Is it safe to crack your knuckles? What causes the popping sound?
Dear Patients & Friends:
Is it safe to crack your knuckles? And what causes the popping sound when a chiropractor makes an adjustment?
In 1998 Donald Unger published the funniest—if not the most definitive—research on knuckle cracking to date. Writing in Arthritis and Rheumatism, Unger wrote that, “During the author’s childhood, various renowned authorities (his mother, several aunts and, later, his mother-in-law [personal communication]) informed him that cracking his knuckles would lead to arthritis of the fingers.” This led him to undertake an experiment to test this hypothesis upon his graduation from medical school: “For 50 years, the author cracked the knuckles of his left hand at least twice a day, leaving those on the right as a control. Thus, the knuckles on the left were cracked at least 36,500 times while those on the right cracked rarely and spontaneously.” Upon his retirement from medical practice he had a radiologist friend make and review x-rays of his hands. The blinded radiologist concluded that, “There was no arthritis in either hand, and no apparent differences between the two hands.” Unger happily concluded that, “there is no apparent relationship between knuckle cracking and the subsequent development of arthritis of the fingers.”
More serious definitive research on the question comes from a recent investigation that concluded that osteoarthritis of the hand was associated with family history of arthritis and with aging, but was not associated with knuckle cracking, sex, nor with heavy manual labor. This study generally confirmed results of a prior study showing no association between arthritis and knuckle cracking.
But what happens when joints crack or pop? Scientists refer to these sounds as cavitations, and when they occur during chiropractic adjustments we refer to them as the “audible release,” because early research on knuckles revealed that carbon dioxide gas in the joints is dispersed causing “microbubbles,” perhaps as a result of a collapsed gaseous nucleus. During the next 15 to 60 minutes after a joint manipulation or adjustment, the gas is resorbed during a “refractory period,” after which another manipulation can again trigger cavitation.
In one CT study, a manipulation that caused at least one cavitation did not result in a change in gas within the joints that were seen in other studies. But two MRI studies of adjustments of the lower back reveals that joint gapping does indeed occur after manipulation, is associated with treatment of the target area, and during side posture treatment occurs mainly in the upside of the back.
While we don’t know yet what chemical and biological changes occur during this process, the strong and growing evidence for chiropractic manipulation for spine related pain suggests some as yet unconfirmed biomechanical effects may be involved. Some studies suggest for example that receptors in muscles are signaled by adjustments even when the pop does not occur, and there is evidence as well that patients may get better even when joint cavitation does not occur. Research at present is not yet definitive, but clearly there is no evidence that cavitation is inherently dangerous, even though it may not always be necessary to achieve a good treatment effect.
So in the meantime, when you see a chiropractor or crack your knuckles you don’t have to be afraid of the worst, and indeed you may be receiving the best treatment outcome.
Robert A. Leach, DC, MS, CHES, FICC(h)
Donald Unger’s hysterical if not thought provoking single subject anthropological research is in the form of a letter to the editor submitted to Arthritis and Rheumatism, and was reviewed by Scientific American: http://www.scientificamerican.com/article.cfm?id=crack-research
Knuckle cracking is not associated with osteoarthritis according to this retrospective study of 135 patients with osteoarthritis and 85 control subjects: http://www.jabfm.org/content/24/2/169.full.pdf+html
No increase in osteoarthrosis, but increased hand swelling, lower grip strength associated with manual labor, biting nails, smoking and drinking alcohol in this retrospective study (12 of 74 knuckle crackers and 36 of 226 non-knuckle crackers had osteoarthrosis, not different between groups): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1004074/
Landmark original research published by Unsworth, Dowson and Wright in Annals of Rheumatic Diseases revealed that the cracking sound heard when knuckles pop (i.e., in the metacarpophalangeal joints) involved gaseous exchange (mainly carbon dioxide) within the synovial fluid: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1005793/pdf/annrheumd00004-0008.pdf
The AHCPR of the National Institutes of Health in 1994 after a review of 3 decades of research, determined that over the counter non—prescription drugs, and spinal manipulation such as provided by chiropractors, were the most “safe and effective” treatments for acute low back pain: http://www.ncbi.nlm.nih.gov/books/NBK52408/ and should be combined with ice, gradual return to normal activity, and gradual increase in exercise for best results.
Pickar and other chiropractic scientists published animal model research in Spine that indicates manipulative thrusts evoke more responses from muscle spindles in the muscles next to the spine, that may be expected to over-ride pain signals and stop spasms: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2075482/pdf/nihms32771.pdf