Chapter 94Electrophysical Agents in Physiotherapy
Electrotherapy has been a component of physiotherapy practice since the early days, but its delivery has changed remarkably and continues to do so. All electrotherapy modalities involve the introduction of some physical energy, which brings about one or more physiological changes that are used for therapeutic benefit. To appropriately select the most suitable modality, it is necessary to:
Electrotherapeutic Windows
The effectiveness of electrotherapeutic treatment is influenced by a number of factors, including the time after the injury at which treatment is applied, the “dose” administered, the amplitude or strength applied, and the frequency. An energy delivered at a particular amplitude has a beneficial effect, whereas the same energy at lower amplitude may have no demonstrable effect. Laser therapy provides a good example—one level will produce a distinct cellular response, whereas a higher dose may be destructive. A modality applied at a specific frequency (pulsing regimen) may have a measurable benefit, whereas the same modality applied using a different pulsing profile may not achieve equivalent results.
Therapeutic Ultrasound
Ultrasound (US) is a form of mechanical energy, not electrical energy, and therefore, strictly speaking, is not electrotherapy but falls into the group of electrophysical agents.1 Mechanical vibration at increasing frequencies is known as sound energy. In children and young adults, the normal audible sound range is 16 Hz to 15,000 to 20,000 Hz. Higher frequencies are known as US. The frequencies used in therapy are typically between 1.0 and 3.0 MHz (1 MHz = 1 million cycles/sec).
Sound waves are longitudinal waves consisting of areas of compression and rarefaction. When exposed to a sound wave, particles of a material oscillate about a fixed point rather than move with the wave itself. Any increase in the molecular vibration in a tissue can result in heat generation; thus US can be used to produce thermal changes in the tissues, although current therapeutic usage does not focus on this phenomenon.1,2 The vibration of the tissues may also have nonthermal effects. As the US wave passes through the tissue, the energy levels within the wave decrease as energy is transferred to the tissues.1
Therapeutic Ultrasound Waves
US waves are characterized by frequency, wavelength, and velocity. Frequency is the number of times a particle experiences a complete compression/rarefaction cycle in 1 second. The wavelength is the distance between two equivalent points on the waveform in the particular medium. In an “average tissue,” the wavelength at 1 MHz is 1.5 mm and at 3 MHz is 0.5 mm. The velocity is the speed at which the wave (disturbance) travels through the medium. In a saline solution, the velocity of US is approximately 1500 m/sec compared with approximately 350 m/sec in air (sound waves can travel more rapidly in a denser medium). The velocity of US in most tissues is thought to be similar to that in saline.
These three factors are related but are not consistent for all types of tissue. Typical average therapeutic US frequencies are 1 and 3 MHz, although some machines produce additional frequencies (e.g., 0.75 and 1.5 MHz), and the “long wave” US devices typically operate at 40 to 50 kHz, a much lower frequency than “traditional US” but still beyond human hearing range.
Ultrasound Waveform
The US beam is not uniform and changes in its nature with distance from the transducer. The US beam nearest the treatment head is called the near field. The behavior of the US waves in the near field is not regular, and areas of interference and energy can be many times greater than the output set on the machine (up to 12 to 15 times greater).
Ultrasound Transmission through the Tissues
Tissues present impedance to the passage of sound waves. The specific impedance of a tissue is determined by its density and elasticity. For the maximal transmission of energy from one tissue to another, the impedance of the two tissues needs to be as similar as possible. An air gap between the generator and the skin will result in the majority of the US energy being reflected rather than transmitted to the underlying tissues.
Coupling media, water, various oils, creams, and gels, are used to bridge the air gap. Ideally, the coupling medium should be fluid so as to fill all available spaces, be relatively viscous so that it stays in place, have an impedance appropriate to the media it connects, and allow transmission of US with minimal absorption, attenuation, or disturbance. At present, the gel-based media appear to be preferable to the oils and creams. Water is a good medium and can be used as an alternative. The addition of active agents (e.g., antiinflammatory drugs) to the gel is widely practiced but remains incompletely researched.
Studies3,4 have considered the effect of animal hair on the transmission of US to the underlying tissue and report that best penetration is achieved by clipping the hair. In addition to the reflection that occurs at a boundary because of differences in impedance, there will also be some refraction if the wave does not strike the boundary surface at 90 degrees. Essentially, the direction of the US beam through the second medium will not be the same as its path through the original medium; its pathway is angled. The treatment head is ideally placed perpendicular to the skin surface (i.e., at 90 degrees). If the treatment head is at an angle of 15 degrees or more from the perpendicular to the plane of the skin surface, the majority of the US beam will travel through the dermal tissues (i.e., parallel to the plane of the skin surface) rather than penetrate the tissues as would be expected.
Contraindications
Treatment Record
Records should be maintained of the machine used, the machine settings (frequency, intensity, time, and pulse parameters), the area to be treated (size and location), and any immediate or untoward effects.
Absorption and Attenuation
The absorption of US energy follows an exponential pattern: more energy is absorbed in the superficial tissues than in the deep tissues.7,8 Thus as the US beam penetrates further into the tissues, a greater proportion of the energy has been absorbed; therefore there is less energy available to achieve therapeutic effects. The half value depth represents the depth in the tissues at which half the surface energy is available and is different for each tissue and US frequency. Tissues with high protein content are greatest absorbers of US energy. Those with higher water and low protein content absorb little. This is very important when electing US as a suitable modality.1 It is impossible to know the thickness of each tissue layer in an individual horse; therefore average half value depths are used for each frequency: 3 MHz = 2.0 cm; 1 MHz = 4.0 cm. Tissues with absorption are tendon, ligament, fascia, joint capsule, and scar tissue. In cartilage and bone, a majority of US energy striking the surface is likely to be reflected.7-9
Therapeutic Effects of Ultrasound
The therapeutic effects of US are influenced by the treatment parameters chosen and are commonly divided into thermal and nonthermal.
Thermal
Therapeutic US may produce heat,10 especially in periosteum, collagenous tissues (ligament, tendon, and fascia), and fibrotic muscle. If the temperature of the damaged tissues is raised to 40 to 45° C, there is hyperemia, which may be therapeutic and help resolve chronic inflammation.11 However, nonthermal effects are probably more important.
Nonthermal
The nonthermal effects of US are from cavitation and acoustic streaming.7,12 Cavitation, the formation of gas-filled voids within tissues and body fluids, occurs in two types—stable and unstable—which have different effects. Stable cavitation occurs at therapeutic doses of US and is the formation and growth of dissolved gas bubble accumulation. The “cavity” acts to enhance the acoustic streaming phenomena. Unstable (transient) cavitation is the formation of bubbles at the low pressure part of the US cycle. These bubbles then collapse very quickly, releasing a large amount of energy that is detrimental to tissue viability. However, this does not occur at therapeutic levels if good technique is used.
Acoustic streaming is a small-scale eddying of fluids near a vibrating structure such as cell membranes and the surface of a stable cavitation gas bubble and affects diffusion rates and membrane permeability.11 Sodium ion permeability is altered, resulting in changes in the cell membrane potential. Calcium ion transport is modified, which in turn leads to an alteration in the enzyme and cellular secretions.
The result of the combined effects of stable cavitation and acoustic streaming is that the cell membrane becomes “excited” (up-regulates), thus increasing the activity levels of the whole cell. The US energy acts as a trigger for this process, but it is the increased cellular activity that is in effect responsible for the therapeutic benefits of the modality.1,2,13
Some US machines offer variable time: typical pulse ratios are 1 : 1 and 1 : 4. In 1 : 1 mode, the machine offers an output for 2 ms followed by 2 ms of rest. In 1 : 4 mode, the 2-ms output is followed by 8-ms rest period.
Ultrasound Application in Relation to Soft Tissue Repair
The process of tissue repair is a complex series of cascaded, chemically mediated events that lead to the production of scar tissue.
Inflammatory Phase
US induces the degranulation of mast cells, causing the release of arachidonic acid and its subsequent cascade.9,13 Thus therapeutic US is proinflammatory rather than antiinflammatory. The benefit of US may not be to “increase” inflammation, although if applied too intensely at this stage, it is a possible outcome; rather, US should act as an “inflammatory optimizer.”1 The inflammatory response is essential for effective tissue repair, and the more efficient the process, the more effectively it can advance to the next phase (proliferation). Studies have failed to demonstrate an antiinflammatory effect of US, and results suggest US is ineffective.14 US may be effective at normalizing inflammatory events and may have therapeutic value in promoting overall repair.1,7
Proliferation
During the proliferative phase (scar production), US is proproliferative and stimulates (cellular up-regulation) fibroblasts, endothelial cells, and myofibroblasts.1,9 US may maximize efficiency, producing the required scar tissue in an optimal fashion. Low-dose pulsed US increased protein synthesis and enhanced fibroplasia and collagen synthesis.15-17 Recent work has identified the critical role of numerous growth factors in relation to tissue repair, and there is some evidence for an effect of US on growth factors and heat shock proteins.13,17
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