What goes ‘crack’?

Something a fair number of us do it. And it isn't necessarily good if it is overdone. But oh, it feels so good. What am I talking about? Cracking your knuckles of course. Obviously, you are not really cracking your finger bones, so where does the sound come from? In a word, gas.

Cavitation

More precisely, the sound of gas, released from the sinovial fluids in the joints, popping. Gas bubbles form when the fluid pressure drops too rapidly, not giving it enough time to 'stabilise'. When we bend our finger joints, we stretch the joint capsules (see image). 

The amount of gas that dissolve in fluids obey a gas law known as Henry's Law. Henry's Law states simply that the amount of gas dissolved is directly proportional to the amount of pressure on the fluid (under constant temperature). So when we stretch the finger joint, we expand the size (volume) of the joint. But since the amount of synovial fluid is the same, the pressure on the fluid drops very rapidly. This very sudden drop of pressure causes the rapid formation of gas bubbles, which then pops. This same effect happens to deep sea divers that rise up from the sea too quickly. 'Bends' or decompression sickness is caused by bubbles forming in the body when the diver comes up from a high-pressure depth to a lower pressure surface depth too quickly. Divers are advised to make stops at certain depth as they rise gradually to prevent this.

Why can't I crack the my finger repeatedly without pause?

You would have noticed that you can't crack the same knuckle twice in a row. You need to wait for a while, usually somewhere between 15 to 20 minutes. This is called the 'refractory period'. During this time, the gas released within the joint is gradually dissolving back into fluid. So there isn't enough gas to pop out again until later.

Does it lead to arthritis? 

There hasn't been any proper study showing that joint cracking leads to arthritis. Arthritis or the more common form, osteoarthritis, is where the cartilage of the joint is worn out. Cracking the joints don't cause any significant wear and tear to the cartilage. 

But if overdone, joint cracking can stretch the joint capsule and the surrouding ligaments that support the joints.