What causes that sound when I crack my knuckles? Is it bad?

Stan Megraw
23 January 2012

Above: Image © iStock, Staras

The HKC may be your best friend, your annoying brother or a complete stranger. Maybe you yourself are a HKC. In any case, the HKC has likely been told on more than one occasion that knuckle cracking can lead to arthritis or some other type of damage to their joints. This is because of the popular misconception that the cracking sound is caused by the scraping of bone or cartilage.

To understand what causes the "cracking" in knuckle cracking, we need a short lesson in anatomy, along with a bit of basic chemistry. Let's start with the anatomy of a knuckle or, more precisely, the metacarpo-phalangeal joint.

Anatomy of the Knuckle

Joints in your fingers consist of two adjacent bones, the ends of which are each covered by cartilage. Strong, fibrous tissue called ligaments as well as a joint capsule hold the two together (see: http://www.lrn.org/Popup/Skeletal/figure5_8.html for the anatomy of a knuckle).

Between the bones is a space filled with a small amount of viscous liquid known as synovial fluid (produced by the synovial membrane). This fluid acts a lubricant to protect the joint.

Researchers have found there are dissolved gases in the synovial fluid, consisting mostly of carbon dioxide (~80%), along with some nitrogen and oxygen.

And this is where we turn to the chemistry side of things.

Chemistry of Gases

There are many so-called "chemical laws" dealing with gases... Boyle's Law, Avogadro's Law, Charles's Law, etc. As it turns out, one of these (Henry's Law) can be used to explain the cracking sound we hear when knuckles are cracked.

According to Henry's Law, the concentration of a gas in solution is directly related to the pressure of the gas above that solution.

Let's look at a simple illustration (see image above).

When a gas is under low pressure, there are few gas molecules colliding with the surface of the liquid and going into solution. If we were to double the pressure above the liquid, the number of collisions with the surface would also double; and the concentration of gas in solution would increase two-fold.

Putting it all Together

At this point you may be saying to yourself "That's all very interesting, but what has any of this to do with cracking knuckles?"

Well, think about the how a HKC performs their "special talent". It may involve bending the fingers towards the back of the hand, squeezing the fingers into the hand, or pulling each finger away from the hand. Any of these can cause the knuckle ligaments to suddenly stretch, the two bones to separate from each other, and the joint capsule to expand.

The sudden expansion of the capsule increases the volume of the space holding the synovial fluid. As a result, there is a decrease in the gas pressure inside the joint capsule. With the drop in pressure inside the joint capsule, the concentration of dissolved gases in the synovial fluid must also decrease, as we learned from Henry's Law.

Now we get to the interesting part ("it's about time..." you may be muttering).

What happens next is that the gases will start to come out of solution and form a bubble on the surface of the fluid. When the ligaments and knuckle joint push back into place, this bubble is burst and we hear the "crack" that sounds like bones being broken.

CURIOCITY NOTE: for more on gas and pressure see "The Chemistry of Cola" in the Home-works section.

And, if you're a HKC, have you ever wondered why you can't crack your knuckles repeatedly, but must wait for a while? It's because it takes about 30 minutes for the released gases to re-dissolve into the synovial fluid and get ready for their next "performance".

In Closing...Is it bad?

Well, other than the fact that many people find it annoying, it's apparently safe to say that a HKC is not likely to develop any serious physical effects from cracking their knuckles. At worst, the scientific evidence suggests they may only experience some loss of strength in the grip of their hands as they get older. As for the supposed link between knuckle cracking and arthritis (inflammation of a joint, resulting in pain, stiffness and swelling), I've found no proof to support that claim.

We would like to thank Dr. Stan Megraw for his help and expertise in answering this question. Stan is a writer/researcher specializing in science, technology and medicine. He has more than 30 years experience as contributing author for research institutes, universities, government agencies and non-profit organizations.

Stan Megraw

Stan is a writer/researcher, a PhD graduate of McGill University and was a member of the CurioCity team for several years. As a kid he dreamed of playing hockey in the NHL then becoming an astronaut with NASA. Instead, he ended up as an environmental research scientist. In his spare time Stan enjoys working on DIY projects, cooking and exploring his Irish roots.

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