- Cracking joints
Cracking joints is the action of moving joints to produce a sharp cracking or popping sound. The most common form of this occurs during deliberate knuckle-cracking. It is possible to crack many other joints, such as those between the back and neck vertebrae, hips, wrists, elbows, shoulders, toes, ankles, knees, jaws, and the Achilles tendon area.
To deliberately produce the clicking sounds, many people bend their fingers into unusual positions. These positions are usually ones that their own muscles are unable to achieve. However, cracking a joint that has been exercised recently is generally recognised to be palliative. For example, bending a finger backwards away from the palm (into extension), pulling them away from the hand (abduction), compressing a finger knuckle toward the palm (into flexion), or twisting a finger about (torsion).
The physical mechanism is uncertain, and cracking may possibly arise from several different causes. Suggested causes include:
- Cavitation within the joint—small cavities of partial vacuum form in the fluid and then rapidly collapse, producing a sharp sound. This explains the popping that can occur in any joint, such as during spinal manipulation. Synovial fluid cavitation is the most likely theory and substantial evidence exists in support of it. Cracking knuckles releases gases from the joints.
- Rapid stretching of ligaments.
- Intra-articular (within-joint) adhesions being broken.
Of these hypotheses, perhaps the most popular is cavitation. When a 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. This process is known as cavitation. 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", which can range from a few seconds to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.
The common advice that "cracking your knuckles gives you arthritis" is not supported by any evidence. A recent study 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 lower 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 criticizied for not taking into consideration the possibility of confounding factors, such as if the ability to crack one's knuckles is associated with impaired hand functioning. As such, it remains unclear if knuckle cracking is in itself associated with any impaired hand function.
Medical doctor Donald Unger regularly cracked the knuckles of his left hand for fifty years while not manipulating those of his right. No arthritis or other ailments formed in either hand, and he was awarded 2009's Ig Nobel Prize in Medicine.
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