Extracorporeal shock wave therapy (ESWT)

Extracorporeal Shock Wave Therapy Gladys L.Y. Cheing, PhD 1 Hua Chang, MSc 2 Extracorporeal shock wave therapy (ESWT) is an emerging treatment modality for managing pain caused by various musculoskeletal disorders. According to recent systematic reviews by Crawford et al 3 and Odgen et al, 12 evidence is accumulating to support the use of ESWT as an effective treatment for heel pain. The US Food and Drug Administration has, in fact, approved the use of the electrohydraulic OssaTron (High Medical Technology, Kreuzlingen, Switzerland) for the treatment of plantar fasciitis. The results of a meta-analysis by Odgen et al 15 demonstrated that, of various applications of ESWT on musculoskeletal conditions, the use of ESWT for treating plantar fasciitis was the most credible. As ESWT may become a new and popular treatment for managing heel pain and possibly other musculoskeletal disorders, 14 the purpose of this paper is to review the background knowledge of ESWT and to describe the technical details of this new modality in treating heel pain. ESWT The extracorporeal shock wave is an acoustic wave characterized by high positive pressures of more than 1000 bar (100 MPa), which can be developed within an extremely short rise time (10 9 seconds) and followed by a low pressure phase of tensile stress equivalent to 100 bar (10 MPa). Because the pulse duration of the shock wave is extremely short (3 to 5 µs) and is generated at low frequencies, it is minimally absorbed by the tissues and therefore no thermal effect is generated. 19 There are 3 mechanisms for generating shock waves: electrohydraulic, electromagnetic, and piezoelectric (Figure 1). A thorough review of these 3 mechanisms and the principles of shock wave therapy is provided by Ogden et al. 16 What follows is a brief summary for each type of device. The electrohydraulic mechanism is similar to that of the spark plug in a car engine. High voltage from a charged capacitor is applied across the electrode tips (spark plug), generating a gas bubble filled with vapour and plasma. The expansion of this bubble produces a sonic pulse and the subsequent implosion of a reverse pulse will generate the shock wave. The shock wave is then reflected from the surface of an ellipsoid and focused at the focal point, which is adjusted to correspond to the desired anatomical region. 16 Electrohydraulic shock wave devices are usually characterized by fairly large axial diameters of the focal volume. Focal volume is defined as the area in which 50% to 100% of the maximal energy is reached. 20 Electromagnetic shock wave devices pass a strong electric current through a flat coil, whereby a magnetic field is induced. At the same time, another magnetic field is induced in a metal membrane overlying the flat coil. As similar poles repel each other, the magnetic field generated by the membrane repels the field generated by the coil. An acoustic lens is used to focus the shock wave and the focal therapeutic point is determined by the focal length of the lens. A piezoelectric mechanism is another way to generate shock waves. Numerous piezocrystals are mounted on the inner surface of a sphere and receive a rapid electrical discharge. This causes contraction and expansion of the crystals. A pressure pulse is induced in the surrounding water and produces a shock wave. 16 The focal volume is determined by the geometric arrangement of the crystals inside the sphere. For any of the above types of generation of ESWT, ultrasound gel is used as a contact medium between the cylinder and the skin. 24 1 Assistant Professor, Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. 2 Senior Physiotherapist, Physiotherapy Department, China Rehabilitation and Research Center, Beijing, China. Send correspondence to Gladys Cheing, Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mail: rsgladys@polyu.edu.hk Applications of ESWT for Musculoskeletal Disorders Shock wave therapy has been successfully used in urology to disintegrate kidney and ureteric stones for more than 15 years. Valchanou and Michailov 28 were the first to use ESWT to treat musculoskeletal condi- Journal of Orthopaedic & Sports Physical Therapy 337

FIGURE 1. The 3 types of devices used to generate shockwaves for clinical application are shown: electrohydraulic (A), electromagnetic (B), and piezoelectric (C). (Reproduced from Odgen et al 16 with the permission from Lippincott Williams & Wilkins.) tions in 1991. They applied a high-energy ESWT (defined by Rompe et al 22 as greater than 0.6 mj/mm 2 ) to treat delayed union or nonunion of long bone fractures in humans. 28 The high-energy shock wave is usually produced by electrohydraulic devices and 1 treatment session is usually adequate. After a single session of 1000 to 4000 shock wave impulses at a pressure of 1000 to 1700 bars (100 to 170 MPa), 28 radiological examination showed that shock waves can break up the sclerotic bone ends and produce numerous small detached or partly attached fragments of bone. This stimulates osteogenesis and speeds up the union of the fracture. In addition to promoting bone healing, Ogden et al 15 also suggested that the high-energy shockwave is effective in reducing pain. High-energy devices require fewer treatment sessions than low-energy devices to produce similar pain relief and result in a higher percentage of positive patient response. However, an anesthetic agent is required for such a high energy output of ESWT, therefore, at present, ESWT can only be used by physicians in the United States. Electromagnetic and piezoelectric machines tend to generate lower-energy shock waves than electrohydraulic devices. Though multiple treatments are required, patients usually do not need an anesthetic agent. More and more studies have demonstrated that low- to medium-energy ESWT units (Rompe et al 21 defined low energy as being equal to 0.08 mj/mm 2 and medium energy as up to 0.28 mj/mm 2 ) are also effective in managing various painful conditions such as heel pain and tennis elbow, and calcifying tendinopathy of the shoulder. 9,11,19,20,24 In particular, Rompe et al 22 found that ESWT was significantly more effective than a placebo treatment in reducing pain and improving walking ability for patients with plantar fasciitis. Other studies also showed that ESWT is effective in managing heel pain due to calcaneal spurs or plantar fasciitis. 7,8,12,29,30 For lateral epicondylitis, research studies using approved Food and Drug Administration protocols are in progress. According to our experience, patients request no local anesthesia when the dosage of ESWT is below 0.37 mj/mm 2. Therefore, it is possible for physical therapists to offer ESWT for the treatment of musculoskeletal conditions. In fact, physical therapists in some Asian countries (eg, China) have started to use ESWT units that generate low- to medium-energy output. By the time more evidence is accumulated to show its effectiveness, these sorts of ESWT units may become a standard treatment modality used by physical therapists. But safety issues should be addressed as well. Below are some of the potential side effects of ESWT that have been listed in the review by Odgen et al. 16 Cavitation is defined as the generation and movement of bubbles in a fluid or tissue, which may cause tissue damage. When a shock wave hits a cavitation bubble and the bubble collapses, there is an inflow of water (jet stream). 17 These inflowing water masses can reach velocities between 400 and 700 m/s. The collapse of the bubbles can produce local effects that result in needle-shaped hemorrhages in tissues. At a cellular level, free radicals produced by cavitation may affect the cellular antioxidative defense status or even damage the tissue, particularly cell membranes. 25 As cavitation usually occurs at the interface of 2 different media, extra attention should be given when delivering treatment for shoulder disorders, because there is a possibility of damaging lung tissue if the ESWT beam is directed toward the chest. Direct exposure of organs and tissues to shockwaves may damage kidney, liver, heart, neural, or vascular structures. However, these are only potential side effects. So far, there is no report of such side effects in clinical applications of ESWT. Nevertheless, some contraindications have been proposed for the use of ESWT: 1. ESWT would be contraindicated for use in patients suffering from hemophilia because it may cause microvascular disruption; 2. ESWT should not be used as a treatment for the clavicle or the first rib, to avoid exposing lung tissue to the ESWT beam; 338 J Orthop Sports Phys Ther Volume 33 Number 6 June 2003

3. extra precautions should be taken with regard to malignancy and growth plate as the effects of ESWT on malignancy and growth plate are unknown. Therapeutic Effects of ESWT When a shock wave passes through human tissues, it produces physiological effects. It is postulated that 4 phases are involved. 5 The first is the direct (mechanical) effect of the shock wave. Due to extracellular cavitations, ESWT ionizes the molecules and there is an increase of membrane permeability. The second phase is the physical-chemical phase. This involves the interaction of diffusible radicals with biomolecules. ESWT may affect lysosomes and mitochondria, and interfere with metabolism in the cell. 16 The third phase is the chemical phase, which may be accompanied by intracellular reactions and molecular changes. A high temperature is developed locally during cavitation, which leads to the development of radicals. The fourth phase is a biological phase. Physiological responses take place in this phase when the changes from the first 3 phases persist. These 4 phases include the most important therapeutic effects of shock waves. However, only limited research has investigated the actual biological mechanisms of clinically used shock waves on human and animal tissues. 16 Clinically, shock wave therapy has been demonstrated to be effective in managing pain due to musculoskeletal disorders. However, the analgesic mechanisms of ESWT are unclear. Various hypotheses have been proposed. Some suggest that shock waves destroy nerve endings. 4 It has also been suggested that ESWT causes nociceptors to emit nerve impulses at high frequencies during nerve transmission, which prevents pain transmission according to the gate control theory. 4 Another hypothesis is that the chemical medium surrounding the nociceptor is changed and disturbs pain transmission. 4 It is also suggested that a locally generated hyperaemia intensifies the degradation of the inflammation mediators, which reduces the irritation of nerve endings and therefore results in less pain. 18 In addition, ESWT may trigger a microtrauma on soft tissues, which initiates a healing process by promoting the release of local growth factors and the recruitment of appropriate stem cells. 27 Further studies are needed to verify these hypotheses. METHODS Instrument Figure 2 shows an extracorporeal shock wave therapeutic unit (electromagnetic device) with an ultrasound imaging unit (Siemens, Sonocur Plus, Munich, FIGURE 2. Extracorporeal shock wave device (Siemens AG Medical Solutions, Sonocur Plus, Munich, Germany). The inflation or deflation of the acoustic lens (B) of the treatment head (A) adjusts the penetration depth of the shock waves. The focus of the shock wave source is continuously monitored by an ultrasound scanner (C). Germany). The acoustic lens of the treatment head of the shock wave unit is filled with water. The acoustic lens can be inflated or deflated by adjusting the water inflow. This, in turn, alters the depth of penetration of the shock wave into the body part. The depth of penetration of the shock wave delivered by the unit can be up to 50 mm, which is deeper than that of an ordinary ultrasound therapy unit (approximately 30 mm, depending on body composition). The treatment head is supported by an articulated arm which allows movement in 3 planes. During treatment, a thin layer of ultrasound gel is applied between the treatment head of the unit and the skin for better transmission of the acoustic wave. 24 The shock wave can be focused on the targeted tissue by an in-line ultrasound scanner. With the use of the ultrasound scanner, the position and the focus of the shock wave can be continuously monitored during treatment. 26 Patient Preparation Most people usually report a sharp pain sensation during the application of ESWT, but require no anesthetic agent. Therefore, the energy density must be within tolerable limits for the patients. A pinprick test is performed prior to ESWT to ensure that the patient has normal sensation. In this paper, the technical details of the application of ESWT are described for the treatment of plantar fasciitis as an example. During the application of ESWT, the patient lies prone on the treatment table. Because the ankle position affects the tension of the plantar fascia, an ankle splint can be used to maintain the ankle in an anatomically neutral position. Figure 3 illustrates that A B C J Orthop Sports Phys Ther Volume 33 Number 6 June 2003 339

FIGURE 3. A patient lies in a prone position with the ankle maintained in a neutral position by an ankle splint. The treatment head of the shock wave therapy unit is targeted at the area causing pain. the treatment head is targeted at the heel, with the ankle maintained in neutral position by the ankle splint. To ensure the application of ESWT on the same location across treatment sessions, each patient is given a transparent plastic card. On the first treatment session, the plastic card is placed over the plantar surface of the foot to map the contour of the heel and the location of the most tender spot (Figure 4). The most tender spot on the heel is identified by palpation with the assistance of an ultrasound scanner and a mark is made on the plastic card. Then the card is carefully taken away without causing any displacement of the treatment head. Shock wave therapy is then focused on the same localized spot during each subsequent session according to the mark. In addition, an ultrasound scanner is used to monitor the penetration depth of ESWT. The inflation or deflation of the acoustic lens of the ESWT device enables focusing the ESWT on the target tissue. These procedures are to ensure the repeatability of the treatment area for each treatment session. Dosage Pulse duration is usually fixed at 3 to 5 µs and the calculation of dose (total energy delivered) depends on the choice of energy level and the total number of shock wave impulses delivered. In the literature, energy flux density is usually reported in mj/mm 2. This refers to the shock wave energy flow through an area perpendicular to the direction of propagation. But the total amount of acoustic energy (number of pulses multiplied by the energy per pulse) is crucial in producing therapeutic effects. For the shock wave therapeutic unit manufactured by Siemens (Sonocur Plus, Munich, Germany), the energy flux density in the focal region can be adjusted FIGURE 4. A plastic card is placed over the plantar surface of the foot. Each patient has a plastic card. The exact location of the painful area is identified and marked on the scale of the plastic card in the first treatment session. Shock wave therapy is focused on the same mark on the plastic card across treatment sessions. The plastic card is removed before the treatment begins. through 8 energy level selections ranging from 0.04 mj/mm 2 to 0.5 mj/mm 2. The frequency of the shock impulses can be selected from 1 Hz to 4 Hz. 26 The user s manual 26 of the Siemens therapeutic unit recommends that the energy level be set at 3 (0.28 mj/mm 2 ) for the treatment of plantar fasciitis. A pulse frequency of 4 Hz and the use of 100 to 1500 pulses are also recommended. It is also suggested that 3 treatment sessions delivered at 1-week intervals should be given. Unfortunately, there are great variations in the treatment protocols used in published studies, including the energy level, number of ESWT impulses, number of treatment sessions, and the use of anesthesia. There seems to be no consensus on the dosage of shock wave therapy for managing heel pain. The Table summarizes the dosages used in recent studies that report the use of ESWT in treating heel pain. The studies included in this table were mostly selected based on the recent meta-analysis by Ogden et al, 15 which rated these studies as being of acceptable quality and demonstrating positive outcomes. 340 J Orthop Sports Phys Ther Volume 33 Number 6 June 2003

TABLE. Treatment protocols reported in previous studies of shock wave therapy on heel pain. Rompe et al 21 * Conditions Energy Level Number of Impulses Chronic plantar fasciitis 0.08 mj/mm 2 ESWT group: 1000 Placebo group: 10 Number of Treatment Sessions 3 sessions at weekly at 6, 12, and 52 wk after last treatment Rompe et al 23 * Plantar fasciitis 0.08 mj/mm 2 3000 3 sessions at weekly at 3, 6, 24, and 52 wk Jakobeit et al 8 Calcaneal spurs High-energy group: started with 0.3 mj/mm 2, increased by 0.3 mj/mm 2 after every 100 shock waves until reaching 22 mj/mm 2 Kristchek et al 10 * Chronic plantar fasciitis with heel spur Low-energy group: started with 0.3 mj/mm 2, increased by 2 mj/mm 2 after every 500 shock waves until reaching 5 mj/mm 2 0.08 mj/mm 2 Group 1: 500 Group 2: 100 Results Treatment group produced significantly greater pain reduction and improvement of function than placebo group in all follow-up sessions. By the 1-year follow-up session, pain level of the treatment group reduced by 75%. Also, significant improvement in movement and walking period reported. 88% of the subjects satisfied with treatment. 1600 1 session High-energy group showed a more rapid pain relief. Pain relief often occurs after only 1 session. 3 sessions at weekly at 1.5, 3, and 12 mo Hammer et al 6 Painful heel 0.12 mj/mm 2 3000 3 sessions at weekly at 5 6 mo after last treatment Wang et al 29 * Heel pain 14 kv (equivalent to 0.18 mj/mm 2 ) 1000 2 3 sessions; follow-up at 6 and 12 wk after last treatment Group 1 demonstrated significantly better results than group 2. It showed pain relief on manual pressure and while walking. Also, an increase in walking ability from 10 min before the treatment to 2 and 3 h after 12 mo. 70% of patients with heel pain had excellent or good results after treatment. At 12th week follow-up session, 26.8% of patients complaint free and 53.6% felt significantly better. J Orthop Sports Phys Ther Volume 33 Number 6 June 2003 341

TABLE. Treatment protocols reported in previous studies of shock wave therapy on heel pain (continued). Buch and Siebert 1 * Ogden et al 13 * Chen et al 2 * Conditions Energy Level Number of Impulses Number of Treatment Sessions Chronic heel pain 0.24 mj/mm 2 2000 3 sessions; at 1-wk at 1.5 and 3 mo Chronic proximal plantar fasciitis Painful heel syndrome *Studies of acceptable quality according to Ogden et al. 15 Additional selected studies. 18 kv (0.23 mj/mm 2 ) 1500 1 2 sessions; follow-up at 4, 8, and 12 wk 14 kv (0.18 mj/mm 2 ) 1000 1 2 sessions; follow-up at 6, 12, and 24 wk after last treatment Results 42% of patients reported significant pain reduction after the first session, 44% after the second, and 60% after the third session. 56% more of the treated patients attained all success criteria when compared with the patients treated with placebo treatment. At 12-wk follow-up, 20.6% of patients were pain free, 52.9% of patients significantly better. At 24-wk follow-up, 59.3% of patients complaint free, 27.7% of patients significantly better. DISCUSSION ESWT is a relatively new treatment modality for musculoskeletal conditions. It is a noninvasive but powerful treatment providing a deeper penetration of up to 50 mm of human tissue, 24 which is deeper than that of conventional ultrasound therapy. To date, research has focused on clinical efficacy in treating various conditions, but there is a lack of evaluation of the effect of underlying biological mechanisms acting on human tissues such as nerves, bone, and soft tissue. This modality utilizes relatively higher energy levels compared to traditional modalities now used by physical therapists, yet appears to have minimal complications associated with its use. It is important to encourage further studies to determine the clinical efficacy, as well as optimal treatment parameters, for this treatment. This would further enhance the potential for ESWT to become a new modality used by physical therapists. REFERENCE 1. Buch M, Siebert W. Shockwave treatment for heel pain syndrome: A prospective investigation. In: Coombs R, Schaden W, Zhou SS, eds. Musculoskeletal Shockwave Therapy. London, UK: Greenwhich Medical Media; 2000:73 77. 2. Chen HS, Chen LM, Huang TW. Treatment of painful heel syndrome with shock waves. Clin Orthop. 2001;387:41 46. 3. Crawford F, Atkins D, Edwards J. Interventions for treating plantar heel pain (Cochrane Review). In: Cochrane Library, Issue 4. Oxford, UK: Update Software Ltd: 2001: 4. Haist, J. Einsatzmoglichkeiten der analgetisch wirksamen extrakorporalen stosswellentherapie an der schulte. Orthopadische Praxis. 1995;9:591 593. 5. Haist J, von Keitz-Steeger D. Shock wave therapy in the treatment of near to bone soft tissue pain in sportsmen. Int J Sports Med. 1996;17:S79. 6. Hammer DS, Rupp S, Ensslin S, Kohn D, Seil R. Extracorporal shock wave therapy in patients with tennis elbow and painful heel. Arch Orthop Trauma Surg. 2000;120(5 6):304 307. 7. Hammer DS, Rupp S, Kreutz A, Pape D, Kohn D, Seil R. Extracorporeal shockwave therapy (ESWT) in patients with chronic proximal plantar fasciitis. Foot Ankle Int. 2002;23(4):309 313. 8. Jakobeit C, Welp L, Winiarski B, et al. Ultrasoundguided extracorporeal shock wave therapy of tendinosis calcarea of the shoulder, of symptomatic plantar calcaneal spur (heel spur) and of epicondylopathia radialis et ulnaris. In: Siebert W, Buch M, eds. Extracorporeal Shock Waves in Orthopaedics. Berlin, Germany: Springer-Verlag; 1997:165 172. 9. Krischek O, Hopf C, Nafe B, Rompe JD. Shock-wave therapy for tennis and golfer s elbow 1 year follow-up. Arch Orthop Trauma Surg. 1999;119(1 2):62 66. 10. Krischek O, Pompe JD, Hopf C, et al. Extracorporeal shockwave therapy in epicondylitis humeri ulnaris or radialis a prospective, controlled, comparative study. Z Orthop Ihre Grenzgeb. 1998;136(1):3 7. 342 J Orthop Sports Phys Ther Volume 33 Number 6 June 2003

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