Fig. 1

Working principle of focused (on the left), defocused (in the middle), and radial (on the right) extracorporeal shock wave technology. In case of focused shock waves, single acoustic pulses are generated either with a spark-gap (electrohydraulic principle), a technology similar to a loudspeaker (electromagnetic principle), or piezocrystals (piezoelectric principle) (black bars represent shock wave generators; details are provided in Fig. 2). By means of reflectors of certain shape and/or the use of acoustic lenses, the acoustic pulses are converted into a focused acoustic pressure wave/shock wave with a point of highest pressure (red arrow) at the desired target (green dot) within pathological tissue. By changing the shape of the reflector (and/or the acoustic lens), the acoustic waves emitted from a focused shock wave generator can be converted into a slightly convergent, parallel, or even divergent acoustic pressure wave/shock wave (“defocused shock wave”). In case of radial shock waves, a projectile is fired within a guiding tube that strikes a metal applicator placed on the patient’s skin. The projectile generates stress waves in the applicator that transmit pressure waves into tissue. The point of highest pressure is found at the tip of the applicator. It is of note that any disturbance in the pathway of the acoustic pulses between a focused shock wave source and the target within tissue (such as bone, calcifications, etc.; red dots in the figures) may result in some parts of the acoustic pulse not reaching the target and, thus, weakening the shock wave energy (i.e., the energy density) at the target. The same disturbances would not impact the energy of radial shock waves at the target (for defocused shock waves, it is unknown to what extent they are weakened by disturbance in the pathway of the acoustic pulses between the shock wave source and the target within tissue). This is most probably the reason why in muscle tissue, the energy of focused shock waves was found to be decreased by > 50% compared to measurements in water, whereas for radial shock waves measurements in muscle tissue and water were consistent [36]