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. Furthermore, Michailov P. and Valchanou V. reported the use of "High Energy Shock Waves In The Treatment Of Delayed And Nonunion Fractures" in 1991. 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
Acoustical shock waves were introduced in medicine for kidney stone lithotripsy in the early 1980's. High pressure amplitudes with rise times of a few nanoseconds and a frequency spectrum ranging from the audible to the far ultrasonic level are one of the unique characteristics of shock waves (3). Shock waves are the result of phenomena that create intense changes in pressure such as explosion, lightning, or supersonic aircraft. The intense changes in pressure produce strong waves of compressive and tensile forces that can travel through any elastic medium such as air, water, or certain solid substances.
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
Today there remains limited knowledge regarding the biological effects of shock waves on tendinous tissue and bone. However, as a result of scientific studies, three therapeutic theories have emerged related to applied energy density, total energy and number of shocks transmitted to target tissue. These theories essentially reinforce the significance of rise time and the distribution ratio of positive and negative energy in the application of extracorporeal shock wave therapy. Moreover, these theories and their relationship to direct and indirect shock wave effects at the cellular level, are key factors when considering "ESWT" to treat specific orthopaedic indications.
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.
Further reading:
Extracorporeal shockwave therapy for chronic proximal plantar fasciitis.
Strash, Walter W, Perez Richard R. Clin Podiatr Med Surg. 2002 Oct;19(4):467-76. Review.