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The last word– WHAT’S THE CRACK? I regret to say that I have a habit of cracking my knuckles. I’ve 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?
● There are a couple of theories regarding what causes the “pop” 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 – a process called cavitation – 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. 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 – there wasn’t. Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
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’s quite satisfying, isn’t it? For more on knuckle cracking, visit www.tinyurl.com/ysjoyu David Farnsworth Shipley, West Yorkshire, UK ● 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
“As a test, one doctor went so far as to crack the knuckles on just one hand for 50 years” the extreme of its range of motion. After the “pop”, the joint capsule is temporarily enlarged, which also increases the neural firing in the joint’s proprioception receptors.
These signals transmit over the local nerve root. This increased neural activity inhibits pain signals from smaller nerve fibres in the same dermatome – 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’s purse: chiropractic spinal treatments rely on eliciting the sound – and offering brief relief – 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 – Ed
THIS WEEK’S QUESTIONS Something eats wasps I spotted this amazing sight (see Photo) in Croatia in July 2007. In light of New Scientist‘s book Does Anything Eat Wasps? I’d like to know what’s going on here. Which insect is eating which, and is it common? Richard Garner UK Worm baiting While sitting on a bench beside a local green, I noticed a gull performing an excellent version of Riverdance. Then it stopped and scrutinised the grass around its feet. This sequence was repeated for about 15 minutes. I assume the gull was trying to attract worms to the surface with its rhythmic dance. Was it? If so, how does the strategy work? Danny Hunter Dublin, Ireland Rose-tinted vision If I look out of my left eye, everything is tinted slightly green. From my right eye, everything is tinted slightly red. When I look out of both eyes at the same time, the image produced is uniform and untinted. Why is this? Philip Oddie Kingston upon Thames, Surrey, UK All alone Do polar bears get lonely? I’m not being flippant, just attempting to find out why animals such as humans or penguins are gregarious while others, such as polar bears and eagles, live more solitary lives. Frank Anders Amsterdam, The Netherlands
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK , by email to [email protected] or visit www.newscientist.com/lastword.ns (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
Last Words past and present, plus questions, at www.newscientist.com/lastword
The last word– WHAT’S THE CRACK? I regret to say that I have a habit of cracking my knuckles. I’ve 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?
● There are a couple of theories regarding what causes the “pop” 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 – a process called cavitation – 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. 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 – there wasn’t. Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
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’s quite satisfying, isn’t it? For more on knuckle cracking, visit www.tinyurl.com/ysjoyu David Farnsworth Shipley, West Yorkshire, UK ● 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
“As a test, one doctor went so far as to crack the knuckles on just one hand for 50 years” the extreme of its range of motion. After the “pop”, the joint capsule is temporarily enlarged, which also increases the neural firing in the joint’s proprioception receptors.
These signals transmit over the local nerve root. This increased neural activity inhibits pain signals from smaller nerve fibres in the same dermatome – 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’s purse: chiropractic spinal treatments rely on eliciting the sound – and offering brief relief – 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 – Ed
THIS WEEK’S QUESTIONS Something eats wasps I spotted this amazing sight (see Photo) in Croatia in July 2007. In light of New Scientist‘s book Does Anything Eat Wasps? I’d like to know what’s going on here. Which insect is eating which, and is it common? Richard Garner UK Worm baiting While sitting on a bench beside a local green, I noticed a gull performing an excellent version of Riverdance. Then it stopped and scrutinised the grass around its feet. This sequence was repeated for about 15 minutes. I assume the gull was trying to attract worms to the surface with its rhythmic dance. Was it? If so, how does the strategy work? Danny Hunter Dublin, Ireland Rose-tinted vision If I look out of my left eye, everything is tinted slightly green. From my right eye, everything is tinted slightly red. When I look out of both eyes at the same time, the image produced is uniform and untinted. Why is this? Philip Oddie Kingston upon Thames, Surrey, UK All alone Do polar bears get lonely? I’m not being flippant, just attempting to find out why animals such as humans or penguins are gregarious while others, such as polar bears and eagles, live more solitary lives. Frank Anders Amsterdam, The Netherlands
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK , by email to [email protected] or visit www.newscientist.com/lastword.ns (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
Last Words past and present, plus questions, at www.newscientist.com/lastword