Shockwave Therapies – The Essentials
A SHOCKWAVE is essentially a PRESSURE DISTURBANCE that propogates 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.
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. Shockwave Therapies
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. A very readable but succinct history of the development of shock waves for medical applications can be found in Thiel (2001).
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 the most commonly employed generation method is based on the pneumatic system, and the key reason for this is that a radial (dispersive) wave results. The focussed waves are essential for ‘surgical’ interventions, but given their destructive nature, they are less appropriate for therapeutic uses. Focussed waves are sometimes also referred to as ‘hard’ shockwaves, the radial or dispersive wave sometimes called a ‘soft’ shockwave (another twist in the terminology). Shockwave Therapies
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)
Shockwaves are divided in terms of their energy content and although there is some controversy, it is generally accepted that the following groups would be reasonable (after Rompe et al, 1998):
LOW (up to 0.08mJ/mm2)
MEDIUM (up to 0.28mJ/mm2 – though some authorities elect for a higher value)
HIGH (over 0.6mJ/mm2)
Though almost all authors, manufacturers and others divide the range into these energy bands, there is (as yet) no universal agreement with regards the boundary values.
Physiological Effects and Mechanisms of Action
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 of 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), and thus the destructive effects are eliminated. There is no evidence of tissue destructive effects at therapy level doses. Shockwave Therapies
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.
The full details of physiological and therapeutic mechanisms are yet to be identified, though a range of effects have been confirmed and several others postulated. Some of the effects relate to an increase in local blood flow which has been clearly evidenced, even in relatively avascular tissues. It is suggested that the beneficial effects are partly also due to a stimulation of an inflammatory response – therefore enhancing tissue repair responses, which is especially relevant when dealing with recalcitrant tissues, such as some chronic tendinopathies and delayed and non unions in bone.
One of the strongest arguments for the use of shockwave in therapy is that it effectively takes a Tissue from a more chronic to a more acute state, and in doing so, provides a stimulus (trigger) to a ‘stalled’ repair sequence. This is actually consistent with other approaches employed in therapy – such as some manual therapies (e.g. transverse frictions), some exercise based approaches (e.g. eccentric loading) and some electrotherapy interventions (e.g. provocative ultrasound or laser treatments). The following are the most strongly established treatment effects at therapy shockwave levels.
Increased local blood flow
Increase in cellular activity – release of substance P, prostaglandin E2, NO, TGF β, VEGF, and almost certainly other inflammatory cytokines
Transient analgesic effect on afferent nerves
Break down calcific deposits (primarily, but not exclusively in tendon)
Energy Levels for Detrimental Effects
High energy shockwave (considered to be over 0.6 mJ/mm2) have been shown to have detrimental effects in soft tissues, though it is proposed that this is not a dose that would normally be employed in therapy, and is likely to require at least some form of local analgesia to be able to tolerate the treatment! There is some evidence that energy densities greater than 0.4 mJ/mm2 may have detrimental effects, though this has yet to be confirmed. In tendon *(using an animal model), shockwave at 0.6 mJ/mm2 was demonstrated to have a damaging effect on local blood vessels (Rompe et al 1998).
Provided that the applied energy levels are in the therapy range (LOW and ?possibly MEDIUM), there have been no significant adverse effects reported. Some reports of pain or discomfort during, and sometimes after the treatment, but this usually subsides within a relatively short period (1-2 days). It is worth advising the patient of this possibility when discussing the treatment, prior to application. There can be minor skin irritation, and sometimes numbness or paraesthesia, but all are temporary. The potential for and the incidence of adverse effects is included in the Wang et al (2012) review. Shockwave Therapies
Treatment Dose Issues
In addition to the applied energy (mJ/mm2) – in therapy we are using the LOW (up to 0.08mJ/mm2 ) and possibly the MEDIUM (up to 0.28 mJ/mm2) energy levels, the other significant factors are
a) number of shocks and
b) number of treatment session repetitions
Shock number usually between 1000 and 1500, though some authorities suggest up to 2000
Some research has tried as few as 100 and also 500
500 more effective than 100
1000 – 1500 have been used in the clinical trials with the best (most significant) outcomes
Anecdotally, 1000-2000 shocks per session appears to be the most commonly applied range
Number of Treatment Sessions
Some evidence for a single session BUT only for High level treatment – using local anaesthesia – not physiotherapy
Most clinical research has used between 3 – 5 sessions at low energy levels (typical therapy application), suggested up to 7 may be needed in the more recalcitrant lesions
There have been no RCT trials yet to determine the maximally effective therapy session number (or interval)
Typically 3 – 5 session appears to be effective for the majority of patients, spaced such as to let the tissue ‘reaction’ at least partly subside from the first session before the next treatment is delivered. Optimal treatment spacing has yet to be identified in the published research evidence. Shockwave Therapies
In terms of specific lesions that have been supported by the research evidence, the tendinopathies are certainly the most frequently reported in the literature, though open wounds and bone union (delayed and non union) problems are also found. Of the chronic and especially recalcitrant tendon lesions, those with strongest research and anecdotal support include :
Tennis and Golfers elbow (medial and lateral epicondylalgia)
Other include trochanteric bursitis, though it is appreciated that this is clearly not a tendinopathy type lesion.
Interestingly, this ‘problem’ list closely mirrors the clinical presentations identified by Poltawski, Watson and Byrne (2008) as those identified by therapists as being ‘highly problematic’.
Shockwave – Research Evidence
The attached reference list identifies a range of research and review papers which have been published. Not all are strongly conclusive, and some of the reviews are less enthusiastic than others – but that largely depends on when the review was carried out, which literature was included, and conversely, which papers were excluded from the review.
The weight of the evidence is more supportive of the intervention than not, with the anecdotal evidence being even stronger. If one were to deliver an evidence based approach to the clinical management of a chronic tendinopathy, it would be difficult to justify the exclusion of shockwave as a therapy. It is not suggested that it is best used instead of other (effective) interventions, but as an adjunct to overall management (see for example the papers by Rompe et al with regards Achilles problems, shockwave and eccentric loading). Shockwave Therapies
Contraindications, Dangers and Precautions
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. This list is compiled from the best (currently) available evidence and expert advice/opinion. It may be that this is an over conservative approach, but as with many ‘new’ or ’emerging’ therapies, it is normal to err on the side of caution in the initial stages of clinical application.
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. Shockwave Therapies
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.
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