What causes the Noise when you crack the knuckles?

Most people crack their knuckles every now and then, as it feels comfortable and one can enjoy more mobility for a while after cracking. This does make some people habitual or even addictive of cracking, while kids might do it just to produce the ‘popping sound’. Other joints that can crack are knees, ankles, back and neck.  Let’s have a closer look at our joints to explain the physical mechanism behind the cracking, grinding and snapping sounds produced by our joints.

We have synovial joints all over our body. A synovial joint is the most common and most movable type of joint in the body. In a healthy joint the ends of the bones are covered with the smooth hyaline (glass-like) cartilage and surrounded by a synovial capsule, which secretes the synovial fluid to lubricate and nourish the joint. The capsule and fluid also function to protect the cartilage, muscles and connective tissue and also serve to function as the shock absorber.

One of the causes of the sound production is ‘formation of bubbles’. As the synovial fluid acts as a lubricant, it contains variety of gases like oxygen, nitrogen and carbon dioxide. This gas rapidly gets released and forms bubbles when we stretch, twist or compress the joint while popping or cracking. It takes some time for the gases to return to the synovial fluid and the same knuckle can again be cracked thereafter.

Another possible factor is the movement of joints, tendons and ligaments. As a result of joint movement, tendon’s position changes and moves slightly out of place. As the tendon ‘returns’ to its original position, a snapping sound may be produced. Another phenomenon commonly observed in the knee or ankle is the production of a cracking sound as the ligaments may get tensed during the movement of joint.

Moreover, some pathology may produce abnormal sounds accompanied by pain. Some patients with arthritis (inflammation of joints, usually painful) notice sounds due to loss of smooth cartilage and the roughness of joint surface. Other conditions like bursitis, tendinitis may also produce sharp cracking sounds caused by snapping of irregular, swollen tissues of the joint.

As far as harmful effects of cracking are concerned, if cracking is accompanied by pain, there could be underlying abnormalities of the structures of the joint, such as loose cartilage or injured ligaments and one should seek a health care professional. In terms of knuckle cracking, the common claim that cracking one's knuckles causes arthritis appears unsupported. In fact, there is evidence that cracking induces muscle relaxation and slightly increases the mobility for a while.

Some studies show that knuckle cracking probably does not cause any serious harm while another study suggests that pathological addiction to knuckle popping may affect the joint soft tissue (higher risk of damages and wear) and worsen hand grip strength. Thus we can safely conclude that cracking does not cause any significant harm until done in moderation, while wild and unrestrained cracking habits can actually prove harmful for the joint.

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