Recent developments in SWL physics research

Two projects in our laboratory highlight some recent developments in shockwave lithotripsy (SWL) physics research. In the first project, we developed a prototype of a piezoelectric annular array (PEAA) shockwave generator that can be retrofitted on a Dornier HM-3 lithotripter for active control of cavitation during SWL. The PEAA generator, operating at 15 kV, produces a peak positive pressure of approximately 8 MPa with a -6-dB beam diameter of 5 mm. The shockwave generated by the PEAA was used to control and force the collapse of cavitation bubbles induced by a laboratory electrohydraulic shockwave lithotripter with a truncated HM-3 reflector. With optimal time delay between the lithotripter pulse and the PEAA-generated shockwave, the collapse of cavitation bubbles near the stone surface could be intensified, and the resultant stone fragmentation in vitro could be significantly improved. In the second project, high-speed shadowgraph imaging was used to visualize the dynamics of lithotripter-induced bubble oscillation in a vascular phantom. Compared with the free bubble oscillation in water, the expansion of cavitation bubble(s) produced in silicone tubes and a 200-microm cellulose hollow fiber by either a Nortech EHL or a Dornier XL-1 lithotripter was found to be significantly constrained. Rupture of the cellulose hollow fiber was observed consistently after about 20 shocks from the XL-1 lithotripter at an output voltage of 20 kV. These results confirm experimentally that SWL-induced cavitation in vivo can be significantly constrained by the surrounding tissue, and large intraluminal bubble expansions could cause rupture of capillaries and small blood vessels.     

Pressure-release versus rigid reflector for extracorporeal shockwave lithotripsy

PURPOSE: To evaluate the advantages and disadvantages of using a pressure-release reflector instead of a rigid reflector to concentrate shockwaves for extracorporeal shockwave lithotripsy (SWL). MATERIALS AND METHODS: As in all electrohydraulic lithotripters, shockwaves were generated by electrical breakdown of water between two electrodes, located at the focus (F1) closest to a paraellipsoidal reflector. A pressure-release reflector, made out of polyurethane foam, was constructed and tested on a research lithotripter using kidney stone models. Fragmentation data and pressure measurements were compared with those of a conventional rigid reflector tested on the same device. RESULTS: The weight of stone model fragments remaining after shockwave exposure was less with the pressure-release reflector after screening through a 3.0 x 3.0-mm mesh. The residual fragment weight was less with the rigid reflector using 1.0 x 1.0- and 0.6 x 0.6-mm meshes. CONCLUSION: Pressure-release reflectors may maintain acceptable stone fragmentation while offering improved patient safety and should be considered for SWL.     

Evaluation of a bifocal reflector on a clinical lithotripter

PURPOSE: To perform in vitro and in vivo tests using a clinical lithotripter in order to determine whether a bifocal reflector is more efficient and produces the same or less tissue damage than a conventional ellipsoidal reflector for electrohydraulic lithotripters. MATERIALS AND METHODS: A standard ellipsoidal and a novel bifocal reflector were tested on a Tripter Compact lithotripter (Direx Medical Systems, Petach Tikva, Israel). The bifocal reflector was constructed by joining two sectors of two rotationally symmetrical ellipsoidal reflectors having different distances between their foci. The F1 foci of the sectors coincided, creating a separation between the F2 foci. The fragmentation efficiency of the reflectors was compared using kidney-stone models. Shockwave-induced trauma was evaluated in vivo by treating both kidneys of six healthy dogs. One kidney was exposed to shockwaves generated with the conventional reflector, and the other kidney was treated using the bifocal reflector. Pressure measurements were obtained for both reflectors using needle hydrophones. RESULTS: The new design appeared to be more efficient than the conventional reflector in breaking up kidney-stone models. Tissue damage did not increase when using the bifocal reflector. CONCLUSION: The use of bifocal, instead of standard ellipsoidal, reflectors should be considered as an alternative to improve extracorporeal shockwave lithotripsy.     

Prefocal alignment improves stone comminution in shockwave lithotripsy

BACKGROUND: The Dornier HM-3 machine continues to be one of the most effective lithotripters in use. However, tissue damage occurs in most, if not all, shockwave lithotripsy (SWL) treatments. Cavitation appears to contribute to desired stone comminution as well as to undesired tissue damage. Studies of cavitation in electrohydraulic shockwave lithotripters indicate that the greatest cavitation activity occurs, not at the geometric focus, F2, but at a site proximal to F2 by 1 to 3 cm. In clinical practice, however, stones are aligned with F2. MATERIALS AND METHODS: In vitro stone comminution, hemolysis, and free-radical production were assessed along the focal axis, and pig kidneys treated with SWL in vivo were sectioned to determine the extent of hemorrhagic injury along the focal axis. Model gypsum stones received 200 shockwaves in vitro at 18 kV. RESULTS: At F2, the average number of fragments >1.5 mm was 1.3 +/- 0.5, and the weight loss was 11.3 +/- 1.1%. At 2 cm from F2 (F2-2 cm), these values increased to 4 +/- 2.8 and 16.1 +/- 4.2%, respectively. Samples of 10% hematocrit blood were similarly exposed. Hemolysis was equivalent at F2-2 cm (14.7 +/- 2.3%) and F2 (15.2 +/- 3%) but decreased significantly at all other positions. Samples of iodine solution received 1500 shockwaves at 20 kV. Hydroxyl radical production was greatest at F2-2 cm (0.384 +/- 0.035 microM) and decreased significantly distal to this position. The volume of tissue injury in pig kidneys was greatest with prefocal shockwave exposure. CONCLUSION: Stone comminution may be achieved more rapidly without greater tissue damage by a simple shift in stone alignment to F2-2 cm.     

Bifocal reflector for electrohydraulic lithotripters

OBJECTIVE: To describe the design and construction of a bifocal reflector that could be used in electrohydraulic extracorporeal shockwave lithotripters in order to increase their efficiency. METHODS: The new reflector is obtained by joining two sectors of two rotationally symmetric ellipsoidal reflectors having different distances between their foci, which results in a bifocal composite reflector with the F1 foci in coincidence and the two F2 foci separated by a certain distance. As in conventional reflectors, shockwaves are generated by the electrical breakdown of water between two electrodes, located at the focus (F1) closest to the reflector. A prototype was constructed and tested in an experimental shockwave generator of our own make, using two different types of kidney-stone models, one to test the stone fragmentation abilities, and the other to test the stone pitting abilities. Fragmentation data for the new reflector were compared with those of a conventional ellipsoidal reflector tested on the same device. RESULTS: The new design appeared to be more efficient in breaking up both types of kidney-stone models than the conventional reflector. Pressure measurements were obtained with both reflectors using needle hydrophones. The physical background of shockwave reflection on both reflectors is also explained. CONCLUSION: With this new reflector, it could be possible, in principle, to reduce the treatment time of extracorporeal shockwave lithotripsy.     

Percutaneous stone implantation in the pig kidney: a new animal model for lithotripsy research

PURPOSE: This report describes a new animal model for research on the parameters of shockwave delivery and the mechanisms of shockwave action in SWL. MATERIALS AND METHODS: Female pigs (approximately 45 kg) were anesthetized for creation of an upper pole peripheral caliceal access. The tract was dilated with a 30F Nephromax balloon and Amplatz sheath, and a 24F rigid nephroscope was used to guide a gypsum artificial stone into a lower pole calix. An internal ureteral stent was then placed. After a 2-hour recovery period, lithotripsy was performed using an unmodified Dornier HM3 lithotripter. Following SWL, en bloc excision of the urinary tract was performed, and the stone fragments were collected. RESULTS: As observed by nephroscopy, most stones were surrounded by urine that was free of clot or debris. Urine output was >1 mL/kg per minute by the time the animal was positioned for SWL after a 2-hour observation period. When the conditions of shockwave (SW) exposure were 400 SWs, 20 kV, and 120 SW/min, the efficiency of stone fragment recovery was 85% +/- 2% (N = 6 stones). CONCLUSIONS: This procedure provides a minimally invasive method for placement of model stones of clinically relevant size within the pig kidney. Stone implantation is efficient and permits experiments to be conducted in 1 day. Stone fragmentation can be quantitated, and the animal can serve as its own control. Long-term experiments are also feasible. Overall, this new animal model is appropriate for experimentation on the parameters of SW delivery in SWL.     

Potential for cavitation-mediated tissue damage in shockwave lithotripsy

PURPOSE: Shockwave lithotripsy (SWL) injures renal tissue, and cavitation has been reported to mediate some of these effects. Much of the work characterizing the cavitation injury of SWL has been performed in small animals or in vitro. We describe experiments that promote cavitation during SWL and estimate the spatial distribution of the resulting hemorrhagic lesion in a large-animal (porcine) model of clinical lithotripsy. MATERIALS AND METHODS: The lower pole calix of the left kidney in female farm pigs was targeted for SWL with a Dornier HM3 lithotripter. Intraventricular injections of polystyrene microspheres were made before and at intervals during lithotripsy to blanket systemic circulation with cavitation nuclei. Following SWL, the abdominal viscera were inspected and the kidneys were processed for morphologic analysis. RESULTS: Extensive surface hemorrhage occurred over both the targeted and contralateral kidneys, along with widespread petechial hemorrhage over the spleen, intestines, and peritoneum. The targeted kidneys developed subcapsular hematomas. Histology revealed focal and diffuse damage to the targeted kidneys and vascular rupture in both kidneys with complete necrosis of the walls of intralobular arteries and veins. CONCLUSIONS: These results demonstrate the potential for unfocused shockwaves to damage blood vessels outside the focal zone of the lithotripter when the vasculature is seeded with cavitation nuclei. The wide distribution of damage suggests that the acoustic field of a lithotripter delivers negative pressures that exceed the cavitation threshold far off the acoustic axis. The findings underscore that conditions permissive for cavitation can lead to dramatic sequelae during SWL.     

Efficacy of shock wave lithotripsy in management of kidney stones in infants

Introduction and objectivesDespite being uncommon, infantile kidney stone remains a major health problem due to its higher recurrence rate and morbidity. The parents usually notice that their infants have recurrent fever and failure to thrive of unknown origin. Those patients comprise a big challenge for the urologist in management. Therefore, this study aimed to evaluate the outcome of shockwave lithotripsy (SWL) in management of renal stones in infants.Subjects and methodsA retrospective analysis of prospectively collected data performed between January 2009 and December 2012 for infants underwent SWL for single radio-opaque renal stones ≤15 mm at a single stone center. SWL was performed with Dorneir S lithotripter with a maximum of 1500 shocks per session. A single session was indicated for each infant, but a second session was performed when satisfactory disintegration was not achieved. Follow-up based on urinalysis, urine culture and sensitivity, plain X-ray kidney ureter bladder (KUB) and abdominal ultrasonography (US) was carried out 2 weeks post SWL and monthly for 3 successive months. Multislice Computed tomography (MSCT) was performed 3-months post-SWL to confirm the stone-free status.ResultsA total of 87 infants, less than 24 months of age were enrolled in this research. SWL success was defined as absence of any residual fragments on MSCT 3-months after the last session. Stone free rate was 93.1% after the first SWL session and reached 100% after the second session. Rate of retreatment with second session of SWL was 6.9%. Urinary tract infection (UTI) was detected in 10.3%, transient renal obstruction with low grade fever in 4.6% of infants and no major complication had been recorded.ConclusionThe new generation of SWL technology with a precise focal area seems to be safe and effective in management of kidney calculi in infants.     

CT attenuation value and shockwave fragmentation

PURPOSE: To evaluate whether, in principle, the mean CT attenuation values of kidney stones could predict fragmentation by shockwaves. MATERIALS AND METHODS: Four types of artificial kidney stones having different CT attenuation values were tested. Artificial stones were weighed and exposed to 700 shockwaves at 21 kV at the focus of an electrohydraulic lithotripter. Fragments were strained through meshes with 2x2-mm and 3.1x3.1-mm openings. The material left on the meshes after shockwave exposure was dried and weighed on a precision scale. Half of all artificial stones were saturated by immersing them in water several days before fragmentation. Fragmentation coefficients (i.e., percent weight loss) were associated with CT attenuation values using a statistical model. RESULTS: Higher CT numbers resulted in lower fragmentation coefficients. Artificial stone weight was inversely proportional to the percent weight loss. Larger fragments were obtained at lower fragmentation coefficients. Statistical analysis revealed that fragmentation can be predicted knowing the weight and the CT number of a stone before shockwave application. CONCLUSION: Prediction of the number of shockwaves necessary for successful SWL could be possible. Our statistical model proved to fit in vitro fragmentation of artificial stones; however, clinical application requires further research.     

Shockwave lithotripsy: anecdotes and insights

Shockwave lithotripters have evolved considerably since the introduction of the Dornier HM3 machine 20 years ago. Although shockwave lithotripsy (SWL) remains the preferred treatment for the majority of symptomatic upper urinary-tract calculi, newer lithotripters are not as effective and may have a higher risk of side effects. Lack of progress in lithotripter evolution is attributable to inadequate understanding of how and why shockwaves produce effects on stone and tissue. Current knowledge suggests that stones fragment by the mechanisms of compression fracture, spallation, squeezing, and acoustic cavitation, while tissue damage from shockwaves is secondary to cavitation and non-cavitational forces such as sheer stress. It appears likely that most tissue damage from shockwaves is caused by cavitation. As the understanding of SWL matures, new lithotripter designs may emerge that truly represent an improvement on the original Dornier HM3 machine.     

Independent predictors of failure of shockwave lithotripsy for ureteral stones employing a second-generation lithotripter

PURPOSE: To define factors associated with the failure of shockwave lithotripsy (SWL) in the treatment of ureteral stones. PATIENTS AND METHODS: We retrospectively studied 405 men and 283 women (mean age 52.6 years) who underwent SWL with a second-generation lithotripter in the period 1994 to 2001. We evaluated available clinical and radiologic features that might have been related to failure of SWL therapy. RESULTS: Treatment was successful in 502 patients (73%). The 186 patients (27%) in whom treatment failed underwent endourologic alternatives or open surgery. Multivariate logistic regression analysis revealed that unsuccessful outcome was significantly related to: (1) pelvic ureteral stones (odds ratio [OR] 4.02; 95% CI 1.97, 8.19); (2) stone size >10 mm (OR 3.46; 95% CI 2.16, 5.53); (3) obstruction (OR 1.93; 95% CI 0.99, 3.77); and (4) obesity (OR 1.87; 95% CI 0.95, 3.77). Although the predictive value of each individual parameter was relatively low (15.3%-27.9%) the cumulative risk was 82.95% when patients had all four features. The strongest independent predictors of failure were pelvic stones and stones >10 mm (cumulative predictive value 57.3%). CONCLUSIONS: These variables may enable identification of a subgroup of patients who will fail initial SWL. These patients may be candidates for endourologic alternatives as first-line treatment.     

Extracorporeal piezoelectric shockwave lithotripsy of ureteral stones: are second-generation lithotripters obsolete?

BACKGROUND: The role of extracorporeal shockwave lithotripsy (SWL) for ureteral calculi is still being debated. We evaluated our results in a large series to clarify the role of this modality. PATIENTS AND METHODS: A total of 478 patients with solitary ureteral stones were treated by in situ piezoelectric extracorporeal shockwave lithotripsy (SWL) using a Wolf Piezolith 2300 ultrasound-guided lithotripter. Two hundred fifty stones (52.3%) were located in the upper ureter and 228 (47.7%) in the distal ureter. Seventy of the upper ureteral stones were located in the ureteropelvic junction and 180 in the lumbar ureter. The diameter of the stones ranged from 5 to 30 mm. Four hundred sixty-seven patients were followed up for a mean of 4 months. RESULTS: Four hundred forty patients (94.2%) were stone free after in situ SWL alone. Complete removal of all stone fragments was achieved in 95.4% of the 216 patients with calculi of 5 to 10 mm in diameter, in 94.3% of the 229 with stones of 11 to 20 mm, and in 81.8% of the 22 with calculi of 21 to 30 mm. In situ treatment completely removed 61 of 69 ureteropelvic junction stones (88.4%), 166 of 175 lumbar stones (94.8%), and 213 of 223 distal ureteral stones (95.5%). In situ treatment failed in 27 stones (5.8%). After 4 months, 12 stone fragments and 15 unfragmented stones persisted despite retreatments and required endoscopic procedures. The mean number of sessions and shockwaves per patient was 1.8 and 4884, respectively. Morbidity was low. Renal colic in 57 patients (11.9%) was managed successfully by analgesics. In 36 patients, stone fragments obstructed the ureter; in 28 of these 36 (78%), the obstruction was resolved and the patients were stone free after in situ retreatments alone. All these results were achieved on an outpatient basis without sedation or local or general anesthesia. CONCLUSION: Piezoelectric SWL is an effective and noninvasive method for eliminating ureteral stones. Second-generation ultrasound-guided lithotripters are not yet obsolete.     

In-Vivo relation between CT attenuation value and shockwave fragmentation

PURPOSE: To use CT attenuation numbers as a means of determining the susceptibility of an artificial stone to in-vivo fragmentation with extracorporeal shockwave lithotripsy (SWL). MATERIALS AND METHODS: Four types of artificial kidney stones having different CT attenuation values were used. One randomly selected stone was implanted in the renal pelvis of a kidney of 12 young pigs and exposed in vivo to 2500 shockwaves (21 kV) using an electrohydraulic lithotripter. Bilateral nephrectomy was performed after SWL. Fragments were strained through a mesh with a 3.1-mm grid, and the debris left on the mesh was dried and weighed. Fragmentation coefficients (FCs) were associated with CT attenuation values using a statistical model. RESULTS: The relation between FC and CT number was significant, indicating that as CT attenuation increases, FC is reduced. Larger stone fragments were obtained from stones with higher CT numbers. Initial stone weight was not a significant explanation for variations in FC. CONCLUSION: The CT values could be helpful in selecting patients for SWL in the future. However, other parameters such as stone porosity, shape, and roughness also will have to be considered.     

In situ SWL of ureteral stones: comparison between an electrohydraulic and an electromagnetic shockwave source

PURPOSE: To compare the influence of different shockwave emitters on treatment efficacy for SWL of ureteral stones in situ. PATIENTS AND METHODS: From January 1, 1990, until August 31, 1998, we treated 175 ureteral stones in situ 9 to 252 mm(2) (mean 55.2 mm(2)) using X-ray targeting on a Dornier MPL 9000 X lithotripter, a spark gap machine. From February 1996 through December 1997, we operated a Dornier Lithotripter S equipped with a conventional electromagnetic shockwave source, the EMSE 220. The size of the 71 treated ureteral stones at all levels ranged from 6 to 276 mm(2) (mean 47.4 mm(2)). After the introduction of an upgraded electromagnetic shockwave source, the EMSE F150, 33 ureteral stones of 9 to 150 mm(2) (mean 40.1 mm(2)) were treated in situ. After a second upgrade, to the more powerful EMSE F150-P, 50 ureteral stones ranging in size from 16 to 345 mm(2) (mean 62.3 mm(2)) were treated in situ. In all series, treatment strategies were identical. RESULTS: In the first series, auxiliary procedures were performed in 2.3% pre-SWL and 25.1% post-SWL (total 27.4%). The retreatment rate was 23.4%, bringing the effectiveness quotient (EQ) to 67.3. In the second series, the auxiliary procedure rate was 2.8% pre-SWL and 22.5% post-SWL (total 25.3%). The retreatment rate was 19.7%, and the EQ was 70.3. In the third series, auxiliary procedures were performed in 3.0% pre-SWL and 24.2% post-SWL (total 27.2%). The retreatment rate was 18.2% and the EQ 70.2. In the most recent series, no auxiliary procedures were needed before SWL, the post-SWL auxiliary procedure rate was 22%, and the retreatment rate was 10.0%, for an EQ of 75.8. CONCLUSION: The significant improvement in EQ with the EMSE F150-P must be attributed to an improved shockwave source, contradicting the myth that a spark gap source is by definition superior to an electromagnetic one.     

Effect of vibration massage therapy after extracorporeal shockwave lithotripsy in patients with lower caliceal stones

PURPOSE: To determine whether vibration massage influences the results of extracorporeal shockwave lithotripsy (SWL) in patients with lower caliceal stones. PATIENTS AND METHODS: One hundred three patients with lower caliceal stones were entered in the study. Patients were divided into two groups that received either SWL alone (Group A, N = 52) or with vibration massage (Group B, N = 51). There was no statistically significant difference between the two groups in stone size, duration of follow-up, or patient age. The complication, stone-free, and stone recurrence rates of the groups were compared. RESULTS: There was no statistically significant difference between the groups in the number of shockwaves, number of SWL sessions, or shockwave energy. The renal colic rate was higher in Group B than in Group A (P = 0.03). The stone-free rates in Group A and Group B were 60% and 80%, respectively (P = 0.003). The stone recurrence rate was higher in Group A than in Group B (P = 0.0006). CONCLUSIONS: This retrospective study demonstrates that SWL with vibration massage appears to have a beneficial effect on the passage of fragments in patients with lower caliceal stones.     

Primary ureteroscopy for distal-ureteral stones compared with ureteroscopy after failed extracorporeal lithotripsy

BACKGROUND AND PURPOSE: We reviewed our experiences with ureteroscopic pneumatic lithotripsy (URS-PL) for the treatment of distal-ureteral stones and investigated whether failed extracorporeal shockwave lithotripsy (SWL) is a limiting factor for the ureteroscopic procedure. PATIENTS AND METHODS: We retrospectively studied the medical records of 375 patients treated with URS-PL from January 1999 to September 2005 in our clinic. Of these patients, 213 were treated with URS-PL primarily (group 1), whereas the remaining 162 patients had undergone SWL unsuccessfully before URS-PL was performed (group 2). We used 9F or 9.5F rigid instruments and the Vibrolith (Elmed, Ankara, Turkey). RESULTS: In group 1, 206 patients (96.7%) were treated successfully with URS alone, as were 155 patients (95.6%) in group 2. Impacted stones were observed in 21 patients in group 1 (9.85%) and in 57 patients in group 2 (35.1%). The average operating time was 33.19 +/- 9.039 minutes in group 1 and 57.42 +/- 8.757 minutes in group 2. The stone-free rates of the two groups were significantly different on the first postoperative day, but this difference decreased to an insignificant level at the end of the first month. CONCLUSION: When SWL fails, URS-PL is as safe and effective as primary URS. Pneumatic lithotripsy also seems to be effective for impacted stones.     

A prospective multivariate analysis of factors predicting stone disintegration by extracorporeal shock wave lithotripsy: the value of high-resolution noncontrast computed tomography

OBJECTIVE: To assess the value of noncontrast computed tomography (NCCT) as a possible predictor of renal stone disintegration by shock wave lithotripsy (SWL). PATIENTS AND METHODS: The study included 120 consecutive patients (71 males, 49 females; mean age: 42.6 yr) with a solitary renal stone of 0.5-2.5 cm in length. NCCT was performed using a multidetector row CT scanner at 120 KV and 240 mA, with 1.25-mm collimation. A bone window was used to measure stone attenuation values. SWL was performed with an electromagnetic lithotripter. Failure of disintegration was defined as no fragmentation of the stone after three sessions. The impact of patients' sex, age, and body mass index (BMI) and the stones' laterality, location, volume, mean attenuation value, and the skin-to-stone distance on disintegration were evaluated by univariate and multivariate analyses. RESULTS: Failure of disintegration was observed in 15 patients (12.5%). BMI and stone density >1000 HU were the significant independent predictors of failure (p=0.04 and 0.02, respectively). The success rate of extracorporeal SWL at 3 mo was 87.5% (105 of 120 patients); 90 patients were stone free and 15 had residual fragments<4 mm. The only significant predictor of residual fragments was stone density (p<0.001). CONCLUSIONS: Obesity and increased stone density as detected by NCCT are significant predictors of failure to fragment renal stones by SWL. An alternative treatment should be devised for obese patients with stone density>1000 HU.     

Why stones break better at slow shockwave rates than at fast rates: in vitro study with a research electrohydraulic lithotripter

BACKGROUND AND PURPOSE: Stones break better when the rate of shockwave (SW) delivery is slowed. It has been hypothesized that the greater cavitation accompanying a fast rate shields pulse propagation, thus interfering with the delivery of SW energy to the stone. We tested this idea by correlating waveforms measured at the SW focus with cavitation viewed using high-speed imaging. MATERIALS AND METHODS: A series of U30 gypsum stones held in a 2-mm mesh basket were exposed to 200 SWs at 30 or 120 SW/min from a research electrohydraulic lithotripter (HM3 clone). Waveforms were collected using a fiberoptic probe hydrophone. High-speed imaging was used to observe cavitation bubbles in the water and at the stone surface. Results: Stone breakage was significantly better at 30 SW/min than at 120 SW/min. The rate had little effect on SW parameters in the water free field. In the presence of particulates released from stones, the positive pressure of the SW remained unaffected, but the trailing tensile phase of the pulse was significantly reduced at 120 SW/min. CONCLUSIONS: Cavitation bubbles do not persist between SWs. Thus, mature bubbles from one pulse do not interfere with the next pulse, even at 120 SW/min. However, cavitation nuclei carried by fine particles released from stones can persist between pulses. These nuclei have little effect on the compressive wave but seed cavitation under the influence of the tensile wave. Bubble growth draws energy from the negative-pressure phase of the SW, reducing its amplitude. This likely affects the dynamics of cavitation bubble clusters at the stone surface, reducing the effectiveness of bubble action in stone comminution.     

The effect of treatment strategy on stone comminution efficiency in shock wave lithotripsy

PURPOSE: The comminution of kidney stones in shock wave lithotripsy (SWL) is a dose dependent process caused primarily by the combination of 2 fundamental mechanisms, namely stress waves and cavitation. The effect of treatment strategy with emphasis on enhancing the effect of stress waves or cavitation on stone comminution in SWL was investigated. Because vascular injury in SWL is also dose dependent, optimization of the treatment strategy may produce improved stone comminution with decreased tissue injury in SWL. MATERIALS AND METHODS: Using an in vitro experiment system that mimics stone fragmentation in the renal pelvis spherical BegoStone (Bego USA, Smithfield, Rhode Island) phantoms (diameter 10 mm) were exposed to 1,500 shocks at a pulse repetition rate of 1 Hz in an unmodified HM-3 lithotripter (Dornier Medical Systems, Kennesaw, Georgia). The 3 treatment strategies used were increasing output voltage from 18 to 20 and then to 22 kV every 500 shocks with emphasis on enhancing the effect of cavitation on medium fragments (2 to 4 mm) at the final treatment stage, decreasing output voltage from 22 to 20 and then to 18 kV every 500 shocks with emphasis on enhancing the effect of stress waves on large fragments (greater than 4 mm) at the initial treatment stage and maintaining a constant output voltage at 20 kV, as typically used in SWL procedures. Following shock wave exposure the size distribution of fragments was determined by the sequential sieving method. In addition, pressure waveforms at lithotripter focus (F2) produced at different output settings were measured using a fiber optic probe hydrophone. RESULTS: The rate of stone comminution in SWL varied significantly in a dose dependent manner depending on the treatment strategies used. Specifically the comminution efficiencies produced by the 3 strategies after the initial 500 shocks were 30.7%, 59% and 41.9%, respectively. After 1,000 shocks the corresponding comminution efficiencies became similar (60.2%, 68.1% and 66.4%, respectively) with no statistically significant differences (p = 0.08). After 1,500 shocks the final comminution efficiency produced by the first strategy was 88.7%, which was better than the corresponding values of 81.2% and 83.5%, respectively, for the other 2 strategies. The difference between the final comminution efficiency of the first and second strategies was statistically significant (p = 0.005). CONCLUSIONS: Progressive increase in lithotripter output voltage can produce the best overall stone comminution in vitro.     

Optimizing results of lithotripsy using robust electromagnetic probe.

BACKGROUND AND OBJECTIVE: A significant impediment to the measurement of the pressures and forces created by lithotripter shockwaves has been their destructive properties, which have rendered most measuring devices impractical. We have developed and tested a robust electromagnetic probe to measure cavitational forces in vitro in the focal zones of extracorporeal lithotripters. The probe responds to the pressure gradient generated by the radial motion of cavitation bubbles. MATERIALS AND METHODS: The effects of shockwaves from the Dornier MPL 9000 electrohydraulic lithotripter were measured over the lifetime of multiple electrodes. RESULTS: The pulse energy from the electrodes dropped off rapidly after approximately 50% of the lifetime quoted by the manufacturer. The electrodes were more efficient at higher power settings. As a result, we altered our protocol for the treatment of ureteral stones to use a higher kilovoltage and a second electrode whenever necessary. Stone-free rates after shockwave lithotripsy (SWL) in situ for stones < 11 mm have increased from 68.2% to 83.3%, and the retreatment rate has dropped from 23% to 15%. Despite significantly higher power settings (23.7 kV v 18.7 kV; P < 0.0001), the need for sedoanalgesia has remained relatively constant (26% v 31%). CONCLUSIONS: Measurement of cavitational forces from lithotripters using a robust electromagnetic probe is useful in planning treatment strategy. We have demonstrated a clinically measurable improvement since implementing our new treatment protocol. Because the probe responds directly to cavitational forces, it should also prove useful for the objective comparison of different SWL machines.