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Therapeutic Ultrasound

Research indicates that the use of ultrasound in physiotherapy may be causing more harm than good.

Dr Linda Maxwell

Using ultrasound to heat injured tissue for physiotherapy purposes has become increasingly common, but there are indications that such heating could stimulate, rather than diminish, inflammation.

Ultrasound is defined as a form of acoustic vibration with frequencies so high that it cannot be perceived by the human ear. Its application is increasing in medicine for diagnostic and surgical purposes, but it is also extensively used by physiotherapists to treat a wide variety of medical conditions including common sports injuries such as torn ligaments and painful swollen joints.

In the UK, 54% of physiotherapy treatments in the private sector and 22% in the hospital setting involve ultrasound, with the aim of the treatment to provide pain relief, relax soft tissues and accelerate healing.

Despite this widespread use, the practice of applying ultrasound to treat injured tissue has developed largely on a basis of applied experience, rather than from controlled scientific study of its cellular and molecular effects in various stages of the healing process.

Many therapists believe that ultrasound has an anti-inflammatory effect on the injured tissue, relieving pain, reducing swelling and promoting healing. Because tissues have higher acoustic absorption than liquids it is relatively easy to produce significant heating of tissue with an ultrasonic beam.

Ultrasound is preferentially absorbed in the areas, such as superficial bone, scar tissue, tendons and tendon sheaths, where heating is generally considered beneficial. The final temperature achieved depends on the beam intensity and the degree of focusing, as well as how much heat is lost through conduction and blood flow. Many of the so-called therapeutic effects of ultrasound are attributed to this heating, but the heating in itself may lead to further inflammation.

Complex Response to Injury

The response of tissue to injury is complex and involves the microcirculation, many cell types and a great many chemical mediators. Alteration in the structure and function of small blood vessels occurs early in inflammation, with an immediate but short-lived reduction in blood flow to the injured part. This is rapidly followed by dilation of the blood vessels and a marked increase in blood flow into the damaged region. Changes in the permeability of the vessel wall make the vessels leaky, so that plasma and proteins escape from the blood into the tissues. In these ways, inflammation makes the injured part red, hot and swollen.

Recent investigations conducted in my laboratory at Auckland University's School of Medicine have demonstrated that ultrasound applied in a therapeutic manner and dose causes the micro-circulation to respond in a manner similar to that which occurs naturally during acute inflammation. Thus it seems likely that the application of ultrasound during the early stages of inflammation serves to exaggerate rather than diminish the blood flow changes leading to inflammation.

The principal function of the normal inflammatory response is the removal of dead tissue and the killing of microorganisms by special scavenger cells called phagocytic leucocytes. To achieve this, leucocytes must first leave the blood and pass through the wall of the blood vessels to enter the area of damage. Leucocytes first adhere to the endothelial cells which line the vessel wall, then squeeze though gaps that open between the cells. They accumulate in the tissues and secrete a variety of enzymes which destroy dead tissue and microorganisms.

How ultrasound affects these complex processes is poorly understood. Recently, using isolated leucocytes and cultured endothelial cells, we have demonstrated that ultrasound helps leucocytes to adhere to endothelial cells by stimulating the production of adhesion molecules on the cell surface. In this way, ultrasound applied in the first few hours of injury may enhance the cellular response of the tissue to injury.

Once dead tissue has been removed from the injured area, healing can proceed by repair which involves the production of collagen-rich connective tissue. Collagen is a structural protein which makes up the fibrous component of skin, tendons, ligaments, cartilage and bone.

There is evidence that therapeutic levels of ultrasound stimulates protein synthesis during the repair phase, improving the mechanical properties of the scar tissue developing at the site of injury. This may be related either to an increased amount of collagen present or to altered organisation of the collagen fibres.

Clearly much more research is still required to determine the cellular and molecular effects of ultrasound techniques, and thus provide therapists with a more precise basis for this widely used form of treatment. The subject is important, not only because ultrasound is used on a large numbers of patients, but also because the cost of providing this service is substantial.

Dr Linda Maxwell is senior lecturer in pathology at Auckland University's School of Medicine.