When ultrasonic waves travel through tissues, they lose a certain amount of their energy, and this process is known as attenuation. For most tissues, attenuation increases as frequency increases, so a 1.0 MHz signal penetrates more deeply than a 3.0 MHz signal due to lower attenuation in the tissue.
Attenuation in tissues is caused by several mechanisms, which can include absorption, ray divergence and deflection.
Absorption is the main cause of ultrasound attenuation. Ultrasound energy is absorbed by tissues and is then converted into heat. Divergence of the ray is the rate at which the ray is dispersed by the transducer. Divergence of the ray decreases as the frequency increases (a higher frequency signal, therefore, has a more focused ray). Deflection includes the processes of reflection, refraction and dispersion.
An ultrasound therapy appliance consists primarily of an alternated current generator (typically 1 MHz and/or 3 MHz) that uses a cable to power an emitting treatment head in which a transducer (a piezoelectric disc or quartz lamina) is inserted. This transducer converts electrical energy into mechanical energy in the form of acoustic vibrations, which are then transmitted to tissues.
Ultrasound therapy can be administered in two different ways — either by direct contact (using a mobile or fixed head) or by immersion.
In the first case, the area to be treated is positioned in such a way as to be relaxed, naked and dry. Therapeutic gel or cream is applied. If the surface of the area to be treated is uneven from bony growths or painful to the touch (for example, in the case of arthritis in the hands), then the immersion technique. Immersion involves filling a tray with water and placing the applicator around 1 cm above the hands and the water to allow the ultrasounds to travel to the relevant site.
Ultrasound can be used to treat all pathologies of the locomotor apparatus (skeleton, muscles, cartilage, tendons, ligaments, joints and other connective tissue) in which an analgesic effect is desired, such as sciatica and neuritis
In treating scapulo-humeral periarthritis, ultrasound therapy is capable of disaggregating calcifications and favouring the reabsorption of calcium salt deposits. Ultrasound therapy can also be used to treat conditions such as epicondylitis or Dupuytren’s contracture and help to reduce muscle spasms. It can also improve the effectiveness of kinesiotherapy therapy (a core element of physiotherapy/physical therapy), which is the therapeutic treatment of disease by passive and active muscular movements such as massage and of exercise.
Furthermore, ultrasound can assist phonophoresis. This treatment is a non-invasive way of administering medications to tissues below the skin — ideal for patients who are uncomfortable with injections. With this technique, the ultrasonic energy forces the drug through the skin. Cortisone, which is used to reduce inflammation, is commonly delivered in this way.
Ultrasound application in relation to tissue repair
The process of tissue repair is a complex series of cascaded, chemically mediated events that lead to the production of scar tissue and restoration of the damaged tissue.
Local blood flow and swelling initiate the healing process when a tissue such as muscle or ligament is damaged. New tissue (known as ‘scar tissue’) can then be formed and laid down. The fibres that make up the scar tissue are often laid down in an unorganised fashion as the tissue is generated. If the tissue fibres are unorganised or not correctly aligned, they are not as strong or as flexible as the original tissue. If this scar tissue remains unorganised, it can sometimes result in tight and/or weak muscles or ligaments — even once healing is complete.
When tissue is exposed to ultrasound, the sound waves cause a micro-vibration within the tissue. This vibration creates heat energy that increases blood flow to the area, causing an increase in oxygen and chemicals that are essential for healing the damaged tissue. As well as increasing blood flow, ultrasound speeds up the transport of chemicals from the blood into the damaged tissue to promote healing. This process helps to build new tissue and ensure the proper alignment of the tissue fibres so that full strength and flexibility are restored.
The end result is a fast and effective way to heal the body and build new tissue.
Mechanism of action
The interaction of ultrasounds with biological tissues produces four different effects: mechanical, thermal, chemical and cavitational.
The mechanical action (also called ‘acoustic streaming’) is described as a small-scale eddying of fluids near a vibrating structure, such as cell membranes, and the surface of a stable cavitation gas bubble. This effect is caused by the movement of tissue particles when crossed by the ultrasonic wave. The variations in pressure that are produced can determine the movement of liquids in the presence of a lack of homogeneity (microcurrents), an increase in membrane permeability and the breakup of tissues due to the separation of collagen fibres.
Although particle movement is minimal, the variations in pressure it produces are considerable, generating a significant mechanical effect in tissues.
This phenomenon is known to affect membrane permeability and diffusion rates. Sodium ion permeability is altered, resulting in changes in the cell membrane potential. Calcium ion transport is also modified — which, in turn, leads to an alteration in the enzyme control mechanisms of various metabolic processes, especially concerning protein synthesis and cellular secretions.
The mechanical modifications induced by ultrasounds determine:
- Acceleration of the processes of diffusion through cell membranes.
- Fission of complex molecules (proteins, polysaccharides, etc).
- Tissue micro-massage.
Deep heating can be used to increase the ‘stretchiness’ of muscles and tendons that may be tight.
The passage of ultrasounds through ‘soft’ tissues causes an increase in temperature due to absorption linked with viscosity, absorption due to thermal conductivity and chemical absorption. The thermal effect depends basically on two factors — the characteristics of absorption of the biological medium and the reflection of energy in the interface between tissues with different levels of acoustic impedance.
Ultrasound can be used to selectively raise the temperature of particular tissues. This form of treatment is particularly effective in heating the dense collagenous tissues (ligament, tendon and fascia) and will require a relatively high intensity. Where a heating effect is not desirable, such as a fresh injury with acute inflammation, the ultrasound can be pulsed at a lower rate.
The therapeutic effects of ultrasounds are represented by analgesia, fibrolytic action and by trophic effects.
The analgesic effect is caused by the action of heat. This is likely due to the direct action of ultrasounds on sensitive nerve endings.
Ultrasound therapy produces oscillations of tissue particles — determining the breakdown of collagen fibres in fibrous tissues.
After an increase in heat, the blood vessels widen (known as vasodilation), facilitating the removal of catabolites and ensuring nutritional substances and oxygen reach the tissues. In this way, ultrasounds improve trophism in tissues, enabling repair of tissue damage and accelerating the resolution of inflammatory processes.
Through the passage of ultrasonic waves, tissue particles are subjected to considerable forces of acceleration and vibration. This process leads to modifications to local pH, the permeability of cell membranes and molecular changes — known as the ‘chemical effect’.
Ultrasound introduces energy into the body, causing microscopic gas bubbles around the tissues to expand and contract rapidly. This process is called cavitation, and it is theorised that the expansion and contraction of these bubbles help to speed up cellular processes and improve the healing of injured tissue.
Through ultrasound therapy, cavitation generates small dissolved gas bubbles in a fluid, with a subsequent increase in the size of bubbles and their possible explosion. From a histological point of view, this results in irregular cell destruction with a petechial-type haemorrhage. At therapeutic dosage levels, destructive reactions such as haemolysis would only occur in the presence of low cell concentration and low viscosity of the medium, such as in the eye and the uterus.
There are two types of cavitation: stable and unstable. When applying ultrasound to the body, stable cavitation is desired. However, unstable cavitation can be dangerous to the body’s tissues and must not occur during application.
When combined with acoustic streaming (the mechanical effect of ultrasound therapy), stable cavitation can cause the cell membrane to become ‘excited’ and boost cell activity levels. Although the ultrasound energy triggers this process, increased cellular activity is responsible for the therapeutic benefits provided.
There are typically three repair phases which can be targeted through ultrasound treatment: inflammation, proliferation and remodelling.
The application of ultrasound during the inflammatory, proliferative and repair phases is valuable not because it changes the normal sequence of events, but because it has the capacity to stimulate or enhance these normal events — thus, increasing the efficiency of the repair phases. For example, if the tissue is repairing in a compromised or inhibited fashion, ultrasound therapy can enhance this activity. Equally, if the tissue is healing ‘normally’, the application of ultrasound can speed up the process. The effective application of ultrasound to achieve these aims is dose dependent.
Ultrasound has a stimulating effect on the mast cells, platelets, white cells with phagocytic roles and the macrophage during the inflammatory phase. Rather than achieving an anti-inflammatory effect, the aim of therapeutic ultrasound and increased activity in these cells is to generate a pro-inflammatory effect.
Although an inflammatory response is possible at this stage, this mode of action is intended to act as an ‘inflammatory optimiser’ — aiding the inflammatory response which is essential for effective tissue repair. The more efficiently this inflammation process is completed, the more effectively the tissue can progress to the next phase (proliferation).
Ultrasound therapy creates a stimulative effect during the proliferative phase, targeting fibroblasts, endothelial cells and myofibroblasts (the cells that are normally active during scar production).
In much the same way treatment is used in the inflammation phase, ultrasound maximises the efficiency of scar production rather than changing it. In this sense, ultrasound therapy can be considered pro-proliferative as well as pro-inflammatory.
The remodelling process can last for a year or more and is an essential part of quality repair. During this phase, the more generic scar tissue produced in the initial healing stages is refined to behave more like the tissue it is repairing. This is primarily achieved by the orientation of the collagen fibres in the developing scar.
By influencing collagen fibre orientation, ultrasound can be used to enhance the functional capacity of the scar tissues. As a result the scar tissue will benefit from increased strength and mobility.
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