A comparison of stone damage caused by different modes of shock wave generation

Analysis of lithotripsy efficiency and stone retropulsion displacement according to pulse characteristics of Ho: YAG laser with Moses effect

BACKGROUND AND OBJECTIVES

Compared to the conventional Ho: YAG laser, a Ho: YAG laser device has been reported that has a Moses effect to reduce stone retropulsion and increase lithotripsy efficiency. The principle of this equipment is to convert a single laser pulse into two pulses. Most studies on such lasers are limited to lithotripsy efficiency and the prevention of stone retropulsion; studies according to each pulse condition have not been performed. Therefore, the purpose of this study was to quantify the bubble shape, lithotripsy efficiency, and stone retropulsion displacement in a ureteral phantom according to the modulation of the first pulse characteristics of the Moses effect laser under conditions that maintained the total energy and repetition rate.

MATERIAL AND METHODS

In this study, a Ho: YAG laser system (Holinwon Pro, Wontech Inc., Korea) with an emission wavelength of 2.10 μm and a Moses effect was used. To verify the Moses effect based on the changes in the pulse, a water tank was fabricated, and the ureteral phantom was manufactured in a structure that could be easily installed in the water tank. Additionally, a spherical artificial stone in the ureteral phantom was prepared by mixing calcined gypsum (Cacinated Gypsum) and water at a ratio of 3:1. In the ureteral phantom, a high-speed camera (FASTCAM NOVA S12, Photron Inc.) and visible light were used to record pulse-dependent image analysis of bubbles and stone retropulsion.

RESULT

After mounting the artificial stone in the ureteral phantom, the pulse duration and energy of the first pulse of the Moses effect laser were varied; 30 laser shots for 3 s at a repetition rate of 10 Hz were applied to quantify the lithotripsy efficiency and stone retropulsion displacement, and the experimental values were compared. The fragmentation efficiency was confirmed by measuring the mass before and after the laser pulse application, the original position of the stone retropulsion displacement, and the distance moved. The minimum value of stone retropulsion displacement appeared when the pulse duration of the first pulse was 300 μs, the pulse energy was 100 mJ, and the value was approximately 0.28 mm. The highest fragmentation efficiency was observed under the same conditions, and the mass loss of the artificial stone at that time was approximately 3.7 mg.

CONCLUSION

Quantitative indices, such as lithotripsy efficiency and stone retropulsion displacement, were confirmed using ultrahigh-speed cameras to determine the effect of the first pulse energy and duration of the Ho: YAG laser with the Moses effect on stone removal. It was confirmed that the longer the duration of the primary pulse and the lower the energy, the higher the fragmentation efficiency. In this study, the possibility of manufacturing a laser with an optimal stone-removal effect was confirmed according to the first-pulse condition of the laser with the Moses effect.

Performance of brushite plaster as kidney stone phantoms for laser lithotripsy

Artificial phantoms used in photothermal near-infrared laser lithotripsy research generally fail to mimic both the chemical and the physical properties of human stones. Though high-energy, 1 J pulses are capable of fracturing hard human stones into several large fragments along natural boundaries, similar behavior has not been observed in commonly used gypsum plasters like BegoStone. We developed a new brushite-based plaster formulation composed of ≈90% brushite that undergoes rapid fracture in the manner of human stones under fragmentation pulse regimes. Single-pulse (1 J) ablation crater volumes for phantoms were not significantly different from those of pure brushite stones. Control over crater volumes was demonstrated by varying phosphorous acid concentration in the plaster formulation. Fragmentation of cylindrical brushite phantoms was filmed using a high-speed camera which demonstrated rapid fragmentation in < 100 µs during the bubble expansion phase of a short pulse from a high-powered Ho:YAG laser (Lumenis Pulse 120 H). The rapid nature of observed fracture suggests increasing laser pulse energy by increasing laser pulse duration will not improve fragmentation performance of laser lithotripters. Brushite plaster phantoms are a superior alternative to gypsum plasters for laser lithotripsy research due to their better mimicry of stone composition, controllable single-pulse crater volumes, and fragmentation behavior.

Comparison of Broad vs Narrow Focal Width Lithotripter Fields

OBJECTIVE

To investigate the impact of lithotripter focal width on stone fragmentation.

MATERIALS AND METHODS

A modified reflector was used to reduce -6 dB beam size of the HM3 lithotripter, while increasing concomitantly peak pressure. Fragmentation in vitro was assessed with modified and original reflectors using BegoStone phantoms. A membrane holder was used to mimic lithotripsy in vivo, and a matrix holder was used to assess variations of fragmentation power in the focal plane of the lithotripter field. Stone fragmentation in vivo produced by the two reflectors was further compared in a swine model.

RESULTS

Stone fragmentation in vitro after 500 (or 2000) shocks was ∼60% (or ∼82%) vs ∼40% (or ∼75%) with original and modified reflector, respectively (p ≤ 0.0016). Fragmentation power with the modified reflector was the highest on the lithotripter axis, but dropped rapidly in the lateral direction and became insignificant at radial distances >6.0 mm. Stone fragmentation with the original reflector was lower along the lithotripter axis, but fragmentation power decayed slowly in lateral direction, with appreciable fragmentation produced at 6.0 mm. Stone fragmentation efficiency in vivo after 500 (or 2000) shocks was ∼70% (or ∼90%) vs ∼45% (or ∼80%) with original and modified reflector, respectively (p ≤ 0.04).

CONCLUSIONS

A lithotripter field with broad beam size yields superior stone comminution when compared with narrow beam size under comparable effective acoustic pulse energy both in vivo and in vitro. These findings may facilitate future improvements in lithotripter design to maximize comminution efficiency while minimizing tissue injury.

In vitro fragmentation efficiency of holmium: yttrium-aluminum-garnet (YAG) laser lithotripsy--a comprehensive study encompassing different frequencies, pulse energies, total power levels and laser fibre diameters

OBJECTIVE

To assess the fragmentation (ablation) efficiency of laser lithotripsy along a wide range of pulse energies, frequencies, power settings and different laser fibres, in particular to compare high- with low-frequency lithotripsy using a dynamic and innovative testing procedure free from any human interaction bias.

MATERIALS AND METHODS

An automated laser fragmentation testing system was developed. The unmoving laser fibres fired at the surface of an artificial stone while the stone was moved past at a constant velocity, thus creating a fissure. The lithotripter settings were 0.2-1.2 J pulse energies, 5-40 Hz frequencies, 4-20 W power levels, and 200 and 550 μm core laser fibres. Fissure width, depth, and volume were analysed and comparisons between laser settings, fibres and ablation rates were made.

RESULTS

Low frequency-high pulse energy (LoFr-HiPE) settings were (up to six times) more ablative than high frequency-low pulse energy (HiFr-LoPE) at the same power levels (P < 0.001), as they produced deeper (P < 0.01) and wider (P < 0.001) fissures. There were linear correlations between pulse energy and fragmentation volume, fissure width, and fissure depth (all P < 0.001). Total power did not correlate with fragmentation measurements. Laser fibre diameter did not affect fragmentation volume (P = 0.81), except at very low pulse energies (0.2 J), where the large fibre was less efficient (P = 0.015).

CONCLUSIONS

At the same total power level, LoFr-HiPE lithotripsy was most efficient. Pulse energy was the key variable that drove fragmentation efficiency. Attention must be paid to prevent the formation of time-consuming bulky debris and adapt the lithotripter settings to one's needs. As fibre diameter did not affect fragmentation efficiency, small fibres are preferable due to better scope irrigation and manoeuvrability.

Optimal shock wave rate for shock wave lithotripsy in urolithiasis treatment: a prospective randomized study

PURPOSE

We aimed to compare the effects of a fast shock wave rate (120 shocks per minute) and a slow shock wave rate (60 shocks per minute) on the shock wave lithotripsy (SWL) success rate, patient's pain tolerance, and complications.

MATERIALS AND METHODS

A total of 165 patients with radiopaque renal pelvis or upper ureter stones were included in the study. Patients were classified by use of a random numbers table. Group I (81 patients) received 60 shock waves per minute and group II (84 patients) received 120 shock waves per minute. For each session, the success rate, pain measurement, and complication rate were recorded.

RESULTS

No statistically significant differences were observed in the patients according to age, sex, body mass index, stone size, side, location, total energy level, or number of shocks. The success rate of the first session was greater in group I than in group II (p=0.002). The visual analogue pain scale was lower in group I than in group II (p=0.001). The total number of sessions to success and the complication rate were significantly lower in group I than in group II (p=0.001).

CONCLUSIONS

The success rate of SWL is dependent on the interval between the shock waves. If the time between the shock waves is short, the rate of lithotripsy success decreases, and the pain measurement score and complications increase. We conclude slow SWL is the optimal shock wave rate.

A simple method for fabricating artificial kidney stones of different physical properties

A simple method for preparing artificial kidney stones with varying physical properties is described. BegoStone was prepared with a powder-to-water ratio ranging from 15:3 to 15:6. The acoustic properties of the phantoms were characterized using an ultrasound transmission technique, from which the corresponding mechanical properties were calculated based on elastic wave theory. The measured parameters for BegoStone phantoms of different water contents are: longitudinal wave speed (3,148-4,159 m/s), transverse wave speed (1,813-2,319 m/s), density (1,563-1,995 kg/m(3)), longitudinal acoustic impedance (4.92-8.30 kg/m(2) s), transverse acoustic impedance (2.83-4.63 kg/m(2) s), Young's modulus (12.9-27.4 GPa), bulk modulus (8.6-20.2 GPa), and shear modulus (5.1-10.7 GPa), which cover the range of corresponding properties reported in natural kidney stones. In addition, diametral compression tests were carried out to determine tensile failure strength of the stone phantoms. BegoStone phantoms with varying water content at preparation have tensile failure strength from 6.9 to 16.3 MPa when tested dry and 3.2 to 7.1 MPa when tested in water-soaked condition. Overall, it is demonstrated that this new BegoStone preparation method can be used to fabricate artificial stones with physical properties matched with those of natural kidney stones of various chemical compositions.

Early Effects of Extracorporeal Shock Wave Treatment on Osteoblast-like Cells: A Comparative Study Between Electromagnetic and Electrohydraulic Devices

BACKGROUND

Extracorporeal shockwave therapy (ESWT) has been increasingly applied to treat orthopedic and musculoskeletal pathologies. ESWT involves mechanical perturbations that, as with other physical therapies, can result in mechanical stimuli to a large number of cells, including bone cells. The aim of this study was to evaluate the effects of shock waves on osteoblast-like cells (MG63) when using two different generators of shock waves (electrohydraulic and electromagnetic devices), in terms of cell damage, cell viability, osteogenic phenotype expression, and cytokine production.

METHODS

MG63 cells were suspended in 1.5 mL screw-cap cryotubes (1 x 10 cells/mL), containing phosphate buffer solution (PBS), which were maintained at 37 degrees C during all the experimental times. Two levels of energy flux density (EFD) were evaluated for each device: 0.15 to 0.18 mJ/mm2 and 0.40 mJ/mm2. Cells were then cultivated for 72 hours starting from a concentration of 1 x 10 cells/mL, and biological activity and viability were evaluated 24 and 72 hours after treatment.

RESULTS

The results obtained demonstrate that the factors most affecting osteoblast activity involve both the device and the level of EFD selected, and they must be considered all together.

CONCLUSIONS

The use of the electromagnetic device and a level of EFD lower than 0.40 mJ/mm2 would appear to induce fewer immediate cytodestructive effects and better stimulate subsequent proliferation and the synthetic activity of MG63.

Single-Center Experience Using Three Shockwave Lithotripters with Different Generator Designs in Management of Urinary Calculi

PURPOSE

We retrospectively reviewed the treatment outcomes of extracorporeal shockwave lithotripsy (SWL) in a single center using either the Wolf Piezolith 2300 (a piezoelectric lithotripter), the Dornier MPL9000 (an electrohydraulic lithotripter), or the Dornier Compact Delta (an electromagnetic lithotripter) from January 1992 to June 2002.

PATIENTS AND METHODS

A series of 3123 (1449 Piezolith 2300, 780 MPL9000, and 894 Compact Delta) solitary radiopaque urinary stones of < or =15 mm receiving primary SWL were identified. "Stone free" was defined as the absence of evidence of stone on plain radiography. Treatment outcomes were assessed by the stone-free rate 3 months after one treatment session, the retreatment rate, the auxiliary procedure rate, the complication rate, and the effectiveness quotient (EQ). In order to have a better assessment of the efficacy of individual lithotripters, multiple logistic regression was performed to control various factors affecting treatment outcomes, including lithotripter-type, patients' sex and age, history of previous SWL, the stone characteristics (side, site, and size), and the presence of a stent or nephrostomy tube.

RESULTS

There were significant differences in the stone site distribution and mean stone size among the three groups. The overall EQ for the Piezolith 2300, MPL9000, and Compact Delta were 0.345, 0.303, and 0.257, respectively. However, using the multiple logistic regression model, the adjusted odds ratio (AOR) of a patient being stone-free after 3 month for the Piezolith 2300 and MPL9000 (using the Compact Delta as the referent category) were 1.38 (95% CI 1.15, 1.65) and 1.72 (95% CI 1.39, 2.11), respectively. Patients treated using the MPL9000 had significantly less re-treatment (AOR = 0.57; 95% CI 0.48, 0.69) than the other groups. No significant difference in the auxiliary procedure rate and complication rate for the three machines was observed.

CONCLUSION

Based on multivariate analysis results, the Dornier MPL9000 had the best treatment outcomes in terms of stone-free rate and re-treatment rate among the three lithotripters.

IN VITRO COMPARISON OF STONE RETROPULSION AND FRAGMENTATION OF THE FREQUENCY DOUBLED, DOUBLE PULSE ND:YAG LASER AND THE HOLMIUM:YAG LASER

PURPOSE

The frequency doubled, double pulse Nd:YAG (FREDDY) laser (World of Medicine, Berlin, Germany) functions through the generation of a plasma bubble. Upon bubble collapse a mechanical shock wave is generated, causing stone fragmentation. This mechanism of action is in contrast to the holmium laser, which cause stone destruction by vaporization. Observed clinical stone retropulsion and fragmentation with the FREDDY and holmium lasers has prompted a series of in vitro experiments designed to compare laser induced retropulsion and fragmentation with those of a holmium laser and pneumatic lithotrite.

MATERIALS AND METHODS

For retropulsion a hands-off underwater laboratory setup, including a horizontally oriented silicone tube 1.3 cm in diameter and a holder to keep the stone phantom in contact with the quartz laser fiber or pneumatic probe, was used. Previously weighed, cylindrical Bego stone phantoms (Bego USA, Smithfield, Rhode Island) were placed in the apparatus. Stone fragmentation was performed with the FREDDY or holmium laser, or the pneumatic lithotripter. The FREDDY and holmium lasers were tested at similar pulse energy and frequency settings. As a standard for comparison, a pneumatic lithotrite was tested with a semirigid probe and single pulse settings of 100, 200 and 300 kPa. Stone phantoms underwent 30 shocks per setting. Mean net retropulsion, defined as the final resting point of the stone, as determined by direct measurement, was recorded for each setting. For fragmentation plaster of Paris stone phantoms of known weights were used to compare the fragmentation ability of each laser. Stones phantoms were placed in a hands-off underwater setup, consisting of an inverted silicon syringe and holder immersed in tap water. The laser fiber (365 microm for the holmium and 280 microm for the FREDDY) was placed through the tip of the syringe in contact with the stone phantom. A total of 24 stones were divided into 4 groups of 6 per group. Two groups were fragmented with the FREDDY laser at 300 and 400 J total energy. The other 2 groups were fragmented using the holmium laser at 300 and 480 J total energy. Fragmentation efficiency was determined as percent weight loss.

RESULTS

For retropulsion at 160 mJ the FREDDY laser caused stone retropulsion to a mean distance of 7.6, 8.1 and 6.8 cm at settings of 5, 10 and 15 Hz, respectively. At 0.8 J the holmium laser retropulsed the stone to a mean distance of 3.3 and 4.9 cm at settings of 5 and 10 Hz, respectively. The pneumatic device caused stone retropulsion a mean distance of 8.5, 9.9 and 13.8 cm at pressure settings of 100, 200 and 300 kPa, respectively. The FREDDY laser generally caused less retropulsion than the pneumatic device, although this difference was only significant at the highest pneumatic lithoclast setting (p <0.05). At clinically relevant settings the FREDDY laser caused significantly more retropulsion than the holmium laser (p <0.05). For fragmentation at total energy settings of 300 and 400 J the FREDDY laser resulted in 44.9% and 86.8% weight loss, respectively (p <0.05). At settings of 300 and 480 J the holmium:YAG laser resulted in 3.3% and 7.1% weight loss, respectively (p <0.05).

CONCLUSIONS

At lower frequency settings stone retropulsion was significantly greater with the FREDDY laser compared with the holmium laser. However, retropulsion was significantly less than that caused by the pneumatic lithotripter at all settings. Therefore, we recommend the use of an occlusive device, such as the Stone Cone (Boston Scientific, Natick, Massachusetts) proximal to the calculus during intracorporeal ureteral lithotripsy and in the ureteropelvic junction during percutaneous laser nephrostolithotomy. In vitro stone fragmentation was significantly greater with the FREDDY laser than with the holmium:YAG laser, suggesting that the FREDDY may offer a low cost alternative to the holmium:YAG laser lithotrite in select patients.

Reduction of tissue injury in shock-wave lithotripsy by using an acoustic diode

An acoustic diode (AD) was constructed of two acoustic transparent membranes with good initial contact to allow the transmission of the positive pressure of lithotripter shock wave at an almost unaltered level, yet attenuate significantly its negative pressure, was fabricated. It was evaluated systematically on a Dornier HM-3 lithotripter to assess its application potential to reduce vascular injury without compromising stone fragmentation efficiency during shock-wave lithotripsy. By inserting the AD, the maximum compressive pressure, maximum tensile pressure and tensile duration of the lithotripter shock wave were formed to drop from 49.7 to 47.8 MPa, -7.5 to -7.0 MPa and 6.0 to 5.1 micros, respectively. Damage of a 0.2-mm inner diameter vessel phantom (cellulose hollow fiber) was reduced from rupture after 31 +/- 11 shocks to no rupture after 100 shocks. Maximum bubble size in free-field, maximum dilation of the vessel phantom wall and bubble collapse time became smaller with the use of the AD. However, stone fragmentation showed similar results without a statistically significant difference between the case with and without the AD. All these evidences suggest that the use of an acoustic diode may be a feasible approach to reduce tissue injury without compromising stone comminution in shock-wave lithotripsy.

Evaluation of Synchronous Twin Pulse Technique for Shock Wave Lithotripsy: Determination of Optimal Parameters for In Vitro Stone Fragmentation

PURPOSE

The Twinheads extracorporeal shock wave lithotriptor (THSWL) is composed of 2 identical shock wave generators and reflectors. One reflector is under the table and the other is over the table with a variable angle between the axes of the 2 reflectors. The 2 reflectors share a common second focal point, making it possible to deliver an almost synchronous twin pulse to the targeted stone. We studied the optimal parameters for in vitro stone fragmentation.

MATERIALS AND METHODS

Two types of 1 cm artificial stones were used, namely Bon(n)-stones of 3 compositions (75% calcium oxalate monohydrate [COM] plus 25% uric acid, struvite and cystine) and plaster of Paris. The parameters tested were shock wave number (100, 500 and 1,000), shock wave power (8, 11 and 14 kV) and angle between the reflector axes (67, 90 and 105 degrees). After the optimal parameters were determined we studied the disintegrative efficacy of THSWL for 3 types of human urinary calculi, including COM, calcium hydrogen phosphate (brushite) and cystine. Each stone received 1,000 twin shock waves at 14 kV with an angle of 90 degrees between the reflectors. All experiments were done using a rate of 60 twin shock waves per minute. Following lithotripsy stone fragments were processed and sized. The ratio of the weight of fragments greater than 2 mm-to-total weight of all fragments was calculated.

RESULTS

Optimal stone fragmentation results for THSWL were obtained with the maximum number of shock waves (1,000) and full power (14 kV). There was no significant statistical difference in fragment size or the ratio of fragments greater than 2 mm with the use of different angles except for cystine and plaster of Paris calculi, for which the right angle was most effective. At application of the optimal parameters to human stones THSWL produced small fragment size for COM and cystine stones, while brushite stones were not fragmented to the same extent.

CONCLUSIONS

The efficacy of synchronous twin pulse technology improves as the number of shock waves and power increase. A 90-degree angle between the shock wave reflectors is advantageous for certain stones (that is cystine and plaster of Paris) but it is not a factor for other stone compositions. THSWL has satisfactory disintegrative efficacy for human stones, especially COM and cysteine calculi.

In vitro analysis of stone fragmentation ability of the FREDDY laser

BACKGROUND AND PURPOSE

The Frequency-Doubled Double-Pulse Nd:Yag) (FREDDY) laser (World of Medicine, Berlin Germany) is a short-pulsed, double-frequency solid-state laser with wavelengths of 532 and 1064 nm. This low-power, low-cost laser was developed for intracorporeal lithotripsy. We designed an experimental set-up to test its fragmentation efficiency at different energy and frequency settings.

MATERIALS AND METHODS

Forty previously weighed plaster-of-Paris stone phantoms were divided into four groups in order to test fragmentation at 5 and 10 Hz for 2 and 4 minutes. A hands-off underwater laboratory set-up including a holder to keep the stone phantom in contact with the quartz laser fiber was utilized. The 280-microm laser fiber was cleaved and stripped between runs to ensure optimal energy delivery. After fragmentation was completed, all of the stone fragments remaining within the holder were allowed to desiccate for 48 hours and reweighed. Fragmentation was measured as the percentage weight loss.

RESULTS

Stone phantoms fragmented at 5 Hz for 2 minutes sustained a mean 24% loss of weight, whereas the 4-minute treatment at 5 Hz reduced stone weight by 54%. Treatment at 10 Hz for 2 minutes demonstrated results similar to those of stones treated for 4 minutes at 5 Hz, reducing stone weight by 51%. Fragmentation at 10 Hz for 4 minutes revealed a 64% loss of mass, less than expected for these power settings. Fiber deterioration observed at the higher energy settings may be the cause of the reduced stone-fragmentation efficiency.

CONCLUSIONS

Fragmentation with the FREDDY laser in the 5 Hz, 4 minutes and 10 Hz, 2 minutes protocols is comparable, suggesting that stone fragmentation correlates well with the total energy delivered to the stone. The slight drop in fragmentation efficiency at 10 Hz, 4 minutes is most likely explained by fiber damage occurring consistently at these higher energy settings. The safety profile and low investment and running costs of this laser are advantages that suggest the laser warrants further clinical trials.

A comparative review of extracorporeal shock wave generation

Shock waves are specific sound waves produced by shock-wave generators; the generators currently available have different physical properties and represent different technical solutions. The measurement of shock-wave pressure is necessary in laboratory settings to define the physical characteristics of a given shock-wave source. Under clinical conditions other variables, e.g. the stone-free rate or the percentage of complications, are used to describe the efficacy and safety of a lithotripter.

The role of stress waves and cavitation in stone comminution in shock wave lithotripsy

Using an experimental system that mimics stone fragmentation in the renal pelvis, we have investigated the role of stress waves and cavitation in stone comminution in shock-wave lithotripsy (SWL). Spherical plaster-of-Paris stone phantoms (D = 10 mm) were exposed to 25, 50, 100, 200, 300 and 500 shocks at the beam focus of a Dornier HM-3 lithotripter operated at 20 kV and a pulse repetition rate of 1 Hz. The stone phantoms were immersed either in degassed water or in castor oil to delineate the contribution of stress waves and cavitation to stone comminution. It was found that, while in degassed water there is a progressive disintegration of the stone phantoms into small pieces, the fragments produced in castor oil are fairly sizable. From 25 to 500 shocks, clinically passable fragments (< 2 mm) produced in degassed water increases from 3% to 66%, whereas, in castor oil, the corresponding values are from 2% to 11%. Similar observations were confirmed using kidney stones with a primary composition of calcium oxalate monohydrate. After 200 shocks, 89% of the fragments of the kidney stones treated in degassed water became passable, but only 22% of the fragments of the kidney stones treated in castor oil were less than 2 mm in size. This apparent size limitation of the stone fragments produced primarily by stress waves (in castor oil) is likely caused by the destructive superposition of the stress waves reverberating inside the fragments, when their sizes are less than half of the compressive wavelength in the stone material. On the other hand, if a stone is only exposed to cavitation bubbles induced in SWL, the resultant fragmentation is much less effective than that produced by the combination of stress waves and cavitation. It is concluded that, although stress wave-induced fracture is important for the initial disintegration of kidney stones, cavitation is necessary to produce fine passable fragments, which are most critical for the success of clinical SWL. Stress waves and cavitation work synergistically, rather than independently, to produce effective and successful disintegration of renal calculi in SWL

Suppression of large intraluminal bubble expansion in shock wave lithotripsy without compromising stone comminution: methodology and in vitro experiments

To reduce the potential of vascular injury without compromising the stone comminution capability of a Dornier HM-3 lithotripter, we have devised a method to suppress intraluminal bubble expansion via in situ pulse superposition. A thin shell ellipsoidal reflector insert was designed and fabricated to fit snugly into the original reflector of an HM-3 lithotripter. The inner surface of the reflector insert shares the same first focus with the original HM-3 reflector, but has its second focus located 5 mm proximal to the generator than that of the HM-3 reflector. With this modification, the original lithotripter shock wave is partitioned into a leading lithotripter pulse (peak positive pressure of 46 MPa and positive pulse duration of 1 micros at 24 kV) and an ensuing second compressive wave of 10 MPa peak pressure and 2 micros pulse duration, separated from each other by about 4 micros. Superposition of the two waves leads to a selective truncation of the trailing tensile component of the lithotripter shock wave, and consequently, a reduction in the maximum bubble expansion up to 41% compared to that produced by the original reflector. The pulse amplitude and -6 dB beam width of the leading lithotripter shock wave from the upgraded reflector at 24 kV are comparable to that produced by the original HM-3 reflector at 20 kV. At the lithotripter focus, while only about 30 shocks are needed to cause a rupture of a blood vessel phantom made of cellulose hollow fiber (i.d.=0.2 mm) using the original HM-3 reflector at 20 kV, no rupture could be produced after 200 shocks using the upgraded reflector at 24 kV. On the other hand, after 100 shocks the upgraded reflector at 24 kV can achieve a stone comminution efficiency of 22%, which is better than the 18% efficiency produced by the original reflector at 20 kV (p = 0.043). All together, it has been shown in vitro that the upgraded reflector can produce satisfactory stone comminution while significantly reducing the potential for vessel rupture in shock wave lithotripsy.

Efficiency and efficacy of different intracorporeal ultrasonic lithotripsy units on a synthetic stone model

PURPOSE

The efficiency and efficacy of the available intracorporeal ultrasonic lithotripters were compared in a stone model experiment.

MATERIALS AND METHODS

Plaster of Paris (POP) stone phantoms having ratios of 1:1, 1.5:1, and 2:1 with water were fabricated into cubes of various hardnesses weighing an average of 24.6 g. The stones were immersed in water in a plastic container, and continuous irrigation through a rigid nephroscope was used. Ultrasonic lithotripters from ACMI, Olympus, Storz, and Wolf manufacturers were evaluated for efficacy in breaking up the three POP concentrations. Time to complete stone fragmentation, occurrence of probe or tubing occlusion, and probe overheating were evaluated.

RESULTS

Efficiency of fragmentation and time to fragmentation of the Storz lithotripter were significantly different from those of the Wolf (p = 0.01 and p = 0.02, respectively) and ACMI (p = 0.001 and p = 0.02, respectively) lithotripters. Comparison of the efficiency of fragmentation and time to fragmentation of the ACMI and Wolf lithotripters showed significant differences (p = 0.005 and p = 0.03, respectively) in favor of the Wolf device. The Olympus lithotriptor resulted in incomplete fragmentation of phantoms of all POP concentrations.

CONCLUSION

The Storz ultrasonic lithotriptor was found to have the lowest fragmentation time and highest efficiency in the fragmentation of phantom stones.

Dynamic photoelastic study of the transient stress field in solids during shock wave lithotripsy

Photoelastic and shadowgraph imaging techniques were used to visualize the propagation and evolution of stress waves, and the resultant transient stress fields in solids during shock wave lithotripsy. In parallel, theoretical analysis of the wavefront evolution inside the solids was performed using a ray-tracing method. Excellent agreement between the theoretical prediction and experimental results was observed. Both the sample size and geometry were found to have a significant influence on the wave evolution and associated stress field produced inside the solid. In particular, characteristic patterns of spalling damage (i.e., transverse and longitudinal crack formation) were observed using plaster-of-Paris cylindrical phantoms of rectangular and circular cross sections. It was found that the leading tensile pulse of the reflected longitudinal wave is responsible for the initiation of microcracks in regions inside the phantom where high tensile stresses are produced. In addition, the transmitted shear wave was found to play a critical role in facilitating the extension and propagation of the microcrack.

Extracorporeal lithotripsy. Update on technology

The development of shock-wave lithotripsy was a serendipitous event. Fortunately, the significance of this accidental discovery was not overlooked by the engineers at Dornier and their medical counterparts. There are many components that make up a lithotripter, but the heart of the lithotripter is its energy source. These machines often are categorized by the type of shock-wave generator used, and each type of generator has its own advantages and disadvantages. Unfortunately, no quantitative value of a shock-wave generator can be correlated to its qualitative effect. Interestingly, each type of energy source delivers its shock-wave energy with such distinctiveness that even the crater pattern it leaves in a stone is unique. New technology and ideas have transformed lithotripters in form and function so that they bear little resemblance to the original HM-1 prototype. Ongoing research is attempting to improve ESWL in several different ways, and advances in shock-wave generation, shock-wave measurement, and stone localization should result in even more efficient lithotripsy. The application of the time-reversal process to lithotripsy ultimately may enable lithotripters to track stones and electronically steer shock waves toward the target. Advances like these herald a time when ESWL, fortunately or unfortunately, will become automated completely.

Improvement of stone fragmentation during shock-wave lithotripsy using a combined EH/PEAA shock-wave generator-in vitro experiments

To control the collapse of cavitation bubbles induced during shock-wave lithotripsy (SWL), a piezoelectric annular array (PEAA) shock-wave generator was fabricated and combined with an experimental electrohydraulic (EH) shock-wave lithotripter with a truncated HM-3 reflector. The PEAA generator consists of eight individual transducers of 200-kHz resonant frequency. At a discharge voltage of 15 kV, the PEAA generator produces a shock wave with a peak positive pressure of 8.2 MPa, a positive half cycle duration of 2.9 micros, and a -6-dB beam width of 5 mm. The trigger of the PEAA generator was controlled via fiberoptic link with reference to the spark discharge of the EH generator. Hence, the PEAA-generated shock wave could be used to interact with cavitation bubbles induced by the EH source at various stages of their oscillation. The duration of bubble oscillation during SWL was monitored by a 2.25-MHz focused hydrophone, and this information was used to control the release timing of the PEAA generator. Stone fragmentation tests in vitro were carried out, and demonstrated that stone comminution could be significantly enhanced when the shock wave-bubble interaction occurred during the collapsing phase of the bubbles. A maximum increment of 60% to 80% in stone fragmentation was achieved when the PEAA-generated shock wave arrives near the collapse of the bubbles. Under these conditions, much intensified collapse of the bubbles near the surface of the stone, with strong secondary shock-wave emission and increased stress concentration at the impact site of the solid boundary, was observed using high-speed shadowgraph and photoelastic imaging.

Does the rate of extracorporeal shock wave delivery affect stone fragmentation?

OBJECTIVES

To evaluate the effect of the rate of shock wave delivery on stone fragmentation, because the optimal rate of shock wave administration has not yet been established.

METHODS

Standard phantom, ball-shaped, ceramic stones were placed in a net-like basket with a hole size of 2.2 mm and immersed in a specially designed water bath coupled with the Econolith 2000 lithotripter. One hundred eighteen stones (mean diameter 9.5 mm) were used. Shock waves were delivered at rates of 30, 60, 90, 120, and 150 shocks/min and at intensities of 15, 20, and 22.5 kV (electrohydraulic). The number of shocks required for complete fragmentation, determined by all fragmented particles falling through the basket holes, was recorded.

RESULTS

The most effective (fewer shocks needed for complete stone fragmentation) rate of shock wave delivery was 60 shocks/min. A statistically significant difference was demonstrated between the mean number of shocks required for complete stone fragmentation at the rate of 60 shocks/min and faster rates at all energy levels (P <0.01) but not between the rate of 60 shocks/min and the rate of 30 shocks/min at all energy levels.

CONCLUSIONS

The rate of shock wave administration during extracorporeal shock wave lithotripsy seems to influence stone disintegration. We demonstrated that extracorporeal shock wave lithotripsy is most effective when waves are delivered at 60 shocks/min.

Impact of holmium laser settings and fiber diameter on stone fragmentation and endoscope deflection

We compared the impact of various energy settings, frequency, and fiber diameters on the stone fragmentation capabilities of the holmium laser. Stone phantoms, made from plaster of Paris and uniform in weight, were treated with one of two laser fiber sizes: small (200 and 365 microm) and large (550 and 1000 microm). Stones were immersed in water and fragmented for 3 minutes at 0.5, 1.0, or 2.0 J and 5, 10, or 15 Hz. The mean percentage decrease in weight in the two groups was compared using one-way ANOVA. The effect on flexible ureterorenoscope deflection of the small fibers was tested in two different ureterorenoscopes. Raising the energy level when using the small fibers resulted in more weight loss (P < 0.05). Increasing the frequency up to 10 Hz also resulted in a significant increase in weight loss (P < 0.05), yet above 10 Hz, there was no significant additional weight loss noted for either small fiber. There was no significant difference in the weight loss produced by the two fibers unless the energy setting was >1.0 J. Studies with the large fibers demonstrated similar results, with significant increments of weight loss occurring with increased energy (P < 0.05), while nonsignificant differences were seen for the two fiber diameters. Increasing laser frequency up to 15 Hz resulted in a significant increase in weight loss for the large fibers. Loss of ureterorenoscope deflection ranged from 7% to 16% and 18% to 37% for the 200-microm and 365-microm fibers, respectively. Small-diameter fibers, in combination with semirigid or flexible ureteroscopes, should be used to treat upper urinary tract stones. The 365-microm fiber should be utilized for the management of ureteral stones, as minimal endoscopic deflection is required to access these calculi. Because the 200-microm fibers are considerably more expensive, their use should be reserved for fragmentation of intrarenal calculi, where maximum deflection is required during flexible ureterorenoscopy. The ideal energy and frequency settings for the small fibers are <1.0 J and 5 to 10 Hz. Larger fibers can be used for managing bladder or renal calculi, as there is no need for significant fiber deflection. The 550-microm fiber is preferred, as it is comparable in efficacy to the 1000-microm fiber and is less expensive. Energy and frequency can be maximized to 2.0 J and 15 Hz without damage to the fiber, but visibility can be affected by high frequencies. Appropriate fiber selection and energy/frequency settings will allow access to most stones throughout the urinary tract, maximize fiber life, and minimize fiber expense.

Holmium:yttrium-aluminum-garnet lithotripsy efficiency varies with stone composition

OBJECTIVES

To test the hypothesis that holmium:yttrium-aluminum-garnet (YAG) lithotripsy efficiency varies with stone composition.

METHODS

Single pulses of holmium:YAG energy were delivered using 272-, 365-, 550-, and 940-microm optical fibers to human calculi composed of calcium oxalate monohydrate (COM), calcium hydrogen phosphate dihydrate (CHPD), cystine, magnesium ammonium phosphate hexohydrate (MAPH), and uric acid. Energy/pulse settings were 0.2, 0.5, 1.0, and 1.5 J. Stone crater width and depth were characterized with reflectance light microscopy.

RESULTS

For similar energies overall MAPH yielded the deepest and widest craters. CHPD, cystine, and uric acid yielded craters of intermediate width and depth. COM yielded the smallest craters. Within any given composition, increased pulse energy yielded craters of increased width and depth.

CONCLUSIONS

Holmium:YAG lithotripsy efficiency varies with stone composition. The rank order of crater size appears to correlate with thermal threshold for each composition. Increased holmium:YAG energy produces larger craters for all compositions.

Laboratory and clinical assessment of pneumatically driven intracorporeal lithotripsy

A pneumatically driven intracorporeal lithotripter (the Swiss Lithoclast) has recently been approved for use in the United States. We compared its performance in vitro with ultrasonic, electrohydraulic and laser lithotripsy devices using a standard plaster-of-Paris stone phantom. The probe sizes and output settings were identical to those used during clinical treatment. The fragmentation efficiency index (measured as the lithotripsy time needed to reduce the stone phantom to particles <2 mm divided by the initial stone weight) ranged from 5.0 to 8.5 min/g of stone mass, with this value increasing from pneumatic to electrohydraulic to laser and to ultrasonic lithotripsy. We also performed an objective study in a swine model, which showed no adverse consequence of pneumatic lithotripsy. Finally, we evaluated our initial 41 patients who had undergone pneumatic stone fragmentation. We treated 8 patients having 11 renal calculi, 30 patients having 37 ureteral calculi, and 3 patients having 6 bladder calculi employing pneumatic probes ranging in size from 0.8 to 2.0 mm. Stone fragmentation was successful in a single session in 95% of the ureteral calculi and 100% of both renal and bladder calculi. Stone analysis in 23 patients revealed 17 (74%) calcium oxalate monohydrate and 1 (4%) cystine calculi. Our clinical and laboratory assessment of this newly developed pneumatic lithotripsy device further validates its efficacy in fragmenting stone of all compositions and its overall safety associated with clinical application.

Controlled, forced collapse of cavitation bubbles for improved stone fragmentation during shock wave lithotripsy

The feasibility of using controlled, forced collapse of cavitation bubbles for improved stone fragmentation during shock wave lithotripsy was demonstrated using microsecond tandem shockwave pulses. High-speed photography revealed that a secondary shock wave, released in less than 500 microseconds (microsec.) following a lithotripter-generated shock wave, can be used to control and force the collapse of cavitation bubbles toward target concretions. This timely enforced shockwave-bubble interaction was found to greatly enhance the cavitational activity near the stone surface, with a resultant up to 43% increment in stone fragmentation. Since most of the cavitation energy is directed and concentrated toward the target stones and fewer shock waves are needed for successful stone comminution, tissue injury associated with this new lithotripsy procedure may also be reduced. This novel concept of shock wave lithotripsy may be used to improve the treatment efficiency and safety of existing clinical lithotripters, as well as in the design of new shock wave lithotripters.

High-energy v low-energy shockwave lithotripsy in treatment of ureteral calculi

The size of the crater formed in a urinary calculus subjected to shockwave lithotripsy (SWL) is directly proportional to the energy delivered to the stone surface. This study compared the effect of high and low energy levels on the outcomes of ureteral SWL. Ureteral calculi (N = 336) were treated with the conventional low-energy Siemens Lithostar and 62 with the higher-energy (1.02 v 0.5 mJ/mm2) modification of the Lithostar, the Siemens Shock Tube C. Stone locations included all regions of the ureter. The average stone treated with the standard Lithostar measured 8.1 mm in diameter and received 5461 shockwaves (treatment time 45 minutes) at 17.2 kV (range 14.5-19.0 kV). The stone-free rate was 72%, with stents being used in 16% of patients and a retreatment rate of 9%. The typical stone treated with Shock Tube C was 10.4 mm in diameter and received 3528 shockwaves (treatment time 30 minutes) at an average energy setting of 4.1 (range 1.5-8.0). The stone-free rate was 75%, with stents being used in 9.8% of cases, and a retreatment rate of only 1.6% (P < 0.003). In this study, Shock Tube C yielded stone-free rates equivalent to those of the conventional machine. However, the number of shockwaves, treatment time, and retreatment rate were significantly lower with the new shock tube. High-energy lithotripsy is more efficient than low-energy treatment of ureteral calculi.

Acoustic and mechanical properties of renal calculi: implications in shock wave lithotripsy

The acoustic and mechanical properties of renal calculi dictate how a stone interacts with the mechanical forces produced by shock wave lithotripsy; thus, these properties are directly related to the success of the treatment. Using an ultrasound pulse transmission technique, we measured both longitudinal and transverse (or shear) wave propagation speeds in nine groups of renal calculi with different chemical compositions. We also measured stone density using a pycnometer based on Archimedes' principle. From these measurements, we calculated wave impedance and dynamic mechanical properties of the renal stones. Calcium oxalate monohydrate and cystine stones had higher longitudinal and transverse wave speeds, wave impedances, and dynamic moduli (bulk modulus, Young's modulus, and shear modulus), suggesting that these stones are more difficult to fragment. Phosphate stones (carbonate apatite and magnesium ammonium phosphate hydrogen) were found to have lower values of these properties, suggesting they are more amenable to shock wave fragmentation. These data provide a physical explanation for the significant differences in stone fragility observed clinically.