June 15th, 2021
Shockwaves are defined as transient pressure disturbances that are transmitted in a three-dimensional space(1). They have been used in medicine for several decades, and are currently a focus of interest for various specialties. Their effects are related to the capacity of biological tissues to convert mechanical pressure into biological signals that stimulate tissue regeneration in a process called mechanotransduction(2). There are two types of devices that generate shockwaves: radial and focused. We review below some of their technical differences and clinical applicability:
Radial waves are emitted through a pneumatic mechanism: compressed air fires a projectile, which in turn strikes a metal tool called a transmitter. This impact emits a wave that is transmitted radially, dissipating its intensity as it goes through the different tissue layers. For this reason, it is recommended for use on more superficial structures, such as plantar fascia and superficial tendinopathies (tennis elbow, patellar and achilles tendinopathies), and, according to the literature, the use of radial waves for the treatment of these conditions has proved very successful(3,4).
The depth of the impact can be adjusted, although it will always be less than a focused shockwave. For example, the BTL Industries' radial shockwave has several transmitters that allow for a treatment that is more precise and more comfortable for the patient: the 9 mm steel transmitter, which is designed for treating acupuncture points or small finger joints; the 20 mm vibrating transmitter, which is used in myofascial treatment; the 15 mm titanium transmitter, which is designed for treating deeper structures, to name just a few.
Some common and recommended indications for radial shockwave therapy are:
Focused shockwaves can be generated through four mechanisms: electrohydraulic, piezoelectric, electromagnetic, and electroacoustic (the latter being exclusive to BTL). Of the above, the electroacoustic and electromagnetic mechanisms offer an optimal ratio between the intensity (energy flux density) and extension of the focal area (the area where the energy received by the tissues is concentrated). However, those generated by electromagnetic mechanisms tend to have lower durability than those generated by electroacoustic mechanisms. Focused shockwaves are characterized by an energy peak generated in nanoseconds, followed by a period of negative pressure that lasts for several milliseconds(1). Unlike the radial wave, the generated energy is concentrated in a focal point that can go through several centimeters. The depth of the impact can be adjusted using coupling pads for both superficial treatments (skin wounds) and deep pathologies (avascular necrosis of the femoral head, pseudarthrosis in bones such as the femur).
Different studies have compared radial waves with focused waves, showing that they differ in terms of the depth of impact, the technique with which they are generated,(5) and the resulting pressure peak (intensity)(6). This is supported by studies performed on conditions affecting superficial structures, such as plantar fasciitis(7), superficial bone healing(5,8), diabetic foot ulcers(9), tendinopathy, and other knee soft tissue disorders(4), where it has been demonstrated that radial shockwave therapy boasts a similar effectiveness to focused shockwave therapy. In this way, it seems that both mechanical waves generate similar effects on biological tissues, albeit at different depths and intensities.
One of the most relevant biological effects of radial and focused shockwaves is cavitation(10), the effects of which are linked to angiogenesis(11) and vascular endothelial growth factor (VEGF) expression(12). Furthermore, potent long-term and short-term analgesic effects have been described with shockwave application. This would be linked to the reuptake of substance P and the decrease in substance P immunoreactive neurons in the dorsal root ganglia(13).
Other significant biological effects include the increase in transforming growth factor (TGF) beta-1, nitric oxide expression and production, NF-kappa-B activity suppression and pro-inflammatory cytokine production(2), although the latter seem to be specific to focused waves.
Shockwave therapy has proved effective in treating various musculoskeletal conditions and others related to its regenerative potential. Focused devices reach a greater depth than radial devices and they also provide a larger amount of energy, which makes them ideal for treating deeper conditions, such as pseudarthrosis and delayed bone regeneration, among others. For their part, radial shockwaves provide a lower concentration of energy and their impact is more superficial. They are ideal for treating more superficial structures, such as patellar tendon, achilles’ tendon, superficial trigger points, myofascial disorders etc. Both have demonstrated effectiveness in treating a range of conditions, with a large amount of scientific articles available in indexed databases. It will be up to the clinician to determine which of the two types of shockwave is more useful for their daily practice, taking into account the similarities and differences that have been described in scientific literature.