Admittedly, I usually get these questions in response to me cracking my knuckles in the middle of class. It’s a habit that I’ve had since I was a kid, and it is a difficult one to break. Needles to say, I’ve been warned a million times about how I would grow up to have arthritis or huge knuckle joints. Going into this article, I honestly wasn’t sure whether it was true or not, but I thought I knew what was making the sound.
What Makes Your Knuckles Crack?I have always heard that the popping sound you hear when you crack your knuckles is the sound of a gas bubble bursting in your synovial joints. A synovial joint is a movable joint where two bones meet in the body. The periosteum of each bone is covered with cartilage, and the entire space is encapsulated by a synovial membrane forming the synovial cavity. The synovial membrane is then supported by an articular capsule, which inserts into each bone on either side of the joint. The cavity that it forms is filled with a clear to pale yellow viscous liquid called synovial fluid that lubricates the joint, cushions and protects the bones in the joint, and nourishes the cartilage. It is formed when plasma filters across the synovial membrane, and then specialized cells called synoviocytes add various compounds including collagen, fibronectin, lubricin, and hyaluronic acid.
The task of lubricating the cartilage in the joint is performed by lubricin. In fact, one study showed isolated lubricin was able to provide the same amount of joint lubrication as whole synovial fluid. They determined this by isolating just the amount of lubricin that would be found in the synovial fluid of a joint and comparing joint friction to actual synovial fluid.
Hyaluronic acid also plays a major role in the function of synovial fluid. It is the component that gives synovial fluid its viscous texture. In fact, if you were to pinch a drop between your thumb and finger and then slowly separate them, the fluid would string out to 1-2 inches before breaking. That high viscosity causes your synovial fluid to act as a non-Newtonian fluid, allowing for the proper amount of lubrication to be achieved at different movement speeds. When the bones in a joint move quickly, it causes a lot of shearing force, which causes the synovial fluid to become more viscous for added protection. When the shear forces decrease, the fluid gets thinner.
Like other body fluids, synovial fluid has gases dissolved in it including carbon dioxide (CO2), nitrogen gas (N2), and oxygen gas (O2). Most people who crack their knuckles do it one of two ways: pulling them outward or bending them. Either way, it causes the joint cavity to enlarge. Since the cavity is sealed off (remember, articular capsule surrounding it plugs into the bones), the increase in volume causes a decrease in pressure being exerted on the gases (an example of Boyle’s Law). In return, the dissolved gases begin to pool together to form a bubble. Eventually, the bubble will grow too big and burst (check out a gif of the MRI). Now, I have always heard that the pop sound comes from the bursting of the bubble, but there is actually some debate about it.
A 1947 study conducted by Roston et. al. determined that the popping sound occured when the dissolved gases coalesced into one large bubble. They found that it took about 20 minutes before the knuckle would crack, again and figured that was how long it took for to gases to redisperse.
A couple of decades later, a study by Unsworth, Dowson, and Wright claimed that the popping sound was caused when the bubble collapsed, and the refractory period where the knuckle won’t pop again was due to the fact that the bones in the joint don’t immediately go back to their previous positions. Instead, they stay farther apart for a little while; and, since they already have an enlarged cavity between them, the attempt to pop the knuckle again isn’t able to create enough space to decrease the pressure enough to cause the bubble to pop.
But, an even newer study conducted in 2015 seems to indicate that we may have had it right the first time. They watched using an MRI machine while knuckles were popped and found that the popping sound consistently occurred before the bubble popped on screen. So, as you can see, the conclusion is… inconclusive. We don’t even know why the popping sound rings out so loudly!
But, Does It Cause Any Long Term Damage?According to many, yes it does! But, is this the truth, or just a way to convince people to quit making that annoying sound? It seems like it may be the latter. No evidence exists that links habitual knuckle cracking to arthritis, although one study showed that it could lead to decreased grip strength. There was even one researcher named Donald Unger who spent 60 years habitually popping the knuckles on one of his hands but not the other, and he found no difference in arthritis pain in one hand versus the other.
So, there ya have it! Crack away without fear of damage! Just don’t try to force a knuckle to crack. Over extending the knuckle may stretch tendons or damage cartilage, which could cause problems down the road.
In ConclusionEverything you know about cracking your knuckles is wrong. The sound probably comes from the formation of an air bubble in your synovial fluid rather than the popping of the bubble, and it is probably perfectly safe to crack away as long as you’re no HULK SMASHing them.