258 LITHOTRIPSY

LITERATURE, MEDICAL PHYSICS. See MEDICAL

PHYSICS LITERATURE.

LITHOTRIPSY

ALON Z. WEIZER

GLENN M. PREMINGER

Duke University Medical Center

Durham, North Carolina

INTRODUCTION

The clinical introduction of shock wave lithotripsy (SWL) by Chaussy in 1980 has revolutionized the way in which patients with renal and ureteral calculi are treated. Shock wave lithotripsy is a noninvasive method of fragmenting stones located inside the urinary tract. Since its initial introduction, SWL technology has advanced rapidly in terms of the means for shock wave generation, shock wave focusing, patient coupling, and stone localization. Despite rapid technological advances, most current commercial lithotripters are fundamentally the same; they produce a similar pressure waveform at the focus, which can be characterized by a leading shock front with a compressive wave followed by a trailing tensile wave (Fig. 1). The acoustic fields produced by different lithotripters differ from each other in terms of the peak amplitudes of the pressure waveform, pulse duration, beam size, total acoustic energy, and therefore, their overall performance.

Clinical experience has guided the technical development of second and third generation lithotripters with the aim of providing user convenience and multifunctionality of the device, rather than on further understanding of how SWL fragments calculi or injures surrounding renal tissue. Furthermore, the evolution of lithotripter design thus far has overwhelmingly relied upon the importance of the compressive wave component of the shock wave (positive portion of the sound wave), with almost total neglect of the contribution of the tensile component of the waveform. Consequently,

Figure 1. Pressure–time relationship of typical shock waves. Typical pattern of a lithotripter-generated shock wave. The shock wave is characterized by a leading positive or compressive component (Pþ) followed by a negative or tensile component (P ).

current lithotripters have suffered from inferior fragmentation rates compared to the original HM3 lithotripter.

In contrast, significant progress in SWL basic science research has been made in the past 5 years to improve our understanding of the primary mechanisms for stone comminution (fragmentation) and tissue injury. It is now recognized that the disintegration of renal calculi in a lithotripter field is the consequence of dynamic and synergistic interaction of two fundamental mechanisms: stress wave-induced dynamic fracture in the form of nucleation, growth, and coalescence of preexisting microcracks inside the stone (1) and cavitation erosion caused by the violent collapse of bubbles near the stone surface (2,3). Similarly, two different mechanisms have been proposed for SWLinduced tissue injury: shear stress due to shock front distortion (4) and cavitation induced inside blood vessels, especially the expansion of intraluminal bubbles (5).

To understand how SWL fragments stones and causes tissue injury, the basic components of current lithotripters, the mechanisms behind stone fragmentation and kidney injury, and clinical results of the original electrohydraulic lithotripter in fragmenting kidney stones in patients will be described. In addition, the future directions of SWL will be reviewed based on current research that is investigating ways to make lithotripters more efficient and safer.

HISTORY AND EVOLUTION OF SWL

Physicists at Dornier Systems, Ltd. and Friedrich Shafen, Germany began experimenting with shock waves and their travel through water and tissue in 1963. Throughout the 1970s, numerous experimental lithotripters were developed that used new methods of shock wave generation and focusing as well as different techniques of stone localization. In addition, experimental studies were being performed in vitro and in vivo (in animal models) examining the effects of shock waves on various organs and tissues.

In 1980, Chaussy and associates successfully treated the first human and reported their first series of 72 patients in 1982 (6). Subsequently, > 1800 articles have been published in the peer-reviewed literature, detailing the use of SWL for the management of renal and ureteral calculi. Moreover, numerous second and third generation devices have been introduced and are currently being used throughout the world. To understand how SWL results in stone fragmentation, the fundamentals of this technology are reviewed.

SWL PRINCIPLES

Despite the tremendous number of lithotripters currently available for fragmentation of renal and ureteral stones, all of these devices rely on the same laws of acoustic physics. Shock waves (i.e., high pressure sound waves) consisting of a sharp peak in positive pressure followed by a trailing negative wave are generated extracorporeally and passed through the body to fragment stones. Sounds waves readily propagate from a water bath or water medium into the human body, due to similar acoustic impedances.

As a consequence, all lithotripters share four main features: an energy source to generate the shock wave, a

Ellipsoid reflector and fluoroscopy (focusing)

Water Bath

(coupling)

Spark electrode (F1) (shock wave)

2c

Kidney stone (F2)

Acoustic lens (focusing)

Electromagnetic coil (F1) vibrates sending out shock wave

2b

2d

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Kidney stone (F2)

Spherical focusing of piezo-ceramic elements (F1)

Kidney stone (F2)

Parabolic reflector (focusing)

Electromagnetic cylinder vibrates (F1)

device to focus the shock wave at a focal point, a coupling medium, and a stone localization system. (Fig. 2a) The original electrohydraulic lithotripter utilized a spark plug energy generator with an elliptical reflector for focusing the shock waves. A water bath transmitted the shock waves to the patient with stone localization provided by biplanar fluoroscopy. Modification of the four basic components of this first generation lithotripter has provided the development of second and third generation devices that are currently available.

Shock Wave Generation and Focusing

While all lithotripters share the four aforementioned features, it is the mode of shock wave generation that determines the actual physical characteristics of that particular device. The types of energy sources differ on how efficient they are in fragmenting stones and how many treatments are required to adequately treat the stone. Figure 2 diagrammatically

summarizes the three different shock wave generation sources used in commercially available lithotripters.

Electrohydraulic Generator. In the original Dornier HM3 lithotripter, the electrohydraulic generator (Fig. 2a) was located at the base of a water bath and produced shock waves by electric spark discharges of 15,000–25,000 V of 1 ms duration. This high voltage spark discharge produced the rapid evaporation of water that created a shock wave by expanding the surrounding fluid. The generator was located in an ellipsoidal reflector that concentrated the reflected shock waves at a second focal point, F2, with F1 being the point of origin of the primary shock waves. While the HM3 continues to be the gold standard lithotripter for stone fragmentation, the short half-life of its electrode results in variable pressure between shocks the longer it is used. In addition, minimal displacement of the electrode, as it deteriorates at F1, results in displacement of the F2 resulting in inaccurate focusing of the shock wave on the

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stone. The need for frequent replacement of the electrode increases the cost of electrohydraulic lithotripsy.

Piezoelectric Generator. Piezoelectric shock waves are generated (Fig. 2b) by the sudden expansion of ceramic elements excited by a high frequency, high voltage pulse. Thousands of these elements are placed along the inner surface of a hemisphere at the base of a pool of water. While each of these elements moves only slightly in response to a pulse of electrical energy, the summation of the simultaneous expansion of multiple elements results in a high energy shock wave directed to the focal area, at the center of the sphere. The shock wave is propagated through either a small water basin or a water-filled bag to the focal point, F2. The spherical focusing mechanism of the piezoelectric lithotripters provides a wide area of shock wave entry at the skin surface, which causes minimal patient discomfort, but a very small focal area with the smallest amount of energy at F2 compared to other energy sources (7). The small focal area necessitates greater precision to focus and fragment the stone inside the kidney.

Electromagnetic Generator. In electromagnetic devices (Fig. 2c and d), shock waves are generated when an electrical impulse moves a thin, spherical metallic membrane, which is housed within a cylindrical shock tube. The resulting shock wave, produced in the water-filled shock tube, passes through an acoustic lens and is thereby directed to the focal point, F1 (Fig. 2c). The shock wave is coupled to the body surface with a moveable water cushion and coupling gel (8). Alternatively, when energy is passed through a cylindrical coil, the resulting magnetic field pushes away the surrounding cylindrical membrane producing a shock wave that can be focused by a parabolic reflector (Fig. 2d). While these devices produce reliable shock waves with consistent pressures and focus on F2, they also produce small focal regions that may result in reduced stone fragmentation and higher tissue parenchymal injury.

Shock Wave Focusing. Shock waves must be focused in order to concentrate their energy on a calculus. The type of shock wave generation dictates the method of focusing used. Machines that utilize point sources, such as sparkgap electrodes (electrohydraulic lithotripters), generate shock waves that travels in an expanding circular pattern. All of these machines use ellipsoid reflectors for focusing shock waves at the second focal point, F2.

Since a single piezoelement produces a very small amount of energy, larger transducers with multiple ceramic elements are required for piezoelectric lithotripters. The array of elements is positioned in a spherical dish that allows focusing in a very small focal region, F1. Finally, the vibrating metal membranes of the electromechanical lithotripters produce an acoustical plane wave that uses an acoustic lens for focusing the shock wave at F1.

Coupling Medium

The original Dornier HM3 machine utilized a 1000 L water bath to transmit shock waves into the patient. This method of patient coupling required unique positioning of the patient, since the anesthetized subject had to be lowered

into the tub and the calculus accurately positioned at the second focal point. Second generation lithotripters were designed to alleviate the physiologic, functional, and economic problems of a large water bath. Current models utilize an enclosed water cushion, or a totally contained shock tube, to allow simplified positioning and ‘‘dry’’ lithotripsy (9).

Stone Localization

Stone localization during lithotripsy is accomplished with either fluoroscopy or ultrasonography. Fluoroscopy provides the urologist with a familiar modality and has the added benefits of effective ureteral stone localization. However, fluoroscopy requires more space, carries the inherent risk of ionizing radiation to both the patient and medical staff and is not useful in localizing radiolucent calculi (certain noncalcium-containing stones are not seen on fluoroscopy or conventional radiographs).

Ultrasonography utilizes sound waves to locate stones. These sound waves are generated at a source. When they encounter different tissue densities, part of the sound wave is reflected back and these reflected waves are used to generate a two dimensional image that can be used to focus the shock wave on a calculus. Sonography-based lithotripters offer the advantages of stone localization with continuous monitoring and effective identification of radiolucent stones, without radiation exposure (7). Additionally, ultrasound has been documented to be effective in localizing stone fragments as small as 2–3 mm, and is as good or better than routine radiographs to assess patients for residual stone fragments following lithotripsy (8). The major disadvantages of ultrasound stone localization include the basic mastery of ultrasonic techniques by the urologist and the inherent difficulty in localizing ureteral stones. While there are several systems that utilize both ultrasound and fluoroscopy to aid in stone localization, many commercially available lithotripters now use a modular design in which the fluoroscopy unit is not attached to the lithotripter reducing storage space as well as allowing use of the fluoroscopy unit for other procedures.

MECHANISMS OF STONE FRAGMENTATION

Due to recent advances made in the basic science of shock wave lithotripsy, there is a better understanding of how shock waves result in stone fragmentation. Both the positive and negative portions of the shock wave (Fig. 1) are critical in stone fragmentation and also play a role in renal tissue injury. The four mechanisms described for stone comminution include compressive fracture, spallation, cavitation, and dynamic fatigue. As the calculus develops in vivo, it is formed by both crystallization of minerals as well as organic matrix material. This combination forms an inhomogeneous and imperfect material that has natural defects. When the shock wave encounters this inhomogeneous structure, the force generated in the plane of the shock wave places stress on these imperfections resulting in compression-induced tensile cracking. This mechanism is known as compressive fracture. Spallation, another mechanism of shock wave induced stone fragmentation, occurs when the shock wave encounters fluid behind the

stone and part of the wave is reflected back onto the stone placing tensile stress on the same imperfections.

Shock waves cause bubbles to form on the surface of the stone. These bubbles grow during the negative or tensile component of the shock wave. When the positive component of the next wave passes, these bubbles violently collapse releasing their energy against the stone surface as secondary shock waves and/or microjets. This phenomenon, known as cavitation, represents the third mechanism of stone fragmentation. Dynamic fatigue describes the sum of accumulated damage to the stone that coalesce to result in stone fragmentation and eventually destruction of the stone (10,11).

CLINICAL RESULTS OF SHOCK WAVE LITHOTRIPSY

Because many experts continue to consider the Dornier HM3 (the original electrohydraulic lithotripter, Dornier MedTech America, Inc., Kennesaw, GA) the gold standard in lithotripsy, the available clinical literature comparing the Dornier HM3 (first generation) lithotripter to other commercially available lithotripters is summarized in Table 1. This type of summary does not truly allow a comparison between different lithotripters currently in use. Yet, this comparison demonstrates that modifications to second and third generation lithotripters have traded better patient comfort for a lessening of stone free rates. This current clinical data has been one of the driving forces behind the modifications that are currently being investigated to improve SWL.

Summarized results of five large early series on the clinical efficacy of SWL using the unmodified Dornier HM3 lithotripter demonstrate stone free rates for renal pelvis (RP), upper calyx (UC), middle calyx (MC), and lower calyx (LC) stones were 76% (48–85), 69% (46–82), 68% (52– 76), and 59% (42–73), respectively. Stone free rates in these series were better for smaller stones (0–10 mm) and RP stones with relatively poorer stone free rates for LC stones. In comparison, results of five large published series on the Siemens Lithostar, a lower power machine demonstrated stone free rates for RP, UC, MC, and LC stones of 69% (55– 80), 67% (46–90), 63% (43–82), and 60% (46–73). A comparison of integrated stone free rates stratified by size in a regression model of the HM3 and Lithostar found significantly greater stone free rates across all stone sizes for the HM3 lithotripter (11). While most studies have evaluated the efficacy of SWL for adults, stone free rates with the HM3 are similar for children (21). The limited number of comparative studies of newer machines and the explosion in the number of commercially available lithotripters makes it difficult to assess their clinical efficacy.

While the HM3 has been shown to produce excellent stone free rates for renal calculi, there continues to be debate on the clinical efficacy of SWL for ureteral stones. The main problem is that stones in the ureter are more difficult to locate and therefore more difficult to target with the shock wave. However, several studies have demonstrated stone free rates close to 100% for the treatment of proximal ureteral stones with SWL (22). However, stone free rates appear to decline to 70% for mid-ureteral stones for many lithotripters (23). Treatment of distal ureteral stones with SWL typically involves a prone or a modified

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sitting position to allow shock wave targeting of the stone below the pelvic brim. Stone free rates of distal ureteral stones with the HM-3 lithotripter have been as high as 85–96% (24). Endoscopic methods (ureteroscopy) can also be employed for ureteral stones, especially those located in the distal ureter with equivalent or better stone free rates.

While many of the lithotripters sighted in Table 1 are no longer in clinical use or have been updated, these studies clearly demonstrated several key points. The HM3 continues to provide equivalent and likely superior stone free rates when compared to other lithotripters in studies. While most commercially available lithotripters provide acceptable stone free rates for stones located within the kidney, these stones often require more treatment sessions and adjunctive procedures to achieve the stone free rates of the HM3 device. Additionally, success rates with all lithotripters declines the further the stone progresses down the ureter and poses positioning challenges with alternative methods indicated to remove ureteral stones.

TISSUE INJURY WITH CLINICAL LITHOTRIPSY

Clinical experience treating patients with SWL has demonstrated that while SWL is generally safe, shock waves can cause acute and chronic renal injury. This concept has been confirmed by multiple animal studies and a few human clinical studies (11). Originally, shear stress to the tissue was believed to be the main cause of this renal injury. However, recent studies have sugggested that SWL induced renal injury is a vascular event induced by vasoconstriction or cavitation-induced injury to the microvasculature (25). Additionally, this initial tissue damage may promote further renal injury via the effects of free radical production (26). Acute damage to the kidney appears to be mainly a vascular insult.

While clinicians have long recognized the acute effects of SWL, most have believed that there was no long-term sequela to shock wave treatment. The > 25 years of clinical experience with SWL serves as a testament to its safety and effectiveness. However, several chronic problems may develop as a consequence of SWL. Table 2 summarizes the acute and chronic effects of SWL. Perhaps the most serious long-term problem of SWL is the increased risk of hypertension. A review of 14 studies on the impact of SWL on blood pressure demonstrated that when stratified according to the number of shock waves administered, higher doses of shock waves seem to correlate with a greater likelihood of increased rates of new-onset hypertension or changes in diastolic blood pressure (11). The impact of hypertension on cardiovascular disease including the risk of myocardial infarction, stroke, and renovascular disease make this a serious long-term effect that needs further investigation.

Three mechanisms of SWL induced tissue injury have been reported: cavitation, vasoconstriction, and free radical induced tissue injury. Investigators have demonstrated in vitro that cavitation bubble expansion can rupture artificial blood vessels (5). Other investigators have shown in animal models that cavitation takes place in kidneys exposed to shock waves (27). While cavitation bubbles that form on the stone surface contribute to stone fragmentation,

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Table 1. Literature Comparison of Lithotripters to Gold Standard Dornier HM3

Table 2. Acute and Chronic Injury with SWL

their formation in other locations (tissue, blood vessel lumen) is an unwanted end product that results in tissue injury.

Recent investigations have elucidated yet another potential mechanism of renal injury secondary to high energy shock waves. Evidence suggests that SWL exerts an acute change in renal hemodynamics (i.e., vasoconstriction) that occurs away from the volume targeted at F2, as measured by a transient reduction in both glomerular filtration rate (GFR) and renal plasma flow (RPF) (28). Prolonged vasoconstriction can result in tissue ischemia and permanent renal damage.

Vasoconstriction and cavitation both appear to injure the renal microvasculature. However, as the vasoconstriction induced by SWL abates, reperfusion of the injured tissue might also result in further tissue injury by the release of free radicals. These oxidants produced by the normal processes of cellular metabolism and cellular injury cannot be cleared and injure the cell membrane destroying cells. Free radical formation has been demonstrated in animal models (26).

It appears that the entire treated kidney is at risk of renal damage from SWL-induced direct vascular and distant vasoconstrictive injury, both resulting in free radical formation. Although previous studies have suggested that the hemodynamic effects are transient in nature in normally functioning kidneys, patients with baseline renal dysfunction may be at significant risk for permanent renal damage (28). Patients of concern may be pediatric patients, patients undergoing multiple SWL treatments to the same kidney, patients with solitary kidneys, vascular insufficiency, glomerulosclerosis, glomerulonephritis, or renal tubular insult from other causes.

SHOCK WAVE LITHOTRIPSY ADVANCES

Based on a better understanding of cavitation in stone fragmentation as well as the role of cavitation, vasoconstriction, and free radical formation in SWL-induced tissue injury, several groups are investigating ways in which SWL can be clinically more effective and safe. In general, these advancements involve changes to the shock wave itself, by modifying the lithotripter, alterations in treatment technique, improvements in stone fragmentation / passage or the reduction in tissue injury through medical adjuncts, and improved patient selection.

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Changes to the Lithotripter

There are two major mechanical modifications that can improve stone comminution, based on our current understanding of acoustic physics. One is to enhance the compressive component of the shock wave. The original HM3 relies on a high energy output and thus the compressive component to achieve stone fragmentation. The downside of this effect, from clinical experience, is patient discomfort and potential renal tissue injury. Alternatively, one can improve stone comminution by altering the tensile component of the shock wave and thus better control cavitation. Below, several ways are describe in which investigators are modifying the shock wave to improve comminution, with decreased renal tissue injury.

Several investigators have modified lithotripters to alter the negative portion of the shock wave that is responsible for cavitational-induced stone fragmentation and tissue injury. In one study, a reflector insert is placed over the original reflector of an electrohydraulic lithotripter to create a second shock wave that arrives behind the original shockwave, thus partially canceling out the negative component of the shock wave. These investigators found that this modification reduced phantom blood vessel rupture, while preserving stone fragmentation in vitro (29). Similarly, an acoustic diode (AD) placed over the original reflector, has the same impact as this modified reflector (30).

However, because reducing the tensile component of the shock wave weakens that collapse of bubbles at the stone surface, two groups have designed piezoelectric inserts into an electrohydraulic lithotripter that send small, focused shock waves at the time of bubble collapse near the stone surface thus, intensifying the collapse of the cavitation bubble without injuring surrounding tissue (29).

Another way in which investigators have modified the shock wave is by delivering shock waves from two lithotripters to the same focal point, F2. Dual pulse lithotripsy has been evaluated by several investigators both in vitro, animal models and in clinical trials. Several investigators have demonstrated in an in vitro model that the cavitation effect became more localized and intense with the use of two reflectors. Also, the volume and rate of stone disintegration increased with the use of the two reflectors, with production of fine (< 2 mm) fragments (31). In both animal models and clinical studies, dual pulse lithotripsy has been shown to improve stone fragmentation with reduced tissue injury.

Modifications to Treatment Strategy

The original Dornier HM3 lithotripter rate was synchronized with the patient’s electrocardiogram so that the shock rate did not exceed the physiologically normal heart rate. Experience with newer lithotripters has revealed that ungating the shock wave delivery rate results in few cardiac abnormalities. As a result, there has been a trend to deliver more shock waves in a shorter period of time. However, increasing doses of shock wave energy at a higher rate may have the potential to increase acute and chronic renal damage.

As a result, several investigators have evaluated ways in which the treatment strategy can be modified to optimize SWL treatment. One proposed strategy is altering the rate of shock wave delivery. Several investigators have reported that 60 shocks min 1 at higher intensities resulted in the

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most efficient stone fragmentation than 120 shocks min 1. This has been confirmed in vitro, in animal models and also in randomized clinical trials. These studies speculate that at increased rates, more cavitation bubbles were formed in both the fluid and tissue surrounding the stone that did not dissipate between shocks. As a result, these bubbles scattered the energy of subsequent shocks resulting in decreased efficiency of stone fragmentation (32).

In order to acclimate patients to shock waves in clinical treatment, lower energy settings are typically used and gradually increased. A study investigating whether increasing voltage (and thus increasing treatment dose) impacted on stone fragmentation has been performed in vitro and in animal models. Stones fragmented into smaller pieces when they were exposed to increasing energy compared to decreasing energy. The authors speculate that the low voltage shock waves ‘‘primed’’ the stone for fragmentation at higher voltages (33). In addition, animals exposed to an increasing voltage had less tissue injury than those kidneys exposed to a decreasing or stable dose of energy. While this treatment strategy has not been tested clinically, it might be able to improve in vivo stone comminution while decreasing renal parenchymal injury (34).

In the same vein, several studies have reported that pretreating the opposite pole of a kidney with a low voltage dose of shock waves (12 kV), prior to treating a stone in the other pole of a kidney with a normal dosage of shock waves, reduced renal injury when as little as 100 low voltage shocks were delivered to the lower pole. It is believed that the low voltage shock waves causes vasoconstriction which protects the treated pole of the kidney from hemorrhagic injury (35).

Other SWL treatment modifications being tested include aligning the shock wave in front of the stone in order to augment cavitation activity at the stone surface or apply overpressure in order to force cavitation bubble collapse. While these techniques have only been investigated in vitro, these alterations in shock wave delivery, as well as the previous treatment strategies demonstrate how an improved understanding of the mechanisms of SWL can enhance stone comminution and potential reduce renal tissue injury (36,37).

Adjuncts to Improve SWL Safety and Efficacy

Antioxidants. A number of studies have investigated the role of antioxidants in protecting against free radical injury to renal parenchyma (38). Table 3 summarizes the results of various in vitro and in vivo studies on the use of antioxidants to protect against SWL-induced renal injury due to free radicals. While these studies are intriguing, further clinical trials will be needed to evaluate potential antioxidants for use in patients undergoing SWL.

Improving Stone Fragmentation. Another potential way to improve stone fragmentation is to alter the stone’s susceptibility to shock wave energy. One group has demonstrated that after medically pretreating stone in an in vitro environment, one could achieve improved stone fragmentation. These data suggest that by altering the chemical environment of the fluid surrounding the stones it is possible to increase the fragility of renal calculi in vitro (49). Further studies are warranted to see if calculi can be

Table 3. Investigated Antioxidants Providing Protection against SWL-Induced Free Radical Injury

clinically modified, prior to SWL therapy, in the hopes of enhanced stone fragmentation.

Improving Stone Expulsion. Several reports have demonstrated that calcium channel blockers, steroids, and alpha blockers all may improve spontaneous passage of ureteral stones. (ref) Compared to a control group, investigators found improved stone clearance and shorter time to stone free in patients treated with nifedipine or tamsulosin following SWL compared to the control group. Additionally, retreatment rates were lower (31%) for the medical treatment group compared to the control group (51%). While expulsive therapy appears to offer improved outcomes following SWL for ureteral stones, confirmation with a randomized controlled study is needed (50). Another intriguing report from the same group involves the use of Phyllanthus niruri (Uriston) to improve stone clearance of renal stones following SWL. This medication is derived from a plant used in Brazilian folk medicine that has been used to treat nephrolithiasis. Again, stone free rates were improved with the administration of Uriston following SWL compared to a control group with apparently the greatest effect seen in those patients treated with lower pole stones (51).

Improving Stone/Patient Selection for SWL. Another way to enhance the efficacy of SWL is improve patient selection. Advances in computed tomography (CT) have allowed better determination of internal stone architecture (52). As a consequence, a few studies have demonstrated that determining the Hounsefield units (i.e., density unit of material on CT) of renal stones on pretreatment, noncontrasted CT could predict stone free rates of patients treated with SWL (53). Current micro-CT and newer multidetector CT scanners have the potential to identify stone composition based on CT attenuation. Therefore, stone compositions that are SWL resistant, such as calcium oxalate monohydrate or cystine stones, can be identified and those patients can be treated with endoscopic modalities, therebye avoiding additional procedures in these patients (54). Clinical trials utilizing this concept will need to be performed.

Other factors, such as the distance of the stone from the skin, weight of the patient, and other imaging modalities, are being investigated to help determine who is likely to benefit the most from SWL and which patients should be treated initially with other modalities.

CONCLUSIONS

Shock wave lithotripsy has revolutionized the way in which urologists manage urinary calculi. Patients can now be treated with minimal discomfort using an outpatient procedure. While all lithotripters rely on the same fundamental principles of acoustic physics, second and third generation lithotripters appear to have traded patient comfort and operator convenience for reduced stone free rates, as compared to the original HM3 lithotripter. In addition, mounting evidence demonstrates that SWL has both acute and chronic impact on renal function and blood pressure as a result of renal scarring.

Basic science research has provided insight into how SWL results in stone comminution as well as renal tissue injury. While the compressive component of the shock wave causes stone comminution, it is apparent that the tensile component plays a critical role in creating passable stone fragments. These same forces cause tissue injury by damaging the microvasculature of the kidney. This knowledge has resulted in several novel modifications to improve both stone free rates as well as SWL safety. Mechanical modifications to lithotripsy have focused on controlling cavitation. Preventing bubble expansion in blood vessels while intensifying bubble collapse near the stone surface has been demonstrated to achieve improved stone comminution with decreased tissue injury in vitro and in animal models. Many of these designs could be adapted to conventional lithotripters. Modification of treatment techniques have also stemmed from our better understanding of SWL. Slowing treatment rates may limit the number of cavitation bubbles that can interfere with the following shock wave. Voltage stepping and alterna- tive-site pretreatment with low dose shock waves, may cause global renal vasoconstriction that prevents cavitational injury to small vessels during treatment. In addition, our understanding that free radicals may be the end culprit in parenchymal damage has suggested that pretreatment with antioxidants may prevent SWL-induced renal injury. Finally, improved CT imaging may allow us to predict which stones and patients are best suited for SWL versus endoscopic stone removal. Further advances will continue to make SWL a major weapon in the war against stone disease for years to come.

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