β Why do joints crack?β
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Because they must be so flexible, synovial joints have lots of lubrication in the cavities of joints to keep things slick and moving. Fun fact: This lubrication, known as synovial fluid, has the consistency of thick egg whites.
There are gases, including oxygen, nitrogen, and carbon dioxide, found naturally dissolved in this synovial fluid. When you pull on the joints to crack them, the joint cavity with the synovial fluid expands which creates a low-pressure environment. In this low-pressure state, the gasses make a bubble in the synovial fluid.
Here's a demonstration of how pulling to expand the volume of the container of liquid causes gas bubbles to form:
It's hypothesized that the sound of cracking your joints is caused by either the rapid formation of these bubbles or the rapid collapse of these bubbles.
Researchers have managed to capture to formation and collapse of knuckle bubbles (great band name) on camera using an MRI. See the actual MRI imaging from the 2015 study below. In this GIF, you can watch as a black spot forms within the joint cavity. That black spot is a gas bubble.
Unfortunately, the MRI machine doesn't record the images fast enough to determine whether the cracking sound is occurring as the bubbles are forming or as they are collapsing. But now we do know that the sound of joints cracking is due to gas bubbles in the synovial fluid.
Reason #2: Your ligaments and/or tendons are rolling over each other.
Gas bubbles aren't the only reason you hear snap, crackle, pops as you bend your joints. Another common reason for creaky joints is due to your ligaments and tendons rolling over each other.
Quick anatomy lesson: Ligaments and tendons are both connective tissues that help hold your joints together. The main difference is ligaments attach bones to bones, and tendons attach muscles to bones.
As you move your body around, these tissues can snap or slide over bones and other ligaments and tendons. This slip and slide can produce a cracking sound. For example, if you've ever had your knee crack as you went to stand up, that would be because of a ligament or tendon moving around.
One way to tell the difference between a crack caused by a gas bubble or a crack caused by tendons and ligaments if checking whether it is quickly repeatable. Gas bubbles need time to accumulate again in the synovial fluid, so if you can keep producing a snapping sound every time your roll your wrist, that would be because of the connective tissues.
There are a handful (sorry for the pun) of reasons why we crack our joints:
No. Despite the common warning that that cracking your knuckles causes arthritis, there is no evidence to support this. But we do have some evidence to support the notion that cracking your knuckles is harmless.
Determined to prove "his mother, several aunts, and, later, his mother-in-law" wrong, medical doctor Donald Unger ran a knuckle-cracking experiment on himself. For 50 years, Unger cracked only his left hand's knuckles every day, which totaled up to at least 36,500 knuckle cracks. At the end of the experiment, Unger found that there was no arthritis in either hand. Talk about a knucklehead.
That being said, if your knuckle cracking is accompanied by any pain, swelling, numbness, or stiffness, it's probably best to take a beat and go see a doctor.
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