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Shockwaves
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Introduction In
1986 European researchers conducted animal experiments, which revealed
shock waves have the potential to stimulate bone formation by activation
of osteoblast cells. The
first positive results were reported after application of shock waves to
artificial humerus fractures in rats, by Gerald Haupt
et.al (1). Furthermore,
Michailov P. and Valchanou V. reported the use of "High Energy Shock
Waves In The Treatment Of Delayed And Nonunion Fractures" in 1991
(2). In this study,
seventy-nine patients suffering with shock waves resulting in seventy
patients achieving bony consolidation.
Thus, the new concept of orthopaedic "Extracorporeal Shock
Wave Therapy" or "ESWT" was introduced. Shock
Waves: Physical Principles Extracorporeal
shock waves used in medicine today (urology and orthopaedics) are created
as a result of Electrohydraulic (Spark Gap), systems Electromagnetic (EMSE)
or Piezoelectric (Piezo)
generation). Each is based
upon its own unique principles as follows: A.
Electrohydraulic (Spark Gap) systems incorporate an electrode
(spark plug), submerged in a water-filled housing comprised of an
ellipsoid and a patient interface. The electrohydraulic generator initiates the shock wave by an
electrical spark produced between the tips of the electrode, much like the
spark plug in an automobile. Vaporization
of the water molecules between the tips of the electrode produces an
explosion, thus creating a spherical shock wave.
The shock wave is then reflected from the inside wall of a metal
ellipsoid to create a focal point of shock wave energy in the target
tissue. The configuration
(size, shape) of the ellipsoid controls the focal size and the amount of
energy produced within the target tissue. B.
Electromagnetic (EMSE) systems utilize an electromagnetic coil and
an opposing metal membrane. A high current pulse is released through the coil to generate
a strong magnetic field, which induces a high current in the opposing
membrane. The magnetic field in turn induces a high current in the
opposing membrane and accelerates the metal membrane away from the coil. These electromagnetic forces induce a slow and low acoustical
pulse that is focused by an acoustic lens to direct the shock wave energy
to the target tissue. The
lens controls the focal size and the amount of energy produced within the
target tissue. C.
Piezoelectric (Piezo) systems form acoustical waves by mounting
piezoelectric crystals to a spherical surface.
When a high voltage pulse is applied to the crystals the
immediately contract and expand thus generating a low- pressure pulse in
surrounding water. The pulse
is focused by means of the geometrical shape if the sphere. In
general, electrohydraulic (Spark Gap) systems have faster shock wave rise
times and provide focused energy over a broader area, thereby delivering a
greater amount of positive shock wave energy to targeted tissue.
Meanwhile, electromagnetic (EMSE) systems have slower rise times
and produce significant amounts of tensile energy across the focal area,
resulting in lower shock wave energy delivery to targeted tissue.
Furthermore, findings from clinical studies involving the use of
shock waves in both kidney lithotripsy and orthopaedics, suggest the need
for re-treatment is significantly reduced with the use of electrohydraulic
systems. Additionally,
electrohydraulic systems have greater utility in orthopaedics when
compared to both electromagnetic and piezoelectric (low shock wave energy)
systems. This is due to the
higher quality shock wave energy delivered, which improves capability to
stimulate osteogenesis. It is
known that low shock wave energy application doesn't stimulate new bone
formation. It is theorized that the distribution of rise time, ratio of
positive to negative energy and energy density across the focal area, are
the most important physical parameters in the treatment of orthopaedic
diseases. Shock
Waves: Biological Effects At
the cellular level, extracorporeal shock waves create what are known as
"direct and indirect shock wave effects". The creation of high tensile forces on the surface of cell
membranes and the reflection, transmission and absorption of shock wave
energy into cells are termed "direct shock wave effects".
Meanwhile, the generation of transient cavitation bubbles and their
collapse during the tensile phase of a shock wave
are known as "indirect shock wave effects".
It is theorized that direct shock wave effects and indirect (cavitation)
effects cause hematoma and some cell death which are responsible for the
stimulation of new bone formation. In
general, "direct and indirect" shock wave effects account for
the energy setting and number of shocks to be administered when treating
soft tissue or bone indications. This
is due to the significant difference in "acoustical impedance"
(material density and sound velocity) between soft tissue and bone.
Thus, for treatment indications involving bone, the highest energy
settings and maximum number of shocks are used.
Meanwhile, treatment involving soft tissue (e.g. tendonitis),
requires approximately ten percent of this energy. Comparison
of Extracorporeal Shock Waves and Ultrasound Both
shock waves and ultrasound are widely used in medicine.
However, the medical application of both of these acoustic
waveforms is different. The
typical use of ultrasound is diagnostic while the use of shock waves is
therapeutic. In orthopaedics
and traumatology, ultrasound is sometimes used to identify the location of
plantar fasciitis, epicondylitis, tendonitis calcarea or other
tendonopathies. Whereas shock
waves are used to treat the patient in efforts of alleviating pathologies
related to these indications. The
physical characteristics of these two types of acoustic waveforms are
distinctly different. An
ultrasound wave has a sinusoidal waveform, with a slow rise time and
low-pressure amplitude. The
tensile portion of the wave has approximately the same magnitude as its
positive amplitude. When compared to the waveform of an extracorporeal
shock wave , it can be seen that an ultrasound wave cannot generate the
same magnitude of mechanical stress.
This is due to the extracorporeal shock wave's high positive
pressure amplitude and faster rise time. As
noted previously, in orthopaedics and traumatology, extracorporeal shock
wave therapy involves the generation of sufficient amounts of mechanical
stress in target tissue or bone in efforts of producing micro bleedings
and micro hematomas (i.e. micro injuries) to an extent that they result in
healing. Meanwhile, pulsed
low energy ultrasound has been used to treat non-unions.
A few clinical reports have described this method of
ultrasound application. However,
to achieve union, daily twenty-minute treatment over a nine-month period
was required. In contrast,
European clinical studies involving use of the OssaTron for the same
indication found one treatment sufficient with a success rate of 80% -
90%. In
summary, ultrasound and shock waves are both acoustic waves with different
physical characteristics, acoustical parameters and medical application.
The optimal use of ultrasound is diagnostic applications, whereas
optimal use of extracorporeal shock waves is therapeutic.
Furthermore, the combined use of both technologies has proven very
useful, ultrasound for localization and extracorporeal shock waves for
treatment. Conclusions Orthopaedic
application of extracorporeal shock wave therapy (ESWT) should consider
the type of shock wave device to be used, electrohydraulic (Spark Gap),
electromagnetic (EMSE) or piezoelectric (Piezo) relative to shock wave
generation and device design parameters.
Based upon various studies, the most significant technical
specifications for consideration include: applied energy density, total
energy, number of shocks transmitted to target tissue (soft tissue or
bone), the distribution of rise time and the ratio of positive to negative
energy across the focal area. These
factors are believed to have direct influence upon treatment outcome,
efficacy and efficiency. Until
the introduction of the OssaTron (R), most extracorporeal shock wave
devices used for orthopaedic indications have been designed primarily for
urologic lithotripsy. The OssaTron is the first and only extracorporeal
shock wave device designed specifically for orthopaedic use. Its' electrohydraulic (spark gap) design optimally
incorporates the essential technical and physical properties necessary for
successful orthopaedic extracorporeal shock wave application. It is the standard insofar as the use of extracorporeal shock
wave therapy (ESWT) in orthopaedics.
As a result, the FDA recently approved use of the OssaTron for the
treatment of chronic proximal plantar fasciitis. ______ 1)
Haupt, Gerald (1997): Use Of Extracorporeal Shock Waves In The
Treatment Of Pseudoarthrosis, Tendinopathy And Other Orthopaedic Diseases.
Jour Of Urology. Vol.
158 No.1 2)
Valchanou V.,P. Michailov (1991): High Energy Shock Waves In
Treatment Of Delayed and Nonunion Fractures.
Int Ortho 15: 181-184 3)
Schultheiss, Reiner (1998): Basic Physical Principles Of Shock
Waves. HMT High Medical
Technologies, Kreuzlingen, Switzerland
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