Therapeutic ultrasound

Indications for therapeutic ultrasound:
– joint contracture
– tendinitis/tenosynovitis
– musculoskeletal pain
– degenerative/inflammatory arthritis
– carpal, ulnar and/or tarsal tunnel syndrome
– acute/subacute injuries
– wound healing
– heel spur/enthesitis

The human ear perceives sounds with frequencies from about 20 to 20,000 Hz. All waves above the upper limit are called ultrasound. Ultrasound requires a medium and direct contact with the tissue. That is why water, gels and creams, which may contain medicinal substances, are used. When a medicinal substance is used, this type of ultrasound is called sonophoresis. The ultrasound beam can be focused, reflected and refracted. Therapeutic frequencies range from 0.3 to 3 MHz, and the most commonly used are 0.8–1 MHz, because they offer the best balance between penetration into the tissue and focus.

How does ultrasound affect the body?
The most significant effect is heating, while the non-thermal effects include cavitation, movement of the medium (acoustic streaming) and standing waves. Cavitation produces bubbles that can change tissue through oscillation and movement. With stationary ultrasound in a confined field, standing waves change the pressure in the tissue and can also cause physiological effects.

How deep does ultrasound penetrate the tissue?
The depth of a clinically significant effect depends on the power applied, the nature of the tissue and the direction and frequency of the ultrasound beam. Ultrasound penetrates soft tissue much more deeply, so at least 50% of the beam reaches 7–8 cm into the tissue, while in bone it goes no deeper than 1 cm. The direction also matters: if ultrasound is applied transversely, in the direction of the muscle fibres, the beam can sometimes penetrate more than 8 cm, while with a more vertical application the waves penetrate less deeply, about 2–3 cm. Frequency has a significant effect: penetration of the ultrasound beam can be up to 85% lower when a higher frequency within the 0.3–3 MHz range is used. Higher frequencies penetrate much less, and in practice therapeutic ultrasound at a frequency of 0.8–1 MHz can raise tissue temperature by 4–5 °C at a depth of 8 cm.

Where do the most intense temperature changes occur during therapeutic ultrasound?
Ultrasound heats the skin, fat, muscles and bones, with the greatest effect at the boundaries between different types of tissue. The boundary between bone and soft tissue heats up the most. That is why ultrasound is not applied over a laminectomy site (where the lamina of a vertebra has been removed) or, in general, over any fracture. Caution is needed: like any method, this one should be supervised by a specialist doctor with extensive experience in treating such conditions, while the therapist applying it must watch for any changes in the tissue.

How is therapeutic ultrasound applied?
Acoustic gel, mineral oil or a cream with or without a medicinal substance is applied to improve contact and penetration into the tissue, and the applicator head is moved in circles. Uneven body surfaces require ultrasound through water: the part of the body is immersed in water, which improves the contact surface. Because the tissue may heat up, the therapist must be careful, and the patient should tell the therapist if they feel any changes. A treatment can last 1–15 minutes, depending on the tissue and the diagnosis it is used for. Application can be pulsed or continuous. The dose depends on the treatment site and the therapeutic goal set by the specialist physiatrist after diagnosis. Non-thermal effects require much lower intensities.