There are a lot of preconceived ideas about chiropractic adjustment - and, many people are afraid of the “cracking” sound that often accompanies a classic manual manipulation. Rest assured, the crack you hear is NOT bones breaking, but is a result of gas bubbles being released from the fluid in your joints - this sound actually has a name: cavitation.
An analogy I often use to describe what’s happening is that of a soda bottle. You may have noticed that when you open a bottle of pop, bubbles seem to form out of nowhere. The carbonation process has infused CO2 into the liquid; kept under pressure, it will stay there. When the volume of the container holding the liquid increases (or the cap is opened), those CO2 molecules (which would rather be in gas form) now have somewhere to go, and come out of solution forming gas bubbles. If you open the bottle slowly, the bubbles form slowly, fizzing to the surface of the liquid; if opened quickly, they “pop” out of solution all at once.
This is very similar to what happens in our joints when we stretch them. Our joints are filled with nutrient rich liquid called synovial fluid. There is nitrogen gas dissolved within this fluid. Unlike a pop bottle, our joints are flexible structures enclosed by a soft capsule that can stretch. As our joints move the volume of the space changes allowing these nitrogen molecules to move into and out of solution on a regular basis. If the joint moves slowly, these bubbles will form slowly and no noticeable noise will follow. But if stretched quickly enough the gas bubbles will pop out of solution making a cracking sound.
This being said, you should never agree to a procedure that you are not comfortable with. If you are afraid, there are other ways to get the joints moving that don’t require a crack. So talk to your chiropractor about different treatment methods that will meet you at your comfort level.