Whats the crack or pop with a osteopathic manipulation

When a osteopath manipulates the vertebra of the body it makes a noise which is very distinct and a common question that often arises is:
What causes the pop or crack when a manipulation is performed?

A simple description could be as follows;

The spine consists of vertebras separated by discs and enclosed in a membrane that contains fluid, when a manipulation is performed by an osteopath there is a mobilizing of the joint creating a cavitation, the gases within the joint change with the pressure which gives a popping sound relieving a lot of pressure in the joint and restoring its movement.

Now a detailed description reference the Department of Biomechanics, Michigan State University, East Lansing 48824, USAA extract from the service of the US National Library of Medicine and the National Institute of Healthhttp://www.medicalnewstoday.com/articles/40748.php

The audible release is caused by a cavitation process whereby a sudden decrease in intracapsular pressure causes dissolved gasses in the synovial fluid to be released into the joint cavity. Once a joint undergoes cavitation, the force-displacement curve changes and the range of motion of the joint increases. The gasses released from the synovial fluid make up about 15% of the joint volume and consist of approximately 80% carbon dioxide.
Habitual joint cracking does not correlate with arthritic changes, but does correlate with loss of grip strength and soft-tissue swelling.
During the "crack" associated with a joint manipulation, there is a sudden joint distraction that occurs in less time than that required to complete the stretch reflexes of periarticular muscles. Theories on the cavitation mechanism were reviewed and new information on the cavitation process is introduced. In this paper, it is proposed that the cavitation process is generated by an elastic recoil of the synovial capsule as it "snaps back" from the capsule/synovial fluid interface. CONCLUSIONS: Because the sudden joint distraction during a manipulation occurs in a shorter time period than that required to complete the stretch reflexes of the periarticular muscles, there is likely to be a high impulse acting on the ligaments and muscles associated with the joint. This is an important conclusion, because others have proposed that reflex actions from high threshold periarticular receptors are associated with the many beneficial results of manipulation. This suggests that the cavitation process provides a simple means for initiating the reflex actions and that without the cavitation process, it would be difficult to generate the forces in the appropriate tissue without causing muscular damage.

If you have any question or worries about the noise just ask you osteopath during your consultation and they will go into detail as required.