Joint cracking

The pressure inside the joint cavity drops and the dissolved gas suddenly comes out of solution and takes gaseous form which makes a distinct popping noise.

To be able to crack the same knuckle again requires waiting about 20 minutes before the bubbles dissolve back into the synovial fluid and will be able to form again.

[3][4] For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was uncertain.

Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion.

[16] This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it.

MRI of a cracking finger joint depicting cavitation between the bones
Static images of the hand in the resting phase before cracking (left). The same hand following cracking with the addition of a post-cracking distraction force (right). Note the dark, interarticular void (yellow arrow).