CHAPTER 116 Extracorporeal Shock-Wave Therapy and Radial Pressure-Wave Therapy

Over the last 10 years, shock-wave equipment has evolved quickly. The original dry lithotripters were large with solid matriculating arms and complex aiming devices. The adaptation of shock waves to equine therapy has necessitated equipment with more flexibility and portability. Most equine applications do not require the fluoroscopic and ultrasonographic aiming devices that were included in the original equipment. A number of machines that meet these needs of the equine practitioner are now available. Concurrently with the increased popularity of shock-wave therapy, radial pressure-wave therapy has also become more common for equine applications. This technology evolved from equipment used in mechanical testing equipment.

These devices are differentiated by the type of waveform they create. A shock wave is a pressure wave with defined characteristics. Shock waves have an abrupt transition in pressure in both time and space, with a total duration of less than 1 microsecond (μs), peak pressures of 10 to 100 megapascals (MPa), and a rise time of less than 10 nanoseconds. Shock waves currently used for medical applications are focused on a point in front of the generator. Planar shock waves are shock waves that are not focused. They are relatively new to medical applications and are being considered as a potential mechanism for treating superficial musculoskeletal and skin wounds.

Even within true shock-wave generators, there are differences in the waveforms created. Shock waves can have different peak positive and negative pressures, rise times, and bandwidth. At this time the components of the wave that affect tissue are unknown. In the future, it may be possible to modify the wave to select the desired outcome. For lithotripsy applications, it is easy to determine whether a stone breaks. For musculoskeletal applications, the outcomes are not immediately obvious. Musculoskeletal targets are heterogeneous and acoustically complex. How this affects the shock wave is not yet known. A good example of how differences in waves affect outcome has been identified in the lithotripsy field. Originally it was thought that precise focusing on the stone at a rapid pulse rate would be most effective. However, less acute focusing, so that the focal point encompasses the entire stone at a slower frequency of 2 Hz or less, is more effective. At this time electrocardiographic triggering is frequently used in lithotripsy to control for breathing and cardiac motion.

Radial pressure waves have a rise time consistently around 1 us, and the highest peak pressure is about 8 MPa. None of the waveforms has a shock front. A notable difference between the pressure waveforms is the ability of the wave to propagate with minimal loss of energy. Shock waves can propagate through a fluid medium with minimal loss of energy, which permits a large amount of energy from the shock wave to penetrate the body. Radial pressure waves decrease in energy proportional to the square of the distance from the applicator head. Clearly the machines are different (Table 116-1 and Figure 116-1). The mechanism of action of shock waves and radial pressure waves is not fully understood and may be different, but both can induce biological effects. Conflicting results among studies suggest that further evaluation of equipment and energy settings is required.

Table 116-1 Comparison of shock and Radial Pressure Waves

Variable	Shock Waves	Radial Pressure Waves
Wave form
Rise time	5-10 nanoseconds	50 microseconds
Focusing	Yes	No∗
Maximum pressure location	At focal point	On surface
Energy loss	Minimal through fluid and tissue	Loss proportional to the square of the distance
Peak pressure	≈100 MPa	≈10 MPa
Energy flux density	0-3 mJ/mm2	0-0.3 mJ/mm2

∗ Includes “focused” endpiece

Figure 116-1 Schematic diagram depicting the three types of pressure waves. Focused shock waves concentrate the pressure at a focal point that can be centered up to approximately 75 mm deep. Planar shock waves are not focused and have a more diffuse pattern with less penetration. Radial pressure-wave generators create waves with a lower maximum pressure and deposit the maximum pressure on the surface.

SHOCK-WAVE GENERATORS

The equipment used in the veterinary market is essentially the same as that used in the human field. Electrohydraulic, electromagnetic, and piezoelectric generators are used to create shock waves for medical applications. Each type of generator has some differences in waveforms such as differences in rise times and tensile components. The waveform can be further manipulated by the reflector shape, the medium in which the shock wave is generated, and many other factors. At this time, it is not known which waveforms are better for specific outcomes.

To prevent reflection of the shock, the shock wave is generated in a fluid medium, such as water, and coupled to the skin with a fluid medium. To select a focal point depth, the hand piece is moved closer or farther from the surface. This can be done with interchangeable offsets, by using probes with different built-in offsets, or by inflating or deflating the end of the handpiece to move it closer to or farther from the target.

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