First off, let us clear one thing up. It is not a crack. Nothing is cracked. "You crack eggs, not spines".
Between each joint in the spine is lubricant called synovial fluid. Synovial fluid consists of dissolvable gas particles that can be manipulated during an adjustment (cavitation) producing sound. During an Osteopathic adjustment, the joint is opened, creating a vacuum effect that sucks the gas out; causing the noise of an audible "pop or click".
There is never a time during an Osteopathic adjustment when an actual pop, click or crack of your spine occurs, as some may believe. It is nitrogen gas filling the space created by the vacuum effect during the adjustment.
A very similar affect to when you take a cork out of a bottle.
No! Not hearing an audible click or pop noise does not mean the adjustment (cavitation) has not worked. It means that there was no sound from the gas escaping the joint.
Yes! Osteopaths can preform many different types of adjustment, based techniques, not all of which cause an audible click or pop sound. Each patient is different and therefore, the Osteopath will assess you for what techniques will work be for you.