Video: Study Finally Shows Why Joints Crack

In frame C, you can see a black spot corresponding to a void, just after the joint popped. Kawchuk et al / PLOS ONE

You might expect that a simple phenomenon like joint-cracking would be well understood. But there isn't actually a consensus as to why knuckles and backs and the like pop when bent in a certain way or pulled upon.

Many scientists have thought that the audible cracking of joints—what you hear when you pull on your finger or press your knuckles together—is caused by the popping of bubbles in the fluid between the joint, based on an influential 1971 study. But new research suggests that's wrong.

In a study published today in the journal PLOS ONE, scientists got a volunteer to pop each of his 10 finger joints under an MRI. The scans show that the popping sound actually is caused by the formation of a bubble-like gap or void in the synovial fluid between the joints. So the 1971 study got it exactly backward: The popping sound is the creation of a "bubble," rather than the popping of one.

This phenomenon, in which a separating force between objects can cause an instantaneous separation and void, is "consistent with tribonucleation, a known process in which opposing surfaces resist separation until a critical point where they then separate, rapidly creating sustained gas cavities," the authors write.

This void is created when pressure—from bending or pulling—causes gases in the fluid (like nitrogen and oxygen) to come out of solution and form a bubble, or cavity. This gap can be seen in the MRI video above as a black spot between the bones. These bubbles of gas then gradually return to the synovial fluid; when they do, the knuckles can be cracked again.

Is cracking your joints harmful? Generally, studies have suggested that it's not associated with arthritis, according to various studies and the Beth Israel Deaconess Medical Center. So it's probably pretty benign. Then again, one study showed that the forces exerted during popping are theoretically enough to cause harm. So if you're into cracking your knuckles, you may err on the side of moderation.