Ultrasound is one of the most commonly used modalities in clinical practice. It has been suggested that ultrasound can produce biophysical effects, which can be defined as cellular functions that are enhanced or catalyzed by ultrasound application.¹ The biophysical effects are the result of sound wave energy being converted to a source of superficial or deep heat to healing tissues. Often treatment goals include decreasing pain, increasing soft tissue extensibility, or increasing tissue temperature to ready the tissue for stretching or therapeutic activity. Current literature appears to differ on the overall effectiveness of ultrasound as it relates to the goals of treatment. Common clinical uses of therapeutic ultrasound include pain control, tissue extensibility, wound healing, fracture repair, enhanced tendon and ligament healing, and resorption of calcium deposits.^1,3

Alternate biophysical effects may also be achieved non-thermally, applying the modality for enhanced tissue healing without the production of heat. Non-thermal application is best suited for acute injury care to improve cellular permeability, increasing macrophage activity and enhancing protein synthesis.^1,2

Ultrasound Parameters and Basic Terms

The process of getting ready to apply an ultrasound can vary among clinicians. Applying a moist hot pack, a cold pack, or performing tissue massage as a means of preparing the tissue for ultrasound application can differ even though the goal or clinical reasoning for using ultrasound during treatment is the same. Further variations include deciding whether the ultrasound head should be warmed, if the gel should be warmed, and if the tissue should be stretched.

The literature suggests that terms and application techniques taught in the classroom are often forgotten after a little time on the job. Effective radiating area, best defined as the area of the transducer over which the energy radiates, is a prime example. ERA more directly indicates the size of the crystal found within the transducer and does not always equal the size of the transducer’s head. This is not always taken into consideration when applying ultrasound to the treatment area. In fact, clinicians often treat an area two times the size of the transducer.² Plus, the size of the transducer does not always correlate with the size of the ERA, as is evidenced in a study that compared 12.6 cm² and 8 cm² transducers. The authors found that both transducers had a 5 cm² ERA. It is important to emphasize that the treatment area should not be more than two to four times the size of the ERA, not two to four times the transducer head area.^2,12-14 Clinicians using the 12.6 cm² transducer may have been misguided into thinking that a 25 cm² to 50 cm² area or greater could be treated with the larger transducer. In actuality, the treatment should be no greater than a 10 cm² to 20 cm² area. Treatment could be ineffective if spread over too large of an area. The ultrasound may not cause adequate heating because the energy source is dispersed over too wide of an area for too short of a period of time. It is important for the clinician to refer to the manufacturer’s labeling to determine the correct ERA and consider this when choosing the treatment area.

Frequency, measured in Hertz, is the number of compressions-rarefaction cycles per unit of time1 of the crystal found within the transducer. The frequency determines the depth of penetration of the energy source. An increase in the frequency of 1 MHz to 3 MHz decreases the depth from 5 cm to 2.5 cm respectively. Although this appears to be a well-understood parameter in the application of ultrasound, a 2007 survey issued to licensed practitioners found a variation in ultrasound treatment depths among those who responded. The survey was issued to 457 licensed PTs who had earned orthopedic clinical specialty certification. Of the 203 PTs who responded, 6% said they would use a 3 MHz frequency to treat deep tissue pathologies for pain, inflammation, tissue extensibility, swelling and soft tissue remodeling; 15% said they would use a 1 MHz frequency for similar superficial pathologies.3 Past research indicates that 1 MHz frequency ultrasound can reach up to 5 cm in depth, while 3 MHz frequency remains more superficial at 2.5 cm.^1,12-15 Understanding the depth of the tissue being treated and the frequency required so that the energy source penetrates to the proper tissue depth could certainly enhance therapeutic ultrasound effectiveness. One could hypothesize that this example either further indicates that clinicians may be misapplying ultrasound parameters or that they only have access to 1 MHz applicators, without the option for a 3 MHz applicator.

Beam non-uniformity ratio is the ratio of the spatial peak intensity to the spatial average intensity.¹ For clarification purposes, the average intensity is the intensity selected by the clinician (0.75 W/cm² to 2.5 W/cm²). Because of the somewhat irregular energy output, the ultrasound can reach a peak intensity much greater than the average intensity during ultrasound application. Most transducers’ BNR falls between 5:1 and 6:1. If the BNR is listed at 5:1 and the intensity chosen by the therapist is 1 W/cm², there is the opportunity for the spatial peak to reach upwards of 5 W/cm² during treatment.¹ In a survey, 8% of respondents reported that they would choose an ultrasound intensity greater than 2 W/cm.^2,3 In the above example, if the BNR is 5:1, an intensity of 2 W/cm² could produce a peak intensity of 10 W/cm². The peak intensity could have a direct effect on tissue healing; it could overheat tissue, increase healing time, and even cause pain during ultrasound application.

The final term to review is duty cycle, which is the proportion of the total treatment time that the ultrasound is on and the transducer is emitting energy.¹ A continuous ultrasound, or 100% duty cycle, will allow for tissue heating to occur during the treatment time because the transducer is producing a sound wave continuously over the time of the treatment. Pulsed ultrasound is a non-thermal application that is usually a 5:1 on:off ratio, or 20% duty cycle. A pulsed ultrasound will not cause tissue heating because the transducer is only producing sound waves for a portion of the application.

Some authors dispute the biophysical effects of pulsed ultrasound treatment and the overall effect it has on the course of healing tissue.¹⁶ They suggest that the added effect of increased cellular permeability and other intracellular reactions either do not occur or have no added effect to healing tissue.¹⁶

Others believe that therapeutic levels of ultrasound (1 MHz, 3 MHz, 45 kHz) stimulate cellular and molecular effects within cells that are centrally involved in the inflammatory and healing process.¹⁷ One author defines these biophysical effects as being caused by acoustic streaming and cavitation. He says these ultrasound “injuries” cause tissue growth retardation that leads to an increase in protein synthesis that he terms a “cellular recovery.”¹⁷ This reaction at the cellular level may better explain the implications for enhanced tissue healing, yet the author concedes that “no clear guidelines exist that provide the clinician with protocols directing when in the injury and healing response, ultrasound should be administered, nor are there guidelines on frequency, intensity, treatment times or number of treatments required for efficacy.”¹⁷ In spite of these findings, the jury is still out on the overall ability to enhance cellular function without added heat and how a pulsed ultrasound treatment will allow for a more effective intervention for acutely injured tissue and more successful treatment outcomes.

Treatment Parameters and Terminology  

According to the current literature, PTs vary in their choice of treatment parameters.^3-5 In a review of PTs’ parameter selections for common pathologies, researchers found “considerably different beliefs as to what is an acceptable dosage.”⁵ For example, in a 1992 study highlighting the ultrasound parameters used to treat osteoarthritis of the knee, the parameters were listed as 1 MHz, 2.5 W/cm² and continuous duty cycle for 3 minutes, performed over 100 cm² area. Although the researchers found the study to be “methodologically acceptable,” the parameters chosen may not have produced viable physiological effects on the pathological tissue. The duration of treatment at 3 minutes may not have been enough time to increase tissue temperature, and the 100 cm² treatment area may have been too large of an area for adequate heating. This type of parameter selection may be the reason why some of the current research has suggested that ultrasound is an ineffective modality for the treatment of pain, shoulder pathology, and tendinopathy and why some practitioners question its viability for soft tissue extensibility.^4-11

Literature Review

Does ultrasound have an effect when used for pain treatment? A literature review indicates the following:

• Robertson, et al, say that ultrasound had no significant effect when used for the treatment of pain in the following musculoskeletal pathologies: pressure ulcers, calcific tendonitis, osteoarthritis of the knee, and epicondylitis. The ultrasound parameters varied between each review, including frequencies of 0.89 MHz, 1 MHz and 3 MHz in pulsed and continuous duty cycles.⁵
• Gursel, et al, failed to find sufficient evidence to merit wide use of 1 MHz ultrasound, continuous at 1.5 W/cm² in combination with other interventions in the management of painful shoulder conditions.¹⁸
• A Philadelphia-based panel suggested that ultrasound application has not been shown to provide a clinical benefit in the treatment of neck pain, acute low back pain, non-specific shoulder pain or knee pain.^6-8
• Baker, et al, debated the biophysical effects of non-thermal ultrasound and suggested that these effects in no way enhanced tissue healing.¹⁶

The literature cited above suggests that ultrasound application is ineffective in treating pain. Gursel, et al, and Alexander, et al, discuss the application of ultrasound as being no more effective when treating the painful shoulder than similar treatments without ultrasound.

However, looking more closely at these reviews, the literature makes comparisons of several tissue pathologies that classify as shoulder pain. For example, a painful shoulder bursitis is compared with the treatment of a painful rotator cuff tendonitis and calcific deposits. Several authors also discuss the wide range of treatment parameters used between studies. The clinical debate should not only be whether therapeutic ultrasound is an effective modality; it should also be focused on proper application, appropriate parameter selection, and its use to enhance treatment. It is important to point out that while most of the authors cited in this module are suggesting that ultrasound application does not appear to enhance treatment, they also concede that there is a need for more well-controlled studies to determine proper parameters and efficacy of ultrasound treatment.

In 1995, Draper and Ricard successfully supported the use of ultrasound for increasing tissue temperature. This study involved inserting a 23-gauge hypodermic needle microprobe into the medial gastrocnemius, then heating the tissue to 5 C above baseline. Their research suggested that tissue should be heated >3 C above baseline to achieve vigorous heating. Tissue that has reached the vigorous heating point is then ready to be stretched. Increasing tissue temperature to 4 C above baseline increases the viscoelastic properties of collagen, allowing for greater tissue extensibility.15 Draper, et al, and Chan, Myrer, Measom and Draper determined that once the tissue was heated >3 C above baseline, there was a limited therapeutic window at which the tissue temperature remained for maximum therapeutic gains. In these two studies involving the 3 MHz and 1 MHz frequency, the results suggest that clinicians have a three- to four-minute window of opportunity at which the tissue remains heated for therapeutic purposes. The authors further recommend that if tissue extensibility is the treatment goal, the area being heated should be placed in a position that stretches the tissue. If the patient or clinician is going to continue stretching the tissue, that should be completed immediately following the ultrasound treatment, taking advantage of the increased tissue temperature and viscoelastic properties of heated tissue.^10,12,15 In a time of evidence-based interventions, it is important to revisit the literature, starting with Draper’s work from 1995, and build a base that will provide parameters in which ultrasound application can be clinically effective.

Choosing Draper, et al, as the initial building block for successful ultrasound intervention, the frequency and intensity can be used to determine the length of time the ultrasound needs to be applied to reach a temperature >3 C above baseline or the vigorous heating point.

Clinical example: The ultrasound treatment goal is to raise the tissue temperature to allow for tissue extensibility.

If the goal of treatment is to heat the tissue >3 C above baseline to allow for greater extensibility, a 1 MHz ultrasound at an intensity of 1 W/cm² (4 C/0.2 C/min) should be applied continuously for 20 minutes to obtain the vigorous heating point. If the literature suggests that ultrasound is ineffective, and the studies reviewed contain a wide range of treatment parameters (frequency, intensity, time of treatment), it may be that we are not correctly choosing the treatment parameters or applying these parameters long enough to obtain the proper therapeutic effects.

When considering vigorous heating as a treatment goal, the clinician must remember that the application of ultrasound should not be painful. Patients should expect to feel mild warmth during treatment. The ERA and BNR must be reviewed for each unit available to the clinician. Using the 1995 Draper publication as a guideline better prepares the clinician for proper ultrasound application. Other citations that may be used as building blocks include Gallo, et al (2004), Knight (2001) and Chan, et al (1989).

Draper and colleagues performed their tissue-heating research with ultrasound units from one manufacturer. Ultrasound units from different manufacturers produce varied heating effects with similar parameters. Johns, Straub and Howard anonymously purchased and examined 66 ultrasound transducers and appropriate ultrasound generators. In short, their research suggested a 16% to 35% intramanufacturer and intermanufacturer variance in spatial average intensities after proper calibration. The variability of effects must be accounted for when selecting the treatment parameters. Draper’s early research can certainly be used as a guide for appropriate parameters, but referring directly to the manufacturer labeling and monitoring the treatment outcomes may be the best option to establish effective parameters.

Discussion

Therapeutic ultrasound is one of the most widely used modalities in clinical practice. The debate appears to be whether its use will enhance physical therapy intervention, progressing the patient toward clinical goals. The literature suggests that when comparing pathologies, ultrasound does not have an effect on a patient’s pain level.^6-9,16,18,19 The clinical question becomes: Can ultrasound be used for a painful joint, muscle, tendon or ligament?

As clinicians, we are well aware of the benefits of heating injured tissue. Increased tissue metabolism, circulation, and tissue extensibility, and a decrease in pain are all benefits of heating tissue before beginning exercise or a manual intervention.^1,10,15 If the heat applied happens to be a moist heat pack, the pain control mechanism may be more in line with the gate control theory of pain modulation.¹ The patient’s perception or sensory response to the warming tissue overrides the painful sensory response of the injured tissue, achieving a pain control mechanism.

In that manner of pain control, ultrasound may appear ineffective because the patient only experiences a feeling of mild warmth over a small area. The area that ultrasound has been applied to does not provide that sensory override to give the patient the perception of less pain. Conversely, a hot pack provides the sense of warming a larger area, having a direct effect on the patient’s perception of pain. After 20 minutes of direct heat, the sensation of warmth overrides the pain sensation, resulting in a sense of pain relief. So clinically, there is some relevance to the bodies of literature that support ultrasound’s inability to decrease pain or at least control pain.

Examining this process further, can other properties of heating tissue be applied as the clinical rationale for use of therapeutic ultrasound when treating pathological tissues? Using the example of a chronically inflamed, painful arthritic knee with a mild flexion contracture to discuss ultrasound application and the treatment principles may help illustrate the clinical rationale behind ultrasound use.

One of the options for treatment is to address the flexion contracture. Heating tissue increases the tissue’s extensibility, so choosing ultrasound to address this limitation is a viable option. Applying ultrasound while the patient is in a prone position to allow for gravity-assisted stretch can add to the overall effectiveness. The increase in posterior capsule of the knee’s tissue temperature achieved during the ultrasound treatment will enhance range of motion by increasing soft tissue extensibility while the tissue is in a stretched position. The parameters suggested for this intervention would be a 3 MHz frequency, 1.0 W/cm², continuous duty cycle, applied for six to seven minutes (or 0.5 W/cm² for 13 minutes) with the expectation of reaching vigorous heating. If the contracture in the above example is due to a hamstring muscle shortening, the parameters would be different. Patient positioning would be the same, allowing for a gravity-assisted stretch, and ultrasound would be applied to the hamstring muscle belly, which is much larger and at a greater depth than the joint capsule. The suggested treatment parameters would be 1 MHz frequency, 1 W/cm², continuous duty cycle, for 20 minutes (or 1.5 W/cm² for 13 minutes). If the flexion contracture is the source of the patient’s knee pain, we’ve addressed the range of motion loss and decreased the patient’s pain level.

If chronic inflammation is the source of the patient’s pain, ultrasound application may provide for a more viable healing environment. An increase in tissue temperature will increase circulation, promoting an exchange of oxygen-rich blood to the tissue while increasing tissue metabolism to enhance tissue healing.¹ In this example, ultrasound can be applied thermally: 3 MHz frequency, 1 W/cm², continuous duty cycle, applied for six to seven minutes (or 0.5 W/cm² for 13 minutes) while the limb is elevated to best promote the fluid exchange. Non-thermally and rationalizing the promotion of increased cellular permeability as a means of reducing the chronic inflammation, the parameters could be applied as 3 MHz frequency, 0.75 W/cm² applied for 10 minutes, setting the duty cycle to 20%.¹⁷ In each of the applications (thermal or non-thermal), the cause of the patient’s pain, capsular restriction, muscle tissue contracture or the chronic inflammation is addressed with the application of ultrasound. The ultrasound parameters are not chosen to directly treat pain; they are chosen to enhance treatment of the patient’s impairment. While the primary goal of treatment is to increase joint range of motion, the end result could be an increase in range of motion and a less painful joint.

Conclusion

The literature suggests that therapeutic ultrasound may not be an effective modality for treating painful pathologies; however, the previously cited articles compare the use of ultrasound on multiple pathologies and therefore may not be a fair comparison. The challenge is set to revisit the terms discussed in the classroom, clinically analyze ultrasound use, and examine whether the parameters chosen enhance the clinical outcomes for the patient. Ultrasound should be applied long enough and at an average intensity high enough to heat the tissue, if that is the goal of the treatment. Referring to the manufacturer’s listed BNR and ERA, understanding the frequency, and knowing the tissue that is at fault will certainly help in choosing safe and effective ultrasound parameters. Further research and well-controlled studies are needed to better determine parameter selection for more effective use of ultrasound during the treatment of musculoskeletal injuries.