What’s the crack? This is a common question from patients when we use manipulation techniques called High Velocity Thrusts (HVT). A lot of people assume it is the sound of bones colliding together but in reality, the cause is less brutal and the sound is simply from gas bubbles within a joint popping.

A synovial joint is formed when bones are joined and held together with a fibrous capsule, which is a ligament-like material. On the inner side of this capsule is a synovial membrane which seals synovial fluid into the gap between the bones forming the joint. This synovial fluid helps reduce friction in the joint and also helps supply nutrients for the cartilage within the joint. This synovial fluid also has normal air gases (carbon dioxide, nitrogen etc) dissolved within it, which will be important in a moment…

When we manipulate a joint, it changes the shape of the capsule slightly and because it is a “sealed unit”, a drop in pressure is created within the synovial fluid. If the pressure drop is enough, that part of the fluid will turn to vapour and essentially create a bubble called a cavitation. This bubble immediately pops as the pressure of the synovial fluid normalises and…that’s the crack!