I’ve never been able to crack my knuckles. Growing up, cracking your knuckles was a sign that you were a cool kid – similar to double-jointedness or being able to wiggle your ears. Then, once about the third grade hit, teachers started telling us cracking your knuckles was bad – that it would eventually lead to arthritis and an inability to have normal-functioning hands. That same belief carried over to when our knees started popping when we squatted down. If knuckle-cracking was bad, the knee-popping must be bad, too, right?
Which joints crack for you? Your knuckles? Knees? Shoulders? Hips? Ankles? Neck?
What is it that actually causes the cracking sound inside your joints? Is it bad? Should you be concerned? Let’s explore…
What Causes the “Cracking” Sound?
Over 100 years ago, in 1911, a few researchers observed gas bubbles inside joints1. This got them thinking that perhaps it’s the release of these bubbles that causes joint cracking sounds.
60 years later, in 19712, a contraption was invented that measured the amount of separation between the knuckles (MPC joints) of 17 men as the machine put traction on the joint. Attached to this device was an x-ray machine that captured the exact amount of separation between the two bones at the point when a “crack” took place.
Based on this data, a model of the finger (MCP) joint was made and synovial (joint) fluid was injected between the two model bones. As separation of the joint was simulated, a high-speed camera captured what occurred.
Here is a summary of the findings:
Synovial fluid within a joint contains about 15% gas. After the joint surfaces are close together, large amounts of pressure can be produced as they’re separated. Under this amount of pressure, synovial fluid vaporizes and gas is released. The collapse of the vapor cavities gives rise to the noise we hear when joint cracking takes place. This phenomenon is referred to as “cavitation”.
Therefore, the gas bubbles aren’t the cause but the effect of the “cracking” noise. The fluid “cavitation” is responsible for the actual noise heard.
And why is it that after a joint cracks once, it doesn’t crack again until awhile later?
The gas removed during the period of low pressure, returns to normal (ambient) pressure but does not reabsorb for another 20-30 minutes. This is why most joints, once “cracked”, won’t crack again for about 20-30 minutes.
Joint cavitation – the collapse of the vapor cavity – has been historically what we attribute the joint cracking sounds to. It is worth noting, though, that a 2015 study shows with real-time MRI imaging that joint cracking sounds are more directly related to the formation of the cavity, rather than the collapse3.
Should You Be Concerned?
Either way, whether the sounds come from cavity formation or collapse, any concern you may have that joint cracking is bad or causing damage should be put at ease.
Studies have shown that habitual knuckle cracking has no correlation whatsoever with an increase in osteoarthritis in that joint4.
It has been shown that habitual knuckle-crackers are more likely to have hand swelling and lower grip strength when compared to a sample of non knuckle-crackers. However, in this same sample, the knuckle-crackers were also more associated with manual labor, smoking, and alcohol consumption. Therefore, in my opinion, the correlation between joint cracking causing any functional impairment is weak at best5.
Based on the evidence: if there is no pain or instability associated with the cracking or popping in your joints, there is no need to worry – in these instances, joint cracking is normal.
Other Causes of Cracking Sounds
Rather than going into full detail about all of the potential causes of popping and clicking sounds in your body – as each source would require a full article to explain – I thought it more appropriate to list a few sources of cracking sounds based on body region. I also provide a link to further reading if it interests you.
This list is not exhaustive, but contains some common areas that may produce clicking noises:
- Spinal Manipulation – What Happens when you back cracks?
- Intra-Articular Loose Bodies
Again, I want to emphasize that cracking sounds in the absence of pain or instability is normal and should not be a cause for concern.
If you do have pain or instability with a “pop” or “click”, this list is intended to guide you to further investigation. Some sources are harmless, some can be more serious. In such cases, you may want to see your doctor to guide you to the right care.
In many instances, a Physical Therapist can help!
Question: What Joints of Yours Snap, Crackle, or Pop? You can leave a comment by clicking here.
- Fick R. Zum Streit um den Gelenkdruck. Anat Hefte. 1911;43(1):397. ↩︎
- Unsworth A, et al. ‘Cracking Joints’. Ann. Rheum. Dis. 1971;30:348. ↩︎
- Kawchuk CN, et al. (2015) Real-Time Visualization of Joint Cavitation. PLoS ONE 10(4):e0119470. Doi:10.1371/journal.pone.0119470. ↩︎
- DeWeber K. et al. Knuckle Cracking and Hand Osteoarthritis. Journal of the American Board of Family Medicine. 2011; 24(2):169-174. ↩︎
- Castellans J, Axelrod D. Effect of habitual knuckle cracking on hand function. Annals of the Rheumatic Diseases. 1990; 49:957. ↩︎