waves that transmit in a non-sinusoidal motion pattern devoid of thermal and micro lesion effects, and achieve peak pressure amplitudes over a thousand times greater than that of an ultrasound wave [26, 28, 34, 76, 78, 79]. In 1997, an International Consensus Conference (1997) established the physical characteristics of an SW (Fig. 1).
Shockwaves utilized in medicine bear these characteristics (Fig. 1) and are propagated utilizing electrohydraulic (Fig. 2a), electromagnetic (Fig. 2b), or piezoelectric (Fig. 2c) technology [35, 76, 78, 79]. SWs created by each of these 3 sources are primarily propagated by a contained high-voltage discharge within a three-dimensional (3D) fluid-filled chamber that causes an ephemeral high pressure disturbance, where each discharge propagates a biphasic sonic impulse (Fig. 1) within the chamber. The accelerated and sudden rise from ambient pressure within the chamber creates an extremely short duration broad frequency spectrum (16–20 MHz) SW, that rises to its peak pressure (100 MPa) and implodes (–10 MPa) within nanoseconds (Fig. 1) of its lifecycle [26, 28, 35, 76, 78, 79].
Fig. 1. Characteristics of a shockwave: phase 1: high pressure wave rise-time from basic ambient value, to a pressure value of approximately 100 MPa within <10 nanoseconds (ns). Phase 2: wave implosion to a negative pressure value of approximately –10 MPa within microseconds. Image adapted from [8].
In order to ensure minimal attenuation of the shockwave’s energy at the refraction point (entry point onto target tissue), ultrasonic gel is utilized for maximal force transmission. Modern SW devices are capable of producing both focused and unfocused SW impulses of varying penetration depths and energy flux densities in order to cater to multiple treatment parameters. Pertinent factors to consider when comparing SWT technology are; pressure distribution, focal zone area, energy flux density, and the total energy concentration at the second wave (focal refraction) zone [28, 29, 34]. More recently the introduction of radial pulse devices have emerged, and due to the lower economic cost associated with radial type devices, its use has increased in popularity. It is of great importance to note that radial pulse devices often referred to as “radial shockwave therapy” produce a wave that has completely divergent physical characteristics (Fig. 3) from those of medical shockwaves as classified by the International Consensus Conference 1997 and as described in Figure 1 [28, 78, 79, 81, 84, 85]. Radial pulses are propagated by either compressed air or by a magnetic motor, where a metallic projectile within a barrel chute in the applicator is rapidly accelerated linearly within the chute. The ensuing ballistic energy occurring at the tip of the applicator (refraction point) is placed onto the skin of the target region, and this energy is then transferred onto the target tissue region as spherical radial pulse waves [28, 78, 79, 81, 84, 85]. The following are the key factors of wave divergence between medical shockwave treatment (SWT), and radial pulse therapy (RPT): principle of stimulus propagation, wave length, maximal energy pressure, wave speed, penetration depth, focal zone size, and maximal energy at the secondary focal (refraction) region. Although SWs, ultrasound waves, and radial pulses are considered as being acoustic waves, they each have completely divergent characteristics (Fig. 4), and will each have a unique action, influence on tissue, and clinical outcome.
Fig. 2. a Shockwave (SW) generated by an electrohydraulic device. At the focal point (1), an electrical discharge is released by an electrode causing heat water vaporization within the chamber. This generates a gas bubble to be rapidly filled with water vapor and plasma. The result of this extremely rapid expansion of the bubble is a sonic pulse, and the subsequent implosion of this bubble causing a reverse pulse, creating an SW. Wave reflection occurs with the assistance of a reflector (2), and the SW is converted and propelled forward as an acoustic pressure pulse. The point of highest pressure occurs at the secondary wave region (3). The secondary wave region or the wave refraction point is where the SW is aimed and transmitted into the desired treatment region. Image adapted from [8, 28, 35, 76]. b SW generated by an electromagnetic device. An adjustable magnetic field is generated by passing potent electric current via a coil (4), causing a high current in an antithetical metal membrane (5). The surrounding liquid in the adjacent membrane (6) is forced rapidly away (reflection site). Due to the high conductivity of the adjacent membrane, the liquid is forced away rapidly, and the ensuing compression of the surrounding liquid generates a SW. An acoustic lens (7) focuses the SW and transmits the wave to the secondary wave region (8). The secondary wave region or the wave refraction point is where the SW is aimed and transmitted into the desired treatment region. Image adapted from [8,
