We’re just completing joints in Anatomy, and I had several students ask me about cracking or popping joints. The best answer I found in Scientific American. Here’s a summary:
The popping sound comes from stretching the synovial capsule so wide it reaches its limit. If you stretch it even more, dissolved gases in the synovial fluid come out of solution – they become gas. This gas is mostly carbon dioxide, and making it a gas rather than being inside the liquid allows the joint to stretch out even more.
They have taken x-rays of joints after cracking, and they can see the gas bubble inside the joint. You may notice after cracking a joint, you can’t crack the joint again for a while – until the gas dissolves back into the synovial fluid.
Does it cause damage? The Scientific American article states: There are actually little scientific data available on this topic. One study found no correlation between knuckle cracking and osteoarthritis in the finger joints. Another study, however, showed that repetitive knuckle cracking may affect the soft tissue surrounding the joint.
The 2nd way joints pop is by shifting of a tendon. Students may remember that tendons cross over a joint to TIE a muscle to its bone attachment. It is not uncommon for a tendon to shift to a slightly different position, followed by a sudden snap as the tendon returns to its original location with respect to the joint.
Now you know.