Does Cracking Your Knuckles Cause Arthritis? Case Cracked

Somewhere between an estimated 25% and 54% of Americans crack their knuckles, including me. When I was in elementary school, one of my friends warned me in a very matter-of-fact tone, “cracking your knuckles will make them all swollen and fat when you’re older.” Somewhat concerned, I turned to one of the few sources I had access to when it came to old people's knuckles – I asked to see my grandma’s hands. Sure enough, her knuckles were quite large. As a good scientist does, I gathered data to test my friend’s hypothesis:

“Grandma, did you crack your knuckles when you were younger?”

“No.”

I took her answer to mean that genetics had probably already predestined me for large knuckles, and kept cracking away.

While my own investigation of knuckle cracking lacked serious merit, there has been a surprising amount of research in the area. As opposed to fat knuckle claims, however, researchers primarily focus on a more pressing concern – the theory that cracking your knuckles can lead to arthritis later in life.

Before we can take a look at the relationship between knuckle cracking and arthritis, we first have to understand the mechanics of both processes.

Zooming into the logistics of knuckle cracking, we arrive at the finger joints. Importantly, joints are full of joint fluid called “synovial fluid,” which is there to lubricate and provide nutrients to the cartilage. Synovial fluid also contains dissolved gases. Making a quick detour to high school physics class, we remember two key things. First, volume and pressure are inversely related – this means that as volume increases, pressure decreases. Second, when pressure decreases, dissolved gases “escape” from liquid (when a soda can is opened and the pressure seal is broken, carbonation escapes as the “bubbles” of the soda). This process is called cavitation.

Back to our fingers. In order to get that satisfying (or annoying) pop, one first has to stretch their finger joints. This can be accomplished by pulling your fingers or by crunching them in towards your palm. When you pull your finger, it increases the space at the joint, where your finger connects to your palm. Similarly, when you crunch your finger in towards your palm, the hinge-point between your finger and your palm extends outward – again, increasing the amount of internal space within the joint. This stretching of the finger increases the space (aka volume) inside. When the volume inside of the finger increases due to stretching, the internal pressure decreases. This causes (you guessed it!) cavitation, as tons of tiny air bubbles resurface throughout the synovial fluid. Within moments, those tiny bubbles combine to form large bubbles. Then, as the joint reaches its stretching extent and synovial fluid rushes to fill the empty space, the larger bubbles collapse back into microscopic bubbles. It is this collapsing of large bubbles that is responsible for that classic snap, crackle, pop. The microscopic bubbles then take around 20 minutes to re-dissolve into the synovial fluid, creating a refractory period and preventing the knuckle from being re-cracked for a short while. Once that recharge period is over, your knuckles are ready to begin the cracking cycle again!

Given this process, one might wonder whether repeated joint cavitation contributes to arthritis. Before placing our bets, take a look at what happens in our hands during the development of arthritis.

Arthritis is medically known as Osteoarthritis (OA) and is a debilitating condition that occurs as the result of cartilage degradation, bone remodeling, and various anatomical and physiological changes in joints. People can develop OA in any of their joints, but common spots include knees, hips, ankles, feet, and – of course – hands. In fact, hand osteoarthritis is the most prevalent joint disorder in the Western world. The biomechanics of hand arthritis are not entirely understood, however there are some well established theories about the development of the disease. Most models of arthritis development include the key term loading, which refers to the “load” that we put on our joints and bodies as we move and use them. Essentially, over time, our bodies are repetitively subjected to loading as we use our joints throughout our daily lives. In response, sometimes the bones underneath our joints remodel their internal structures to resist the recurring load and stressors that they face. As a result, the bones become stiffer and, in turn, they work less effectively to absorb shock. This puts the shock and stress of our load onto our cartilage, which eventually breaks down over time. The degradation of cartilage results in the joint degeneration that is known as hand osteoarthritis. These degenerative changes are painful and often limit a person’s ability to use their impacted joints. And, while prevalence rates vary based on location, in the United States, clinical estimates suggest that around 39.8% of adults will develop symptoms of hand arthritis in their lifetime. With those already high odds, it makes sense that scientists have investigated whether knuckle cracking increases the risk of OA. Let’s see what the science has to say about it!

Several studies have looked for a connection between knuckle cracking and hand arthritis, but the data suggests there is none.

We will begin our investigation with a trip down memory lane, to a time when research standards were vastly different and led to nuance results. In 1990, a report was published about a study including 74 knuckle crackers and 226 non knuckle crackers. The study looked for associations between knuckle cracking and arthritis rates, but it also looked for differences in grip strength and swelling. The researchers found no association between knuckle cracking and arthritis (woohoo!). However, they warned against knuckle cracking as they claimed it was associated with weaker grip strength and greater hand swelling. The caveat here is that they didn’t report how they measured grip strength, whether the researchers knew which group was which during evaluation, or even which joints were cracked. So, while arthritis rates are a binary (yes/no) diagnosis and can be more reliably accepted at face value from that study, the swelling and grip strength claims leave more to be explored.

In a recent 2017 study, researchers investigated the relationship between knuckle cracking and grip strength, as well as “metacarpal cartilage thickness” (think, chubby fingers). They matched 35 knuckle crackers (who cracked their knuckles at least 5x per day) with 35 non knuckle crackers, while controlling for age, gender and BMI (we’ll see why BMI might matter soon). They found no difference in grip strength between the two groups, but they did find a difference in cartilage thickness. The knuckle crackers had thicker cartilage, albeit by only about 0.05 millimeters (slightly larger than the width of a human hair). Alas, there may be a morsel of truth to what my friend told me about knuckle cracking and chubby fingers years ago.

Bringing it back to recent findings related to arthritis, a team of researchers in 2011 aimed to examine the long-term effects of knuckle cracking by asking a large pool of adult participants (both with hand OA and without hand OA) about their histories of knuckle cracking. This retroactive approach (notably similar to my own childhood investigation) allowed the researchers to investigate whether a history of knuckle cracking is predictive of arthritis outcomes. 215 participants aged 50-89 were sampled from a database of people who had all received hand radiographs – allowing the researchers to objectively measure and confirm arthritis status. 135 patients with hand OA and 80 controls (without hand OA) answered various survey questions about their knuckle cracking habits. The researchers found that OA rates did not differ between those who cracked their knuckles (18.1%) and those who did not (21.5%). Interestingly, this sample actually trended in the opposite direction with slightly higher rates in the non knuckle crackers, but that difference was not statistically meaningful.

To summarize our findings, research has not found any correlation between knuckle cracking and arthritis nor grip strength, but there is a possibility that the pesky habit can make your fingers a hair-width chubbier.

While that is likely relieving for about 25-54% of Americans, it raises a more critical question – are there things that actually can predispose a person to developing osteoarthritis? Given the prevalence of OA and the pain that it causes, there has been significant interest in these lines of research as well. Here’s what the science says about risk factors, starting with non-modifiable risks, then looking at what we can do to protect our hand health:

Rates of OA are impacted by a variety of factors that are out of our control. For example, rates of OA significantly increase after the age of 50. OA also has a genetic heritability of around 60%, and women are around 2.6 times more likely than men to develop hand OA. While these risks may be interesting, they don’t exactly provide us with useful information for promoting our health. So, let’s look at some factors that are “modifiable” to some degree.

Obesity: Obesity is the most well-established risk factor for OA, with some studies showing that obesity increases risk of OA by nearly three-fold. This is because high levels of body weight increase the “load” that a person’s joints take. Thanks to our understanding of the development of arthritis, we now know that this increases the pressure placed on a person’s bones and cartilage – a pressure that can degrade joints.

Diet: Some research shows improved arthritis outcomes in individuals with diets high in vitamin D, vitamin C, and vitamin K, however there is unclear data between studies and more research is needed to draw conclusions. The possible benefit of these vitamins is likely due to the strengthening effects that they have on bones, cartilage, and various other bodily structures. For example, one study found that Vitamin D is associated with better metrics of cartilage.

Smoking: Interestingly, some studies show a negative correlation between smoking cigarettes and hand OA, such that more smoking seems to be associated with less hand OA. However, that research was observational in nature (not experimental), thus preventing any causal claims from being drawn. The implications of this observed phenomenon are not well understood and practitioners do not support the use of cigarettes as a preventative measure.

Occupation: This is another one of the most influential factors associated with OA. Research shows that occupations including repetitive movements (ex: clothing manufacturing, construction, farm work) are positively correlated with rates of OA. In fact, even repetitive movements such as chopstick use have been demonstrated to correspond with higher rates of OA. Movements of this nature involve repeatedly putting load on the same joints day after day. This is thought to exacerbate the need for bone structure remodeling, as those bones, cartilage and joints are repeatedly under stress.

Notably, knuckle cracking is nowhere to be found among exhaustive lists of known factors that can increase a person’s risk for developing hand osteoarthritis. So, where does the “knuckle cracking causes arthritis” myth come from?

Dr. Donald Unger, M.D., gives us a bit of insight. Throughout Dr. Unger’s childhood, his mother, aunts, and adult relatives warned him that cracking his knuckles would lead to arthritis – a familiar tale for knuckles crackers everywhere. Curious and in an epic act of science, Dr. Unger decided to run an experiment on himself that would continue for decades. For 50 years, Dr. Unger cracked the knuckles on his left hand twice a day, while leaving his right hand uncracked as a “control.” 36,500 cracks and 50 years later, Dr. Unger compared his hands, looking for symptoms of arthritis. He found no difference. In his publication of those results, Dr. Unger left us with insight into the origins of the myth, “This result calls into question whether other parental beliefs, e.g., the importance of eating spinach, are also flawed. Further investigation is likely warranted.”

While Dr. Unger’s sarcasm reads clearly, he brings up a pertinent point. Generational knowledge is primarily preserved by word of mouth, and is not often subjected to rigorous scientific investigation. As we grow up, we often take it upon ourselves to investigate some of the wives tales that surround us – sometimes on purpose, sometimes by accident. We eat a watermelon seed and after a nerve wracking week, we discover that nothing is growing inside us. We have a friend hit us on the back of the head while we cross our eyes, and we breathe a big sigh of relief when they uncross. We turn 15 and a half, take the driver’s permit test, and discover that there is no law preventing us from turning on a car light at night. For a college writing class, we write an essay about popular misconceptions, and during our initial brainstorming phase we discover that our heads do not, in fact, lose more heat proportionally than the rest of our bodies. Inevitably, however, some things slip through the cracks. Instead of uncovering the truth, we unknowingly pass along misinformation by warning our daughter that her fingers are going to get fat if she cracks her knuckles. And then, she tells her friend at school the next day.

 References: 

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