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Invited Commentary

VJ Robertson, PT, PhD, is Professor, Teaching & Research Unit, Gosford Hospital, Bldg 14, Corner of Holden Rd and Racecourse Rd, Gosford, New South Wales 2250, Australia.

Address all correspondence to Dr Robertson at: Val.Robertson{at}newcastle.edu.au

Ultrasound offers many clinical users considerable potential. Many current uses of ultrasound (US) made by physical therapists, however, probably contribute little to patients’ outcomes. In years to come, the findings of the survey by Wong et al¹ will provide an interesting reality check for physical therapists: When did we accept how little evidence there is that supports how physical therapists use US? The findings suggest that, instead of adjusting practice according to research evidence, many physical therapists persist with long-discredited ideas and practices that are not evidence based. It also is not apparent that they are adopting practices justified by evidence.

Wong et al¹ chronicled the self-reported uses of US by physical therapy experts for 6 impairments. The 205 contributing experts (44.9% effective return rate, with 457 physical therapists surveyed) were all orthopaedic certified specialists (OCS), and the specified impairments were "pain, soft tissue inflammation, decreased tissue extensibility, delayed tissue healing, soft tissue swelling, ... and scar tissue remodeling." The authors indicate that they chose common impairments and provided opportunities for OCS respondents to add "other" options. The most frequently reported "other" impairments that the respondents identified managing with ultrasound were muscle spasm, calcium deposits, and hematoma. Other uses of US that received only one mention each included to increase blood flow and enhance bone healing.

The article also provides a summary of the most frequently reported parameters used (Tab. 4). To summarize, a frequency of 3 MHz was used for superficial lesions and a frequency of 1 MHz was used for deep lesions (not defined). A typical intensity, apparently irrespective of frequency, was 1 to 1.5 W/cm². The space-averaged, time-averaged intensity (I_SATA) (which Wong et al¹ called the "temporal average intensity") varied from 0.02 to 3.3 W/cm² for superficial tissues and from 0.10 to 2.5 W/cm² for deep tissues. These findings suggest little understanding by the OCS respondents of the physical properties of US, including the effects of frequency on the absorption of US energy.^2(p262)

More than 50% of the OCS respondents reported US as clinically important (somewhat important, essential, or very important) for treating 3 of the 6 impairments: soft tissue inflammation, decreased tissue extensibility, and when scar tissue remodeling was needed. The absence of supporting peer-reviewed evidence for this finding suggests that practice possibly is unduly altered by reimbursement pressures or the need to meet patients’ expectations of what a physical therapy treatment should include.

There are 2 ways of interpreting the findings of the survey by Wong et al.¹ One way is with some optimism that maybe the nonresponding experts (55.1% of those surveyed) recognized the contradiction in investigating uses of a modality to manage conditions for which the researchers recognized there is no evidence to support its effectiveness. The alternative is further confirmation that introducing evidence-based change to practice is very difficult. The present commentary will assume this latter interpretation: nearly 50% of experts in a relevant area reported uses that are difficult to justify on the basis of peer-reviewed research evidence. With this in mind, I will focus on 3 issues: (1) how the findings of the survey by Wong et al contribute to practice development, (2) the effectiveness of therapeutic US, and (3) dosage issues.

	Practice Development

 
There is an underlying contradiction in the study by Wong et al¹: as the authors report in the "Discussion" section, "there are very few controlled trials that have found US to be clinically effective." Despite this, the researchers investigated the use of US for 6 conditions for which the authors indicate there is little or no high-quality evidence that it improves the outcomes.^3–5 Why, when, and how is investigating a seemingly ineffective practice of using a modality such as US justified?

Investigating practices known as unlikely to be effective can be very informative. From a "big picture" perspective, results such as those reported by Wong et al¹ can provide readers with insights into whether practice is adopting uses supported by research.

Wong and colleagues’ findings,¹ therefore, are quite disappointing. Their findings suggest that research published over the past 10 years has had little effect on clinical practices in physical therapy that involve using therapeutic US (as distinct from diagnostic US). The same practices appear to be continuing, apparently with few new options being adopted.

The questionnaire that Wong et al¹ used in their survey was not provided, but the data and description of the study suggest that the researchers missed an ideal opportunity to monitor the adoption of new uses of US by physical therapists, including for diagnostic imaging and for biofeedback during muscle retraining. It is difficult to accept that the study by Wong et al will contribute, as they suggested in the last sentence of their abstract, to helping "researchers prioritize needs for future research on the clinical effectiveness of US." Rather, the study suggests a need to investigate how to provide clinicians with more ways of accessing and integrating knowledge into practice. This need likely extends beyond US, electrical stimulation, and other modalities.

	Effectiveness of Ultrasound

Top Practice Development Effectiveness of Ultrasound Dosages Summary References

 
The second issue related to the study by Wong et al¹ is: When, and for what, is there evidence that applying US is effective? It is used frequently for imaging a growing list of tissues as the technology improves, for guiding needles during procedures,⁶ for biofeedback, and for mixing compounds. Ignoring those and many other related uses, there also is clear evidence that US can promote healing in some types of tissues under some circumstances.

A number of articles published around the turn of the century attest to the value of using US to manage fractures and delayed union.^7–9 The outcomes and the possible mechanisms were well described in an article written for sports medicine clinicians.¹⁰ That same article also canvassed other possible uses of US for managing soft tissue problems. They included promoting the repair of ligaments, tendons, cartilaginous tissues, and muscle. None of these conditions were included or mentioned by Wong et al.¹

Laboratory researchers have investigated additional possibilities. A cursory search retrieved many current and relevant studies. For example, at the cellular level, pulsed US can promote proteoglycan synthesis in primary bovine chondrocytes.¹¹ This effect is more than just that induced by the heat produced by US. Such findings may result in future clinical uses in humans, or they may not be relevant in an in vivo context. In rabbits, osteochondral defects treated with US for 40 minutes a day improved faster than those treated for any lesser time.¹²

Surgically induced damage at the bone-to-tendon junction of the patella-patellar tendon complex of rabbits repaired better if treated with low-intensity pulsed US.¹³ By week 4, those rabbits treated with US showed evidence of more advanced repairs, and by week 16, this difference was clear. At that stage, biomechanical testing demonstrated a higher failure load and greater strength in the bone-to-tendon junction in those rabbits treated with US than in the control group. It is unclear whether the control group was handled identically and sham ultrasound also was applied. This possible confound was managed in a study using rats and examining, in part, the effects of US on knee ligament healing.¹⁴ Again, the findings showed that US applications produced, even after 2 weeks, an increased ultimate strength and stiffness in the US-treated ligaments.

These results from animal studies suggest that US holds considerable potential for clinical uses in physical therapy. Some caution is warranted despite the findings being consistent with evidence that low-intensity pulsed US promotes fracture union in humans: controlled in vivo studies in humans have not yet been reported, and the parameters and dosages that Wong et al¹ reported physical therapists now use are very different.

Fracture repair is promoted by low-intensity pulsed US. The parameters usually used are a frequency of 1.5 MHz, a beam nonuniformity ratio of 2.16, an effective radiating area of 4 cm², and an I_SATA of 0.03 W/cm².² In addition, the dosage is usually daily, 20-minute applications, using a stationary applicator. The parameters are produced by a system specifically designed and used to treat fractures (Exogen). The extent to which these parameters may be varied is not yet clear, but this dosage and system of usage raises more questions.

By definition, low-intensity pulsed US has an I_SATA of less than 0.1 W/cm².¹⁵ Warden et al¹⁵ investigated whether using a standard, conventional therapeutic US unit could produce the same outcome as that produced by the purpose-designed US equipment known to promote bone union. They used rats with surgically created femoral fractures. After 40 days, more healing had occurred in those rats treated with the active US than in those rats treated with the sham low-intensity pulsed US. The parameters used included a frequency of 1.0 MHz, pulsed so that the I_SATA was 0.1 W/cm²; a beam nonuniformity ratio of less than 6; an effective radiating area effective radiating area of 5 cm²; and daily 20-minute applications 5 times a week with a stationary applicator. The machine output was tested and confirmed weekly during the study.

Questions remain: How effective is conventional equipment for managing human fractures and bone damage? To what extent can US improve the rate and extent of recovery of a range of types of tissue damage in humans? This includes ligament and cartilaginous damage, injuries for which there currently is little convincing evidence.

Another issue is, how realistic is it to expect US to improve the rate of healing of normal tissues? The conditions for which there is supporting evidence typically concern tissues that are slow to heal, where healing has been delayed for various reasons, or detailed biomechanical studies are done after sacrificing an animal. The initial stages of acute soft tissue healing normally take less than 2 weeks, and patients may not want to make return daily visits for additional weeks to get an unseen benefit. This also raises an interesting problem for clinical users of US: If a condition with a usual rate of initial repair of 14 days is speeded up by, say, 10% by using US, would the outcome measures used clinically detect this change? Similarly, with changes in the ability of ligaments to respond to lengthening, will it ever be possible clinically to demonstrate a faster rate of strength development in vivo?

Fractures are typically managed with US for 20-minute sessions once per day. Few physical therapy clinicians apply US this often and for this long to a dosage area. Given the parameters that Wong et al¹ reported as favored, this is a blessing. Skin burns of possibly considerable depth would be inevitable with either frequency of US, especially if applied with a stationary applicator.

Existing clinical and laboratory findings suggest that considerable change is needed to existing uses of US made by physical therapists. Disappointingly, only one respondent in the study by Wong et al seemed aware of the use of US for promoting bone healing (ie, from among 205 clinical experts).

Another issue with US is, when should this modality be preferred? For example, as a method of increasing tissue extensibility, when should it be preferred over an application of heat, of heat and stretching, or of stretching alone? The relevant component of US for increasing tissue extensibility is accepted as heat, but this can be provided by other means. Shortwave diathermy (SWD) and microwave, for example, are known to be effective methods of deep heating. Even in young, uninjured people, applying SWD can increase the range of movement available in an adjacent joint.¹⁶ The mechanism is not understood, but the effect is readily demonstrated, and SWD enables heating of a large tissue volume. By contrast, the temperature of only small volumes of tissue can be effectively heated using therapeutic US.

There is no evidence that US is effective for managing pain, and little is known about the comparative effectiveness of modalities including heat and cold for relieving different types of pain or how well they compare with different types of medication. Similarly, the relative benefits of different modalities and electrical stimulation (eg, pulsed current from a transcutaneous electrical nerve stimulation machine) are not known. Physical therapists need evidence-based guidelines for making choices from a range of specific modalities. Clinicians are best placed to investigate these types of questions, particularly experts who would usually treat a higher number of patients with relevant presenting problems.

In summary, in vitro and some in vivo research shows that US can improve some outcomes for patients. However, these uses were not those investigated by Wong et al,¹ nor did more than 0.4% (1/205) of the experts who responded indicate knowledge of the types of findings for which research-based evidence exists. This is despite a recent article¹⁵ in this same journal reporting the possible relevance of standard therapeutic machines for managing bony damage in vitro. Neither do the self-reported types of uses made of US suggest that many of the therapists surveyed recognize that alternatives to US might be more effective. Pain and reduced tissue extensibility, for example, are possibly better managed by alternatives to US.

	Dosages

Top Practice Development Effectiveness of Ultrasound Dosages Summary References

 
The third issue to address in this commentary is dosage. Dosage problems have dogged studies of therapeutic US. One problem concerns the lack of stability of output produced by US equipment. This problem has been well documented in locations ranging from North Wales¹⁷ to Texas¹⁸ to New Zealand¹⁹ and has been known for a long time^19,20 and their implications discussed often and in many places.²¹ More recent evidence shows that, although the tested equipment met Food and Drug Administration regulations and was consistent with the manufacturer's advice, considerable differences could exist in the space-averaged intensity produced by transducers with the same frequency.²² A serious omission in the survey by Wong et al¹ was a question asking clinicians how often they have their equipment tested for electrical safety and the output calibrated. The results of that question might have been very enlightening.

Ultrasound has been used clinically for nearly 70 years,²³ well prior to the 1950s as Wong et al¹ suggested. Despite that, no dose-response relationship has been identified. This makes it difficult to understand why therapists choose the dosages they use. The patterns of absorption of US at frequencies of 1 and 3 MHz are different. Dosages should reflect this difference, but they do not, suggesting that some users lack an understanding of the basic properties of US. To apply US at a frequency of 3 MHz and an intensity of approximately 1 W/cm² (I_SATA range=0.02–3.3 W/cm²) over a superficial lesion seems excessive unless there is no underlying bone and the applicator is moved rapidly. Otherwise, the dosage does not appear to account for the high rate of absorption of 3-MHz US in a small volume of tissue if over a bone.² Even without bone directly underneath a superficial lesion, this is still a high dosage and not seemingly consistent with using US with a frequency of 1 MHz at only slightly different intensities, from 0.1 to 2.5 W/cm², over deep tissue. These assumptions may be incorrect, because Wong et al collapsed the 1- and 3-MHz US dosages but indicated that respondents preferred them for deep and superficial lesions, respectively. This leaves readers in a conundrum: Do respondents not know and understand the differences and modify dosages accordingly, or is this interpretation a product of how the results are presented? Hopefully, the higher intensities reported were for US applied at a frequency of 1 MHz, not 3 MHz, and a few survey responses skewed the findings unduly.

Variations in dosages reported in studies of US as used by physical therapists are always intriguing. Robertson and Baker, in an earlier systematic review of clinical uses of US, commented on this, saying, "the dosages of ultrasound used in the studies we reviewed varied considerably and for reasons that were not always clear."^5(p1348) The findings of the survey by Wong et al imply that nothing has changed and that even experts use a very wide range of dosages. Admittedly, in this instance, users were asked to self-report, no checking was possible, and respondents had to imagine an ideal patient for each of the 6 conditions included.

Taken as a whole, the dosages reported by Wong et al¹ should concern the profession, given they suggest a limited understanding of the relevant biophysical properties of US. This is compounded by long-identified problems with US equipment. The question was not asked, so we can only assume that much of the equipment used is not necessarily in optimal condition and recently tested for electrical safety and output.

	Summary

Top Practice Development Effectiveness of Ultrasound Dosages Summary References

 
The study by Wong et al¹ raises concerns as to how well physical therapists integrate current research findings and use them to change practice. The results suggest that a considerable number of experts use practices that the researchers themselves reported as not supported by evidence. Whether this was intentional or a by-product of pressures such as for reimbursement and patient expectations is not known.

The near 50% nonresponse rate might suggest that many experts recognized that the questions asked by the researchers were only exploring uses known not to be effective rather than the effect of new findings on the profession. Perhaps, but the findings also suggest unjustifiable differences in dosages, another fallout from the lack of supporting peer-reviewed evidence and the continuing absence of a dose-response relationship.

I am still left with 2 major puzzles. First, why did Wong et al¹ investigate patterns of usage of a modality for which there is little evidence of effectiveness? If this was a test of integration of research into practice, then discussion of this issue and the findings could have been very interesting. Second, why claim to investigate the "conditions [for which US is] ... most often used" and then constrain respondents to the 6 categories with only an option of "other"? This was an ideal opportunity to identify what OCS therapists use US for and why.

The main contribution of the article by Wong et al¹ will remain a reality check for those concerned with the integration of peer-reviewed evidence into practice, and the findings provide cause for concern as this study was of clinical experts in physical therapy.

	References

Top Practice Development Effectiveness of Ultrasound Dosages Summary References

 

1. Wong RA, Schumann B, Townsend R, Phelps CA. A survey of therapeutic ultrasound use by physical therapists who are orthopedic certified specialists. Phys Ther. 2007;87:986–994.[Abstract/Free Full Text]
2. Robertson VJ, Ward AR, Low J, Reed A. Electrotherapy Explained. 4th ed. London, United Kingdom: Elsevier Science Ltd; 2006.
3. Speed C. Therapeutic ultrasound in soft tissue lesions. Rheumatology. 2001;40:1331–1336.[Abstract/Free Full Text]
4. Gürsel K, Ulus Y, Bilgi A, et al. Adding ultrasound in the management of soft tissue disorders of the shoulder: a randomized placebo-controlled trial. Phys Ther. 2004;84:336–343.[Abstract/Free Full Text]
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8. Takikawa SC, Matsui N, Kokubu T, et al. Low-intensity pulsed ultrasound initiates bone healing in rat nonunion fracture model. J Ultrasound Med. 2001;20:197–205.[Abstract/Free Full Text]
9. Busse JW, Bhandari M, Kulkarni AV, Tunks E. The effect of low-intensity pulsed ultrasound therapy on time to fracture healing: a meta-analysis. Can Med Assoc J. 2002;166:437–441.[Abstract/Free Full Text]
10. Warden S. A new direction for ultrasound therapy in sports medicine. Sports Med. 2003;33:95–107.[CrossRef][ISI][Medline]
11. Kopakkala-Tani M, Leskinen JJ, Karjalainen HM, et al. Ultrasound stimulates proteoglycan synthesis in bovine primary chondrocytes. Biorheology. 2006;43:271–282.[ISI][Medline]
12. Cook SD, Salkeld SL, Popich-Patron LS, et al. Improved cartilage repair after treatment with low-intensity pulsed ultrasound. Clin Orthop Rel Res. 2001;(391 suppl):S231–S243.[CrossRef]
13. Lu H, Qin L, Fok P, et al. Low-intensity pulsed ultrasound accelerates bone-tendon junction healing. Am J Sports Med. 2006;34:1287–1296.[Abstract/Free Full Text]
14. Warden S, Afin K, Beck E, DeWolf M, et al. Low-intensity pulsed ultrasound accelerates and a nonsteroidal anti-inflammatory drug delays knee ligament healing. Am J Sports Med. 2006;34:1094–1102.[Abstract/Free Full Text]
15. Warden S, Fuchs R, Kessler C, et al. Ultrasound produced by a conventional therapeutic ultrasound unit accelerates fracture repair. Phys Ther. 2006;86:1118–1127.[Abstract/Free Full Text]
16. Robertson VJ, Ward A, Jung P. The contribution of heating to tissue extensibility: a comparison of deep and superficial heating. Arch Phys Med Rehabil. 2005;86:819–825.[CrossRef][ISI][Medline]
17. Lloyd JJ, Evans JA. A calibration survey of physiotherapy ultrasound equipment in North Wales. Physiotherapy. 1988;74:56–61.
18. Artho PA, Thyne JG, Warring BP, et al. A calibration study of therapeutic ultrasound units. Phys Ther. 2002;82:257–263.[Abstract/Free Full Text]
19. Chapman R. A Survey of Output Calibrations of Ultrasound Therapy Equipment Used in Physiotherapy Practice. Christchurch, New Zealand: Department of Health; 1985.
20. Pye SD, Milford C. The performance of ultrasound therapy machines in the Lothian region. Ultrasound Med Biol. 1984;20:347–359.[CrossRef]
21. Robertson VJ. Therapeutic ultrasound: re-evaluating the evidence. Physiotherapy Singapore. 2003;6(2):28–35.
22. Johns L, Straub S, Howard S. Analysis of effective radiating area, power, intensity, and field characteristics of ultrasound transducers. Arch Phys Med Rehabil. 2007;88:124–129.[CrossRef][ISI][Medline]
23. Bélanger A. Therapeutic Physical Agents. Baltimore, Md: Lippincott Williams & Wilkins; 2002.

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