The “Shock”-ing Truth about Pressure Waves
Updated: Feb 27
When extracorporeal shock wave technology was adopted in the medical field, the race for developing more performant and least costly devices started immediately. Due to this initiative, the shock wave devices were changed from expensive “water-bath” to “dry” devices. “Water bath” devices required the patient to be immersed in a bathtub filled with degassed water, while the newer “dry” devices were much more cost effective by allowing a therapy head filled with water to become the delivery system for shock waves while the patient was laying on a regular medical table. The change generated a significant price drop (80% to 90%) for the “dry” devices when compared to the “water-bath” systems.
The initial medical devices used a spark-gap discharge to produce pressure shock waves. These systems are known as “electrohydraulic” devices. The newer designs produced medical pressure waves by using one of three methods, “electromagnetic”, “piezoelectric”, or “concussive”. The electromagnetic devices use an electrical coil in close proximity to a metal plate. Piezo-crystal vibration was the next advancement in the field and led to the creation of piezoelectric devices. Concussive designs created an expanding wave front away from the concussion plate as a “radial wave”, sometimes also known as “soft”, ”unfocused/non-focused”, or “dispersive” waves. The new designs did not improved the performance and in some cases, it even dropped. However, their price was more attractive to physicians and patients, which constantly drove acceptance. At the forefront of the price drop are the radial devices, due to their relatively simple construction and durability, as they last large number of pulses without refurbishment.
As seen from above pictures, radial waves are created by accelerating a metal projectile inside a cylinder, known as an applicator body. The accelerated metal projectile then suddenly hits an impact body, concussion plate, or radial head that vibrates. These vibrations generate radial and unfocused pressure waves. Based on Physics principles, the energy decreases with the increase in radius from the source, which makes these radial waves have maximum pressure immediately at the exit from the applicator head at the impact body or point of origin. Therefore, upon entering the human body, they rapidly attenuate, thus the maximum effects of radial pressure waves are at skin surface. The radial waves can generate slight or strong side effects ranging from skin reddening to hematoma, depending on the energy/pressure generated by the radial shock waves.
Similar to ultrasound, the radial pressure waves can be described as ripples created when someone drops a rock in water. The intensity of the ripples, height of the ridges and their total number, depends on the size of the rock. The heavier the rock the higher the intensity.
Just as the intensity of ripples-effect in water, for radial pressure waves the higher the pressure in their epicenter or origin, the larger their penetration and action in the tissue will be. The same principle of action applies for High Strikers, as another analogy to better understand the action and functioning of radial pressure waves.
The punch’s intensity delivered by the hammer on High Striker moves the needle to various heights, which is similar to penetrations of radial pressure waves. Just to have a little fun with this analogy, strikingly enough, if the force or pressure is sufficient to get to the “1” mark for the High Striker, it can be equated to a penetration depth of 1 cm for radial waves. The “1” mark for the High Striker indicates a very weak guy or a “wimp”. By continuing the analogy with the High Striker, for a 2 cm penetration of radial waves, the Striker puts a person in the “daisy” category. A penetration of 3 cm gives a “big boy”, 4 cm translates in “big dog” and a 5 cm penetration makes you “he man”. Based on this analogy, the higher the initial pressure of radial waves, the more impressive the results should be, if not for the restrictive side effects, which might limit the radial waves to the “wimp” or “daisy” status of the High Striker.
Correspondingly, the radial waves in their epicenter can be compared to a boxing punch delivered during training to the punching bag.
In order to avoid debilitating side effects, the intensity of the radial pressure waves should be careful tuned down to dodge a “knockout punch” that can end the good effects of the radial pressure waves, or continuing with analogies, ending a boxing match.
Based on what we learned so far, the question that comes to mind is regarding the nature of radial pressure waves. Hence, are the radial waves truly shock waves or not?
To answer the question, manufacturers of such radial devices say “yes”, but the scientists say “no”. The debate reached the German Scientific Committee for Physics and Technology and the International Society for Extracorporeal Shock Wave Therapy. Based on these two scientific forums, a fundamental characteristic of medical shock wave devices is their ability to be focused, for achieving the maximum therapeutic effects at deep penetrations inside the human body and within their focus. Since the requirements for each treatment indication are different, it should be possible to adjust the depth of penetration for the focus to where the maximum shock wave energy should be found.
As seen from the above pictures, the radial pressure waves are not focused and have limited pressure or energy in order to not produce severe side effects such as a hematoma, which makes them NOT A SHOCK WAVE.
The German Scientific Committee for Physics and Technology and the International Society for Extracorporeal Shock Wave Therapy debated also about the specific parameters that characterize or ”makes” a shock wave and distinguish a shock wave from a pressure wave. Such parameters are the wave’s “rise time”, its “travel speed”, and the temporal duration of a pressure signal generated in the treatment zone, also known as “pulse duration”.
The shock wave standards and theory states that pressure waves reach the shock wave status when the shock waves’ “rise time” gets very short (tens of nanoseconds). As seen from the above picture, the radial waves have a rise time of few microseconds (or few thousands of nanoseconds), which is enormous when compared to tens of nanoseconds for shock waves. This large “rise time” shows that radial waves are slow-developing events when compared to shock waves.
The “travel speed” for shock waves is constantly the speed of sound, which is 300 m/s (0.186 mile/s) in air, 1500 m/s (0.932 mile/s) in liquids and up to 9000 m/s (5.592 mile/s) in solids. In general, the amplitude of a pressure wave dictates its non-linear distortion during propagation, which can speed up the wave enough to reach the speed of sound. As seen in the above picture, the radial waves can produce pressure of few MPa to reduce side effects, which based on theory will require tens of meters for traveling distance in order to reach the speed of sound. Clearly that will not happen in reality and thus the radial pressure waves used in medical field do not reach speeds equal to the speed of the sound.
The third parameter, “pulse duration”, shows a fast or a slow event. Shock waves have a “pulse duration” less than 10 microseconds, whereas the “pulse duration” for radial waves is more than 1000 microseconds, which points out a very slow event for radial waves when compared to shock waves.
Based on these main parameters characteristics that distinguish a shock wave from a pressure wave, the radial pressure waves are NOT SHOCK WAVES.
Furthermore, the following figure illustrates the difference between an extracorporeal shock wave therapy (ESWT) device and a radial device in terms of energy distribution and depth of penetration.
Due to the radial propagation, there is no focus for radial pressure waves. Radial pressure waves are the strongest at the tissue entrance and dissipate very fast afterwards. Based on the above graph, the radial pressure waves are efficiently used only for less than 1 cm penetration. This result is also confirmed by the graphic display or mapping of the pressure distribution produced by a radial device with a flat impact body, or concussion plate, or radial head. The largest pressures of 1 MPa generated by the lowest energy setting for the radial device are found in a region that has a 10 mm radius (corresponding to the radial head radius) and below 1 cm (10 mm) in axial direction from applicator head (positioned at the left side of the map).
In an effort to produce better penetrations, the radial devices’ manufacturers created a design for the impact body, or concussion plate, or radial head so that it has a concave surface to produce some focusing.
In this case, the largest pressures of 7 MPa generated by the highest energy setting for the radial device are found in a region that has a radius of 5 mm (corresponding to the concave portion radius of the radial head) and below 2 cm (20 mm) in axial direction from the applicator head (positioned at the left side of the map). It means that the radial pressure waves’ penetration can be increased by using a higher energy setting and by designing the impact body, or concussion plate, or radial head with a concave surface to produce some focusing. However, the radial pressure waves are still have the highest impact at the entrance into the patient skin and decrease quickly after 1-2 cm of penetration.
The significant differences between acoustic pressure shock waves and radial pressure waves can also be seen from the output-effects produced by these technologies, as cavitation generated by the negative portion of their respective pressure profile in the treatment zone. As seen before in one of the previous pictures, the pressure signal created by a radial device has a much smaller region of negative pressures when compared to the large region produced by the pressure shock waves, which should translate in less cavitational activity for the radial devices.
The above high-speed pictures of the cavitation bubbles confirm that for radial devices the cavitation phenomenon is greatly reduced and it is localized to the applicator head. In contrast for shock wave devices, the cavitation happens in a much larger volume known as focal region and at deeper penetration.
The German Scientific Committee for Physics and Technology and the International Society for Extracorporeal Shock Wave Therapy summarized in the following table their findings regarding the comparison of radial pressure waves (unfocused pressure wave therapy –UDWT) with the shock waves (extracorporeal shock wave therapy – ESWT).
Extracorporeal shock wave therapy (ESWT) is designed as a therapeutic modality in which focused shock waves are used. The maximum energy is in the therapeutic zone and the depth of penetration can be adjusted. Radial pressure wave therapy does not meet these criteria and therefore it is not an ESWT. In conclusion, the “shock”-ing truth is that the radial devices cannot produce any proper shock waves and by designating the radial pressure waves as “shock wave therapy” is misleading.
Keywords: ESWT, extracorporeal shock wave technology, extracorporeal shock wave therapy, wound care, diabetic foot ulcers, DFU, shockwave therapy, amputation prevention, dermaPACE, chronic wounds, SANUWAVE, PACE Technology