Countless times, I have heard people say that they are “afraid to get cracked,” whether because of the scariness of the sound itself, or the assumption that the sound could be coming from some sort of damage or breakage of bones.
So what is actually happening when we hear that crack sound?
Well, firstly, the crack sound is not coming from bones at all, but rather, from joints. Joints can make lots of different sounds for a variety of reasons, but the distinctive “crack” often heard during chiropractic adjustments has its own very interesting explanation. To understand it, we first need to understand what lies within our joints. Every joint in the body is surrounded by a joint capsule. Within that capsule are the 2 surfaces that move against one another, as well as some slippery, soap-like fluid called synovial fluid. This fluid hydrates the joint and allows for smooth gliding, and also helps nourish the joint surfaces. For a long time, it was assumed that the sound heard during a chiropractic adjustment was caused by a bubble within the soapy synovial fluid popping. A 2015 study sought to examine this phenomenon, and the results demonstrated that the reality is almost the exact opposite.
The study revealed that, during chiropractic manipulation, joint surfaces are rapidly separated. The nature of synovial fluid and the surround joint capsule is to resist this separation until a critical moment when they suddenly, rapidly separate. As the synovial fluid stretches rapidly, the fluid undergoes a change in chemical state from liquid to gas, as pressure is suddenly decreased by the increased volume of the synovial space. This means that an air bubble is actually forming during a high velocity, low amplitude (aka chiropractic) adjustment. The study termed this process of the formation of vapor cavities that do not collapse “tribonucleation” (Kawchuk, et al., 2015). Another paper describes tribonucleation as the mechanism of producing gas nuclei (bubbles) by making and breaking contact between solid bodies that are immersed in liquid (Ike’s, 1970). The results of the MRI study allowed researchers to determine that the sound heard when a joint “cracks” is actually the formation of a bubble, or vapor cavity, within the synovial capsule.
This fueled a completely new understanding for the chiropractic world, and may change the way you see adjustments. If you feel hesitant about chiropractic care, or know someone who is afraid to go to a chiropractor because they fear breaking something, please share this information! There is no breakage occurring, but rather new space forming which allows for greater freedom of motion within the joint and across the surrounding tissues.
Kawchuk, G. N., Fryer, J., Jaremko, J. L., Zeng, H., Rowe, L., & Thompson, R. (2015). Real-time visualization of joint cavitation. PloS one, 10(4), e0119470. https://doi.org/10.1371/journal.pone.0119470
Ikels, K.G. (1970). Production of gas bubbles in fluids by tribonucleation. Journal of Applied Physiology, 28, 524-527.
