I am often asked what the popping or cracking noise is when adjusting a patient. Patients also wonder why you do not always hear a popping or cracking noise with each adjustment.
To answer, I first want to give a brief explanation of how an adjustment is performed. An adjustment is performed on restricted joints, discovered with palpation. Once the restricted segment or joint is determined the chiropractor can then apply a chiropractic adjustment using a technique that is specific for that joint and also appropriate for the patient. The actual adjustment is a highly controlled procedure, the chiropractor starts with a tissue pull to accurately target a specific joint. Next, a 'lock-out' of the joint must be achieved by bringing the joint to the end of its passive range of motion. Active range of motion is when you use your muscles to move a joint in each direction and passive range of motion is that extra motion that the joint can move if you push it with your hands. It takes years of technique training to learn how to properly bring each joint to the end of passive range of motion. Once the joint is brought to the proper position, the doctor then performs a high velocity low amplitude adjustment. In doing so, the Doctor pushes the joint a short distance past its passive range of motion and as a result, restoring proper motion in the joint.
When doing so, there is often a popping or cracking sound, called a cavitation. The noise is actually a collapse of gas bubbles that were in the joint fluid. While the adjustment is being performed, a change in pressure is created within the joint which leads to the release of gas bubbles. The noise is not made from bones, there would have to significant lack of normal anatomy for that to occur! There are a number of structures that separate bony structures within the body including ligament, connective tissue and fluid.
Some patients feel it is necessary to hear an audible "pop" in order for the adjustment to be successful. This is not the case, and in fact there is no scientific physiological data that suggests that is necessary in order to restore proper motion in the joint being adjusted. The "pop" or cavitation is just a sound created during the adjustment that a number of patients associate with a successful adjustment and find it satisfying.
•ACA Frequently Asked Questions: https://www.acatoday.org/level3_css.cfm?T1ID=13&T2ID=61&T3ID=152#popping