Ultrasound is a therapeutic modality that has been used since the 1940s. Ultrasound is applied using a probe that is put in direct contact with the patient’s skin, separated by a conductive medium (ultrasound gel). Therapeutic ultrasound is in the frequency range of about 0.8-1.0 MHz.
The waves are generated by a piezoelectric effect caused by the vibration of crystals within the head of the probe. The sound waves that pass through the skin cause a vibration of the local tissues. This vibration causes a deep heating to the area however; typically no sensation of heat will be felt on the skin. In situations where a heating effect is not desirable, such as a fresh injury with acute inflammation, the ultrasound can be pulsed rather than continuously transmitted.
Ultrasound can produce many effects other than just the potential heating effect. It has been shown to cause increases in tissue relaxation, local blood flow, and scar tissue breakdown. The effect of the increase in local blood flow can be used to help reduce local swelling and chronic inflammation, and, according to some studies, promote bone fracture healing.