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Valentina Carlile Osteopata
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  • Writer's pictureValentina Carlile DO

What happens when it cracks?

What happens when it cracks?

To understand what happens when you 'crack' your knuckles or other joints, you need to make a brief excursus on the joints of the body. The type of joints that can crack are called diarthrosis. They are the most common type of joints. They are composed of two bones that connect through their cartilaginous surfaces; these surfaces are surrounded by a joint capsule inside which there is a lubricant, known as synovial fluid, which also serves as a reserve and resource of nutrients for the cells that maintain the articular cartilage. In addition to nutrients, synovial fluid also contains dissolved gases, including oxygen, nitrogen and carbon dioxide. The easiest joints to crack are those of the fingers (interphalangeal and metacarpophalangeal). As the joint capsule is stretched, its expansion is limited by several factors. When small forces are applied to the joint, one of the factors that limits movement is the volume of the joint. This volume is given by the amount of synovial fluid contained in the joint. Synovial fluid cannot expand without the pressure within the capsule decreasing (falling) to the point where dissolved gases can leave solution; when these gases come out of solution, they increase the volume and mobility of the joint.

The noise, the crack, is thought to be caused by the rapid exit of these gases from the solution, which allows the capsule to stretch a little more. The stretching of the joint is immediately contained by the length of the capsule. If you do an x-ray of the joint after the crack, you may see a gas bubble inside the joint. This gas increases joint volume by 15-20% and is made up mostly (80%) of carbon dioxide. The joint in question cannot be cracked again until the gas has dissolved back into the solution, which explains why we can't crack our knuckles repeatedly.

But how could the release of such a small amount of gas create all this noise? For this, there is not yet a good answer. The researchers evaluated the energy levels of the noise produced by using accelerometers to measure the vibrations created during the crunching. The amount of energy involved is very small, of the order of 0.1 mJ/mm3. Some studies have also shown that there are two sound peaks during crack, but the causes of these peaks are unknown. It could be that the first peak is related to the dissolution of the gas from the solution and the second is caused by the capsule reaching its length.

A common question on this topic is 'Is cracking bad?'. There is currently some small scientific data on this point that has shown that repetitive cracking can affect the soft tissues surrounding the joint. Other studies, however, have found no correlation between cracking the knuckles and possible osteoarthritis of the finger joints. However, this habit tends to create swelling in the hand and a decrease in grip strength.

In the human body, other sources of cracks come from tendons and ligaments around the joint. Tendons must cross a joint in order to produce movement, but when the joint moves, the position of the tendon in relation to it is necessarily modified. It's quite common for a tendon to slide into a slightly different position, and then snap as it returns to its original position. These noises are often heard in the knees and ankles when standing up from a sitting position or going up and down stairs.

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