Why Does My Knee Pop? | 3 Reasons It Happens and When You Should Worry

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Knee specialists often receive concerned people worried about their knee joint, wondering: “Why does my knee pop? Is the cartilage ok? Is there any damage?”

These noises – or “crepitus” as they’re called in sports medicine– are fairly common and contrary to popular belief, they don’t always mean joint damage. Studies suggest that 99% of knees make some sort of noise.(1,2)

Common reasons include gas bubbles and tissues rubbing against each other. Depending on the type, circumstance, and location of the sound, you might be able to tell whether the sound is healthy or not. 

3 causes of knee popping in healthy joints

Some sounds are present in healthy knees and mean no harm. As long as there’s no pain or problem with the function of the knee, this crepitus isn’t a cause of concern.

Here are 3 causes that make a healthy knee joint pop:

1) Gas bubbles

Comparison of a knee without gas bubbles and another knee being stretched to show gas bubbles that cause popping sounds when you bend your knee

The space between two bones is filled with a viscous liquid called “synovial fluid”.

It cushions the bones and helps them glide past each other. It also helps keep the joints healthy.

Among other things, this fluid also contains lots of dissolved gas molecules which can cause popping in 4 steps:

  1. When you bend your knee, the tissues stretch but the amount of synovial fluid stays the same.
  2. This creates a low-pressure zone that pulls those dissolved gas molecules together, forming a “bubble”.
  3. The surrounding fluid presses on it until it collapses, usually with a “pop”.
  4. This scatters the gas molecules inside the fluid, which then dissolve again.

2) Rubbing of tissues against each other

A leg showing where the iliotibial band rubs against the side of the knee and why it causes the knee to pop

There are +15 structures inside the tight space of the knee: menisci, ligaments, tendons, bones, fluid, soft tissue…

They rub onto one another any time you move your knee, and sometimes that rubbing produces crepitus.

A common example is feeling a snap on the sides of the knees. This could be the iliotibial band rubbing on the bony structures of the femur.

You can fix this easily by changing the position of your feet and/or your hips. Experiment until you find the one that doesn’t produce the sound.

3) “Stick-Slip” phenomenon

Side view of the knee showing an uneven articular cartilage that explains why knees pop

There’s also “cartilage” behind your kneecap, which helps it glide on the femur when you bend or extend your knee.

Some people have cartilage with uneven surfaces (perhaps because of scar tissue) which makes the glide a little bumpy.

This might be the cause of that fine grating, sometimes loud sensation behind the kneecap.

Examples of knee sounds and their meaning

Doctors showing an x-ray of the knee to explain to their patient the difference between normal and abnormal joint sounds that cause knees to pop

Normal joint sounds

  • The sound isn’t painful or started after a trauma.
  • Sudden cracking/popping: Air bubbles within the joint.
  • A single snap in a specific moment while bending/extending the knee: Probably a tendon snapping over a bony prominence.
  • A fine gritty or grating sound, usually very loud and present throughout all the knee movement: Due to the “stick-slip” phenomenon described above.
  • You’ve had that sound on your joint for as long as you can remember.

Consult with a physical therapist if…

  • The knee pain increased progressively or it affects your daily activities.
  • The noise is like a creaky door. It could indicate an early stage of knee osteoarthritis, best treated with exercises and physical therapy.
  • The noise appeared suddenly, with or without pain. This could be a sign of patellofemoral pain syndrome or arthritis.
  • If any of the sounds in the “normal” section suddenly become painful.

See a doctor if…

  • It’s painful and/or appeared after a specific trauma/injury.
  • There’s a loud pop present in the moment of trauma. You could have an injured anterior cruciate ligament.
  • You have a single click, that repeats in a specific moment of the movement after an injury. Possible meniscus tear.
  • The noises appeared after a knee replacement or surgery.
  • The noises are associated with swelling of the knee or other symptoms.
  • The noises disappear and pain begins. This could be a sign of arthritis.

When to worry about it

A doctor examining his patient's knee to find out why his/her knees crack

Consult with a knee specialist if these other symptoms are present:

  • The noises appeared suddenly or after a knee injury.
  • Pain.
  • Loss of motion.
  • Tissue swelling.
  • Gait problem.
  • Instability.
  • Right after a direct injury to the knee.

FAQs

What if it hurts when my knee pops or grinds?

If your knee hurts when it pops, it could be a sign that you need to seek medical attention.

it might be arthritis, torn ligaments, or other injuries.

Consult with a knee specialist, whether it’s a physical therapist or a physician, to check what’s going on.

Is there any treatment for it?

Some will perceive fewer knee noises after losing some weight, or after starting an exercise regime.

In healthcare, we usually treat the noises when there’s joint pain or an injury involved.

For example, if the noise is due to a ligament tear, we treat the tear. Once it’s managed, the crack or pop should disappear.

Related: Remedies For Knee Pain at Home

How do I stop my knees from popping?

This depends on the cause of popping.

Try these ideas if the knee crack didn’t start during or after a trauma, and there’s no pain associated with it:

  • Change the position of your feet, or bend a little more (or less) your hips. This could reduce the popping if it comes from rubbing tissues or the “stick-slip” phenomenon described above.
  • Strengthen your legs, preferably under the guidance of a personal trainer.
  • Walk more throughout your day.

For some people, the popping doesn’t go away no matter what they try. If it’s painless, this should be normal.

However, knee pops can also be linked to a number of pathologies. These noises will stop when you manage the underlying cause.

Can knee cracks cause osteoarthritis?

No, knee cracks don’t cause osteoarthritis.

Doctor Donald Unger proved this when he tested this myth with his own knuckles.(6)

He cracked the knuckles of his left hand twice a day for 50 years without cracking the ones of the right hand.

36,500 cracks later, both hands were still free of arthritis.

How to keep knees healthy?

If you’re between 18-64 years old, do a variety of exercises regularly to strengthen your legs and knee joints.

Aim for(7):

  • 150-300 minutes of moderate-intensity aerobic activity, AND
  • Strength training twice a week.
  • Do it under the guidance of a personal trainer to prevent injuries.

Also, try to keep your BMI below 25. The higher the BMI, the lower your knee health.(8)

Combine this with a healthy lifestyle for best results; eat nutritious foods, sleep well, and take care of your mental health.

If you have knee osteoarthritis, please follow the indications of your healthcare provider to prevent it from getting worse.

Conclusion: Why do knees pop?

Most people with knee popping find it alarming and worrying. However, these sounds are VERY common and not associated with damage.

Crepitus happens because of one of these 3 causes:

  • Gas bubbles within the joint.
  • Rubbing of tissues with each other.
  • “Stick-slip” phenomenon.

These are not harmful, they’re part of how our bodies work.

But if you have pain and/or trauma associated with the sound, make an appointment with a knee specialist to check what’s going on.

Resources

  1. Mccoy, G., Mccrea, J., Beverland, D., Kernohan, W., & Mollan, R.A. (1987). Vibration arthrography as a diagnostic aid in diseases of the knee. A preliminary report. The Journal of bone and joint surgery. British volume, 69 2, 288-93 . https://doi.org/10.1302/0301-620X.69B2.3818762
  2. Song, S. J., Park, C. H., Liang, H., & Kim, S. J. (2018). Noise around the Knee. Clinics in orthopedic surgery10(1), 1–8. https://doi.org/10.4055/cios.2018.10.1.1
  3. Kawchuk, G. N., Fryer, J., Jaremko, J. L., Zeng, H., Rowe, L., & Thompson, R. (2015). Real-time visualization of joint cavitation. PloS one10(4), e0119470. https://doi.org/10.1371/journal.pone.0119470
  4. Roscher, M. Knee Crepitus. Physiopedia. Retrieved on April 21, 2021 from: https://www.physio-pedia.com/Knee_Crepitus
  5. Nelsen, E. Why do your knuckles pop? TedEd. Retrieved on April 21, 2021 from: https://ed.ted.com/lessons/why-do-your-knuckles-pop-eleanor-nelsen
  6. Unger D. L. (1998). Does knuckle cracking lead to arthritis of the fingers?. Arthritis and rheumatism41(5), 949–950. https://doi.org/10.1002/1529-0131(199805)41:5<949::AID-ART36>3.0.CO;2-3
  7. How much physical activity do adults need? [Last reviewed: October 7, 2020]. Centers for Disease Control and Prevention. Retrieved on May 6, 2021 from: https://www.cdc.gov/physicalactivity/basics/adults/index.htm
  8. Raud, B., Gay, C., Guiguet-Auclair, C. et al. (2020). Level of obesity is directly associated with the clinical and functional consequences of knee osteoarthritis. Scientific Report. 10, 3601. https://doi.org/10.1038/s41598-020-60587-1