1 Biliary Tract Surgery
Extracorporeal Shock Wave Lithotripsy for Biliary Stones
Jay B. Prystowsky, MD, * and David L. Nahrwold, MDt
HISTORY OF EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY The fundamental principles of extracorporeal shock wave lithotripsy were developed from investigations by the Dornier Corporation of the Federal Republic of Germany into the causes of surface pitting often seen on the outer shells of its spacecraft and supersonic airplanes. 21 Dornier discovered that when these aircraft collided with raindrops at high speeds, shock waves were produced that created stresses inside the material structures of the aircraft. Further studies demonstrated that shock waves could exert a destructive effect on brittle solids yet pass harmlessly through organic tissues. Finally, Dornier researchers developed the hemiellipsoidal dish -to focus shock waves on a fixed point. 22 The initial application of extracorporeal shock wave lithotripsy was for the fragmentation of kidney stones. The first in vitro experiments began in 1972, and in 1980, the world's first kidney stone lithotripsy treatment was performed. 10 Since then, extracorporeal shock wave lithotripsy has dramatically altered the treatment of urolithiasis and gained worldwide acceptance as a noninvasive therapy for kidney stones. The application of this technology to gallstones commenced in the early 1980s, and the first patient to undergo gallstone lithotripsy was treated in 1985 at the Klinikum Grosshadern in Munich, Federal Republic of Germany. 36 The treatment of gallstones has received increasing attention in recent years. A variety of new modalities, including oral dissolution agents,' contact dissolution with methyl tert-butyl ether, and percutaneous techniques of laser, electrohydraulic, or ultrasonic destruction serve as examples of the directions this rapidly changing field has taken. Extracorporeal shock wave *Assistant Professor, Department of Surgery, Northwestern University Medical School, Chicago, Illinois tLoyal and Edith Davis Professor and Chairman, Department of Surgery, Northwestern University Medical School; and Surgeon-in-Chief, Northwestern Memorial Hospital, Chicago, Illinois
Surgical Clinics of North America-Vol. 70, No.6, December 1990
PRYSTOWSKY AND DAVID
lithotripsy for gallstones is a promising noninvasive technique that is undergoing intense scrutiny.
EXTRACORPOREAL SHOCK WAVES: PRINCIPLES OF GENERATION Shock Waves Shock waves are sound waves with a large amplitude or, in other words, shock waves are high-energy sound waves. Like sound waves, shock waves can be described as longitudinal pressure fronts that occur as a single pulse with a rapid rise time, measured in nanoseconds, and a duration measured in microseconds. Each wave consists of a positive-pressure segment followed by a negative-pressure portion that is much smaller in amplitude but longer in duration (Fig. 1). The behavior of sound waves can be described by linear or geometric acoustics. Sound waves have a single frequency and propagate without deformation at a constant velocity. Lithotripters employ a variety of physical principles to generate a pressure front. A shock wave is created when the velocity of the pressure front exceeds the speed of sound. Strong shock waves deviate from the laws of linear or geometric acoustics. Shock waves have multiple frequen~
900/0 - -
1 500/0 - - --
Rise Time = 200 n;
100/0 - -
I~ r -
Figure 1. Shock wave hydrophone response at focal zone (F2). (From Appendix: Manufacturers' presentations. In Ferrucci JT, Delius M, Burhenne HJ (eds): Biliary Lithotripsy. Chicago, Year Book Medical Publishers, 1988, p 268; with permission.)
EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY
cies. Components of a strong shock wave with a higher pressure also have a higher velocity of propagation. These parts of the wave propagate faster than the parts with lower pressure or amplitude. The movement toward the head of the wave steepens the shape of the wave with increasing length of propagation to a step-like shock front (Fig. 2). Thus, a "shock wave can be defined as a single, step-like increase in pressure, density, temperature, and particle velocity, which is always propagating faster than the sound velocity of the medium ahead of the shock front. "18 Shock waves must propagate in a medium, with water being ideal. Because the body is composed primarily of water, shock waves can propagate in living tissues if the shock wave source is coupled adequately to the body. Shock waves pass through organic tissue with minimal deflection and minor side effects. However, shock waves produce a large negative-pressure deflection at an interface with air, resulting in tissue damage. Therefore, precautions must be taken to avoid passage of shock waves through lung or gas-filled intestine. Shock Wave Generation All lithotripters generate shock waves in water and focus those waves at a distance from the generator. Lithotripters differ in the mechanism by which the shock waves are generated, the means by which they are focused, the method of coupling the patient to the lithotripter, and the system used to image the gallstone(s). Shock waves are initiated by one of four basic generating devices, conveniently categorized as spark gap, electromagnetic, piezoelectric, or microexplosive (Fig. 3 and Table 1). Spark gap generators utilize electrodes that are mounted within a water-filled hemiellipsoidal reflector. Electrical energy is discharged from a capacitor and passes across the spark gap electrode, resulting in vaporization of the water surrounding the electrode. The rapidly expanding gases (or plasma) propagate faster than the speed of sound in water, thereby creating a shock wave. The shock wave reflects off the walls of the hemiellipsoid and converges at a remote, fixed point from the electrode. This focal zone, or F2, is generally defined as the area or volume that exhibits more than 50% of the maximum peak pressure generated by the shock wave lithotripter. Electromagnetic generators employ an elongated, water-filled tube that has an electromagnetic coil mounted at its base. Electrical current is
Figure 2. Steepening of a high-pressure wave by higher velocity of sections of the wave with higher pressure. The direction of wave propagation is from right to left. (From Delius M, Muller M, Vogel A, et al: Extracorporeal shock waves: Properties and principles of generation. In Ferrucci JT, Delius M, Burhenne HJ (eds): Biliary Lithotripsy. Chicago, Year Book Medical Publishers, 1988, p 10; with permission.)
PRYSTOWSKY AND DAVID
Ellipsoid Spark Gap -,
" Piezoelectric Crt,tals "
Figure 3. Extracorporeal shock wave lithotripters. A, Spark gap type. B, Electromagnetic type. C, Piezoelectric type. (From Delius M, Muller M, Vogel A, et al: Extracorporeal shock waves: Properties and principles of generation. In Ferrucci JT, Delius M, Burhenne HJ (eds): Biliary Lithotripsy. Chicago, Year Book Medical Publishers, 1988, p 14; with permission.)
passed through the coil, causing it to expand. An electromagnetic field is created that deflects an adjacent thin metallic membrane, resulting in a shock wave within the tube. The shock wave is focused by an acoustic lens, and the energy is concentrated at F2. Piezoelectric generators use several hundred to several thousand piezoceramic crystal transducer elements, which are fixed to the base of a hemispherical dish submerged in water. Electrical pulsing causes these elements to expand, which produces a shock wave. The elements are arrayed in a mosaic fashion so that the shock wave converges at a fixed, remote point. Hence, piezoelectric generators are autofocusing. Microexplosive generators employ lead azide pellets that are located within a hemiellipsoidal reflector and made to explode, producing a shock wave. The shock waves converge at F2, in a manner similar to the spark gap generators. Shock waves can propagate through living tissue because of its high water content. However, the body must be adequately coupled to the lithotripter. This process requires the patient to be in contact with the water medium in which the shock wave propagates. Direct contact with the water can be accomplished through a porthole in the table on which the patient lies or by placing the patient in a shallow pool of water. Alternatively, the patient may lie on a flexible membrane which, in turn, is in contact with the water. The surface of the membrane is coupled to the patient with ultrasound gel. In most lithotripters, the water is degassed, deionized, and temperature controlled for optimum propagation of shock waves. Ultrasound and plain radiography are the two available methods for stone imaging. However, as most gallstones are radiolucent, ultrasound
Table 1. Second-Generation Extracorporeal Shock Wave Lithotripters SHOCK WAVE MANUFACTURER
Wolf (W. Germany) Technomed International (France) EDAP (France) Siemens (W. Germany) Medstone International (USA) Nitech (Denmark) Dornier Medizintechnik (W. Germany)
Direx (Israel) Northgate (USA) Diasonics (USA) Storz Medical (W. Germany) PE stones.
Piezolith 2300 Sonolith 3000 LT 01 Lithostar Lithostar Plus 1050 ST HM 4 MPL 9000 MFL 5000 Tripter Xl SD 3 Therasonic Modulith SL 10 Modulith SL 20
PE SGE PE EM EM SGE SGE SGE SGE SGE SGE SGE PE PE PE
Spherical array Ellipsoid Spherical array Acoustic lens Acoustic lens Ellipsoid Ellipsoid Ellipsoid Ellipsoid Ellipsoid Ellipsoid Ellipsoid Spherical array Spherical array Spherical array
US US US X-ray X-ray + US X-ray ± US US X-ray US X-ray X-ray + US US X-ray + US US X-ray + US
Mini-tank Mini-tank Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag Waterbag
GB GB GB CBD GB + CBD CBD (+GB) GB CBD GB CBD GB + CBD GB GB + CBD GB GB + CBD
= piezoelectric, SGE = spark gap electrode; EM = electromagnetic; US = ultrasound; GB = gallbladder stones; CBD = common bile duct
From Vergunst H, Terpstra OT, Brakel K, et al: Extracorporeal shockwave lithotripsy of gallstones: Possibilities and limitations. Ann Surg 210:566, 1989; with permission.
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imaging is the method of choice for gallbladder stones. Bile duct stones are not easily seen with ultrasound, and their localization requires contrast filling of the duct or placement of a nasobiliary catheter adjacent to the stone. Stones can then be imaged using plain radiography or fluoroscopy. Interaction of Shock Waves with Gallstones "Cavitation" is the term used to describe the generation and movement of gas bubbles in a fluid. 2, 13, 25 Cavitation bubbles may be generated by optical or electrical energy or by acoustic waves of negative pressure: "expansion waves. "19 Expansion waves are part of the shock waves produced by lithotripters. In addition, expansion waves may occur when a shock wave is reflected at a water-air interface. When a shock wave encounters a cavitation bubble, the bubble will collapse asymmetrically from the side where the shock wave approaches and generate a jet that penetrates the opposite side of the bubble" (Fig. 4). If the bubble is in contact with a surface, the jet formation resulting from bubble collapse can create a surface crater and thus induce material damage (Fig. 5). If the bubble is in contact with another bubble, a shock wave is generated from the first bubble that can repeat the same process in the second bubblc.!" Lithotripters generate cavitation bubbles and also collapse them. Although there is strong in vivo evidence to implicate
Figure 4. Dynamics of a laser-produced single spherical bubble near a solid boundary. The size of each frame is 7.2 x 4.6 mm'', the boundary is located at the bottom of the frame, and the framing rate is 75,000 per second. The sequence runs from upper left to lower right. A bubble is generated (upper left), expands, and collapses (up to the third row). Jet formation in the direction of the surface is observed at the end of the third and in the fourth row. (From Lauterborn W, Hentschel W: Cavitation bubble dynamics studied by high-speed photography and holography I. Ultrasonics 23:260, 1985; with permission.)
EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY
Figure 5. Craters from cavitation bubble collapse on a polymethylmethacrylate surface. (From Vergunst H, Terpstra OT, Brakel K, et al: Safety and efficacy in biliary lithotripsy. In Ferrucci JT, Delius M, Burhenne HJ (eds): Biliary Lithotripsy. Chicago, Year Book Medical Publishers, 1988, p 26; with permission.)
cavitation as the mechanism for tissue damage from lithotripsy, there is only indirect evidence that cavitation is the mechanism for stone fragmentation. 15 Shock waves can be transmitted in tissue because the acoustic impedances of tissue and water are approximately equal. However, when a shock wave encounters a stone whose acoustic impedance differs from that of surrounding tissue, energy is reflected from the stone, and a compressive wave propagates into the stone. Also, a tensile wave is induced after shock wave reflection from the back surface of the stone. Compressive and tensile waves are the direct results of shock wave interaction with a stone and may produce sufficient shear and tear forces to fragment stones. 15 Thus, shock waves fragment gallstones by the indirect effects of cavitation and the direct effects of compressive and tensile waves. The relative contributions of these mechanisms to stone destruction remain an important subject of investigation for optimization of extracorporeal shock wave lithotripsy of gallstones.
EFFICACY OF FRAGMENTATION AND TECHNICAL CONSIDERATIONS IN GALLSTONE LITHOTRIPSY Clinical data from Munich suggest that the probability of complete clearance of fragments from a gallbladder after lithotripsy is inversely related to the diameter of the largest fragment and that optimal treatment should result in fragments less than 3 mm in diameter" (Table 2). The efficacy of stone fragmentation is dependent on a variety of factors, including certain characteristics of the lithotripter, characteristics of the stoners) and the surrounding medium, stone volume, operator skill and experience, clarity of stone imaging, and adequacy of patient coupling to the shock wave generator.
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Table 2. Percentage (Actuarial) of Patients with Stone-Free Gallbladder as a Function of Largest Fragment Determined by Ultrasound Within 1 Day of Biliary Lithotripsy MONTHS SINCE TREATMENT
LARGEST FRAGMENT (MM)
:53 4-5 >5
51 9 8
70 14 30
86 43 67
89 67 78
A significant difference was observed between the three groups in the probability of complete clearance (P < 0.001). From Sackmann M, Sauerbruch T, Holl J, et al: Fragmentation clearance after extracorporeal biliary lithotripsy. In Burhenne HJ, Paumgartner G, Ferrucci JT (eds): Biliary Lithotripsy, Vol 2. Chicago, Year Book Medical Publishers, 1990, p 75; with permission.
Shock Wave Generators Important lithotripter features influencing fragmentation are the size of the focal zone and the maximum peak pressure of the generated shock waves. 30 These are the key determinants of the total beam energy produced at the focal zone. The maximum peak pressure can be varied by changing the voltage discharged in the generation of a shock wave. Increasing the voltage will increase the energy delivered to the focal zone, but at a cost of more patient discomfort. The number of shock waves administered per session varies with the type of lithotripter employed and is usually based on thresholds for tissue damage as determined by animal experiments." 17 In general, the spark gap systems produce the highest energy at F 2 , and the piezoelectric systems produce the lowest.'! The higher energy at F 2 causes more pain for the patient, but the efficacy of fragmentation may be greater, and the need for retreatment may therefore be lower. As yet, there are no clearcut data that indicate the clinical superiority of one type of lithotripter over another. Acoustic Coupling and Shock Wave Attentuation Acoustic coupling of the patient to the shock wave generator is accomplished by placing the patient in a waterbath or by allowing the patient to recline on a compressible water cushion to which he or she is coupled with ultrasound gel. The mobile water cushion does allow for simplified treatment of patients and more variable shock wave entry. However, adequate coupling of patients to the water cushion can be difficult, especially in patients with atypical anatomy. Although quantification of the importance of this variable is difficult, it is a consideration in achieving a successful result. Shock wave attentuation is higher in living tissue than in water because the components of tissue are not homogeneous and have different densities. Patients generally are treated in the prone position in an effort to minimize the amount of tissue intervening between the shock wave generator and the gallstone(s). Stone Size, Number, and Composition Clinical and in vitro data suggest that the adequacy of gallstone fragmentation and eventual stone clearance is inversely related to the size
EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY
of the stone as described by its greatest diameter or volume (LJ Schoenfield et aI, unpublished data). 4, 9, 20, 28,31, 32, 38 In general, patients who have solitary stones less than 20 mm in diameter have the greatest probability of adequate fragrnentation and eventual stone clearance. In vitro experiments have determined that stone volume is a more important determinant of adequate fragmentation than is stone number. 3, 38 However, clinical data have consistently found suboptimal fragmentation and lower stone-free rates in patients with multiple stones (LJ Schoenfield et al, unpublished data). 9, 20, 31, 32 The discrepancy may be attributable in part to inadequate imaging of multiple stones. The pressure threshold for fragmenting stones of differing compositions is not known. Conflicting in vitro data exist regarding the importance of stone composition, especially as it relates to stone calcium content. 3, 28, 38 However, clinical data strongly suggest that calcified stones are much less amenable to lithotripsy than are noncalcified stones (LJ Schoenfield et al, unpublished data). 31 In addition, patients suspected of having pigment stones are generally excluded from lithotripsy, because such stones are not amenable to adjuvant oral bile acid dissolution.
Bile Composition and Gallbladder Size Delius and others have found that the medium surrounding the stone could be an important determinant of stone fragmentation. 16 However, there is no evidence that a change in bile composition has a significant effect on stone fragmentation. Adwers and Strauss have reported preliminary data that suggest that successful fragmentation is more likely when treating stones in a "small" gallbladder.' Although bile composition and gallbladder size could be altered (i. e., the postprandial versus the fasting state), the clinical significance of bile composition and gallbladder size has yet to be fully elucidated. Adjuvant Oral Litholytic Therapy In vitro and clinical evidence suggests that adjuvant oral litholytic therapy plays a crucial role in clearing a gallbladder of its stone burden after lithotripsy (LJ Schoenfield et al, unpublished data);" Lithotripsy creates a greater surface-to-volume ratio and fragments the noncholesterol outer layer of gallstones, thereby exposing a large cholesterol surface to the unsaturated bile produced by the dissolution agent. Ursodeoxycholic acid is well tolerated by most patients and is devoid of the complications of chenodeoxycholic acid, especially intolerable diarrhea." Likewise, a combination of the two bile acids does not cause diarrhea. It is not known 'whether ursodeoxycholic acid alone or the combination of agents is more efficacious, nor has the optimal length of bile acid therapy been determined (see previous article). CLINICAL EXPERIENCE WITH EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY FOR GALLBLADDER STONES Selection Criteria The selection criteria for patients undergoing extracorporeal shock wave lithotripsy were derived from the initial experience in West Ger-
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many.:" The present criteria are based on theoretical considerations and undoubtedly will be modified as clinical data accumulate. Patients must have a history of biliary colic, and gallstones must be capable of being imaged by ultrasound. The criteria for the number and size of stones differ among protocols, but generally, patients are treated if they have one, two, or three stones between 4 to 5 mm and 20 to 30 mm in diameter. As passage of gallstone fragments out of the gallbladder is an important mechanism for stone clearance, an oral cholecystogram or nuclear scintigraphic study is required to assure patency of the cystic duct. Patients with heavily calcified or pigment stones are usually excluded from treatment. Therefore, only patients who have radiolucent stones or stones with no more than a 3-mm rim of calcium as detected by plain abdominal roentgenography are treated. Likewise, patients with conditions suggestive of pigment stones, such as cirrhosis or jaundice, are generally excluded. Pregnancy, coagulopathy, vascular aneurysms, cysts, acute cholecystitis, cholangitis, pancreatitis, bile duct obstruction, or bile duct stones constitute contraindications to gallstone lithotripsy. In addition, the path of the shock wave has to avoid lung and bone. Anesthesia Initially, all patients were treated under general anesthesia. However, after Sackmann and others reported the successful performance of gallstone lithotripsy without general anesthesia, intravenous analgesia (spark gap and electromagnetic lithotripters) or no analgesia (piezoelectric lithotripters) has become routine. " Treatment on an outpatient basis is likewise becoming routine, especially when general anesthesia can be avoided. Clinical Results Since the introduction of gallstone lithotripsy in 1985, more than 600 patients with gallbladder stones and more than 90 patients with bile duct stones have been treated at the Klinikum Grosshadern in West Germany. The first 175 patients (Table 3) were treated with a prototype spark gap lithotripter (GMI; Dornier Medizintechnik GmbH, Munichi.i" The selection criteria were as previously described. Overall, 83% (145 of 175) of patients had a solitary stone, and 50% of patients (88) had a solitary stone less than 20 mm in diameter. Each patient received oral chenodeoxycholic acid and ursodeoxycholic acid (6-9 mg/kg per day). Adjuvant litholytic therapy was started 12 days before treatment and continued for 3 months after the disappearance of stones. An average of 1200 shock waves was administered per treatment. Fragmentation of stones was achieved in all patients, with 80% of the patients harboring no fragment greater than 3 mm in diameter as assessed by ultrasound 1 day after lithotripsy. The retreatment rate was 5.1%. Of patients examined, 70 of 112 (63%) were stone free at 4 to 8 months, 53 of 68 (78%) were stone free at 8 to 12 months, and 31 of 34 (91%) were stone free at 12 to 18 months. Evaluation of patients with solitary stones less than 20 mm in diameter revealed stone-free rates of 78% (47 of 60) at 4 to 8 months, 86% (36 of 42) at 8 to 12 months, and 95% (21 of 22) at 12 to 18 months.
Table 3. Results of ESWL for Gallbladder Stones ORAL
48 67 80