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Application of Shock Waves in Medicine

Thiel, Michael, PhD

Clinical Orthopaedics and Related Research®: June 2001 - Volume 387 - Issue - p 18-21
SECTION I SYMPOSIUM: Extracorporeal Shock Wave Therapy in Orthopaedics

Extracorporeal-generated shock waves were introduced approximately 20 years ago to disintegrate kidney stones. This treatment method substantially changed the treatment of urolithiasis. Shock waves have become the treatment of choice for kidney and ureteral stones. Urology, however, is not the only medical field for the potential use of shock waves for problems. Shock waves subsequently have been used in orthopaedics and traumatology to treat various insertional tendinopathies (enthesiopathies) and delayed unions and nonunions of fracture. Shock wave application also has been used in the treatment of tendinopathies in veterinary conditions (race horses). The concept of orthopaedic disorders is that shock waves stimulate or reactivate healing processes in tendons, surrounding tissue and bones, probably through microdisruption of avascular or minimally vascular tissues to encourage revascularization, release of local growth factors, and the recruitment of appropriate stem cells conducive to more normal tissue healing. The current author will give an overview of history and basic research of the application of shock waves in medicine.

From HMT High Medical Technologies AG, Lengwil, Switzerland.

Reprint requests to Michael Thiel, PhD, HMT High Medical Technologies AG, Kreuzlingerstrasse 5, CH-8574 Lengwil, Switzerland.

The application of extracorporeal shock wave therapy in orthopaedics and traumatology still is in its infancy. In the past few years, extracorporeal shock wave therapy has gained extensive acceptance within the European orthopaedic community. Physicians now have an overview of several thousand patients who were treated worldwide. 7 Nevertheless, the healing mechanism of extracorporeal shock wave therapy for treatment of the established indications such as calcific tendinitis of the shoulder, epicondylitis, plantar fasciitis, delayed unions, and nonunion fractures is not understood completely. Advantages of extracorporeal shock wave therapy are avoidance of surgery, safety, and effectiveness. Compared with open surgery, the costs of extracorporeal shock wave therapy are reasonable. However, it still is necessary to determine the basic biologic and medical effects of shock waves on musculoskeletal tissues through cooperative studies initiated among institutes, hospitals, physicians, societies, and manufacturers of extracorporeal shock wave therapy devices.

Shock waves basically are acoustic waves that accompany daily life, often without being noticed. The sound of thunderstorms and bangs of an explosion or of an applauding crowd are typical examples in which shock waves play an obvious role. An earthquake (macroscopic) and the collapse of gas bubbles in a liquid (microscopic) generate shock waves. By means of shock waves, energy can be transmitted over extensive distances. An airplane breaking the sound barrier generates a very loud, audible bang, which can lead to the jingle or breakage of glasses in a faraway cupboard. The shock wave has transmitted energy from the airplane to the glasses.

The opportunity to transmit mechanical energy with shock waves leads to applications that can be classified in two groups. 9 The first group is destruction of material structure. Examples of this group are crushing of hard material such as concrete, the crushing of glass in a recycling process, and removal of deposits in pipes. Extracorporeal shock wave therapy belongs to this group.

Another group is the application of shock waves as a source of signal. For example, in deep sea investigation shock waves are used to measure distances because of the low energy loss over wide distances.

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Historic Overview

During World War II, it was observed that the lung tissue of castaways was disrupted because of the explosion of waterbombs (depth charges), even though no external symptoms of violence existed. This was the first time that the influence of shock waves on human tissue was documented. 9 In the 1950s, the first systematic investigations for the use of shock waves were undertaken. 9 It was found that electrohydraulically-generated shock waves were able to crush ceramic plates immersed in water. The first patent of an electrohydraulic shock wave generator was granted in the United States to Frank Rieber in New York (Patent No. 2.559.277). 9 At the end of the 1950s, the physical properties of electromagnetic-generated shock waves also were described. 3,9

In 1966, the effect of shock waves on humans was observed accidentally during experiments with high-velocity projectiles when an employee touched the plate at the very moment that the projectile hit the plate. 8 He described feeling something in his body comparable with an electric shock. Measurements, however, showed that no electricity was present. The generated shock wave traveled from the plate through the hand into the body.

From 1968 until 1971, the interaction between shock waves and biologic tissue in animals was investigated in Germany through a program financed by the Department of Defense (of Germany). 8,9 This particular study showed that high-energy shock waves caused effects in a biologic organism over long distances. The effects of interfaces in the organism and the difference and damping of the shock waves as it coursed through living tissue were investigated. Another field of interest was the transition of the shock wave into the body. It was observed that shock waves created low side effects on the way through muscles, fat, and connective tissue. Intact bone tissue remained unharmed under shock wave burden. The danger for the lung, brain, and other organs (abdominal, thoracic) was part of the investigation in this program. The best transition media for the shock wave was water and gelatin because of the similarity in the acoustic impedance to the tissue. 8

These investigations led to the concept of disintegrating kidney stones with extracorporeal-generated shock waves. In the beginning, the technical and medical realization of the idea was not very clear. In 1971, Haeusler and Kiefer 4 reported the first in vitro disintegration of a kidney stone with shock waves without direct contact to the stone. Additional in vitro experiments of contact-free stone disintegration followed. As reported previously, 8,9 the Department of Research and Science of Germany financed a research program entitled “Application of ESWL” in 1974. In 1980, the first patient with a kidney stone was treated in Munich with a prototype of the Dornier Lithotripter HM1. 8,9 In the following years, in vivo and in vitro experiments using extracorporeal-generated shock waves to disintegrate gallstones were conducted. In 1983, the first commercial lithotripter (HM3, Dornier) was installed in Stuttgart, Germany. 9

In 1985, the first clinical treatment of a gallbladder stone with extracorporeal shock wave lithotripsy was done in Munich, Germany. One year later, a prototype of a lithotripter without a water bath and with dry coupling was tested in Mainz, Germany. 8,9 Modern lithotripters work without a water bath and often without the patient under anesthesia. For localization of stones, lithotripters are equipped with xray and/or ultrasound localization systems. Until 1995, more than 2 million patients have been treated worldwide. 2 Although shock wave therapy is safe and effective, inappropriately applied shock wave therapy has the potential to cause severe tissue damage.

Urology is not the only field in medicine where shock waves were used successfully. In 1986, the first experiments were done to investigate the influence of shock waves on bones as reported by Haupt. 5 The impetus for this research was the apprehension that shock waves could damage the hip as a result of shock wave therapy on lower ureteral stones. However, these experiments showed that an intact bone was not affected detrimentally. Additional animal experiments showed that shock waves had osteogenetic potential and stimulated fracture healing. Histologic investigations confirmed the influence of shock waves on the activation of osteoblasts. 5

In 1988, the first shock wave treatment of nonunion fracture in a human was done successfully in Bochum, Germany as reported by Haupt. 5 At the same time Valchanov and Michailov 11 described shock wave therapy for nonunions and delayed unions. Their success rate was 85% but the study was controlled poorly. An additional clinical study reported success rates between 60% and 90%. 5 Two essential circumstances exert influence on the success of the shock wave therapy on nonunions or delayed unions. The influence of shock waves on hypertrophic nonunions seems to be more effective than on atrophic nonunions. The stabilization of the fracture after shock wave therapy seems to be an essential condition for the success of the therapy. Local hematomas, petechial hemorrhage, and local swelling have been described as minor side effects. 5 These side effects disappeared within a few days and the patients did not have any long-term complications.

The first investigations and treatments on humans were done with lithotripters which were designed for the requirements of shock wave application in urology. Because of the anatomic decentralization of the therapy areas (shoulder or foot), it was necessary to develop a special orthopaedic shock wave device. In 1993, a dedicated orthopaedic shock wave device, OssaTron (HMT AG, Lengwil, Switzerland), became available.

At the beginning of the 1990s, the first reports about shock wave therapy on calcific tendinitis (shoulder) were published. 5,7 Additional investigations led to successful nonoperative treatment of epicondylitis and plantar fasciitis. An increasing number of scientific evaluations of the application of extracorporeal shock wave therapy for orthopaedic diseases are becoming available. 5,7

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Effects of Shock Waves

Shock waves are able to disintegrate kidney stones and to cure nonunions and certain soft tissue disorders. The effect of the shock wave in urology and orthopaedic applications seems to be different. Currently, at least two different mechanism of the shock wave are noted. Shock waves are characterized by high positive pressure, a rise time lower than 10 ns and a tensile wave. The positive pressure and the short rise time are responsible for the direct shock wave effect and the tensile wave for the cavitation, which is called the indirect shock wave effect. 1,2,9 Interfaces between two different materials with different acoustic impedance influence the shock wave, which is traveling through the interface 1,2,9 (Fig 1). Reflection, refraction at the interface, and damping inside the material leads to energy loss of the shock wave. The fast pressure transition of shock waves (high pressure, short rise time) cause very high tension at the interfaces so that the structure of the material cracks. This effect depends on the plasticity of the material. The energy of the shock wave that is sufficient to disintegrate a kidney stone has minimal to no effect on an intact bone.

Fig 1.

Fig 1.

The tensile part of a shock wave corresponds to a local lowering of the pressure so that cavitation bubbles will be created, with these bubbles growing under the influence of the tensile wave. If the pressure is normal, the bubbles collapse uncontrolled and a large energy concentration occurs. This leads to additional generation of shock waves. The interaction between shock waves and gas bubbles leads to water jets. The positive part of the shock wave compresses a gas bubble with 1 mm radius within a few microseconds to 0.5 μm. The pressure and the energy inside the bubble increase strongly. A water jet originates that moves in the direction of the pulse. If the water jet comes across a surface, a crater will be created on the surface. 1,2 The disintegration of a kidney stone is a combination between direct and indirect shock wave effect. Figure 1 shows the different mechanism of a shock wave that disintegrates a kidney stone. The mechanism of the shock wave for orthopaedic diseases is being investigated. Currently, it is not clear which effect is dominant, or whether it is a combination between direct and indirect shock wave effects.

Shock wave therapy for nonunions and delayed unions, calcific tendinitis of the shoulder, and epicondylitis and plantar fasciitis currently is under active outcome evaluation. Besides these indications, additional orthopaedic diseases and disorders will be investigated clinically. In particular, the preliminary results of the shock wave therapy on avascular (ischemic) necrosis of the head of femur are very promising. 10 Osteochondritis dissecans, 6 patellar tendinitis, and Achilles tendinitis are orthopaedic indications with promising preliminary results. 5

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Section Description

John A. Ogden, MD; and Richard R. Alvarez, MD, Guest Editors

© 2001 Lippincott Williams & Wilkins, Inc.