Ultrasounds in physiotherapy

Ultrasound therapy is commonly used in physiotherapy treatments. It is also called sonotherapy. The frequencies typically used in physiotherapy are 1MHz and 3MHz. The actual frequency of ultrasound applicators operation deviates from these values up to 20%, which is not a matter of great importance due to the way of its impact.

Ultrasounds – general information

Ultrasounds are mechanical waves with a frequency exceeding 16 kHz (20 kHz). The frequencies used in physiotherapy are much higher – 1MHz and 3MHz. Mechanical wave due to its physical properties is transferred to subsequent mediums. The waves are divided into transverse and longitudinal. At the medium boundary (of substances with different densities) the division of energy occurs. Part of the energy is reflected, part of it is absorbed, and remain part is transferred to the subsequent medium. In the inelastic medium, absorption occurs. Whether the medium is elastic or inelastic depends on the wave frequency. Elastic mediums such as rubber in the case of 1MHz frequency or more, becomes inelastic and absorbs the energy of the wave. Another example of inelastic medium is a cork. Echosounder is an example taken from everyday life which includes the use of ultrasound. Dispersion velocity of mechanical wave in the tissue is 1500m/s.

Ultrasounds – physical phenomena

Mechanical waves are subjected to reflection, absorption and transmission. Individual values (energy) depend on the parameters (of medium material). In relation to the interested application, total reflection occurs in head/air circuit.

As a result of the absorption phenomenon, the heat is generated. The reflection phenomenon may enhance the absorption effect because the wave may be “trapped” in some layer.

Waves with higher frequencies get closer to the surface, with smaller ones – deeper.

According to Wiedau at a dose of 2W/cm2 and f = 800 kHz wave range in tissue is 8cm, and the half-depth (the depth at which the energy is halved) equals 3 cm.

Effects of the ultrasounds on the human body

Mechanical effect

Mechanical activity of ultrasounds is classified as fundamental effect. The wave penetrates into the tissue and deflects particles. It is something similar to a massage. Hence the name “internal tissue massage”. This is of course a different form of massage than manual massage as the frequencies of “massage” are various so the effect is also different. As a figure of speech, it is said that ultrasounds make tissues soften. We have some medical examples confirming this phenomenon. Ultrasound therapy is applied eg. to change the structure of scar tissue, tendons, etc

Thermal effect

Thermal activity of ultrasounds is classified as fundamental effect. The absorption in tissues results in heat generation. Muscles move the mechanical wave, while the bones absorb it.

Table of absorption coefficients in various tissues:

The values of the coefficients indicate:

• high absorption in the bones
• high absorption in the cartilage (US applied through the muscles warm up joints)
• small absorption in the muscles
• small absorption in the nervous tissue (clear medium boundary so there is an impact)
• small absorption in the adipose tissue (thick layer of fat tissue is not a limitation)

In the case when due to the reflection phenomenon, the ultrasound wave will be “trapped” between the periosteum and the bone, we might have to deal with the shear phenomenon. As a consequence of this phenomenon, the waves alter from the longitudinal to transverse and it may lead to adverse consequences, namely the damage of connection between the periosteum and bone. Therefore, caution should be exercised when carrying out treatments in the bone area. In this case, we should operate on smaller doses or apply the pulse mode.

Chemical effect

Ultrasounds stimulate chemical reactions in the body. It may be depict as shaking of the test tube with mixed chemicals in order to accelerate the reaction.

Bilogical effect

Ultrasounds increase the permeability of cell membranes. This facilitates the transportation of fluids and nutrients to tissues.

Ultrasound influence – secondary reactions

Ultrasounds affect nerve endings and thus they stimulate the sympathetic nervous system. This has a beneficial effect on the heart, blood pressure, respiratory system, etc.

Effects of the ultrasounds on the human body – summary

As a result of combination of all above mentioned phenomenon, ultrasounds demonstrate the following effects on the human body:

• analgesic effect
• increase the blood supply to the tissues
• reduce muscle tension
• increase the flexibility of connective tissue (scar, conditions after injuries of muscles and joint capsules)
• accelerate wound healing