There is no proof that cracking your knuckles harms your joints

You sent us a message via WhatsApp (to +34 666 908 353) asking us whether “cracking your knuckles is bad for you”. The popular belief that this practice increases the risk of arthritis has prompted scientific research in order to elucidate the associated risks.

The habit of cracking one’s knuckles has traditionally been associated with a higher risk of developing arthritis. But this popular belief – the origins of which we could not determine – is not supported by scientific research, as pointed out by Harvard University and the Johns Hopkins Arthritis Center, both in the United States.

Research on the topic is limited. The studies have focused on comparing the incidence of osteoarthritis, the most common type of arthritis, in groups of people who crack their knuckles on a regular basis and others who do not. None of the studies have found any association between the two factors, but the studies have always had a limited number of participants (the two biggest ones involved 300 and 215 people, respectively) or have been based on the memory and statements of the people studied, which may distort the results, as we explained here.

The study of the myth introduced us to the curious story of Donald Unger, an allergist who cracked the knuckles of only his left hand twice a day over the course of 50 years, leaving his right hand untouched in order to compare what would happen in both cases. According to his analysis, he did not experience any noticeable differences between the two hands, as he pointed out in a letter to the editor of Arthritis & Rheumatology magazine in 1998. This research was what helped him win the Ig Nobel Prize in Medicine in 2009, a satirical award in the scientific community.

However, it is important to differentiate the sound of cracking from the sound of crackling, which occurs when the joint produces a sound “like a piece of Velcro being ripped off, or a fire when it crackles”, Marcos Paulino, president-elect of the Spanish Society of Rheumatology (SER) and head of the department at General University Hospital of Ciudad Real. This phenomenon is indeed indicative of a loss of cartilage and the resulting friction between two bones, and “indicates that it can develop into arthrosis with time”.

But there is “no proof that [cracking one’s knuckles] produces any kind of injury to joints”, Paulino of the SER declares. One paper from 1990 suggested that the habit could lead to the loss of strength in the hand and cause inflammation, but that conclusion was criticised in a letter to the editor for not showing a causal relationship, and refuted by a subsequent paper that led to opposing results.

“If you do it in a reasonable way, it does not have any effect”, specifies the expert, who says that it would require doing it all day long without stopping to “get to a point in which you have a higher risk that a ligament is more relaxed, or that the joint returns to being a little more unstable”, he concludes.

Gas bubbles

The conclusions are generalisable to any joint, because the mechanism by which the sound is produced, which people have been studying for over 100 years, is the same. Joints are full of synovial fluid, a thick fluid that minimises friction between the bones. The fluid has a mix of gases (mainly nitrogen and carbon dioxide) dissolved in it which forms bubbles when the joint is stretched, and the bubbles, in turn, cause the popping sound. The question that is still unanswered is whether it is the formation of the bubbles or their bursting that produces this characteristic sound, although “it is not of any interest from a scientific perspective”, given that “they are physiological sounds that do not have a pathological significance”, states the president-elect of the SER.