Cracking Joints - Source

Source

The physical mechanism is uncertain, and cracking may possibly arise from several different causes. Suggested causes include:

• Cavitation within the joint—small cavities of partial vacuum form in the fluid and then rapidly collapse, producing a sharp sound. This explains the popping that can occur in any joint, such as during spinal manipulation. Synovial fluid cavitation is the most likely theory and substantial evidence exists in support of it.
• Rapid stretching of ligaments.
• Intra-articular (within-joint) adhesions being broken.

Of these hypotheses, perhaps the most popular is cavitation. When a manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low-pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution, making a bubble, or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. This process is known as cavitation. The contents of the resultant gas bubble are thought to be mainly carbon dioxide. The effects of this process will remain for a period of time known as the "refractory period", which can range from a few seconds to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.