Have you ever had a cold glass of something sitting on a coaster and when you pick up the glass the coaster comes up with it? And in my case usually drips on my lap. This is due to the surface tension created by water as it touches glass. Now imagine holding down the coaster and yanking the glass off very quickly straight upwards. If you do this just right you will hear a pop. What is happening is that when you quickly lift the glass and separate it from the coaster for a split second there is nothing in the space between the glass and the coaster, a vacuum. But just as fast air rushes in to fill the space and the pop is from the sound of the air molecules slamming into each other from all sides.
One theory about the source of the crack is similar to the glass story. Sometimes when a chiropractor adjusts his patient he does what is called gapping the joint. He causes the joint to separate just a little bit which creates a gap or an empty space just like with the glass and coaster. Since there is not any air to fill this sudden vacuum like with the glass and coaster something else must happen. As it turns out we have a great deal of nitrogen gas dissolved in our bodies just sitting around. So this nitrogen gas comes out of solution and fills the vacuum made by the gapping and as the molecules slam into each other we hear the crack. That’s one theory and the most prevailing explanation from science.
However, it does not tell the whole story. You see, most of my adjustments do not gap the joint. If anything, it brings the joints closer together, not further apart. And, in fact, if you listen carefully, most often you will hear 3 cracks with each adjustment. This is because there are three joints between each pair of vertebrae. Two are the little facet joints similar to finger joints and about the same size. They are the gliding or guiding joints that control spinal motion. The third joint is the intervertebral disc, a piece of cartilage separating each vertebra from its neighbor. And if you listen even more carefully, the sounds coming from the adjustment are often described as 2 clicks and a clunk, the clicks coming from the facets and the clunk from the disc. And if you ask how the facet joints click when they aren’t gapped and how the disc makes any noise at all no one knows. So what’s that noise? Your guess is as good as mine. But even though we don’t know what makes some of the noises we do know that the adjustment is still very good for you.