1 EXTRACORPOREAL SHOCK WAVE THERAPY : What ? Why ? Safety ?

What are shock waves? Extracorporeal shock waves are pressure waves generated outside the body that can be focused at a specific site within the body. Shock waves are characterized by high positive pressures, up to 100 MPa (over 100 times atmospheric pressure), and negative pressures of 5-10 MPa. They have a rapid rise time of 30-120 nanoseconds (10-9) and a short, 5 microsecond (10-6) pulse duration. 1 The pressure waves travel through fluid and soft tissue and their effects occur at sites where there is a change in impedance, such as the bone-soft tissue interface. The common use for shock waves is to break kidney stones into fragments that can then be passed. 2 Shock waves are deflected at the border zones of tissues with different acoustic impedances including reflection and refraction of the wave. 1 This results in release of kinetic energy at the junctions, which can cause tissue alterations. For example, a kidney stone can be cracked by a certain amount of shock wave energy, whereas in bone, the same amount of shock wave energy does not result in fragmentation. The release of kinetic energy at interfaces of different acoustic impedances is crucial in planning ESWT. Shock waves must never be focused on gas-filled cavities like lung or intestine. The acoustic impedance of air is markedly lower than the acoustic impedance of soft-tissue such as muscle. Thus, virtually all acoustic energy is reflected at the border zone. As a consequence, maximum pressure at the border zone may turn into rarefaction pressure up to twice the extent of the former pressure wave and may result in considerable tissue damage at the border zone.