The act of cracking joints means bending a person's joints to produce a distinct cracking or popping sound, often followed by a feeling of satisfaction or relaxation to the person. It is sometimes done as part of a joint adjustment/mobilization routinely performed by a chiropractor, osteopath or physical therapist.
According to traditional belief, the popping of joints, especially knuckles, can lead to arthritis and other joint problems. However, medical research has yet to conclusively demonstrate a connection between knuckle cracking and long-term joint problems. The cracking mechanism and the resulting sound is caused by carbon dioxide cavitation bubbles suddenly partially collapsing inside the joints.
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:
- Formation of bubbles of joint air as the joint is expanded.
- Cavitation within the joint—small cavities of partial vacuum form in the synovial fluid and then rapidly collapse, producing a sharp sound.
- Rapid stretching of ligaments.
- Intra-articular (within-joint) adhesions being broken.
There were several theories to explain the cracking of joints. Synovial fluid cavitation had 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 (which are naturally found in all bodily fluids) 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. 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. But in 2018, an improved mathematical model and further experimentation and simulation demonstrated that the cavitation bubbles only partially collapse during cracking, and explained that the 2015 experiments were observing the smaller remaining bubbles.
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 (aged 50 to 89) 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. An earlier study also concluded that there was no increased preponderance of arthritis of the hand of chronic knuckle-crackers; however, habitual knuckle-crackers were more likely to have hand swelling and lowered grip strength. Habitual knuckle-cracking was associated with manual labour, biting of the nails, smoking, and drinking alcohol and was suggested to result 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.
- Crepitus—sounds made by joints
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