Therapeutic ultrasound (US) is one of the most commonly used, and misused, biophysical agents in physical therapy and other rehabilitative professions. Wong et al1 reported that certified orthopedic clinical specialists use therapeutic US for decreasing soft tissue inflammation and pain and for increasing tissue extensibility, scar tissue remodeling, and healing acute soft tissue injuries. The most common diagnoses for which US is used by rehabilitation professionals include back, shoulder, knee, and neck pain and difficulty in walking and other gait abnormalities.2 US represents more than 5.8% of all line items submitted to the Centers for Medicare and Medicaid Services (CMS) for payment.2 A recent study3 reported that 82.4% of physical therapists use US—and 36.4% use it daily. Most US use occurs in private practice settings (58.4%) and in the musculoskeletal area of practice (74.8%). Therapists use it on an average of 40% of their patients. Although quite high for a given modalities treatment, this rate of use is a considerable decline from the 94% that reported daily US use two decades ago.4
Despite its use, the evidence for the effectiveness of therapeutic US has not been well documented in the rehabilitation literature. In fact, most studies have shown it not to be effective when compared to placebo controls. A critical analysis of the literature shows, however, that this level of negative evidence may be more related to problems in study design and to the variability of patient responsiveness to US rather than to the ineffectiveness of US. Interestingly, Amijo-Olivo et al3 report that only 13% of therapists using US make their clinical decisions using research evidence while 40% use their own clinical experiences.
PHYSICAL PRINCIPLES OF ULTRASOUND
US is high-frequency mechanical waves delivered using acoustic energy. Sound waves exert their mechanical action by pressing the initially vibrating molecules into adjacent molecules, which in turn causes them to vibrate. These newly vibrated molecules will then transmit their energy into vibrating molecules adjacent to them. When no molecules are present, such as in a vacuum, there would be no transmission of sound energy. Conversely, in substances (or tissues) with higher molecular densities, transmission of US would be more efficient because there are more molecules per given volume. For example, human speech at normal volumes can be heard only a few meters in air while whale vocalizations can be heard over many kilometers in water because water is a denser medium than air. Conversely, ultrasonic energy can be dissipated (attenuated) more quickly in denser substances because denser substances offer more resistance to molecular motion (i.e., acoustic impedance). Because of the poor transmission of US waves through air, a coupling agent is needed. Generally an aqueous gel is used between the US applicator and the skin to enhance the efficient transmission of ultrasonic energy to body tissues from the US applicator.
Sound waves are transmitted ...