What’s poppin’?

“What’s that popping sound when you adjust me?” you ask. It’s gas (usually)! Specifically, gas bubbles located inside a synovial joint. This popping sound you hear is sometimes called cavitation and sometimes called tribonucleation. Both terms refer to the formation of gas bubbles within a liquid, but differ after that.

This is a synovial joint! A synovial joint consists of two bones connected by a joint capsule. The joint capsule consists of two layers: a fibrous outer layer and an inner layer called the synovium. Within the joint capsule is synovial fluid, this is where the gas bubbles form.

Synovial joints are the most common type of joint throughout the body. These joints include the facet joints of the spine, the joints between your finger bones, your knees and many more. There are two other structural joint classifications: fibrous and cartilaginous. Fibrous joints include the sutures in your skull and the connective tissue (syndesmosis) between your radius and ulna in your arm. These types of joints do not pop when adjusted. Cartilaginous joints can be found in the spine (the discovertebral joint) and the pelvis (pubic symphysis). Growth plates are also considered cartilaginous joints, but these will eventually disappear. Cartilaginous joints may produce a popping sound, but not as commonly as a synovial joint.

During an adjustment the popping sound that may be produced is not indicative of a “good” adjustment. There are a number of reasons why a joint may not produce a popping sound when adjusted, here are some:

1. The type of joint (see above).

2. The health of the joint. Arthritic joints most likely have a reduced joint space and may not produce as much synovial fluid. This can affect the ability of a joint to cavitate.

3. You “self-adjusted.” There is a refractory period between “pops.” If you “self-adjust” before your adjustment, there’s a good chance we won’t hear any “pops” during the adjustment.

4. The technique used. There are a variety of adjustment techniques in chiropractic and not all of them require a thrust that would result in an audible being produced. (If you don’t like being “popped” ask about these other techniques!)

While some studies show that following a joint cavitation there may be an increase in range of motion and a decrease in pain. This can occur without cavitation. Why would this be? Located within the joint capsule layers are receptors called mechanoreceptors. These receptors respond to pressure and movement. When we adjust a joint the mechanoreceptors are stimulated. According to some theories, this results in a blockage of pain signals from that area. This blockage occurs because the mechanoreceptors and pain receptors (nociceptors) use the same pathways to the brain. How cool is that?!