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What causes bones to crack?

I'm 18 and I have extremely creaky bones and joints, especially in my hands. I crack my knuckles all the time ? is that bad? I've heard about 20 different explanations for cracking joints, so I don't know what to believe. What exactly causes cracking joints?
The popping sound that is created when you 'crack' your knuckles or your back or really any other joint in your body is related to the the anatomy of a synovial joint and the physical property of the gases dissolved in the synovial fluid within that joint. The majority of the joints in your body are known as synovial joints, the anatomy of a synovial joint is as follows: at the opposition of two bones (a joint) there is a small collection of lubricating fluid known as synovial fluid that is contained within a connective tissue structure known as a joint capsule. This synovial fluid acts like the 'oil' that allows the bones within the joint to slide back and forth on one another without creating a great deal of friction. when you 'pop' a joint, you are effectively stretching out the joint capsule (as you pull on the joint) and thereby slightly increasing the volume inside the joint. from the basic physical property that volume is inversely related to pressure (ie if you increase the volume any container, you decrease the pressure inside that container), as the volume of the joint increased, the pressure inside the joint space decreases and the partial pressure of the dissolved gas inside the synovial fluid also decreases. as a physical property of a gas in solution, when the pressure is decreased, it will bubble out of solution (think of opening the cap on a bottle of coke and all the bubbling that ensues). the main dissolved gas in your synovial fluid is nitrogen, and so when you lower pressure within the joint space, the nitrogen forms bubbles out of the solution and bursts--creating the popping sound. that nitrogen must then be redissolved back into the synovial fluid when the pressure returns to it normal level (i.e. when you stop manipulating the joint by pulling on it), this process can take up to 30 minutes--which is why you can't immediately pop a joint right after it has just been popped. hope this helps!

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