“So, what exactly is that noise I hear when you crack my back?”
The term for the noise is “cavitation.” A simple definition for this term in general is the formation of bubbles in a liquid. How this applies to chiropractic adjustments is a combination of anatomy and a good ol’ high school physics concept.
Let’s start with the anatomy. Most joints in the body are surrounded by a joint capsule. Think of it as a balloon around the ends of the two bones that form the joint. Inside the joint capsule is a liquid, called synovial fluid. It’s a thick, clear liquid similar in appearance to raw egg whites, that helps to lubricate the joint. There are all sorts of different chemicals and substances that make up this fluid, some of which are gasses dissolved in it (think of a soda bottle before it is opened- no bubbles, but C02 is dissolved in the drink to make it bubble once it’s opened).
Next comes the physics portion. When you have a certain amount of gas inside a container, the pressure of that gas is directly proportionate to the size of the container. If you change the size of the container, the pressure of that gas also changes. To get super nerdy for a second, it’s called Boyle’s Law, and it describes how the pressure of a gas tends to increase as the volume of the container gets smaller, and vice versa.
Finally, to tie it in to the chiropractic adjustment. When we perform the adjustment, we are restoring movement to the joint. In doing so, we momentarily change the shape, and therefore size, of that joint capsule. This causes a momentary change in the pressure of the fluids/gasses inside of that joint capsule, creating a “bubble” (to go back to that soda bottle analogy, the bottle gets opened and the pressure changes, allowing the carbonation to start bubbling/fizzing). A split second later the adjustment is complete, the joint capsule resumes its usual shape and size and the bubble dissolves back into the synovial fluid.
The formation of this bubble (hence the term “cavitation”), and its subsequent disappearance, creates the “pop” or “crack” that is perceived during an adjustment.
Pretty cool, huh?
There are some other sounds that joints can make that are not a cavitation, so if you have any questions about any snaps, crackles, or pops, please bring them up to the doctors at your next appointment.