Joints can make a number of different sounds. The most familiar is the single, rather loud, popping sound of joints cracking. Joints like your knee are surrounded by a fluid-filled capsule that helps lubricate the joint. Like other liquids, joint fluid contains dissolved gas, in this case the gas is mostly carbon dioxide. Movements that increase the volume of the joint capsule reduce the pressure on the fluid and allow the dissolved gas to come out of solution and form bubbles. When the bubbles form, the gas inside them is under very low pressure. The joint fluid flows into the area of low pressure collapsing the bubble and releasing energy as the cracking sound you hear. The technical name for this process is cavitation and the bubbles formed can actually be seen on x-rays. The bubbles form rapidly but even after they collapse the gas re-dissolves into the joint fluid slowly, so it takes 15 – 20 minutes before a joint can be ‘re-cracked’.
An important issue is whether cracking injures joints. The energy released when joints crack is very small. Too small, in the opinion of experts, to directly damage joint structures. Studies of people who repeatedly crack their knuckles have not shown an increase in arthritis later in life, although this fact has not been established with certainty, and people have injured their joints while attempting to crack them. Your joints need to function trouble-free for many years, so this practice, which serves no good purpose (and which many find annoying), is best avoided. As with anything involving your body, your doctor is the best source of advice for concerns you have about unusual sounds or other changes in your body.
Mark D Baustian
West Middle School