Extracorporeal shock wave therapy (ESWT). Physical and biological mechanisms. Types of shock waves. Therapeutic parameters, indications, contraindications, and safety considerations

At a glance

1. A shock wave is a single acoustic pulse: a steep rise within nanoseconds to a high pressure (tens of megapascals), a duration of microseconds, then a short negative (tensile) phase. Its effects are mechanical, not thermal.
2. Focused shock waves (electrohydraulic, electromagnetic or piezoelectric) concentrate energy at a point several centimetres deep. Radial pressure waves (ballistic, compressed air) are strongest at the skin and treat superficial targets.
3. The pulses stimulate repair through mechanotransduction: new vessels, growth factors, collagen and bone formation. They also break up calcium deposits and relieve pain.
4. Dose: energy flux density in mJ/mm² (low below about 0.08, high above about 0.28), number of pulses (about 1000 to 3000), pulse rate, and usually 3 to 5 sessions a week apart.
5. Standard indications: chronic plantar fasciitis, calcific shoulder tendinopathy, lateral epicondylopathy, Achilles and patellar tendinopathy, greater trochanteric pain, delayed union and non-union. Results vary by condition.
6. Never with a malignant tumour, a fetus or a pacemaker in the field. Never aim high energy at lung, brain, spinal cord or the growth plates.

01Physical characteristics

Extracorporeal shock wave therapy (ESWT) sends pressure pulses into the body from a device outside it ("extracorporeal"). A shock wave is what a thunderclap or a whip-crack is: not a steady tone but one sudden jolt of pressure.

positive peak pressure (the course gives 50 to 100 for focused waves)

10–100MPapositive peak pressure (the course gives 50 to 100 for focused waves)

rise time to the peak

10–100nsrise time to the peak

duration of the pulse

1–10µsduration of the pulse

the tensile phase that follows

−5 to −10MPathe tensile phase that follows

Both are sound, so both need gel to pass from the device into the skin.

Where it came from

1. 1

   1940s

   Lung injuries in people near underwater and air explosions show that blast waves tear tissue at air boundaries.

2. 2

   1980

   The first patient has a kidney stone broken by extracorporeal shock wave lithotripsy (ESWL) in Munich; urology adopts it worldwide.

3. 3

   Late 1980s

   Animal studies show that shock waves set off new bone formation.

4. 4

   1991

   Valchanou and Michailov in Sofia report high-energy shock waves healing delayed unions and non-unions: 70 of 82 fractures united.

5. 5

   1990s onward

   Tendinopathies, plantar fasciitis and calcific shoulder deposits; radial devices follow, then wound and spasticity treatment.

02Types of shock waves

Focused shock waves

The wave is made in water inside the head and gathered to a focal point, like sunlight through a magnifying glass. Energy is highest at the focus, which can lie several centimetres deep (the course gives 0 to 6 cm, depending on the device), and lower at the skin.

Radial pressure waves

Compressed air (about 1.5 to 5 bar) fires a small projectile, like a bullet in a tube, against a metal applicator on the skin. The applicator passes the blow on as a pressure wave that spreads out like a ripple from a stone dropped in a pond. Its energy is greatest at the skin and falls quickly with depth (the course gives 0 to 3 cm). The rise is slower and the peak lower than in a focused shock wave, so physically it is a pressure wave, not a true shock wave.

03Physical and biological mechanisms

Physical

• Positive phase: the steep compression pushes and shears tissue, most where tissues of different density meet (tendon and bone, calcium and tendon, tissue and gas), because part of the wave reflects there.
• Tensile phase: the brief pull makes small gas bubbles grow and then collapse, cavitation, sending tiny jets of fluid that hit nearby surfaces. In a kidney stone this cracks it; in soft tissue at therapeutic doses it is a strong local stimulus.

Biological

The core idea is mechanotransduction: cells sense mechanical force and answer by changing which genes they read. Bone that is loaded grows stronger, a tendon that is loaded remodels; a shock wave is a very concentrated load.

These come mostly from laboratory and animal work. In people the exact mechanism is not settled; too much energy can damage tissue instead of stimulating it.

04Therapeutic parameters

Energy flux density (EFD) is the energy passing through each square millimetre at the focus, in mJ/mm². It is the main dose of focused waves. Radial devices are set by air pressure in bar instead.

The cut-offs differ between authors; there is no agreed international scale. Low and medium energies are used for tendinopathies and spasticity, high energies for calcium deposits and bone.

The total dose is the energy per pulse times the number of pulses, so a few strong pulses or many weak ones can deliver similar energy.

Technique

1. 1

   Assess

   Confirm the diagnosis, check contraindications, and explain that treatment is uncomfortable and may flare pain for a day or two.

2. 2

   Locate

   Find the target: by the most tender point with the patient's guidance (clinical focusing), or with ultrasound or X-ray for calcium deposits and non-unions.

3. 3

   Couple

   Gel on the skin; the applicator held at right angles to the surface.

4. 4

   Treat

   Start at low energy and raise it as tolerated; keep the focus still on a deposit or non-union, or move the radial applicator over the painful area.

5. 5

   After

   Paracetamol if needed rather than anti-inflammatory drugs; continue the loading exercise programme. Reassess after the course; the full effect often takes weeks to months.

A local anaesthetic is usually not given: it removes the patient's guide to the painful spot and some trials found it weakened the effect.

05Indications

ESWT is for chronic problems, usually after three months or more of failed standard care, especially exercise.

ISMST also lists uses tried widely but less proven, such as non-calcific rotator cuff tendinopathy, myofascial trigger points, knee osteoarthritis and mild carpal tunnel syndrome. The course places ESWT in the recovery phase of tendon and soft tissue injuries, alongside loading exercise.

What the evidence shows

• Chronic plantar fasciitis: several trials and meta-analyses show benefit, with both focused and radial devices; among the best-supported uses.
• Calcific shoulder tendinopathy: high-energy focused treatment helps deposits resolve in several trials, but a Cochrane review that pooled all rotator cuff disease found it probably gives little benefit over a sham.
• Lateral elbow pain: a Cochrane review found little or no benefit over a sham.
• Achilles and patellar tendinopathy, greater trochanteric pain: moderate evidence, best combined with loading exercise.
• Non-union: in selected cases, union rates in series approach those of surgery, without an operation.

06Contraindications

The logic: shock waves tear where tissue meets gas, may stimulate unwanted growth, and can disturb what lies at the focus.

The course adds, as contraindications or precautions:

• pregnancy (the course avoids treatment in pregnancy; ISMST rules out only the fetus in the field);
• bleeding disorders and anticoagulation, relative for low energy (bruising), firmer for high energy;
• infection in the treated area, and open wounds there except when the wound itself is treated by a specialist;
• growth plates in children, which are kept out of the focus;
• the eyes, major nerves and vessels, and gas-filled bowel, which are kept out of the field.

Many clinicians also avoid treatment soon after a corticosteroid injection into the area, which weakens the tendon, because of the rare risk of rupture.

07Safety considerations and side effects

Shock waves are safe in the right place and dose. The common effects are local and short-lived:

• pain during treatment and a flare for a day or two;
• redness, swelling, petechiae and small bruises;
• brief numbness or tingling if a nerve is near.

Rare but serious: tendon rupture (known from case reports), and damage to lung, bowel or nerves if the field is misplaced. Treatment is stopped for severe pain or unexpected nerve symptoms.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1What sets a therapeutic shock wave apart from therapeutic ultrasound?

It is an electromagnetic wave rather than a sound waveIt works mainly by heating the tissueIt is a single pulse with a very steep rise to a very high pressure, followed by a short negative phaseIt needs no coupling gel

2A 50-year-old has a 15 mm calcific deposit deep in the supraspinatus tendon. Which device fits best?

Radial pressure waves at the highest air pressureFocused shock waves, with the focus set on the deposit using ultrasound imagingRadial waves at low energy, many sessionsShock waves are not used for calcific tendinopathy

3Which of these is a contraindication to high-energy focused ESWT listed by ISMST, but not to low-energy treatment?

Lung tissue in the shock wave fieldA malignant tumour in the fieldA fetus in the fieldA pacemaker in the field

4Before radial shock wave therapy for plantar fasciitis, a patient asks for a local anaesthetic injection. What is the usual advice?

Always inject; the treatment is unbearable otherwiseInject only for the first sessionAvoid it; it stops the patient guiding the applicator to the painful spot and may weaken the effectGive a corticosteroid injection instead

5The day after a session for Achilles tendinopathy, a patient has redness, a few petechiae and more pain than before. What is the right response?

Stop the course; this is a serious adverse reactionReassure him that these are common, short-lived effects, give paracetamol if needed and continue the planned courseStart an anticoagulant to prevent thrombosisImmobilise the ankle in a cast for two weeks