A cracked knuckle is caused by bubbles that burst in the synovial fluid (which helps in the lubrication of the joints). The bubbles pop when the bones are pulled apart, either by stretching the fingers or bending them backward, creating a negative pressure.
Cracking Knuckles: Causes and Side Effects
The synovial fluid, which lubricates the joints, contains bubbles that explode when people snap the knuckles, which produces a "popping" or "cracking" sound.
