Shockwave Theory : What is Shockwave Therapy?
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 in tervention of increasing popularity, most especially for the recalcitrant lesions of these tissues.
There are basically three different way to produce the 'shock wave', which, without getting technical about it are : piezoelectric, electromagnetic and electrohydrualics. The wave that is generated will vary in its energy content and also will have different penetration characteristics in human tissue.
Although becoming much more popular (especially in Europe and to some extent, in the States), 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. 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 very readable but succinct history of the development of shock waves for medical applications can be found in Thiel (2001).
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, reported 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.
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 a normal ultrasound wave (see other pages in the web site for details), 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.
The characteristics of a shock wave are (typically) :
Peak pressure - typically 50-80MPa (according to Ogden et al, 2001) and 35 - 120MPa (according to Speed, 2004)
Fast pressure rise (usually less than 10 ns (nanoseconds)
Short duration (usually about 10 microseconds)
Narrow effective beam (2-8mm diameter)
(more detailed descriptions can be found in Ogden et al 2001, Speed, 2004)
The pressure wave causes direct effects (as one would expect) and also 'indirect' effects associated with the subsequent low pressure part of the cycle (often referred too as the tensile phase), and during this phase, cavitation will occur (as with therapeutic ultrasound). The collapse the these cavitations (bubbles) is in part at least, responsible for the efficacy of the therapy. The waves are focused in order to achieve the effects in a volume limited zone of tissue, though the focus does not actually come to a 'point' in therapy devices - more like a zone or small volume typically several mm across (2 - 8mm).
As the shock wave travels through a medium and comes to an interface, part of the wave will be reflected and part transmitted. There are equations around for calculating this proportional relationship, but effectively, the dissipation of the energy at the interface is almost certainly responsible for the generation of the physical, physiological and thus the therapeutic effects. Whilst the detailed mechanisms of how these are achieved remains elusive, there is a growing body of evidence that supports the use of the therapy in fracture healing problems (delayed or non union) and numerous soft tissue clinical problems, especially those concerning tendon, ligament and fascial problems. The evidence for these clinical applications of the therapy is reviewed on the 'evidence' page.
Dose :
As suggested above, dosage issues remain an area of some considerable controversy in the literature, though the somewhat general terms 'high' and 'low' dose are frequently banded about - even though there is no absolute consensus as to their definition.
The energy flux density (mJ per mm2) is the amount of acoustic energy that is transmitted through an area of 1mm2 during a pulse. the total pulse energy is (as one might expect) the sum of the energy across the whole pulse (beam) per shock wave. The total energy delivered in a treatment session is usually expressed - and this is simply calculated as the total energy per pulse X the number of applied pulses. Needless to say that there is a bit more to it than I have suggested here, but I am not trying to replicate a physics text - just trying to get the general ideas across! - so if you happen to be a physicist, please don't write to tell me how much I have left out!!! - but by all means let me know if what I have said is not right. A the end of the day, the pressure distribution profile, the energy density and the total acoustic energy delivered appear to be the key characteristics of the therapy. It is interesting that there appears to remain some controversy over which of these parameters is the most important in terms of the 'dose' of treatment (but then the same could still be said for ultrasound, laser, pulsed shortwave and numerous other therapies).
The three different basic generator types (piezoelectric, electrohydraulic and electromagnetic) produce differeng energy profiles. Essentially (so far as I can see at the moment anyway) the electrohydraulic generator appears to be the most problematic, and in terms of the use of the intervention in the therapy world, then the piezoelectric and electromagnetic devices appear to be in the ascendant. The piezo electric appears to have the edge in terms of delivering a smaller energy, though this may mean more visits. the electromagnetic device can operate with a large aperture which means that the surface energy density can be low whilst delivering a reasonable energy concentration at the focal zone.
Speed (2004) reports the (arbitrary) division of doses into low and high, and of course, somebody has also come up with a meduim just to make life complex. I have reproduced the table from Speed (2004) below
| Authors | Level | mJ/mm2 |
|---|---|---|
| Mainz | Low |
0.08 - 0.27 |
Medium |
0.28 - 0.59 |
|
High |
>0.60 |
|
| Kassel | Low |
<0.12 |
High |
>0.12 |
Contraindications, Precautions and Dangers
Whilst not intended to constitute a definitive list, there are several areas/pathologies where concern has been expressed with regards the use of shockwave, and until further clarification has been obtained, some of the key issues are identified below. There have been several sources that I have consulted in the process of generating this list, including Ogden et al 2001 . . . . . .
Lung tissue appears to be damaged unequivocally and should be avoided
The epiphysis has been considered and whilst some experiments demonstrate a detrimental outcome, others do not. Whilst clarification is being obtained, it would make sense to avoid epiphyseal regions
Patients who are haemophiliac or who are on anticoagulant therapy are best not treated with shockwave given that some visible tissue damage (skin petechiae and disruption of the microvasculature) has been noted in several studies.
Malignancy remains on the contraindication list, though, as with other modalities, some experimental work is ongoing whereby shockwave therapies are being employed to try and minimise the growth and spread of malignant tissue. Given the unknowns at the moment, it is considered best to avoid such areas.
Metal implants appear to be OK with regards bone based treatments, but implanted cardiac stents and implanted heart valves have not been fully evaluated. If however, one is avoiding the lungs, then they should not be exposed anyway.
Infection in the local area should be treated with strong caution given the as yet unknown effect of the therapy in this field.
Joint replacements - interestingly - come up with a mixed result. Some have used the therapy experimentally as a means to help with the removal of prostheses, making extraction easier. Given this, it would seem wise to avoid cemented implants. On the other hand, it is suggested that several researchers have actually used shockwave as a means to stimulate bone growth around an already lose prosthesis (osseous ingrowth). It would seem prudent to avoid the area given the possible loosening effect which, unless desired, would certainly constitute a detrimental outcome.
