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301
J.J. Talati et al. (eds.), Urolithiasis,
DOI 10.1007/978-1-4471-4387-1_38, © Springer-Verlag London 2012
Introduction
On February 7, 1980, in Munich, Germany, for the fi rst time,
kidney stones were successfully fragmented within a patient’s
body by externally generated shock waves. The mechanical
energy of shock waves was transmitted through the intact
skin and concentrated on the stone without signi fi cant dam-
age of the tissue. The granular fragments were fl ushed out of
the body in natural way, eliminating the need for invasive
surgery. This date marks the beginning of a new era charac-
terized by application of acoustic energy for noninvasive
stone fragmentation and clearing [ 1, 2 ] . The revolutionary
fi nding was that shock waves may pass through living tissue
without signi fi cant injury or side effects while being simulta-
neously strong enough to fragmentize hard urinary stones.
The key reasons are the following: fi rst, elasticity of living
tissue, which may pass high transient pressures up to 100 MPa
(1,000 bars) and more; second, the liberation of transient
forces predominantly at acoustic interfaces with different
acoustic characteristics (acoustic impedance); and third, the
possibility to couple acoustic energy with low intensity into
the body and concentrate (focus) it on the region of interest,
the stone to be fragmented. This option allows the stone to be
exposed to suf fi cient shock wave power for fragmentation
while, simultaneously, all other tissue areas, which are passed
by the shock waves, are only affected insigni fi cantly.
To date, stones in the entire urinary tract are susceptible to
shock wave fragmentation only limited by the occasionally
huge amount of fragments to be passed via naturalis through
the urinary tract.
Abstract
Extracorporeal shock wave lithotripsy (SWL) is a gentle and noninvasive treatment
procedure suitable for a wide range of kidney and ureteral stones. The technique makes
use of extremely short transient (<1 m s) pressure pulses with pressure amplitudes up to
100 MPa (1,000 bars). Brittle stone material breaks into small pieces, whereas biological
tissue passes shock waves without signi fi cant damage. Different shock wave generation
principles (electrohydraulic, piezoelectric and electromagnetic) are presented, and basic
shock wave parameters are discussed with regard to stone fragmentation and side effects.
Modern lithotripsy devices utilize fl uoroscopic and/or echographic imaging methods for
stone localization and targeting. As with all sophisticated technologies, SWL requires
comprehensive technical and anatomical skills to fully bene fi t from this exciting treat-
ment method.
Keywords
Shock waves • Extracorporeal lithotripsy • Shock wave lithotripsy (SWL) • Kidney stones
• Electrohydraulic • Piezoelectric • Electromagnetic
Physics and Technique of Shock
Wave Lithotripsy (SWL)
Othmar J. Wess
3 8
O. J. Wess , Ph.D.
Storz Medical AG ,
Lohstampfestrasse 8 , Product Development, Taegerwilen,
Thurgau 8274 , Switzerland
e-mail: wess.othmar@storzmedical.com
Electronic supplementary material The online version of this chapter
(doi:
10.1007/978-1-4471-4387-1_38 ) contains supplementary material,
which is available to authorized users.
302 O.J. Wess
Nature of Shock Waves
Medically used shock waves are mechanical waves featuring
extremely high-pressure amplitudes in the range of
10–100 MPa (100–1,000 bars). They require an elastic
medium such as gas, fl uids, or soft tissue for propagation and
cannot be transmitted through vacuum. Shock waves appear
in nature as explosive noise generated by atmospheric lighting
or by detonation of explosive material. A typical pressure–
time pro fi le of a shock wave in water is shown in Fig. 38.1 .
Usually, medical shock waves are generated in water propa-
gating with approximately the speed of sound (1,500 m/s eq.
5,400 km/h). Due to the very high-pressure amplitudes, how-
ever, normal linear propagation conditions are exceeded so
that the highest pressure components travel slightly faster than
lower pressure components resulting in a steepening of the
pressure ascend of the wave as shown in Fig. 38.2 . Shock
waves in gaseous or fl uid mediums propagate as longitudinal
waves featuring particle movements in direction of propaga-
tion and reverse. As acoustic waves in general, shock waves
may be re fl ected, refracted, diffracted, or strayed when pass-
ing inhomogeneities with respect to acoustic features (acous-
tic impedance) of the medium. Shock waves may be focused
for energy concentration at speci fi c regions within human
bodies in order to expose selected target areas such as tissue
under treatment or kidney and ureteral stones.
Shock Wave Generation Methods
Electrohydraulic Shock Wave Generation
Shock waves may be generated by explosive processes, pri-
marily, expanding faster than the speed of sound in a particu-
lar medium such as water. Instead of explosive material, the
fi rst clinically used method was based on a high-energy elec-
trical discharge across a 1-mm spark gap ignited in a water
bath. A capacitor bank was charged by approximately 20,000
V (20 kV) and discharged across the electrode generating a
rapidly expanding plasma channel (spark). This plasma
channel pushed the surrounding water by pressures of more
than 100 MPa, which propagated into the medium. Figure 38.3
shows a series of high-speed photographs (frame rate 10
7
frames/s) with a lighting spark between two electrode tips
and a spherically expanding shock wave displayed with
shadow photographic technique. Electrical discharge gener-
ates shock waves in its physical meaning from the origin on
since the expanding velocity of the spark channel is slightly
higher than speed of sound. After a few millimeters of travel,
electrohydraulically generated shock waves are slowed down
to normal propagation velocity (approximately 1,500 m/s).
They may be re fl ected by acoustic mirrors and focused onto
the target position of the stone.
The primary shock wave generated by this method is a
spherically expanding wave, which propagates with the
speed of sound into the surrounding medium. The energy is
dissipated by expansion and needs to be concentrated on the
target stone for fragmentation. Concentration is done by
Shock Wave
time
tr
tw
tf
pressure
p+
p-
Fig. 38.1 Time pro fi le of a medical shock wave ( p + peak positive pres-
sure, p − peak negative pressure, t r rise time, t w pulse width, t f fall time)
pressurepressurepressure
slower at low pressure
distance
distance
distance
steepening
steep flank
faster at high pressure
faster at high pressure
slower at low pressure
Fig. 38.2 Steepening of pressure rise. The pressure rise is compressed
to shorter rise times due to nonlinear propagation (faster with higher
pressure, slower with lower pressure). At high-pressure amplitudes,
shock fronts with steep rise develop with travel distance
30338 Physics and Technique of Shock Wave Lithotripsy (SWL)
re fl ection of the spherical wave at the surface of a hollow
ellipsoidal metal re fl ector (see Fig.
38.4 ).
The spark gap is placed at the fi rst focal spot of the ellip-
soid, and the primarily diverging wave is concentrated at the
second focal point in a distance. By cutting the ellipsoidal
structure at an intermediate level, this second focal spot can
be placed outside the metal structure to be aimed from out-
side the human body at the target stone within the body.
This pioneering type of shock wave generation and focus-
ing has been very successful and motivated engineers to fur-
ther develop shock wave techniques to overcome some of the
disadvantages of the electrohydraulic principle such as fre-
quent change of worn-out electrodes, fl uctuations of energy
output, and excessive noise.
A subgroup of electrohydraulic generators, called “elec-
troconductive shock wave generators,” makes use of a con-
ductive fl uid between the electrode tips in order to reduce
energy fl uctuations of sequential shocks.
Piezoelectric Shock Wave Generation
A more reproducible method of shock wave generation is
based on the piezo effect, which is widely used in ultrasound
diagnostic devices. Making use of the piezo effect, a large
number of piezo ceramic elements are activated simultane-
ously by applying an electrical tension of several kilovolts
(kV). According to the electrical excitation, the elements
expand, and the displacement of the surface generates a pres-
sure wave that propagates into the adjacent medium.
Piezoelectric elements can be arranged on the inner surface
of a spherical calotte to generate a spherical wave directed to
Fig. 38.3 HF photography of spark and shock wave generated in water
(10
7 frames/s). A spark is ignited between two electrode tips in a dis-
tance of approximately 1 mm. The plasma channel between the tips
expands with supersonic velocity and generates a spherical shock wave.
The shock wave separates from the plasma channel as soon as the
expansion velocity of the spark channel drops below the propagation
velocity in water (1,500 m/s)
F2F1
Fig. 38.4 Spherical shock waves generated in the fi rst focal spot F1 of
a semi-ellipsoid are partly concentrated on an area around F2 , the sec-
ond focal spot of the ellipsoidal structure. Part of the primary spherical
wave is radiated as dissipating wave without being focused
304 O.J. Wess
the center of the calotte. A piezoelectric shock wave genera-
tor is shown in Fig.
38.5 .
Whereas the electrohydraulically generated shock waves
originate from a tiny small point-shaped area between the
electrode tips with extremely high pressure and high energy
density, piezoelectrically generated shock waves are emitted
by a relatively large area with low energy density. In order to
provide suf fi cient energy for stone fragmentation within the
focal zone, a relatively large area of active piezo ceramic ele-
ments (several hundred square centimeters) is required. The
acoustic energy is generated as (non-shock wave) pressure
pulse and concentrated to a small area at the center of the
calotte. It becomes a shock wave by steepening on its travel
to the focus as shown in Fig. 38.2 .
Piezoelectric shock wave generators avoid the need for fre-
quent and costly change of electrodes as in the electrohydrau-
lic case and last several million shocks. Due to its complexity,
an exchange of the total shock wave head is more expensive.
Another advantage of piezoelectric generators is the reproduc-
ibility of each single shock and the precise control of the shock
wave parameters. The fragmentation power of piezo elements
is relatively low. It requires quite a large active area to yield
suf fi cient energy for stone fragmentation. There are attempts
to make use of a sandwich structure of two active areas: one at
the inner surface of the spherical calotte and another at the
outer surface. A slight delay of activation between the two
active spheres allows for a certain pulse shaping. By this tech-
nique, the size of the large calottes may be reduced to more
convenient dimensions. Nevertheless, the overall fragmenta-
tion power seems to be inferior to other techniques.
Electromagnetic Shock Wave Generation
The method of electromagnetic shock wave generation is
based on the physical principle of electromagnetic induction,
as used, for example, in loudspeakers. A strong pulse current
running through an electrical coil generates repelling forces
in a conducting membrane attached to the coil separated only
by a thin insulation foil. The conducting membrane is acti-
vated by eddy currents and pushed toward the adjacent
medium (water). Although the displacement of the membrane
measures only a fraction of a millimeter, the surrounding
water is compressed, and the according pressure distortion is
radiated as pressure wave into the medium (Fig. 38.6a, b ).
Two different con fi gurations are used in modern shock
wave lithotripsy (SWL) devices. The traditional electromag-
netic generator utilizes a fl at coil/membrane arrangement
Fig. 38.5 Piezo-calotte. Numerous piezo elements are arranged on a
spherical bowel and activated simultaneously to generate a self-
focussing spherical pulse wave
ab
Fig. 38.6 Flat coil/acoustic lens con fi guration . The electrical fl at coil ( a ) is covered by an insulation foil and a conducting membrane. Primary
plane acoustic waves are focused with the aid of an acoustic lens ( b )
30538 Physics and Technique of Shock Wave Lithotripsy (SWL)
that focuses a primarily plane pressure wave by aid of an
acoustical lens. The most advanced con fi guration has a cylin-
drical coil/membrane and a special rotational parabolic
re fl ector for focusing the primarily cylindrical pressure wave
to a precise focal spot (Fig. 38.7 ). As in the case of piezo-
electric generators, pressure waves generated by electromag-
netic principle turn into shock waves by steepening when
concentrated in the focal zone. The coil con fi guration pro-
vides a central opening for high-precision in-line integration
of X-ray or ultrasound localization devices.
The cylinder con fi guration does not require acoustical
lenses and offers widest apertures for gentle energy trans-
mission into the body and largest focal depth for treatment of
obese patients with extraordinary stone-to-skin distances.
Physical Characteristic of Shock Waves
Medically used shock wave devices may be characterized by
the type of shock wave generator and the spatial and tempo-
ral parameters of the shock wave fi eld. Aperture diameters,
focus distances, lateral and axial focus dimension, peak pres-
sure, and energy fl ux density are parameters affecting frag-
mentation power of a shock wave device as well as pain
sensation and risk of side effects [ 3– 6 ] .
Pressure
The shock wave fi eld is measured by taking time-pressure
records p ( t ) point by point, scanning a small hydrophone
through the three-dimensional shock wave fi eld. For each
spatial position, the peak pressure p + and the time pro fi le p ( t )
of the pressure curve are taken, including rise time and time
duration of each single pulse as shown in Fig. 38.1 . Shock
waves in medicine feature typically peak pressures of
10–100 MPa (100–1,000 bars) and sometimes more. Rise
times are in the range of <10–100 ns (nanosecond), depend-
ing on the type of generation. Time duration of the pressure
curve is approximately 0.5 m s (microsecond). Pulse length in
water/tissue is <1 mm. Each shock wave pulse is associated
with negative (tensile) wave components in the range of
10 % of the peak pressure p + .
−6 dB Focal Zone and 5 MPa Treatment Zone
Shock waves are coupled over a wide surface area into the
body and are concentrated (focused) to a treatment area with
intensi fi ed energy. The transmission pathway is protected
against high acoustic energy reducing side effects while,
simultaneously, effective treatment and possible side effects
are restricted to the region of interest, the treatment zone.
The therapeutic shock wave fi eld thus shows a central peak
of highest pressure p + and a continuously declining pressure
around as shown in Fig.
38.8 .
Taking the peak pressure p + as reference, a three-dimen-
sional zone may be de fi ned with pressure values being equal
or greater than ½ p + , the so-called −6-dB isobar. This zone is
usually taken as focal zone of a lithotripter. Please note that
Fig. 38.7 Cylinder source. The electrical cylinder coil is covered by an
insulation foil and a conducting membrane. The primary cylindrical wave
is re fl ected and focused by a rotational parabolic re fl ector. The hollow cen-
ter may be used for in-line fl uoroscopic or ultrasonic stone localization
Pressure
5 MPa
700650600550500450400350300250200150100
50
0
60 38 26 13 0
Axial [mm]
-13 -25 -38 -60
-10 -5 -6
Lateral [mm]
510
P+
P+
2(-6bB)
Fig. 38.8 Spatial pressure fi eld. The −6-dB zone is characterized by
50 % of the actual peak pressure p + . The area exceeding the pressure of
5 MPa may change with energy settings (different peak pressure)
306 O.J. Wess
this focus de fi nition does not re fl ect the dimensions of the
treatment area itself. It only relates, per physical de fi nition,
to the peak pressure. The peak pressure p + may vary between
different devices and settings signi fi cantly without affecting
the focal size. Therefore, the −6-dB focal zone simply re fl ects
the degree of energy concentration, but is neither a measure
to characterize the power of the shock wave fi eld nor the area
within which a shock wave fi eld may break stones.
To enable a comparison between different energy settings
and different devices, one may de fi ne the zone within a fi xed
isobar—of, for example, 5 MPa—as an area of therapeutic
(fragmentation) ef fi ciency. Even though the de fi nition of 5 MPa
seems to be arbitrary, it re fl ects the in fl uence of the energy set-
ting on the spatial dimensions of the effective zone (Fig. 38.9 ) .
Energy
Although p ( t ) being the primary parameter of a three-dimen-
sional shock wave fi eld, it does not correlate directly with the
fragmentation ef fi ciency, the main goal of a shock wave lith-
otripter. Shock wave energy (not only pressure) is needed to
break stones. The energy may be calculated for a given area
A by taking pressure curves p ( t ) within the area A and acous-
tical parameters of the propagation medium density ( r ) and
sound velocity ( c ), according to the formula:
The acoustical energy of a shock wave pulse is given in
millijoules (mJ). As a rule, several hundreds or thousands of
shock wave pulses are emitted per treatment, so that the total
emitted energy is yielded by multiplication with the number
of pulses.
Only that portion of the shock wave fi eld that hits the
stone will contribute to stone fragmentation. For character-
ization of different lithotripsy devices, a medium-sized stone
with a circular cross section with a diameter of 12 mm is
taken for reference by convention. The energy within this
cross-sectional area A gives a comparable number for the
fragmentation power of a lithotripter device.
Published data on the energy content of the −6-dB focal
zone are just physical parameters that do not characterize the
fragmentation power of a lithotripter (see chapter “-6dB
Focal Zone and 5 MPa Treatment Zone”).
Energy Flux Density (ED)
As previously mentioned, the therapeutic effect of shock
waves depends on how much shock wave energy is deposited
on the stone. The part that is missing the stone cannot con-
tribute to fragmentation. Therefore, not only energy values
()
2
/ ( )d energyEAcpttρ= ∫
low
-6dB
5MP 5MPa 5MPa
-6dB -6dB
5MPa 5MPa
5MPa
50% = -6dB
50% = -6dB
50% = -6dB
medium
high
100
50
10
100
50
10
100
50
10
-6dB
5MPa
Fig. 38.9 A −6-dB focus zone versus 5-MPa treatment zone at differ-
ent energy settings. With increasing energy the treatment area, de fi ned
by the 5-MPa zone, increases accordingly whereas the dimensions of
the physical focus zone (−6 dB) may stay independent in size from the
selected energy setting. Nevertheless, the energy content (see text) of
the focus zone may vary signi fi cantly
30738 Physics and Technique of Shock Wave Lithotripsy (SWL)
are of interest but also the energy fl ux density (ED), a mea-
sure for the spatial energy concentration (energy E /area A ):
The energy fl ux density (ED) is given in millijoules per
square millimeter (mJ/mm
2 ).
The total amount of shock wave energy delivered to the
stone may be calculated by integration of the energy fl ux
density over the cross-sectional area of the stone.
The parameters previously described are usually suf fi cient
to characterize a shock wave fi eld for medical applications.
Shock wave devices that work with different generation
principles can differ in relation to the listed parameters. The
“quality” of the shock waves used in the treatment zone,
however, seems to be independent from the generation prin-
ciple. In other words, the measurement of the above param-
eters in the treatment zone does not allow any fundamental
conclusions to be drawn about the type of generation.
Electrohydraulically generated shock waves are not better or
worse than piezoelectrically or electromagnetically gener-
ated shock waves, although secondary parameters such as
accuracy of repetition, dose control, energy range, and oper-
ating costs for consumables may differ signi fi cantly.
Note that the parameters discussed previously Ohysical
Characteristics of Shock Waves are usually measured in
water. Due to the inhomogeneities in living tissue, however,
deviations from the straight propagation of shock waves
occur. With increasing depth in the body, the peak pressure
as well as ED values may signi fi cantly be reduced by realis-
tic anatomical conditions. Accordingly, focal dimensions
(−6-dB focus) may be signi fi cantly (up to 10 mm) expanded
depending on individual anatomical conditions [ 7 ] .
Medical Shock Wave Effects
Wanted Effects
Although shock waves feature extremely high pressures for
very short time duration, elasticity of living tissue allows for
transmission of high-pressure waves usually without
signi fi cant lesions. Shock wave-speci fi c forces are generated
at acoustic interfaces characterized by different acoustic
impedances (density r × propagation velocity c ). Since
water, where shock waves are usually generated, and soft tis-
sue feature similar acoustic impedances, shock wave forces
at the water/skin interfaces are small as well as at interfaces
between different soft tissues. At the interface between kid-
ney and stone, however, a signi fi cant impedance mismatch
creates strong forces by incident and re fl ected waves.
Fragmentation forces are generated at front and rear surface
of the stone when shock waves are transmitted and re fl ected
(Fig. 38.10a ). Erosion and spallation (Hopkinson effect) at
rear surface are observed (Fig.
38.10b ). There are different
additional fragmentation effects discussed such as squeez-
ing, fatigue by development of micro rupture lines, and oth-
ers including cavitation effects [ 8– 12 ] .
Cavitation is an important mechanism for stone fragmen-
tation. The tensile trail of shock waves tears open small cavi-
tation bubbles (Fig. 38.11 ). They may collapse by generating
a needlelike water jet when close to an acoustic interface
such as the stone surface. Those microjets are directed toward
the interface and may have the velocity of gun bullets of sev-
eral hundred m/s. The impact of these high-velocity micro-
jets contributes to erosion and is an important factor of stone
fragmentation.
Shock waves were introduced into medicine primarily for
noninvasive stone fragmentation. Meanwhile, however,
another important shock wave effect was discovered. Shock
wave stimulation of soft tissue results in regeneration and
improved blood supply and is widely used in a variety of
medical indications, mainly for treatment of chronic pain
diseases such as chronic pelvic pain syndrome (CPPS),
orthopedic pain diseases, and cardiology.
Unwanted Effects
As with any medical method, there are always side effects
(Paracelsus 1493–1541). Fortunately, shock waves are asso-
ciated with predominately bene fi cial features, but certain
unwanted side effects may occur. The stone-breaking power
of shock waves may cause tissue lesions mainly by cavita-
tion effects. The mentioned microjets may punch small ves-
sels and cause microbleedings, which may, rarely, result in
signi fi cant hematomas (Fig. 38.12 ). Administration of anti-
coagulation drugs has to be thoroughly controlled.
Since shock waves may affect the nervous system as well
as the cardiac excitation system, according effects such as
cardiac interference are possible risks. Due to a major mis-
match of acoustic impedances of soft tissue and gas-contain-
ing organs (lungs, intestine), signi fi cant forces may be
generated at interfaces to such organs. Therefore, these organs
should be kept free from shock wave exposure, at least from
the high energetic focal area. Bony structures exposed to
shock waves will distort shock wave propagation, avoiding
successful stone fragmentation. Last, but not least, shock
wave application for stone treatment causes pain. Depending
on the shock wave generation methods and treatment strategy,
pain killers or even general anesthesia may be appropriate.
How to Apply Shock Waves
As a general rule, the mentioned bene fi ts (stone fragmenta-
tion, less invasiveness etc.) have to be balanced against rare,
()
2
/ 1/ ( )d ED energy flux densityEA c p t tρ==
∫
308 O.J. Wess
a
b
Fig. 38.10 ( a ) An arti fi cial stone of plaster (3 × 3 × 1.5 cm) is hit from
the right by a focused shock wave. The front surface ( right ) is eroded
and a crater develops. The whole stone is shattered and pieces break off
the corner. ( b ) Sequential shock waves (incident from right ) hit a 1-cm
cube of plaster. At the rear surface ( left ), spallation due to the Hopkinson
effect is seen, and pieces are expelled from the front surface until the
stone breaks completely
Fig. 38.11 The shock wave at top propagated from the bottom to top
(approximately 75 mm) within 50 m s and generated a cloud of cavita-
tion bubbles. The time history of bubble expansion and the following
collapse can be deduced from their spatial location. Bubbles immedi-
ately behind the shock front were just generated and growing. Bubbles
in the middle have been generated approximately 30 m s before when the
shock front passed this area. They reached maximal diameters and start
to collapse. At the lower third, they collapsed completely after 40–50 m s
and create spherically expanding secondary shock waves ( circles at
lower part of the image )
30938 Physics and Technique of Shock Wave Lithotripsy (SWL)
but possible side effects (tissue lesions, pain, hematoma etc.).
Two aspects have to be considered: (1) skills and diligence of
the operating personnel and (2) technical concepts and
con fi gurations of the shock wave sources.
First: The operator needs medical training for exact diag-
nosis of stone diseases and in-depth anatomical knowledge
to proper selection of the appropriate shock wave passage to
the stone and optimal patient positioning. It does not suf fi ce
to see the stone at the predetermined position by the local-
ization device if the transmission pathway through the tis-
sue is obstructed by bony or gaseous structures. If necessary,
the position of the patient has to be corrected repeatedly.
Since with extracorporeal lithotripsy shock wave energy is
generated outside the body, it needs to be transmitted to the
body without signi fi cant losses. The coupling area of a
shock wave head at the surface of the patient may include
air bubbles that will signi fi cantly reduce effective energy
coupling even if the bubbles seem to be insigni fi cantly
small. Great care is required to perform the coupling pro-
cess to satisfaction. These skills and suf fi cient experience of
the operator are mandatory. They were not always given in
the past and could cause low success rates not to be accused
to the device.
Second: From a technical point of view, the con fi guration
of the shock wave fi eld can be optimized to provide suf fi cient
amount of acoustic energy for stone fragmentation prefera-
bly only at the location of the stone and to keep the energy
exposure low anywhere else. The same rationale holds true
also for the area where bene fi cial (fragmentation) as well as
impairing cavitational effects may occur. The technical
solution is a shock wave head with a wide aperture for soft
and pain-free energy transmission and an appropriately
sized treatment zone, not too big to affect tissue excessively
around the stone and not too small to make positioning
dif fi cult. In spite of the differences between treatment zone
and focal zone (see previous), the ability of a device to focus
precisely is preferred if stone localization is frequently
checked. There is no consensus about the optimal size of the
treatment zone. Larger treatment zones may facilitate posi-
tioning but lack generous power reserves for excessively
hard stones when needed. Additionally, even at lower energy
settings, an increased tissue area is exposed to signi fi cant
shock wave energy far beyond the dimensions of the stone
treated. Whenever the power setting of a device is suf fi cient
for stone fragmentation, a certain risk, usually small, to gen-
erate tissue lesions cannot be excluded. Single shock wave
devices such as the MODULITH® SLX-F2 ( STORZ
MEDICAL), therefore, offer switchable focus capabilities
for an individually matched treatment strategy regarding
size and hardness of stones, localization, pain sensation, and
other parameters.
Device Concepts
Routine lithotripsy procedures demand for ergonomic device
concepts consisting of a shock wave generator, localization
modality such as ultrasound and/or X-ray, a patient support,
and positioning structure. Ultrasound localization provides
real-time control of stone location and progress of fragmen-
tation but requires according skills of the operator that are
not common in all countries. Fluoroscopic targeting seems to
be preferred more widely. Both localization modalities may
be applied in-line, out of the center of the shock wave source,
Fig. 38.12 Collapse of a cavitation bubble (schematic) close to an
acoustic interface (stone surface). Due to asymmetric streaming condi-
tions at the interface, an asymmetrical collapse occurs and generates a
microjet directed toward the interface. Microjets featuring velocities up
to several hundred m/s affect the interface and contribute to erosion and
stone fragmentation as well as tissue lesions
310 O.J. Wess
meaning coaxially with the direction of shock wave propaga-
tion, or off-line, meaning outside the therapy head. The in-
line con fi guration enables direct control of the coupling area
as well as the transmission area within the body allowing for
optimized treatment selection with respect to coupling direc-
tion and quality of energy transmission (Fig. 38.13 ).
For busy stone centers with large numbers of SWL patients,
dedicated lithotripsy workstations, just for lithotripsy pur-
poses, may be the best choice. Today’s multifunctional work-
stations include highly fl exible patient support and positioning
systems as well as high-quality fl uoroscopic imaging and
localization devices to enable endourologic and percutaneous
stone treatment procedures on the same device. A modern
multifunctional workstation is shown in Fig. 38.14 .
Conclusion
Extracorporeal shock wave lithotripsy is an ef fi cient and
gentle stone treatment method less invasive than percuta-
neous or transureteral procedures. Although SWL tech-
niques seem to feature low risks and simple operation,
excellent SWL results require special operational skills as
well as technical and anatomical knowledge in order to
gain full bene fi t of modern lithotripsy technology.
Acknowledgement The author worked from 1979 till 1987 in the
fi eld of extracorporeal shock wave lithotripsy for Dornier Medizintechnik
GmbH, Münich, Germany, and from 1988 till 2009 for STORZ
MEDICAL AG, Taegerwilen, Switzerland. Since 2010, he is consultant
for STORZ MEDICAL AG.
Fig. 38.13 In-line localization. Fluoroscopy or ultrasound localization arranged in-line with shock wave propagation enables precise control of
stone position and quality of shock wave passage
Fig. 38.14 Multifunctional urological workstation for lithotripsy,
endourology, and percutaneous nephrolithotomy ( PCNL )
31138 Physics and Technique of Shock Wave Lithotripsy (SWL)
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