubbles observed in the fluid are the result of a partial collapse. Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion. The snapping of tendons or scar tissue over a prominence can also generate a loud snapping or popping sound.
Effects
The common claim that cracking one's knuckles causes arthritis is not supported by evidence. A study published in 2011 examined the hand radiographs of 215 people and compared the joints of those who regularly cracked their knuckles to those who did not. The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles. A 1990 study also concluded that there was no increased preponderance of arthritis of the hand of chronic knuckle-crackers but that habitual knuckle-crackers were more likely to have hand swelling and lowered grip strength. It claimed further that habitual knuckle-cracking was associated with manual labour, biting of the nails, smoking, and drinking alcohol and suggested it resulted in functional hand impairment. This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it.
