Joint manipulation in humans can produce a sharp cracking or popping sound. This occurs during deliberate knuckle-cracking, but it is possible to crack many other joints, such as those in the back and neck vertebrae, hips, wrists, elbows, shoulders, toes, ankles, knees, jaws, feet, sternum, and the Achilles tendon area. The mechanism that produces the cracking sound is not known.
According to a folk belief, the popping of joints, especially knuckles, leads to arthritis or other joint problems. However, medical research has so far failed to conclusively demonstrate any connection between knuckle cracking and long-term joint problems.
The physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints is uncertain. Suggested causes include:
- Cracking sounds occur when bubbles form in joints as they are pulled apart.
- 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 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 (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.
However, recent evidence demonstrates that the cracking sound is produced when the bubble within the joint is formed, not when it collapses.
The common claim that cracking one's knuckles causes arthritis appears unsupported. 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.
There are many ways people have learned to crack the joints of their fingers, one of the most common ways is to put pressure on the joint between the metacarpals and the proximal phalanges.
Medical doctor Donald Unger cracked the knuckles of his left hand every day for more than sixty years, but he did not crack the knuckles of his right hand. No arthritis or other ailments formed in either hand, earning him the 2009 Ig Nobel Prize in Medicine, a parody of the Nobel Prize.
- Crepitus – sounds made by joints
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