I regret to say that I have a habit of cracking my knuckles. I鈥檝e read somewhere that it does no harm, but I am still far from convinced. Could it be damaging in the long run? If so, why?
鈥� There are a couple of theories regarding what causes the 鈥減op鈥� heard in knuckle cracking. The common view is that it is caused by bubbles of gas within the synovial capsule of the metacarpophalangeal and phalangeal joints. These bubbles form when these joints are stressed 鈥� a process called cavitation 鈥� and then collapse as the pressure changes within the joint. It is the collapse that creates the noise. The energy released by this process has been estimated at just 0.07 millijoules per cubic millimetre. To cause damage to a joint, this figure would have to rise to about 1 millijoule per cubic millimetre. Cumulative damage from these pops cannot be ruled out, however.
The second theory is that the noise comes from the sudden deformation of the fibrous joint capsule itself and that the pop is its sudden slap onto the joint fluid within. This might cause microtrauma which could accumulate over years.
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Regardless of the theories, there is little evidence that knuckle cracking causes arthritis: a survey of knuckle crackers showed no more incidence of arthritis than non-knuckle crackers. One American doctor went so far as to crack the knuckles on just one hand for 50 years to see if there was a difference between that hand and the other 鈥� there wasn鈥檛.
鈥淎s a test, one doctor went so far as to crack the knuckles on just one hand for 50 years鈥�
It is possible to cause acute trauma from the stress required to cause the joints to pop in the first place, of course, but one has to say that it鈥檚 quite satisfying, isn鈥檛 it?
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David Farnsworth, Shipley, West Yorkshire, UK
鈥� The sound is caused by a bubble of nitrogen gas forming in the joint. This occurs through a pressure drop which is created when the joint is forced to the extreme of its range of motion. After the 鈥減op鈥�, the joint capsule is temporarily enlarged, which also increases the neural firing in the joint鈥檚 proprioception receptors. These signals transmit over the local nerve root. This increased neural activity inhibits pain signals from smaller nerve fibres in the same 鈥� the area of skin supplied by the same nerve root.
The only known effect of repeated joint-popping is in the joints of the spine, and results in a reduction in the financial content of the patient鈥檚 purse: chiropractic spinal treatments rely on eliciting the sound 鈥� and offering brief relief 鈥� but later when symptoms recur (presumably as the gas is reabsorbed) the cyclic need for treatment resumes.
Don L Jewett, Professor Emeritus of Orthopaedic Surgery, University of California, San Francisco, US
We expect some chiropractors will disagree with this opinion. Watch this space 鈥� Ed