Evidence Supporting Extracorporeal Shockwave Therapy for Acute and Chronic Soft Tissue Wounds
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Focused, high-energy ESWT is utilized for delayed union and non-union of fractures, as well as lithotripsy; however, defocused, low-energy shockwaves are applied for soft tissue indications.
Shockwaves for use in medicine can be generated using different physical principles: electrohydraulic, electromagnetic, piezoelectric, and radial (Figure 2a). It is important to note that electromagnetic and piezoelectric sources only produce a typical shockwave in the focal area, whereas electrohydraulic systems produce shockwaves outside of the focal area as well. Figure 2b shows the differences in the acoustic pressure waves produced between electrohydraulic and radial shockwave sources.
Electrohydraulic. The original method of shockwave generation (used in the Dornier HM3) was electrohydraulic, meaning that the shockwave is produced via spark-gap technology. Electrohydraulic (Spark Gap) systems incorporate an electrode (spark plug) submerged in a water-filled housing comprised of an ellipsoid and a patient interface. The electrohydraulic generator initiates the shockwave by an electrical spark produced between the tips of the electrode (Figure 3). Vaporization of the water molecules between the tips of the electrode produces an explosion, thus creating a spherical shockwave. The shockwave is then reflected from the inside wall of a metal ellipsoid to create a focal point of shockwave energy in the target tissue.
Electromagnetic. Electromagnetic systems utilize an electromagnetic coil and an opposing metal membrane. A high current pulse is released through the coil to generate a strong magnetic field, which induces a high current in the opposing membrane. The magnetic field, in turn, induces a high current in the opposing membrane and accelerates the metal membrane away from the coil. These electromagnetic forces induce a slow and low acoustical pulse that is focused by an acoustic lens to direct the shockwave energy to the target tissue.
Piezoelectric. The piezoelectric effect produces mechanical stress via application of electricity. Piezoelectric ceramics or crystals, set in a water-filled container, are stimulated via high-frequency electrical pulses. The alternating stress/strain changes in the material create ultrasonic vibrations resulting in the production of a shockwave.
Radial. While focused ESWT is used to produce effects in deeper tissue and deliver higher density flux of energy to the tissue and can be used rather for destruction (0.15–0.6 mJ/mm2), ie, urinary stone lithotripsy, shockwaves indicated for soft tissue application are utilized for treatment of larger areas and delivery of lower energy density flux (0.08–0.25 mJ/mm2). In wound care, typically a larger surface area is necessary to achieve energy transfer via the shockwave therapy, and the head has a parabolic instead of an ellipsoid reflector. Positioning the opposing electrodes at the primary focus (F1) in a parabolic reflector will result in a planar wave, which is emitted after the reflection of the primary spherical wave.