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The Joint-popping FAQ

David A. Nye MD

Q. What happens when I crack my knuckles or other joints?

Distracting (pulling on) the joint produces negative pressure in the joint. When the pressure falls below the solution pressure of gasses dissolved in joint fluid they come out of solution. Once a gas bubble is formed, the resistance to further distraction of the joint falls (the gas bubble can easily be rarified but liquid cannot) and the surfaces move apart rapidly, causing an audible pop.

Q. How do you know that a bubble forms? I hear this is an urban legend.

It's not. You can see the bubble form under fluoroscopy.

Q. Why does it feel better after I crack a joint?

Particularly in the neck and back, joint fluid may decrease in the joint space, most often because tight muscles across the joint force it out. When this happens, the joint does not move as well and becomes uncomfortable. Cracking the joint separates the surfaces and allows the joint fluid back in, providing lubrication again. The joint then feels better for a while until the joint fluid is forced out again. Tightening of the muscles is often due to paimxor stress.

Q. I think I have loose bone in my joints. My neck hurts and it crunches and pops when I turn it.

These sounds are caused by rubbing of the joint surfaces together as well as popping of the joints and result from increased pressure across the joint primarily from tight muscles (see above).

Q. Does cracking one's joints cause arthritis or other injury to the joint?

No. People who crack their knuckles or other joints often have a higher incidence of arthritis, but this appears to be because the discomfort of early joint disease makes people want to crack their joints rather than because cracking joints causes disease. Some joint diseases such as types of arthritis cause underproduction of joint fluid which may lead to an increased desire to pop the joint.

David A. Nye, MD
Midelfort Clinic, Eau Claire, WI

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