Please let me know if this citation (and reference below) is useful to you.

“Why joints pop

Like nearly all the joints in your body, the knee joint is covered by a protective membrane containing synovial fluid. This fluid lubricates the joint, allowing it to move smoothly and easily.

Occasionally, tiny gas bubbles build up in this fluid. When the joint moves, the bubbles are released, causing the nearby ligaments to emit a snap or pop sound. The technical term for this phenomenon is crepitus, which also describes all grinding or crackling sounds and sensations in the body.

When to be concerned about joints popping

Most of the time, this popping and creaking of joints is harmless. However, crepitus is also a symptom of the joint degeneration that leads to osteoarthritis.

 You should worry about joint popping if:

•It's occurring frequently in one location

•It's accompanied by pain

•It's accompanied by joint swelling, tenderness, or stiffness

•You're also having pain as a result of prolonged joint movement, such as when walking

If you're experiencing pain when a joint pops or you have any other of the symptoms listed above, talk with your doctor. If your symptoms and test results indicate it, your doctor may diagnose osteoarthritis and start treatment. Treatments for osteoarthritis can ease pain, improve mobility, and slow disease progression—especially if it's caught early.”

Reference:

https://www.arthritis-health.com/blog/why-do-my-knees-pop

Dennis

Dennis Mazur

Please let me know if this citation (and reference below) is useful to you.

“Why joints pop

Like nearly all the joints in your body, the knee joint is covered by a protective membrane containing synovial fluid. This fluid lubricates the joint, allowing it to move smoothly and easily.

Occasionally, tiny gas bubbles build up in this fluid. When the joint moves, the bubbles are released, causing the nearby ligaments to emit a snap or pop sound. The technical term for this phenomenon is crepitus, which also describes all grinding or crackling sounds and sensations in the body.

When to be concerned about joints popping

Most of the time, this popping and creaking of joints is harmless. However, crepitus is also a symptom of the joint degeneration that leads to osteoarthritis.

 You should worry about joint popping if:

•It's occurring frequently in one location

•It's accompanied by pain

•It's accompanied by joint swelling, tenderness, or stiffness

•You're also having pain as a result of prolonged joint movement, such as when walking

If you're experiencing pain when a joint pops or you have any other of the symptoms listed above, talk with your doctor. If your symptoms and test results indicate it, your doctor may diagnose osteoarthritis and start treatment. Treatments for osteoarthritis can ease pain, improve mobility, and slow disease progression—especially if it's caught early.”

Reference:

https://www.arthritis-health.com/blog/why-do-my-knees-pop

Dennis

Dennis Mazur

good detail reply by Dennis Mazur. 

Dennis gave a great description and good reference.  Another non symptomatic popping can occur at any joint caused by the tendon gliding and the bone groove it sits in.  More common on joints like the ankle where the tendons cross at a more acute ankle but can occur at any joint.

thanks prof. Denis for the answer in details , 

so any painful popping think in damage to the joint 

Dear Dennis

Thank you for your valuable and detailed answer. Also, thank you for the suggested website.

Dear Brian 

Thank you for your valuable answer

..........................

I wonder if there is any published journal articles discuss this issue!

Mansour

This is a normal phenomenon of detachment of articular surfaces, in which nitrogen bubbles are formed. A click is heard when these bubbles burst. Such a phenomenon can be observed at any age. Perhaps this is an inborn quality.

The pubbing sound (cracking) heard in the joint with normal movement like waking, mostly due tiny gas bubbles build up in the joint fluid. When the joint moves, the bubbles transfer under the pressure of the joint movement with pubbing sound. It is different sound from crepitus which is related to osteoarthritis. The pubbing is harmless.

Dear Stepan

Thank your for your useful answer

Dear Salameh

Thank you for your clear and valuable answer

Knee cracking in a young person could be the heralding symptom of the patello-femoral syndrome. This is due to the slippage of the patella in and out of the intercondylar groove of the femur. This would cause a cracking sound on knee flexion like stooping and squatting.

Examination would show evidence of patella retinacular thickening and a clicking sound or sensation on passive gliding of the patella from lateral to medial.

In a young person with these symptoms I would look for some biomechanical abnormality like pes cavus in the feet etc and if present correct the biomechanics and start on a vastus medialis strengthening program.

Nice answer by Terence Babwah

Nice reasons and answers for the joint cracking. Regarding the tiny nitrogen bubbles bursting theory, how does it explain the relaxing and strecthing - comfortable feeling of the joint just cracked, or how does it explain the feeling of needing of the joints, namely in the fingers, to be cracked to feel the joint "free"?

Dear Terence

Thank you for your useful and detailed comment.

Hi Luis

Honestly, I have not found any research articles discuss knee cracking in normal adult who do not suffer from any symptom which was my main concern. However, I have some research articles discuss knuckle (fingers) cracking that may help you to answer your question.  Please find the attached documents. 

Good Luck!

Thanks for information about Knee cracking, frequently i have patient who do not suffer symptom asking me Knee and other articulacion Cracking, and I can´t answer this question.

Similar crackimg may occur in people with general joint laxity. Sometimes the reason may be looked for in periarticular structures, i.e. Coxa saltans, Tendovaginitis stenosans, etc.

Hi,

Claire Robertson @clairepatella on Twitter, has done quite a bit on this.  She has even developed patient information to reduce stress or anxiety around knee crepitus.  Hope that helps.