We’ve all been there. You stretch your fingers or twist your back, and suddenly, pop! That unmistakable cracking sound escapes from your joints. For decades, people have wondered: Why do knuckles crack? Is it harmful? What causes the sound during a chiropractic adjustment? Many believed it was due to a bubble collapsing inside the joint—but new research suggests otherwise.
A while back, I published a post called Ask the Chiropractor: What is that cracking sound when you adjust me? In it, I explained that the cracking sound during a chiropractic adjustment was due to a bubble forming and then collapsing as the joint opened up. However, this is not the case. Recent studies using real-time MRI show that the cracking occurs as the joint opens up, not when the bubble collapses.
This new research, published in PLOS ONE under the title Real-Time Visualization of Joint Cavitation by Gregory N. Kawchuk and his team, sheds light on the actual cause of joint cracking. By using advanced imaging techniques, they debunk the old bubble-collapse theory and reveal the real mechanism behind that satisfying “pop.”
Cracking Joints: The Mystery Unraveled
For years, many assumed that joint cracking was caused by the collapse of a bubble inside the synovial fluid (the liquid that lubricates your joints). According to this theory, when you pull on a joint—such as when you crack your knuckles—pressure inside the joint drops, causing gases in the fluid to form a bubble. When the bubble collapsed, it was thought to create the cracking sound.
However, this bubble-collapse theory had some gaps. Why can’t you crack the same joint again right away? Why does the sound seem so loud if it’s just a tiny bubble? These unanswered questions hinted that something more was happening inside the joint.
A New Approach: Using Real-Time MRI
The latest study by Kawchuk and his team offers a more accurate explanation by using real-time magnetic resonance imaging (MRI). Unlike older techniques like X-rays, real-time MRI captures joint movement frame by frame, allowing the researchers to observe exactly what happens when a knuckle cracks.
In this study, they observed ten metacarpophalangeal joints (the knuckles of the hand) and found that the cracking sound was caused by something called cavity formation, or tribonucleation, not the collapse of a bubble. This is a game-changing discovery, shifting our understanding of joint mechanics.
What is Tribonucleation?
So, what exactly is tribonucleation, and how does it cause joint cracking? Imagine two surfaces (such as the bones in a joint) that are separated by a thin layer of synovial fluid. When you apply pressure or pull on the joint (like when you crack your knuckles), these surfaces resist separation at first. But when enough force is applied, they quickly snap apart, creating a gas-filled cavity in the joint fluid. This cavity formation is the source of the cracking sound.
Unlike the previous bubble-collapse theory, the study found that the gas-filled cavity doesn’t collapse right away. Instead, it remains in the joint for a period before slowly disappearing. This explains why you can’t crack the same joint again immediately—the cavity needs time to vanish.
Busting the Bubble Myth: What This Means
This discovery challenges the long-standing belief that the cracking sound is caused by the collapse of a bubble inside the joint. Instead, it shows that the sound happens as the joint surfaces separate and a cavity forms, not when the bubble collapses. This finding also provides a better understanding of why cracking your knuckles isn’t harmful—despite the noise, there’s no evidence of damage to the joint.
Is Cracking Your Knuckles Bad for You?
One of the most common questions people ask is, “Is cracking your knuckles bad for your joints?” Thankfully, the answer appears to be no. Research shows that knuckle cracking doesn’t cause arthritis or increase the risk of joint problems. Despite the loud sound, habitual joint cracking has not been linked to long-term harm.
This study suggests that the energy released during joint cracking is relatively small, meaning it’s unlikely to cause damage. However, there are still some unanswered questions. Could frequent cracking have subtle effects on the cartilage over time? This study doesn’t provide a definitive answer, but it opens the door for more research on joint health and cracking.
Cine MRI: A Game Changer for Joint Research
One of the most exciting aspects of this study is the use of cine MRI, a type of real-time imaging that allows scientists to observe joint movements in extreme detail. Unlike traditional imaging techniques like X-rays, cine MRI provides a frame-by-frame look at what happens inside the joint when you crack it.
This technology offers new possibilities for studying joint health, injuries, and diseases like arthritis. It could also help researchers better understand how exercise or repetitive movements affect the joints over time.
What Does This Mean for Chiropractic Adjustments?
As a chiropractor, I often get asked, “What’s that cracking sound during an adjustment?” With this new information, we now know that the cracking sound occurs due to cavity formation as the joint surfaces rapidly separate. It’s not a bubble collapsing, as was once thought.
This understanding doesn’t change the way we perform chiropractic adjustments, but it does provide a clearer, more accurate explanation of what’s happening inside the joint. The good news is that the sound doesn’t indicate harm—it’s simply the result of natural joint movement and pressure changes.
Conclusion: A New Understanding of Joint Cracking
For years, the sound of joint cracking has mystified scientists and the public alike. But thanks to this new study using real-time MRI technology, we now have a clearer explanation: the sound comes from the formation of a gas-filled cavity, not the collapse of a bubble. This process, known as tribonucleation, occurs when joint surfaces rapidly separate, allowing gases in the synovial fluid to form a cavity.
For those who love the sensation of cracking their knuckles, the good news is that this process doesn’t appear to cause harm. So next time you hear that satisfying “pop,” rest easy knowing that it’s just your joints doing what they naturally do—without any risk to your health.
And for chiropractors like myself, this study provides a more accurate way to explain the cracking sound patients hear during adjustments. It’s simply a natural part of joint mechanics, and nothing to worry about.
Originally posted on May 17, 2022 @ 4:38 pm