A few weeks ago an 8-year old patient stumped me with a wonderfully refreshing question: is it bad to crack your knuckles? Well, these common-sense questions aren’t taught in medical school, so I dug around and found the answers. First, from the Naked Scientists Discussion Forum is a description of what actually makes the popping sound:
The reason that joints “crack” is that when you pull on them, or put the joint into a certain position, the pressure drops within the synovial fluid that lubricates the joint. This pressure drop encourages a gas bubble (containing mostly carbon dioxide) to form within the joint, taking up about 15% of the joint space. This pushes on the ligaments and other structures that surround the joint making them pop outwards, producing the first of the two noises that you hear when you crack your knuckles.
Next, when the joint is moved again the pressure increases, causing the CO2 bubble to abruptly implode, which produces shockwaves and also sucks back in the ligaments. This we hear as the second cracking / snapping noise. Measurements suggest that the energy unleashed when this happens is only about 7% of the amount you would need to damage cartilage, making knuckle-cracking an unlikely cause of arthritis.
The post, along with other articles including a British Journal of Sports Medicine editorial, review the few studies that looked into this issue, and they agree that cracking your knuckles does not cause arthritis. However, there does seem evidence that it may cause weakness in hand grip strength.
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