Extracorporeal generated shock waves have been introduced for medical therapy almost 40 years ago to disintegrate kidney stones. Since this time shock waves have changed the treatment of kidney stones substantially. Urology is not the only medical field for shock waves in medicine. For more than 30 years shock waves are in use in Orthopedics and treat tendons, ligaments and bones. The idea of the shock wave therapy for orthopedic diseases is the stimulation of healing processes in tendons, surrounding tissue and bones. This is a completely different approach compared to urology where shock waves were used for disintegration. Also in Traumatology shock waves are in use to treat non- or delayed unions, avascular necrosis of the head of femur and other necrotic bone alterations.
In physics, a shock wave as used in medicine is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary sound wave, a shock wave carries energy and can propagate through a medium but is characterized by an abrupt, nearly discontinuous, change in pressure, and density of the medium1.
This might sound a bit unfamiliar but it describes a shock wave quite well, a short acoustic pulse of high amplitude (pressure) traveling through the soft tissue at the speed of sound. In medical application these pulses are repeated several times per second and several hundred up to a few thousand shock waves are applied per therapy session.
To generate a shock wave, an electric high voltage discharge between two electrode tips in water can be used. This discharge generates a gas bubble of partially ionized water which expands rapidly at a speed higher than the sound velocity. The expansion decelerates quickly and a shock wave front is released from the surface of the bubble at the very moment the expansion velocity drops below sound velocity.
To get the shock wave front to the region of interest, a metallic reflector can be used. With an reflector shaped like an half ellipsoid and the bubble generated in the first focus F1 of the ellipsoid, the shock wave front gets reflected and focused to the second focus F2. At F2 the shock wave has its highest intensity and pressure. The region of interest normally is placed around the F2. To transfer the shock wave from the reflector to the body, a silicone membrane is placed between the reflector and the body. With some ultrasound gel or water between the membrane and the body surface, the shock waves can travel with only little losses into the tissue.