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How can popped knuckles be so exceptionally loud? (continued)

Marthinus Roos
Elgin, Moray, UK

The surfaces of human joints fit against each other very well and have rims of synovium, the membrane lining of the joint. Together, these form a very effective suction cup, which work together with the surrounding ligaments and thickening in the fibrous 鈥渟acks鈥� around the joints, known as joint capsules, to hold the bones of the joint in the correct place and orientation.

When a distraction force (such as pulling on a finger) is applied, the ambient pressure in the surrounding tissues, which is equal to the air pressure surrounding the body, keeps the joint surfaces together until the force is big enough to overcome this. A vacuum is then suddenly formed in the joint space when the bones pull apart as far as the ligaments and capsule allow.

The sound is produced when the surfaces part at speed. This is known as a vacuum snap. There is no bubble formation involved (a vacuum isn鈥檛 a bubble).

In my career as an orthopaedic surgeon, I have never seen a patient with problems in their knuckle joints that could be ascribed to them cracking their knuckles. Therefore I deduce that the habit is harmless to the joints, although I can鈥檛 vouch for its effect on the person鈥檚 relationships.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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