Ultrasound therapy (US) is the use of sound waves above the range of human hearing12 to treat injuries like muscle strains or runner’s knee. It is mostly used by physical therapists, and has been one of the Greatest Hits of musculoskeletal medicine since the 1950s.3 There are many flavours of therapeutic ultrasound, using different intensities and frequencies of sound, but all share the basic principle of “stimulating” or even provoking tissue with sound waves.4
Almost everyone seems to assume that ultrasound is proven — good technological medicine — but that just doesn’t seem to be the case.
There is a jarring, bizarre lack of research for such a popular therapy.Unfortunately — although there are some interesting exceptions and tantalizing hopes for some conditions — ultrasound is not a promising therapy for most of the painful problems it is used for. There is a jarring, bizarre lack of quality research for such a popular, mainstream therapy. What little research is available paints a bland picture. Ultrasound therapy isn’t even on good theoretical foundations. At best, it’s more complicated and unpredictable than most therapists believe. At worst, there is no rational basis for US at all.
Although ultrasound is almost certainly useful for some patients, some of the time, it is not a reliable or evidence-based therapy, and enjoys far more credibility than it deserves.
I do not like the principle of using magic machines to treat.
“Nari,” physical therapist, in an internet forum discussion
The vast majority of patients will encounter ultrasound in one of two forms:
Garden-variety therapeutic US is cheap and available virtually everywhere. The machine is small, even portable (you can buy small handheld ones). Treatment is brief and painless, and applied (indiscriminately?) to virtually anything that hurts — almost any common painful musculoskeletal problem.
On the one hand, ESWT is just a “more is better” version of standard US, because it is often used with the same imprecise clinical intention to stimulate/provoke tissues. On the other hand, because it was originally developed for smashing gall stones, ESWT is strong enough to actually disrupt tissue, such as calcifications in tendons — which is a nice precise clinical goal and a whole different kettle of fish.
When I started studying for this article, I was surprised by how little there was to study. There’s hardly any research about ultrasound at all! Every scientific paper about US starts by pointing out there is not enough research on this topic, or at least not enough good research. There are practically more reviews of scientific papers than there are scientific papers to review. One major review of US for a common knee problem in 2001 comically found only a single worthwhile test of efficacy to report on!9
That’s not a lot to go on.
I didn’t think it would be like this. For years now, I’ve been looking forward to delving deeper into this topic, assuming that there had to be a pile of science about it. We’re talking about ultrasound, here: one of the staples of physical therapy! It practically defines the experience of going to a physiotherapists. Everyone has had that cold gel slapped on an injury, and felt that tingling, penetrating … placebo?
The disconnect between the ubquity of the service and the more or less total lack of informative research is troubling. A handful of studies is a joke for a therapy that is worth literally billions of dollars in the marketplace. How can that much therapy be sold without a satisfactory body of evidence that it works? Bizarre! This is the ultimate example of pseudo-quackery — popular treatments that aren’t overt quackery (they are plausible, not obviously at odds with established science) but far well short of validated, scientific medicine and are sold with excessive confidence and usually considered mainstream.
This does not mean that US never works for anyone. It does mean that it has been prescribed and sold to patients for decades with unjustified confidence. And that is not cool.
In most cases I consider ultrasound less than useless — that's 8-10 minutes wasted that could be used doing something that might actually help.
Jason Silvernail, DPT, Board-Certified in Orthopedic Physical Therapy, in an internet forum discussion
It’s not rocket science. Ultrasound is not a difficult therapy to test,10 and if it works reasonably well, then the results should be pretty clear: simply compare results in patients who received real ultrasound to patients who get a fake instead. To a shocking degree, these simple tests have simply not been done adequately. There should be hundreds of them in the archives. Instead there are just a few dozen.
Between 1995 and 2008, the science that has been done was reviewed in only ten papers that seem worthwhile.1112131415161718192021 Nine were unambiguously negative about US, and some of them strongly so. Their authors had almost nothing good to say about ultrasound. Conclusions like this one from Windt et al are typical:
As yet, there seems to be little evidence to support the use of ultrasound therapy in the treatment of musculoskeletal disorders. The large majority of 13 randomized placebo-controlled trials with adequate methods did not support the existence of clinically important or statistically significant differences in favour of ultrasound therapy.
Windt et al, “Ultrasound therapy for musculoskeletal disorders: a systematic review,” Pain, 1999
(I like that phrasing, “did not support the existence of clinically important differences.” Ouch. Ultrasound’s therapeutic effect has an existential crisis!)
Most of these reviews give a nod toward US used for a particular purpose, or used in a particular way. For instance, Windt et al, despite their overwhelmingly negative conclusion, also noted that “findings for lateral epicondylitis [tennis elbow] may warrant further investigation.” But, naturally, that optimism about tennis elbow is contradicted by other studies (Ho, Staples).
In short, it’s all just a discouraging mess, and a classic case (yet another one) of failing to impress. If ultrasound were generally effective, it certainly should have performed much better in the few studies that have been done.
Therapeutic ultrasound … has fallen out of favor as research has shown a lack of efficacy and a lack of scientific basis for proposed biophysical effects.
Baker et al, “A review of therapeutic ultrasound: biophysical effects,” Physical Therapy, 2001
Except it hasn’t fallen out of favour! It’s still widely used. The only professionals it’s fallen out of favour with, I imagine, are a small minority of scientists and unusually alert clinicians.
Not only that, but ultrasound has found new life in the marketplace as shockwave therapy — faster, stronger waves, and a bigger price tag! Consider this marketing language from a Canadian company, Shockwave Institute, specializing in delivering ESWT:
Provided you are a candidate for this type of treatment, clinical studies suggest there is a 80–85% chance this technology will improve your condition.
from the Shockwave Alberta FAQ, as of Nov 30, 2009
Shockwave Alberta certainly doesn’t think ultrasound has fallen out of favour! Here we have an entire company devoted to delivery of therapeutic US, and selling it with the implication that it is not only proven to be effective, but exactly how effective — to within 5%!
Based on the available evidence, do you think it’s actually possible or meaningful to declare that ESWT is exactly “80–85% effective”? Where are the scientific review papers confirming this marvellous triumph of US? Where is the data to support such a specific promise of therapeutic success?
There is nothing a cold slimy prickling ultrasound wand can do that a pair of warm hands can’t do way better.
If only I had a buck for every time a patient or reader has told me that they are skeptical about “that ultrasound thing they always do to you at physiotherapy”!
Patients often express irritation with a common physical therapy business model: working with several patients at once, rotating between rooms or beds, often leaving patients with passive therapies (like a moist hot pack from a hydrocollator — nice enough, but worth a steep fee?) Many patients often go a step further and complain specifically about ultrasound and TENS, skeptical that these treatments really do anything.22 To the patient, these treatments seem unremarkable & also obvious ways for a physiotherapist to get paid while not doing much.To the patient, they seem therapeutically unremarkable and also obvious ways for a physiotherapist to get paid while not doing much. This perception really pushes people’s “I don’t want to be a sucker” button.
And so few patients are singing the virtues of standard US. It not only fails to generate testimonials, but actually generates many annoyed antimonials.
Patients do not (yet) feel the same cynicism about shockwave ultrasound. As a more expensive and painful medicine, ESWT is a hope-generating machine. Having spent their hard-earned dollars and endured the discomfort of treatment, patients are more subject to expectation effects (placebo) — and much less willing to entertain the possibility that it was all a waste. At this stage in their quest to feel better, more people will report ambiguous results if they were positive (“Yeah, I think it did some good!”), and even negative reports will often be toned down (“I didn’t seem to get that much out of it, but I guess it works really well for some people.”) This could go on for years.
The big idea is — this will blow your mind! — that cells and tissues respond “well” to being shaken (not stirred). In theory, ultrasound works by vibrating tissues back to health, which sounds like something you’d hear on an infomercial, or the Dr. Oz Show. What, exactly, does vibration do to tissues? Does anyone actually understand it?
In 2001, Physical Therapy published a review of the biological effects of ultrasound. More than ten years ago, the authors — Baker, Robertson, and Duck — explained that it had already been at least two decades since it was first pointed out that “physical therapists tended to overlook the tenuous nature of the scientific basis for the use of therapeutic ultrasound.”23 They also point out — it’s the point of their whole paper — that the situation had not improved in twenty years (before 2001):
The frequently described biophysical effects of ultrasound either do not occur in vivo under therapeutic conditions or have not been proven to have a clinical effect under these conditions. This review reveals that there is currently insufficient biophysical evidence to provide a scientific foundation for the clinical use of therapeutic ultrasound …
There is a great deal of interesting ultrasound biology to consider, and scientists may yet nail down effects that might be the basis for new evidence-based therapies. But today? For now, the clinical bottom line is that it is complex and unpredictable, and there is no basis for concluding that ultrasound has a basis. The entire enterprise rests on the single, oversimplified idea that “stimulation is good.” Ultrasound is literally just tissue vibration therapy.
Physical therapists often cite the “gate control” mechanism as a justification for US and ESWT (and some other popular treatments, especially TENS). This is nonsense and a great example of why patients should be cautious, especially with the expense of ESWT.
The “gate control” mechanism is an important idea in pain science, proposed in 1965 by Dr. Ronald Melzack and Dr. Patrick Wall, and still accepted today as an explanation for a familiar phenomenon: the way we rub injured body parts for a little pain relief. The idea is that pain signals pass through a “gate” in the spinal column. The state of the gate is controlled by many factors. How much pain can get through depends on competing signals and information from other sources, such as touch and pressure, as well as emotional context.
So the idea with US is that the stimulation closes the gate and thus reduces pain. This may well occur, but it’s nothing to write home about. It’s not a “treatment” — it doesn’t fix anything— and it’s simply ridiculous as a justification for an expensive therapy. It’s a minor and temporary effect, and can be achieved just as easily by rubbing the area yourself! There is no reason to think that any kind of ultrasound closes the gate better or longer than any other stimulus.
The reasons for doing ultrasound are not at all clear, and adding this one is just a way to pad the list in a way that sounds scientific — especially handy when you’re trying to sell expensive ESWT — but is actually almost meaningless. It’s disturbingly marketing-savvy, but not at all medically savvy.
File this one under “intriguing”: Canadian researchers treated trigger points (muscle knots) in 50 patients with either ultrasound or a sham, and the amount of pressure on the trigger point required to induce pain was measured before and after. Trigger points treated with ultrasound were more tolerant of pressure than those that were not treated, at 1, 3 and 5 minutes after treatment.24 The improvement was no longer significant just 10 and 15 minutes later, however — so the effect in this case was brief. The authors concluded:
…low-dose ultrasound evokes short-term segmental antinociceptive effects on trigger points which may have applications in the management of musculoskeletal pain.
They are not necessarily proposing that ultrasound is a useful treatment for myofascial pain syndrome, but producing evidence of an interesting effect that may prove to be clinically significant in time — an important distinction.
Although it may be surprising in contrast to the generally unimpressive evidence of the effectiveness about therapeutic ultrasound, it nevertheless reinforces that ultrasound does indeed do some interesting things to tissues: it’s just not clear exactly what. An important caveat is that there is significant scientific debate about what “trigger points” really are.25 Some would say it’s hard to study the effect of ultrasound on a phenomenon that may or may not even exist!
I am a science writer, former massage therapist, and assistant editor of Science-Based Medicine. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook and Google, but mostly Twitter.
Ultrasound is widely used but poorly studied. This 2007 survey of the usage of ultrasound, the first such American survey for almost 20 years (see Robinson, 1988), “examined the opinions of physical therapists with advanced competency in orthopedics about the use and perceived clinical importance of ultrasound.” They found that “ultrasound continues to be a popular adjunctive modality in orthopedic physical therapy. These findings may help researchers prioritize needs for future research on the clinical effectiveness of US.”BACK TO TEXT