Why Do Joints Crack?
All joints consist of the ends of two bones, which are connected by a strong capsule of ligaments. This capsule has a set space that contains a certain amount of synovial fluid (the ‘oil’ of the joint). The capsule contains just enough synovial fluid to fill the space, whatever it may be. When adjusting a patient, chiropractors separate the space between the bones, which causes the joint capsule to slightly stretch. The net effect is the space inside the capsule increases.
Now the scientific bit, as the space inside the capsule increases, the fluid pressure decreases. As the fluid pressure decreases, substances that are dissolved in the synovial fluid becomes a gas. You see this when you look at a coke bottle- no bubbles appear in the liquid until you open the top (which increases the volume and decreases the pressure to normal atmospheric pressure). The dissolved substances in the coke liquid turns into gas and bubbles are released.
Now that we are up to speed on gasses in fluids, when an adjustment is done, the gentle separation of joint surfaces increases the internal space, which decreases the pressure and releases gas from the synovial fluid, which results in the popping sound. It takes about 20 minutes for the gas to re-dissolve back into the fluid. As the gas dissolves back into the fluid the viscosity of the fluid changes and improves the function of the joint.
Just to let you know, chiropractors prefer patients to describe our treatment as adjusting the spine rather than “cracking” backs- It gives the impression to the general public that we are breaking bones (which of course is not what’s happening). The popping sound is a natural physiologic effect that occurs when the joint capsule is stretched in a certain direction.
Chiropractors mainly help restore the integrity and alignment of spine by adjusting specific vertebrae in a specific direction to remove any irritations to the nervous system. No harm can be done when the popping sound occurs with a proper chiropractic adjustment. The popping sound may also occur when you stretch or move- although this is not an indication that a correction has been made- so please, leave the adjustments to your chiropractor.