Shockwave - The Essentials

A SHOCKWAVE is essentially a PRESSURE DISTURBANCE that propagates rapidly through a MEDIUM. It can be defined thus : A large-amplitude compression wave, as that produced by an explosion or by supersonic motion of a body in a medium  which just a formal version of the first sentence.

Although the adjacent image is from a plane flying at supersonic speed, it clearly illustrates the principle.

Shockwaves generated from supersonic aircraft in flight (CNN / NASA)

Obvious examples of shock waves are the sonic boom from an aircraft, thunder or the sound following an explosion. A shockwave is, put simply, an acoustic wave, as is a means of transmitting energy.

A clinically useful shockwave is effectively a controlled explosion (Ogden et al 2001), and when it enters the tissues, it will be reflected, refracted, transmitted and dissipated like any other energy form. The energy content of the wave will vary and the propagation of the wave will vary with tissue type. Just like an ultrasound wave, the shock wave consists of a high pressure phase followed by a low pressure (or relaxation) phase. When a shock wave reaches a 'boundary', some of the energy will be reflected and some transmitted.

A Brief History

Shock waves were initially employed as a non invasive treatment for kidney stones (from the early 1970's, with treatment proper starting in the 1980's), and it has become a first line intervention for such conditions. In the process of the animal model experimentation associated with this work, it was identified that shockwaves could have an effect (an adverse one initially) on bone.

This led to a series of experimental investigations looking at the effect of shockwaves on bone, cartilage and associated soft tissues (tendon, ligament, fascia) resulting in what is now becoming an intervention of increasing popularity, most especially for the recalcitrant lesions of these tissues, though the clinical uses are expanding and now include wound management, treatment of fractures and numerous additional applications. The use of shock waves to treat bone problems was researched through the early 1980's. with the earliest clinical work (that I can easily identify) being around the middle of that decade on delayed and non unions. By the early 1990's, reports start to appear in the journals and conference papers where shockwave is being employed to deal with soft tissue problems, most commonly calcific tendinitis in the first instance, and then on to a variety of other long term problems in tendon, ligament and similar tissues.

Although becoming much more popular (especially in Europe and to some extent, in the UK), it is still a relatively new technology for musculoskeletal intervention, and although the publication volume is steadily increasing, some of the published trials are of doubtful methodological quality and need to be considered with some caution.

The treatment goes by several names, the most popular being SHOCK WAVE THERAPY or EXTRACORPORAL SHOCKWAVE THERAPY, though, as ever, there are several variations, often linked to the names of particular machines. Some have recently suggested that the therapy version of shockwave therapy might be usefully called RADIAL SHOCKWAVE THERAPY to distinguish the nature of the wave from the focused versions employed elsewhere in medical practice though more recently, it has been suggested that a preferred term would be RADIAL PRESSURE WAVE. A very readable but succinct history of the development of shock waves for medical applications can be found in Thiel (2001).

Shockwave - Principles of Production

There are basically four different way to produce the 'shock wave', which, without getting technical about it are : spark discharge; piezoelectric; electromagnetic and pneumatic (or electrohydrualic). The wave that is generated will vary in its energy content and also will have different penetration characteristics in human tissue. In therapy t