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Author Response

We thank Robertson for her thoughtful review of our article¹ and, more importantly, for an in-depth review of therapeutic ultrasound outcomes.² This article was one of the driving forces behind our original pursuit of an inspection of ultrasound energy output as we tried to determine why clinical therapeutic ultrasound may not be working. We view our response to Robertson's commentary as having 2 goals: (1) to clarify our stance on some of the points made in the commentary and (2) to further emphasize additional points made by Robertson.

Robertson states that "the margin of accuracy for the output (spatial average intensity [SAI]) is permitted to be ±20%." Although many clinicians may assume this to be true, neither the International Electrotechnical Committee (IEC)³ nor the US Food and Drug Administration (FDA)⁴ regulates this parameter. The FDA regulates total power (in watts) with a ±20% error window and requires manufacturers to report an error band around the effective radiating area measurement (often 20%–25%). Thus, the true margin of spatial average intensity (SAI) accuracy between 2 transducers theoretically can range up to 150% variability⁵ while both transducers continue to meet FDA regulations.

We strongly agree with Robertson's comment that there is a lack of evidence of a dose-response relationship for therapeutic ultrasound in clinical treatment. Our biggest issue is that scientifically sound dose-response curves cannot be developed when FDA standards permit variability of up to 150% between 2 treatment units.⁵ Few clinicians would accept a study of the affects of ibuprofen on inflammation if the test subjects were reported to receive 125 mg of ibuprofen but may have received as little as 80 mg or as much as 200 mg. Still, as heath care providers, we consider this to be an acceptable standard for therapeutic ultrasound treatment. A second concern is determining whether current clinical research has developed an effective test model of clinical outcomes for therapeutic ultrasound. Robertson rightfully points out that low-intensity pulsed ultrasound (LIPUS) has been shown to have positive treatment effects on rates of fracture healing, a clear and definitive outcome measure. However, the clinical outcome measures associated with megahertz-frequency ultrasound are less defined: soft tissue healing rates, resolution of inflammation, and so on. For example, in 1992, Maxwell⁶ reported that therapeutic ultrasound resulted in the promotion of cellular metabolic rate and an increase in the viscoelastic properties of collagen. Yet, in 2005, Zammit and Herrington⁷ measured the effects of therapeutic ultrasound on pain, swelling, range of motion, and postural stability and reported no increase in clinical outcome. Does this mean that the viscoelastic properties of the ankle ligaments in question remained unchanged? Are these ligaments now stronger and less likely to tear during the next stress event? Our current model does not allow us to assess these measures. When combining the variability within SAI with the lack of markers associated with soft tissue healing, it is difficult to develop true dose-response relationships.

Robertson views the 1994 study by Pye and Milford,⁸ which found problems with "newly styled transducers," as being dated in that these transducers are now 14 years old. We continue to feel this is a relevant study because the "new style" that was introduced is the now common dual-frequency transducer. We have concern about the ability of a crystal to be run at multiple frequencies and be accurate at all frequencies. More testing still needs to be done in this area. This concern over frequency is again touched upon by Robertson in her concluding statement that users seem unaware of the implications of using different frequencies of ultrasound. When our current article is combined with a previous article by Johns et al,⁵ it is apparent that some ultrasound units work better at one frequency than another. Clinicians may want to determine which frequency best matches their clinical practice and choose a manufacturer based on this decision.

Short of the use of LIPUS for the treatment of fractures, the evidence for improved clinical outcome with the use of therapeutic ultrasound is lacking. There are 2 parts to developing better clinical trials with therapeutic ultrasound. First, all future clinical research studies examining the effects of therapeutic ultrasound should carefully control and report measures of ultrasound field characteristics, power, intensity, and treatment area so that accurate measurements of dosing may be reported and replicated. Second, the scientific community needs to identify biological markers that can be more easily measured in human subjects, which can reflect altered tissue response.

	References

 

1. Straub SJ, Johns LD, Howard SM. Variability in effective radiating area at 1 MHz affects ultrasound treatment. Phys Ther. 2008;87:50–57.
2. Robertson JR, Baker KG. A review of therapeutic ultrasound: effectiveness studies. Phys Ther. 2001;81:1339–1350.[Abstract/Free Full Text]
3. US Food and Drug Administration. Performance standards for sonic, infrasonic, and ultrasonic radiation emitting products. 21 Federal Register1050 (1999).
4. International Electrotechnical Committee. Ultrasonics: Physiotherapy Systems—Performance Requirements and Methods of Measurement in the Frequency Range 0.5 to 5 MHz. Geneva, Switzerland: International Electrotechnical Committee; 1996. (IEC 61689, Annex C)
5. Johns LD, Straub SJ, Howard SH. Variability in effective radiating area and output power in new ultrasound transducers at 3 MHz. J Athl Train. 2007;42:22–28.[Web of Science][Medline]
6. Maxwell L. Therapeutic ultrasound: its effects on the cellular and molecular mechanisms of inflammation and repair. Physiotherapy, 1992;78:421–426
7. Zammit E, Herrington L. Ultrasound therapy in the management of acute lateral ligament sprains of the ankle joint. Physical Therapy in Sport. 2005;6:116–121.[CrossRef][Web of Science]
8. Pye SD, Milford C. The performance of ultrasound physiotherapy machines in Lothian Region, Scotland. Ultrasound Med Biol. 1994;20:347–359.[CrossRef][Web of Science][Medline]

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This Article Full Text (PDF) Submit a response Alert me when this article is cited Alert me when Rapid Responses are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Straub, S. J Articles by Howard, S. M Search for Related Content PubMed Articles by Straub, S. J Articles by Howard, S. M Related Collections Physical Agents/Modalities Social Bookmarking                      What's this?