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Thanks for the e-mail requests to post the info on joint popping from my scuba fitness column, April issue: A certain amount of gas can dissolve in your body fluids, including joint fluids called synovial fluids. When pressure is decreased, the gas can come out of its dissolved form in solution. If that happens quickly, bubbles form. There are easy ways for this to happen, for example, coming up from a dive too fast, popping your knuckles, or squatting down and cracking your knees. Cracking your knuckles doesn't cause or relate to decompression sickness, although the bubble formation can be similar. Exactly how and why is extraordinarily complex. A summary of a few of the basics follow: Gases like carbon dioxide and nitrogen are present in dissolved form in your body fluids. They are held in solution by the pressure of air around us and by gas pressures in your body. If you pull your finger joint so that it enlarges briefly, the slight vacuum formed creates a temporary low pressure cavity. The low pressure pulls gas molecules out of solution forming bubbles in a process called vacuum cavitation. The bubble quickly collapses. Pop. It takes time for the gas to redissolve before the joint can crack again. Another way bubbles have been shown to form is in areas where muscle is crushed against bone, as during knuckle cracking. Pop. Studies have also looked at body tissues that are full of dissolved gas but no bubbles, even where the pressure reduction was large. However when energy was added by moving the tissue, for example with muscle contraction, bubbles readily formed. Pop. Marine propellers cavitate bubbles by similar mechanical forces. The rotating propeller blade rapidly forms low pressure areas. Dissolved gas cavitates out of the water near the blades. When these many tiny bubbles suddenly form then collapse with great vigor against the blade, they make noise and eventually pit the blade. Pop. Pop. Anticavitation devices are important tools to extend the life of propellers and minimize military submarine noise. Not all joint cracking is due to bubbles. Often it involves tendons moving quickly over adjacent bony knobs, making a distinctive noise. Pop. Tendons are tough inelastic fibers that connect a muscle to a bone. Snapping tendons occurs routinely in many people's shoulder, elbow, hip, back, and ankle joints. Occasionally tendon snapping occurs in the knee. This kind of snapping, unlike knuckle popping can be repeated within short intervals. Pop. Pop. Pop. Sometimes things snap as you walk. On rare occasions it is so painful, constant, and detrimental to sports activity that surgery is performed to move the tendon to a slightly different position to avoid the snapping. Another reason for joint popping is the occasional tendon or ligament that gets pulled over a bone as muscles tighten with bad sitting, standing, and lifting postures all day. People often learn that they can 'crack' their back to restore a more comfortable position. Pop. Although it often feels good, it is not a miracle cure. It is better to learn to identify and minimize the muscle stress that caused the need in the first place. How detrimental knuckle and other joint cracking may be to your joints depends on degree. It's probably safe to avoid any vigorous trauma to your joints. Joints are important parts of your body. Joint deterioration is related to the extent of abuse, genetics, and other not well identified factors. Although not a certainty, chronic, traumatic joint stress may be a contributor to early wear and tear that may lead to arthritic degeneration. ------------------------ btw - regarding the question about whether bubbles formed in joints or tissue can pass into your circulation - bubbles, even though tiny, are generally too big to physically pass through blood vessel walls. They may dissolve for passage through vessel walls then reform into bubbles, but they are too big to fit through the vessel walls.
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