In “Man Cracks Knuckles for Decades to Annoy Mother, Science Backs Him”, Mr Joshi made one claim that sounded like a family joke. Once a knuckle has cracked, he said, it can’t be cracked again for twenty minutes. “I have timed it many times,” he said, and his wife confirmed that he had. It turns out to be one of the most reliable findings in the whole strange science of knuckle cracking, and it explains what the crack really is.
Start with a bottle of soda. Before you open it, the drink inside looks almost completely still. There’s a great deal of carbon dioxide in it, but it’s dissolved, spread out between the water molecules, invisible. It stays dissolved because the bottle is sealed under pressure. Open the cap, the pressure falls, and the gas comes rushing out of the liquid as bubbles. Chemists describe this with Henry’s law: the amount of gas a liquid can hold dissolved depends on the pressure. Squeeze harder and more gas dissolves. Release the pressure and the extra gas has to leave.
Your finger joints are tiny sealed bottles. Each knuckle sits inside a capsule filled with synovial fluid, a slippery liquid that lubricates the joint, and like any body fluid, it contains dissolved gases. When you pull a finger or bend it backwards, you try to separate the two bone surfaces. But the capsule is sealed, and no extra fluid can flow in to fill the space, so instead the pressure in the fluid falls. At first the joint resists, and the pressure keeps dropping. Then, past a critical point, the fluid suddenly gives way: in a fraction of a second, a pocket of gas forms, the joint surfaces jump further apart, and you hear the crack.
Engineers at Leeds measured this as early as 1971, pulling on volunteers’ finger joints with a machine and recording the force. The force rose steadily, then dropped abruptly at the moment of the crack, as the joint surfaces leapt apart. In 2015, the MRI videos Mr Joshi plays at family functions showed what happened inside: a dark gas-filled cavity appearing in the joint fluid at the moment of the crack. And here’s the important detail. The cavity didn’t vanish. It was still there afterwards.
That is the twenty minutes. After a crack, the joint contains a pocket of gas, and the fluid around it has lost much of the dissolved gas that made the sudden event possible. Pull the joint again and the bones simply separate a little, smoothly, with no snap, because the cavity is already there to take up the space, and there’s no sudden change left to happen. Before the knuckle can crack again, that gas has to dissolve back into the fluid and the joint has to settle back into its tight, sealed state. Gas dissolves slowly, especially in a fluid that isn’t being shaken, and in practice it takes around twenty minutes, sometimes longer. Mr Joshi’s stopwatch was measuring the speed of a gas dissolving.
What actually makes the noise is still being argued about, as the original piece said. For decades, scientists thought the sound came from bubbles bursting. The 2015 video seemed to show the opposite, the cavity forming, not collapsing. Then, in 2018, two engineers built a mathematical model of the joint and found they could reproduce both the sound and its loudness if the cavity forms and then partly collapses, in a few thousandths of a second. The answer may involve both. It’s a nice reminder that even something you can do with your own hand, a thousand times a day, can still hold a scientific puzzle.
Synovial fluid itself is worth admiring. It’s thickened by a long, chain-like molecule called hyaluronic acid, which gives it roughly the consistency of egg white and makes it a superb lubricant: the cartilage surfaces in a healthy joint slide over each other with less friction than ice sliding on ice. The fact that its dissolved gas can be coaxed into a sudden pocket with a sharp pull is a side effect, harmless as far as the evidence shows, and, as Mr Joshi’s mother might say, extremely irritating.
So next time someone near you cracks their knuckles, and then tries the same knuckle again and fails, you’ll know what you are hearing in the silence. It’s Henry’s law, slowly putting the gas back.