Shockwave therapy: ultrasound reborn! ∞
Last post: analysis of one new study of shockwave therapy for a pathology, very specific!
This post? Very general! This is about the big shockwave picture. What the heck are shockwaves anyway? Are they different from ultrasound? How? What's the point? Do they work? Et cetera.
98% of information about this popular high-tech therapy is published by manufacturers or providers. It is just a 🤏🏻 wee bit biased. But the salamander, of course, is refreshingly free of those vested interests.
Super-duper ultrasound
ESWT is high tech. The main differences for the patient? It ain’t cheap, and it’s much more intense.
The pressure wave therapies are huge in physical therapy: both ultrasound therapy (US) and extracorporeal shockwave therapy (ESWT). Both are used to treat pain and injuries like plantar fasciitis, stress fractures, muscle strains, or runner’s knee.
Therapeutic ultrasound has been one of the staples of physical therapy since the 1950s. It is still widely used in physical therapy clinics today, and you can buy your own DIY ultrasound machine at the drugstore. It’s fancier and more potent cousin, shockwave therapy, has been steadily replacing it in the 21st Century.
Shockwave therapy is ultrasound… reborn!
Although ultrasound and shockwave therapy are different … are they really? All their variants are used in much the same ways for mostly the same conditions. They are both vibration delivery systems, intended to stimulate or even damage tissue — which might, depending on the details, induce regenerative and anti-inflammatory effects in injured bone, cartilage, and tendon. They both belong to a family of vibration therapies with massage guns, jacuzzis, and vibrating exercise platforms, which are all very similar in spirit.
Fast sound waves versus slow shock waves
Shockwave therapy is often considered a new kind of ultrasound, but shockwave therapy isn’t “ultra” sound — it’s infra!
Also not actually “sound.”
Ultrasound uses high-frequency sound waves, above the range of human hearing (> 20,000 Hz) — which vibrates tissue so quickly that it warms up from the friction, like a gentle microwave. The point of ordinary therapeutic ultrasound is that it’s like a heating pad with a more penetrating warmth. It’s mostly a “heat ray.”
It’s cheap, ubiquitous, harmless … and mostly useless.
Shockwave therapy: like ultrasound, but … more
Shockwave therapy is, weirdly, much more like a powerful vibrating massage “gun.” It looks almost exactly like ultrasound: a gadget that you pass over an injury like a magic wand. But it’s the more-is-better cousin of therapeutic ultrasound — more intense, more technologically impressive, more expensive. The price has come down a lot in the last decade, but even now it will run you at least $100 per visit, often much more, with a typical prescription of three to six treatments.
Shockwave therapy generates pressure waves that slap tissues with much higher energy, travelling faster than the speed of sound (in flesh), about 1500 meters per second.
But those waves are also extremely infrequent, compared to ultrasound. They come at a leisurely tempo that you could almost dance to — mostly under 10 Hz, thousands of times lower than ultrasound. In fact, a typical five shockwaves per second (5 Hz) is quite a bit slower than tap dancing, which can get up to about 18 Hz! But it is way below the range of human hearing, which starts around 20 Hz. So shockwave therapy is closer to infrasound than ultrasound.
Shockwaves also aren’t technically “sound” waves: they are high-energy pulses of pressure rather than continuous oscillations — like sharp mountains rather than rolling hills.
A schematic contrasting the overall shape of shock waves, sound waves, and ultrasound waves … not to scale! Ultrasound waves are dramatically more frequent that shock waves, or even low-frequency sound waves: you might only get a few big shock waves per second, but ultrasound machines will pump out more than 20-30 thousand much smaller sound waves in the same second! Any properly scaled diagram that shows one of them clearly will render the other effectively invisible.
What’s the “extracorporeal” about?
“Extracorporeal” just means “generated outside the body,” which seems obvious — you don’t swallow it, and it’s not a surgical implant. Shockwave tech was adapted from a revolutionary method of smashing gall stones and kidney stones with pressure waves, one of the greater victories of modern medicine, back in the 1980s. It was called “extracorporeal” to emphasize a feature that we take for granted today, but was truly amazing at the time: you could deal with the stones without having to cut into someone, or insert a probe up the urethra (which, eek, had been a common technique).
In a physical therapy context, we don’t much care that there’s no scalpel work or urethral violation … so the “extracorporeal” is often dropped. (And sometimes not dropped, because big words make things sound important!)
What’s the point?
It’s not clear why kidney-stone smashing tech was adopted by physical therapy. Why “slap tissues with much higher energy than ultrasound”? What’s the point? The goals of ultrasound and shockwave therapy mostly overlap: to “stimulate” tissue, which is the same vague rationale for nearly all gadget-powered treatments. But there are at least two significant differences:
- Shockwaves are too slow for thermal effects.
- But the pressure waves do have enough energy to cause microtrauma. They can also break up some calcifications, which sounds much more important than it is.
It is certainly a different kind of stimulation. But the evidence for its usefulness is a classic example of “promising” evidence that falls apart when you dig into it. Shockwaves have some excellent medical uses, but vibrating tendinitis? It’s not so clear. Downright speculative, actually!
Like basically every gadget and technique in manual therapy, shockwave therapy is not based on evidence. I review some of that evidence in my full guide to ultrasound and shockwave therapy, but the summary is really simple, and it follows a classic pattern in this business: the strongest evidence is negative, and the positive evidence is the weakest.
I’ve been working on this a bunch lately, levelling up my shockwave knowledge, and this is an adapted excerpt of some of the new material. I’ll share another excerpt soon, to wrap up this short series of posts about shockwave therapy, about it’s sketchiest claim: that shockwaves helps by … harming?