Whether you have been to see a chiropractor or not, you have probably heard that joints can “crack” or “pop” with an adjustment.  So, just what is that sound we are making?  A popular belief is that Chiropractors “crack” bones to get them back into place.  What we are really working on is the joint between two bones.  There are many types of joints in the body, the one we primarily are concerned here with the cracking are synovial joints which at minimum contain two bones and a fluid filled cavity called the joint space.  This space is filled with synovial fluid which helps with lubrication to decrease the friction between the bones with any-and-all movement.  Now, the sound of the crack is actually called a cavitation-the formation of gas bubbles within the synovial fluid of the joint.  During an adjustment, the bones are separated ever-so-slightly until the joint capsule becomes taut, and the pressure within the synovial fluid continues to decrease as the bones of the joint are separated.  Eventually the synovial fluid gets to a point where there is not enough pressure to keep the nitrogen gas within the synovial fluid held in suspension that it forms into small bubbles.  It is this formation and spontaneous collapse of these tiny bubbles back into the liquid that causes the audible sound of the cavitation-the crack.

Other previous theories included the sound coming from ligaments snapping past one another, others came form the process of the bones being smashed together.  But if you look at the small video from a recent study done you can see the nitrogen bubbles form and collapse in the joint as it is adjusted.  You can see the two bones of the finger and as they are separated, as this happens you see a small white cloud form in the joint between then “pop” a black bubble of nitrogen gas appears.  Cool, hey!!