Ultrasound is high frequency sound waves, greater than 20,000 Hz. Therapeutic ultrasound is in the frequency range of 0.9 - 3 MHz.
The utilization of ultrasound has been a 20th century phenomenon. In addition to its use by the military to detect submarines, it was also used in the 1930's for emulsification, and atomization of particles in a gas. Since then, ultrasound has been used therapeutically for its effects of cavitation, stable and unstable bubble formation, and a phenomenon called acoustic streaming or microstreaming.
Ultrasound is used to:
break up scar tissue and adhesions
reduce inflammation, swelling and calcium deposits
create a deep heat to a localized area to ease muscle spasms (much deeper than can be achieved with a hot pack - up to 5 cm)
increase soft tissue extensibility prior to stretching and exercise
facilitate healing at the cellular level
speeds metabolism and improves blood flow
reduces nerve root irritation
at low intensities can speed bone healing
enhance transcutaneous drug delivery by phonophoresis
The main piece of equipment is a high-frequency generator, which provides an electrical current through a cable to a transducer which contains a piezoelectric crystal. This crystal when exposed to the current will vibrate at a given frequency, expanding and contracting, which produces the necessary compression wave. By using a different frequency the therapist can target tissues at different depths for either healing or destruction, or simply use the device to reduce pain. Although simple in principle, the use of ultrasound as a therapeutic modality requires a comprehensive understanding of its effects on the body tissues and of the physical mechanisms by which its effects are produced. The lower the frequency used, the deeper is the penetration of the waves into the body. By varying the frequency from continuous to intermittent, the amount of heat applied can likewise be controlled by the physiotherapist.
For instance, contusions are one of the most frequent and debilitating injuries encountered in sports medicine. Although contusions may be caused by shearing and tension between over-stressed body parts, the most common cause is compression of soft tissue, usually when it is crushed between bone and some hard surface. This almost invariably involves capillary rupture and infiltrative bleeding, followed by edema and inflammation. This usually involves hematoma or "pooling" of blood, and occasionally myositis ossificans can result as a complication if not treated. This is a syndrome in which the body starts laying down painful calcium deposits within the muscle. Quick and effective treatment is crucial in sports injuries. Proper and efficient healing is essential to the health and career of any athlete, regardless of how minor or major the injury. Basic treatment involves the application of ice to contain the immediate inflammation, followed by timely applications of ultrasound to reduce the subsequent edema and further stimulate the healing process.
Ultrasound is effective in treating wounds in both the inflammatory and the proliferative stages. Ultrasound causes a degranulation of mast cells resulting in the release of histamine. Histamine and other chemical mediators released from the mast cell are felt to play a role in attracting neutrophils and monocytes to the injured site. These and other events appear to accelerate the acute inflammatory phase and promote healing.
In the proliferative phase of healing, ultrasound effects fibroblasts and stimulates them to secrete collagen. This accelerates the process of wound contraction and increases the tensile strength of the healing tissue. Connective tissues will elongate better if both heat and stretch are applied. Continuous ultrasound at higher therapeutic intensities provides an effective means of heating deeper tissues prior to stretching them.
Its effectiveness has been enhanced over the years by studies which help determine optimum techniques and patterns of application, and a wide range of injuries have shown to be responsive to this non-invasive therapy.