The versatile effects and usage of shock waves in medicine

Dr Pavel Novak www.storzmedical.com

In nature, shock waves occur during every thunderstorm. They develop their shattering effects when a supersonic jet breaks the sound barrier. This destructive force has allowed the beneficial use of shock waves in medicine for the first time: to shatter kidney stones (Fig. 1). The breakthrough - the first application in humans - came in early 1980. Extracorporeal shock wave lithotripsy, i.e. ESWL, was born.

Shock waves are an acoustic phenomenon, just like ultrasound, which has a significantly longer history of application in medicine. One important difference to ultrasound is that this technology does not involve a continuous wave train but occasionally very intensive, short pulses (Fig. 2). Contrary to therapeutic ultrasound, shock waves have no thermal but only purely mechanical effects.

Ever since the shattering of the first kidney stone, a whole host of biological effects of shock waves has been discovered, scientifically studied and their versatile areas of application exploited. Nevertheless, many issues have still not been fully clarified. In addition to lithotripsy, numerous new applications have been developed in the meantime, some of which, e.g. pseudoarthroses or pain treatment, are now part of standard medical procedures. They are generally referred to as extracorporeal shock wave treatment, ESWT.

In medicine, shock waves are usually generated in an extracorporeal source and directed into the body using a coupling medium (water) at minimal loss and concentrated on the region to be treated. When they pass boundaries between tissue with greatly differing acoustic properties, as is the case with kidney stones and the surrounding tissue or fluid, mechanical energy is released which shatters the stones. Another effect contributing to the shattering effect of shock waves is cavitation. Here, the gas dissolved in the fluid is outgassed. Small bubbles are formed which expand within milliseconds and also have a shattering effect during their subsequent collapse.

Initially, further indications were sought for which it was assumed that the shattering effect of shock waves could also be put to beneficial use: shattering painful calcium depots in the tendon insertions of the shoulder and heel or creating microlesions for non-healing bone fractures which would initiate the healing process. This was successful in both cases, albeit not due to the shattering but mainly due to the stimulating effect, as was later discovered upon closer observation. It was found that this stimulating effect requires far lower shock wave energy levels (Fig. 3). At these low intensity levels, there are virtually no side effects worth mentioning.

It was observed that shock waves stimulate local blood circulation, microcirculation and cell metabolism. This is caused, on the one hand, by the increased permeability of the cell membrane and, on the other hand, by the release of nitrogen monoxide, an important messenger which causes vasodilation. It also triggers a release of numerous growth factors and has an anti-inflammatory effect. The bone growth factor, for instance, is - in addition to the increased metabolism and improved blood circulation - decisive for the successful treatment of pseudoarthroses.

The treatment of calcified tendon insertions resulted in an entirely new shock wave pain treatment method which has in the meantime become widely accepted. Additional effects were observed here, such as the release of substance P, an important neurotransmitter, the reduction of pain-triggering unmyelinated nerve fibers or the deactivation of muscle trigger points, which contribute to the understanding of shock waves' pain-relieving effect.

Pseudoarthrosese and poorly hearing bone fractures are often accompanied by still unhealed wounds. Interestingly, it was observed that the wounds in the area of the shock waves healed better and more rapidly. In the meantime, it has been shown in numerous cases that non-healing wounds which are open for many years can be successfully treated with shock wave therapy. In addition to the above-mentioned effects, the generation of new blood vessels initiated by the shock waves also has an important role to play. This is mainly caused by the release of the vessel growth factor, VEGF.

The generation of new vessels also plays a crucial role for a different indication: the treatment of ischemic regions of the heart muscle. Treating advanced angina pectoris with shock waves has become a scientifically proven treatment method. The regeneration of a heart muscle damaged during a heart attack has already been successfully proven in animal tests.

When using shock waves for pain treatment, it has also been discovered that the skin can experience positive effects as well. Patients report that their skin feels firmer and cellulite-caused skin irregularities (orange peel) diminish. This shock wave effect is also to be credited to the formation of growth factors, in particular the collagen growth factor, the new generation of which contributes to firmer skin and stronger connective tissue.

Finally, to mention also the latest developments in shock wave effects, it has been proven in numerous studies that shock waves influence the expression, proliferation and differentiation of stem cells. This mechanism is probably also not of minor significance for many of the treatments discussed above. Its targeted and optimised effect promises even more discoveries in the future.