There is a lot of controversy regarding the chiropractic crack. Many people will try to let you believe that it is from the release of nitrogen gas in the joint. Some of the pops I have heard in peoples spines are so loud that they could not possibly be coming from a single tiny bubble of gas. After doing hundreds of thousands of adjustments, and from my own neck and back making cracking sounds thousands of times, I am certain that instead something that is stuck is releasing.
There has been a lot of research in the dental area about adhesions forming in the jaw joint. That along with other research I have studied, and my own experience I have come to the determination that there must be something stuck, which is being released. Well, that is my opinion, and I am sure I will here others now, but when the dust finally settles, I bet this will be the truth.