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Physical Principles and Generation
of Shock Waves
Ludger Gerdesmeyer, Mark Henne, Michael Göbel, Peter Diehl
17
Introduction
The effect of extracorporeal shock waves on biological tissue has been known
since observations were made during the second World War. At that time, castaways swimming in the water suffered lethal lung damage when water bombs
were detonated in a wider radius. The impact of such extracorporeal shock waves
caused lungs to tear without any visible injuries on the outside[12]. The properties of such shock waves were utilized to develop new test procedures in material
research. Another area of application is signaling technology. Indeed, shock waves
propagate (in water, for example) over long distances with only minor energy
loss. By measuring acoustic travel time, distances can be deduced as illustrated in
laser distance measurement technology[13, 18].
Since they were first used successfully by Chaussy in the treatment of kidney
stones in 1980, shock waves have held their own in medicine and the number of
applications has risen considerably[2]. The first gall bladder stones were treated
in 1985[15]. Since the study conducted by Valchanov, who described the effect of
shock waves on the healing of bone fractures, the range of indications for ESWT
treatment widened significantly in the area of orthopedics[21]. Today, not only
kidney and gall bladder stones but also salivary gland stones, pancreatic stones,
pseudoarthrosis, lateral epicondylitis humeri radialis, heel spur and calcified
shoulder are treated with varying success[5, 6, 9, 11, 15-17]. A precise description
of shock waves was long thought to be unnecessary. What mattered most in the
treatment of kidney stones was to be able to fragment the calculi without touch11
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EXTRACORPOREAL SHOCK WAVE THERAPY
ing the patient. Side effects such as bleeding or skin lesions were thought to be
acceptable. However, the situation in orthopedics is quite different. As a result of
the multitude of indications, often very different clinical objectives have been
defined. Changing conditions in orthopedic ESWT have brought about a greater
need for information on the shock wave itself but also on the equipment to be
used and on the possibilities to favor the positive effects of ESWT and to reduce
even further any of the rare side effects found to be relevant. To meet such objectives, the information would allow modifying therapy parameters as well as shock
wave emission technologies. What is known today about the effects of shock
waves on the treatment of kidney stones does not suffice to determine the reasons
for the effect of ESWT on orthopedic treatments, to describe which effects are
induced on the cellular level, or to explain the shock wave’s analgesic effect[4, 7,
14]. A precise definition of shock wave-relevant parameters is therefore an indispensable requirement to be able to evaluate any observed clinical effects or findings from basic research.
This chapter gives an overview of the physical parameters, the characteristic
properties and the generation of extracorporeal shock waves.
The definition of physical parameters and their characteristic properties is
based on the international standard proposal IEC 61846 (International Electrotechnical Commission, Geneva, Switzerland) and on the work of the Shock Wave
Therapy Consensus Group. They define which parameters are to be described for
individual units and therefore provide utmost transparence for the user[23].
However, they do not state which of these parameters has medical relevance or
which has specific biological effects. Parameters can be viewed at http://
www.ismst.org/. The hope is to define the relation between dose and effect independently of the equipment used, and to look for parameters that may be responsible for the medical efficacy of the shock wave.
Shock waves are defined as transient pressure oscillations that propagate in
three dimensions and typically bring about a clear increase in pressure within a
very short time. In most units used in medicine, such maximum pressure is
reached within few nanoseconds (ns)[10]. Besides this very rapidly rising positive pressure impulse, shock waves are also characterized by a tension phase with
negative pressure following the pressure phase. Overall, shock waves are characterized by the following properties (Figure 1)[19, 22, 23]:
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Figure 1a. Illustration of a shock wave. Shock wave pressure is shown as a
function of time. A, first portion of the shock wave with positive pressure; B, second portion of the shock wave with negative pressure; P+, positive peak pressure;
P-, negative peak pressure; Tr, rise time; Tw, impulse width; I+, standard time
interval to calculate the shock wave’s so-called “positive energy”; I, standard time
interval to calculate the shock wave’s so-called overall energy.
• Positive peak pressure (P+): P+ is defined as the difference between maximum positive peak pressure of the shock wave and ambivalent pressure.
Depending on equipment type, P+ varies from 5 mega Pascal (MPa) to
120 MPa.
• Negative peak pressure (P-): P- is defined as the maximum negative peak
pressure during the second phase of the shock wave. P- reaches values
between 10% and 20% of P+.
• Rise time (Tr): Tr is defined as the interval in which pressure rises from
10% of P+ up to 90% of P+. Depending on equipment type, Tr can vary
from a few nanoseconds to milliseconds.
• Impulse width (Tw): Tw is defined as the interval between the time when
pressure first exceeds 50% of P+ and the time when pressure (during the
exponential pressure drop within the first phase of the shock wave) is less
than 50% of P+. The duration of Tw is between 200 ns and 500 ns. The
term “full-width-half-maximum” (FWHM) is also used as a synonym for
Tw. The duration of Tw affects directly the energy flow density of extracorporeal shock waves.
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Figure 1b.
Pressure measurement of one radial shock wave impulse.
The values of P+, P-, Tr and Tw of a shock wave depend to a large extent on the
shock wave source and on the setting used. The tension portion of the shock wave
lasts clearly longer than the pressure portion. In contrast, the value of P- is always
less than the value of P+. Also, in contrast with P+, P- is limited in its amplitude
due to physical principles. In the case of higher tension force, cohesion forces of
the surrounding medium may be exceeded whereby gas-filled negative pressure
bubbles - so-called cavitation bubbles - appear[11]. A complete shock wave lasts
from a few microseconds to milliseconds; the frequency spectrum encompasses a
range between 16 Hz and 20 MHz.
Principle of shock wave generation
Different processes have been developed to generate shock waves. Four techniques in particular are used in clinical applications. All methods so used aim to
couple the generated pressure impulse to the tissue while minimizing energy
loss. To this effect, several coupling media are used. The units used in medicine
to generate pressure impulses rely on some techniques that are basically different[18].
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Figure 2a. Illustration of a ballistic shock wave source
One of the latest but also very common methods is the mechanical generation of shock waves. Based on ballistics, compressed air significantly accelerates a
projectile which hits an applicator placed on the skin at very high kinetic energy.
By using a coupling gel such as ultrasound gel or Castor oil, this impact pressure
hitting the applicator can be delivered to the tissue in the form of a pressure wave.
Then, the shock wave continues to propagate in the body in the form of a spherical or ball-shaped wave. This wave travels in a radial fashion, thus the descriptive
term radial shock wave.
The main characteristic of this type of unit is that steepening occurs much
more slowly compared to focused shock wave devices. Focused technologies are
required to treat deep areas. Radial technologies provide no second acoustic
focus. In this type of shock wave generation, the applicator surface constitutes the
geometric point of highest pressure and highest energy density and so it is called
tip-of-the-applicator-focus. Due to radial expansion, the pressure and energy
density of the shock wave drop steadily upon leaving the applicator. The same
shock wave propagation characteristics could be found in focused technologies
behind the acoustic focus. Due to theoretical considerations, they also first appear
to be less appropriate for classic indications such as pseudoarthrosis or calcific
tendonitis located in deeper tissue layers[23]. In contrast, there is no doubt that
indications near the surface are highly appropriate for shock waves that propagate
radially. In the meantime, new and improved techniques have led to a modification in applicator systems used to deliver ballistic shock waves. Special geometry
and changes in applicators allow ballistic shock waves to focus on areas of higher
concentration. Clinical studies will need to demonstrate whether such technical
modifications would also be appropriate to treat conditions such as pseudoarthrosis or calcific tendonitis reserved until now for the “classic” shock wave.
Another common method to generate shock waves is using electromagnetic
currents (Figure 3).
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EXTRACORPOREAL SHOCK WAVE THERAPY
Figure 3.
Illustration of an electromagnetic shock wave source
In this process, local electromagnetic currents are induced with a flat coil in a
thin copper foil. Due to the Lorentz effect on charges in motion, the foil undergoes an explosion-like deflection. In the process, the associated water column is
deflected in proportion to the tension. The pressure impulse so generated is then
coupled and transmitted to the next medium. Additional technical features such
as acoustic cutoff lenses are able to bundle the pressure waves into a defined focus
to be placed in deeper regions of the body. Additional acoustic reflectors can further improve the precision of the focus. Further clinical studies will have to show
whether such features would be suitable as part of orthopedic pain therapy. The
electro pneumatic principle is an old method by which shock waves are generated
by a spark plug located in the primary focus (Figure 4). The high temperatures
reached at the time of spark discharge cause the surrounding liquid to evaporate
and a plasma bubble to occur. These radial shock waves from the primary focus
are then bundled into a second focus by using an elliptic acoustic mirror.
By using appropriate coupling media, shock waves can be transmitted in
selected regions. One of the disadvantages of this process is spark plug wear and
wear-dependent shock wave energy variations. The rather costly exchange of electrodes must be repeated after a certain number of discharges. In addition, the shock
wave generated with this process may fluctuate between shots in terms of energy
and geometry [1] but these variations are believed to be clinically irrelevant.
Another method is the piezoelectric principle[20]. A small pressure impulse
is emitted in the center of a ball cup by pulse-like local electric impulses of individual piezocrystals. Since the crystals are arranged on a half shell, the individual
pressure waves can be bundled in one focus (Figure 5).
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Figure 4. Illustration of an electromagnetic shock wave source
As in the other methods, such focus can then be placed in the body by using
appropriate localization systems. The shock wave is coupled based on the same
principle as in the other emission methods mentioned earlier.
In terms of shock wave generation methods, a detailed discussion about technical equipment parameters is of little use at this time from a medical point of
view. Indeed, today it has not been determined which parameter is significant for
possible biological effects or clinical results[8]. However, for the purpose of technical comparisons, it is useful to identify as many shock wave parameters as possible.
Figure 5. Summary illustration of a piezoelectric shock wave source
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Figure 6.
Illustration of a PVDF hydrophone
Measuring Technique
Just like technical progress has led to on-going equipment modifications and
new techniques to generate shock waves, so have technical advances affected the
processes involved in measuring technology. In principle, shock wave magnitude
can be measured by electrical and non-electrical means. Non-electrical methods
include optical methods and the defined fragmentation rate of model stones[7,
18]. However, there is general consensus that only electrical sensors, so-called
hydrophones, are suitable for the quantitative measurement of shock wave magnitude.
These are sensors for acoustic pressure in liquids which deliver an electric
signal and which are appropriate due to the possibility of calibration for absolute
measurements. Among the older hydrophones are PVDF hydrophones. The first
dynamic pressure measurements of shock waves were performed with them. The
hydrophone used at the time is based on a concept by Chivers and Lewin dating
back to 1981 and originally designed for the low-pressure area. This measuring
technique is based on the piezoelectric properties of a polyvinyl fluoride (PVDF)
foil applied to a thin steel tube. At the end the generated potential is made transparent via an oscilloscope (Figure 6).
A major disadvantage of this hydrophone is the fact that the tensile part of
the shock wave could not be measured precisely due to local cavitation phenomena as well as the limited stability (life) of the hydrophone.
The consensus therefore was to use hydrophones which do not have the
PVDF hydrophones related disadvantage. Among them are fiber-optic hydrophones which register acoustic waves as peaks and convert them to proportional
output voltage.
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Due to the high cost of this technology fiber glass hydrophones have only
been used for the past two years. These more recent measurements produce in
part significantly higher values for the same parameters. As mentioned before,
measurements performed under in vivo conditions raise particular problems[7].
It must be noted once again that until now no link has been established
between measurable shock wave parameters and biological or analgesic effect.
Today, it is not known whether such equipment-specific parameters are clinically
relevant at all or rather serve to animate a technical discussion of lesser clinical
relevance.
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