Claudia Hammond examines what causes the wince-inducing sound, and sifts through the evidence to see whether it actually damages your joints. Some people crack their knuckles by pulling the tip of each finger one at a time until they hear a crack. Others make a tight fist or bend their fingers backwards away from the hand, cracking the lot at once. If you are one of those people who sits and cracks your knuckles while others wince, at some point somebody is bound to have told you that cracking your joints gives you arthritis. For some it’s a nervous habit; for others the sensation brings relief. Depending on which research you read, between 25 and 54% of people do it, with men more likely to do so than women. Whichever method is used, the noise is created in the same manner. The space between the joints increases, causing the gases dissolved in the synovial fluid bathing the joint to form microscopic bubbles. These bubbles merge into large bubbles which then get popped by additional fluid which rushes in to fill the enlarged space. Once the joints have been cracked they can’t be cracked again for about fifteen minutes. This gives the space in the joint time to return to its normal size and for more gases to dissolve in the fluid, ready to form bubbles which can then pop all over again. Taking an engineering approach, cracking the knuckles repeatedly over many decades could in theory damage the cartilage covering the joint. Comparisons have even been made with the mechanical wear and tear accrued over time by ship’s propellers, but the evidence that the same is happening in people’s hands is thin. Close to the knuckle In fact, very few studies have been carried out at all. Perhaps one of the most well-known is the self-inflicted research rewarded with an Ig Nobel Prize in 2009. For more than 60 years, a Californian doctor called Donald Unger cracked the knuckles of his left hand at least twice a day, leaving his right knuckles uncracked. His conclusion? "I'm looking at my fingers, and there is not the slightest sign of arthritis in either hand," he said. There have been some other, perhaps more formalised studies carried out. In 1975, twenty-eight residents in a Jewish nursing home in Los Angeles were asked whether they had ever cracked their knuckles habitually. Those who had were less likely to have osteoarthritis in their hands later on. In a larger study conducted in Detroit in 1990, researchers examined the hands of three hundred people over the age of 45. Knuckle-crackers appeared to have a grip that wasn’t as strong, and 84% of them had signs of swelling in their hands. The authors say this means knuckle-cracking should be discouraged, but I wonder whether the people who crack their knuckles might feel more discomfort in their hands in the first place. Could this indicate a predisposition towards problems later on, rather than a cause? It’s worth pointing out that when it came to the crucial question of whether the joint-crackers had more osteoarthritis the answer was no. The most recent study, published just last year, is the most comprehensive so far, because it looks not only at whether people ever crack their knuckles, but also how often they do it. You might guess that cracking your knuckles every fifteen minutes could have a very different effect from doing it once a day, but again it made no difference to rates of osteoarthritis. In fact, there was no difference in the prevalence of osteoarthritis between those who did or did not crack their knuckles. Crack on So how did the idea of a relationship between knuckle-cracking and arthritis emerge? It’s true that people who already have arthritis sometimes find their joints crack because the cartilage of the surface of the joints has been damaged. However it’s unusual for this to be the first symptom and it seems more likely to be a consequence of damage, rather than a cause. The risk factors of arthritis that have been established are age, a family history of the condition, and previous accidents with hand, or a lifetime of working with your hands doing heavy labour. OK, but could cracking your knuckles cause other forms of damage? There are isolated reports of accidental self-inflicted injury from knuckle-cracking, with injured thumbs and sprained finger ligaments, but these are rare. So if you like cracking your knuckles then by all means carry on, but just bear in the mind the conclusions of the doctors who carried out the research in the LA nursing home. They said: “The chief morbid consequence of knuckle cracking would appear to be its annoying effect on the observer.”