Stone Fragmentation Efficiency Research Articles

Optimization of Treatment Strategy Used During Shockwave Lithotripsy to Maximize Stone Fragmentation Efficiency

Previous studies have demonstrated that treatment strategy plays a critical role in ensuring maximum stone fragmentation during shockwave lithotripsy (SWL). We aimed to develop an optimal treatment strategy in SWL to produce maximum stone fragmentation. Four treatment strategies were evaluated using an in-vitro experimental setup that mimics stone fragmentation in the renal pelvis. Spherical stone phantoms were exposed to 2100 shocks using the Siemens Modularis (electromagnetic) lithotripter. The treatment strategies included increasing output voltage with 100 shocks at 12.3 kV, 400 shocks at 14.8 kV, and 1600 shocks at 15.8 kV, and decreasing output voltage with 1600 shocks at 15.8 kV, 400 shocks at 14.8 kV, and 100 shocks at 12.3 kV. Both increasing and decreasing voltages models were run at a pulse repetition frequency (PRF) of 1 and 2 Hz. Fragmentation efficiency was determined using a sequential sieving method to isolate fragments less than 2 mm. A fiberoptic probe hydrophone was used to characterize the pressure waveforms at different output voltage and frequency settings. In addition, a high-speed camera was used to assess cavitation activity in the lithotripter field that was produced by different treatment strategies. The increasing output voltage strategy at 1 Hz PRF produced the best stone fragmentation efficiency. This result was significantly better than the decreasing voltage strategy at 1 Hz PFR (85.8% vs 80.8%, P=0.017) and over the same strategy at 2 Hz PRF (85.8% vs 79.59%, P=0.0078). A pretreatment dose of 100 low-voltage output shockwaves (SWs) at 60 SWs/min before increasing to a higher voltage output produces the best overall stone fragmentation in vitro. These findings could lead to increased fragmentation efficiency in vivo and higher success rates clinically.

Effect of Pulse Energy, Frequency and Length on Holmium:Yttrium-Aluminum-Garnet Laser Fragmentation Efficiency in Non-Floating Artificial Urinary Calculi

Holmium:yttrium-aluminum-garnet (Ho:YAG) laser lithotripsy is the standard lithotrite in ureteroscopy. We investigated the influence of pulse frequency, energy and length on the fragmentation efficiency of Ho:YAG laser lithotripsy in non-floating artificial stones in vitro. Stone fragmentation efficiency of three different Ho:YAG laser devices were evaluated in vitro at different pulse energy (1.0 and 2.0 J) and frequency settings (5 and 10 Hz), resulting in a standardized output power of 10W, respectively. Where possible, pulse length was modified (350 vs 700 microsec). Each setting was performed with a 273 microm and a 365 microm fiber. Lithotripsy was conducted using non-repulsive stones consisting of soft stone (plaster of Paris) and hard stone composition (Fujirock type 4). Our results showed an increased stone disintegration efficiency at higher pulse energy (2.0 J/5 Hz vs 1.0 J/10 Hz) independently of two fiber diameters and stone types applied in this study (P < 0.05 in 18 of 20 groups). Similarly, reduction of the pulse length from 700 to 350 microsec resulted in a higher stone disintegration (P < 0.05 in 13 of 16 groups). This effect was most prominent when applied to soft stones. Higher fiber diameter was not constantly associated with an increase in stone disintegration. We demonstrate that an increase of pulse energy and a reduction of pulse length at a standardized output power of 10W can improve Ho:YAG laser fragmentation efficiency in vitro in nonfloating stones. These results may potentially affect clinical practice of Ho:YAG laser lithotripsy in impacted or large stones, when retropulsion is excluded.

Treatment Time Reduction Using Tandem Shockwaves for Lithotripsy: AnIn VivoStudy

Reducing extracorporeal shockwave lithotripsy (SWL) time by increasing the shockwave rate of the lithotripter has been tested in the past; however, basic research and treatment outcomes revealed that this is not convenient. The purpose of this study was to use an animal model to demonstrate that SWL treatment time can be reduced significantly without sacrificing stone fragmentation efficiency using tandem shockwaves. A tandem research lithotripter was used to treat 50 artificial kidney stones implanted into the kidneys of 50 rabbits. Standard single-pulse and tandem shockwaves were compared in two different scenarios: Without a fluid-filled expansion chamber and with a fluid-filled expansion chamber surrounding the stone. The presence of fluid surrounding the stone enhances fragmentation in both the standard and tandem modes. No significant difference in fragmentation efficiency was recorded between the standard and tandem SWL with stones surrounded by fluid; however, the treatment time with tandem shockwaves was reduced by 50%. Significantly shorter SWL treatments may be possible in the future using tandem shockwaves on urinary stones that are surrounded by fluid.

Maximizing Stone Fragmentation Efficiency With Ultrasonic Probes: Impact of Probe Pressure and Rotation

We examined the effects of probe rotation and pressure on stone fragmentation in an in vitro percutaneous nephrolithotomy model. The study was a fully randomized, factorial experiment with 20 repeat trials performed at each combination of variables, yielding a total of 300 trials per device for ultrasonic tests and 360 for ultrasonic/pneumatic combination tests. Varying masses were placed on the hand piece of each device to create a probe contact pressure of 400, 1,000 or 2,000 gm. The impact of rotation was tested only at 0 or 90 degrees and rotating only at a frequency of 2 Hz. Statistical analysis was performed using R, version 2.6.2. For the Cyberwand the Tukey HSD test showed that 400 and 1,000 gm probe pressure were significantly more effective than 2,000 gm pressure (p <0.05). The range and frequency of rotation were not statistically significant variables affecting Cyberwand efficiency. For the LithoClast Ultra using only the ultrasonic probe significant differences were found among the 3 pressure levels (400, 1,000 and 2,000 gm, respectively, p <0.05). For rotation 90 degrees were significantly more effective than 0 degrees (p <0.05) at a mean +/- SD stone mass of 0.168 +/- 0.078 and 0.107 +/- 0.09 gm, respectively. For the LithoClast Ultra ultrasonic/pneumatic combination 1,000 gm pressure were significantly more effective than 400 or 2,000 gm (p <0.05). The 6 and 12 Hz pneumatic frequencies outperformed 3 Hz but were not significantly different from each other (p <0.05). Differences in probe manipulation impact stone fragmentation efficiency and procedural success.

Renal injury during shock wave lithotripsy is significantly reduced by slowing the rate of shock wave delivery

To assess the tissue protection afforded by simply reducing the rate of shock wave (SW) delivery, compared with studies in the pig in which SW lithotripsy (SWL)-induced vascular damage was significantly reduced by initiating treatment using low-amplitude SWs. Juvenile pigs (6-7 weeks old) were treated with an unmodified lithotripter (HM3, Dornier Medical Systems, Kennesaw, GA) at either 120 or 30 SW/min. Treatment was to one kidney per pig, with SWs (2000, 24 kV) directed to a lower-pole calyx. After treatment, parenchymal haemorrhage was determined morphometrically and expressed as percentage of functional renal volume (%FRV). Kidneys treated at 120 SW/min had focal to extensive subcapsular haematomas. Parenchymal lesions were found only at the lower pole, but included regions within renal papillae and the cortex. Occasionally, damage extended across the full thickness of the kidney. The lesion in the pigs treated at 120 SW/min occupied a mean (sd) of 4.6 (1.7) %FRV. Kidneys of pigs treated at 30 SW/min showed no surface bleeding. Parenchymal haemorrhage was limited to papillae within the focal volume, and measured 0.08 (0.02) %FRV, a significant (P < 0.005) reduction in injury. Slowing the rate of delivery to 30 SW/min has a dramatic protective effect on the integrity of the kidney vasculature. This finding in our established pig model suggests a potential strategy to improve the safety of lithotripsy. As it was shown that a reduced SW rate also improves the efficiency of stone fragmentation, a slow rate appears to be a means to improve both the safety and efficacy of SWL.

In Vitro Assessment of a Novel Dual Probe Ultrasonic Intracorporeal Lithotriptor

The Cyberwand (Cybersonics, Erie, Pennsylvania) is a novel intracorporeal lithotrite that uses coaxial ultrasonic elements operating at 2 frequencies. We compared this device to the LithoClast(R) Ultra, which we previously noted is the most efficient commercially available intracorporeal lithotripsy device. An in vitro test system was used to assess the efficiency of stone penetration for the Cyberwand and the LithoClast Ultra. The devices were mounted upright with the probe tip up in a modified irrigation sheath. A gypsum artificial stone was centered on the probe tip and a mass was placed atop the stone to provide a constant force. The manufacturer recommended setting was selected for the Cyberwand, while for the LithoClast Ultra a pneumatic frequency of 12 Hz with an ultrasonic power setting of 100% was selected, representing optimal settings in previous in vitro tests. The time required for complete stone penetration was measured. Differences in mean stone penetration times were compared using ANOVA. Mean +/- SD penetration time for the Cyberwand was significantly shorter than for the LithoClast Ultra (4.8 +/- 0.6 vs 8.1 +/- 0.6 seconds, p <0.0001). Neither device showed any difficulties with overheating, occlusion or another malfunction. Initial assessment using our hands-free in vitro test system, in which stone penetration time is not affected by operator bias, suggests that the Cyberwand is an efficient lithotrite. These promising results justify in vivo testing.

Influence of focusing and signal shape on the efficiency of stone fragmentation in vitro under equal energy conditions

To investigate the mechanisms of fragmentation in lithotripsy, simulations and experiments were performed. Wave shapes deduced from measurements near the focus of a piezoelectric source were used as input signals for finite element calculations of stresses and strains in spherical stones of different sizes. The effects of strongly and weakly focused sound fields were compared to those of a plane wave assumption representing lithotripters with large focal widths. The energies of the fields were matched. In targets with diameters less than 6 mm the simulated tensile stresses induced by strong and weak focusing exceed those of the plane wave assumption. For in vitro experiments a double‐layered self‐focusing piezoelectric source was used. To represent lithotripters with different sound fields the signal shape was varied by changing the interpulse delay but maintaining the energy. The total number of pulses needed for fragmentation into particles smaller than 2 mm diameter was counted. At usually applied energies experimental results show that the numbers of pulses needed for complete fragmentation are almost independent of the signal shape. In conclusion, the results indicate that strongly focused lithotripters are in principle more effective than those with wide focal widths in breakage of small stones.

DESIGN AND FABRICATION OF A SHOCK WAVE GENERATOR FOR MUSCULOSKELETAL DISORDERS

In the past ten years, extracorporeal shock waves have been successfully used in orthopedics. The idea of shock wave therapy is the stimulation of a healing process. Electrohydraulic shock wave therapy provides higher energy density flux than electromagnetic and piezoelectric types. But electrohydraulic shock wave generators are less stable than the other two types because of electrode erosion. In this study, a shock wave generator with a controllable spark gap system has been designed in order to give steady output pressures by automatic adjustment of the electrode gap. An ellipsoidal shock wave reflector is equipped with two AC servo motors with drivers. The motor driver actuates the associated motor which is connected to an electrode base by a belt. On a designed image feedback system, a CCD camera is used as the image detection tool to measure the electrode gap. Experimental results show the coincidence of the second focus of the shock wave reflector with the gas-dynamic focus. Moreover, measurements of focused pressure and energy intensity with PCB and PVDF pressure sensors and tests of stone fragmentation efficiency have been carried out to evaluate the performance of the newly designed shock wave generator with a gap-adjusted system and an image feedback system. It is found that the designed electrohydraulic shock wave generator is stable and efficient in pressure output with a low cost of electrodes.

Development of a system of automatic gap-adjusted electrodes for shock wave generators

In this study, a system of automatic gap-adjusted electrodes is developed for electrohydraulic shock wave generators that can be used both for extracorporeal shock wave lithotripsy (treatment of renal calculi) and for the extracorporeal shock wave therapy for musculo-skeletal disorders. This system is composed of three main components: (1) two electrodes and their bases; (2) servo motors and control software; (3) a high sensitivity CCD camera and image processing program. To verify system performance, in vitro fragmentation tests have been conducted using kidney stone phantoms. Results indicate that the efficiency of stone fragmentation using automatic gap adjustment can be increased up to 55.2%, which is twice more than without automatic gap adjustment (26.7%). This system can be applied to any commercial electrohydraulic extracorporeal shock wave lithotriptor or orthotriptor without difficulty.

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 μs, 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. (E-mail: slzhu@duke.edu)

Stone Fragmentation During Shock Wave Lithotripsy is Improved by Slowing the Shock Wave Rate: Studies With a New Animal Model

The current trend toward ungated shock wave lithotripsy means that more patients are being treated with shock waves delivered at a rapid rate (120 shock waves per minute or greater). However, no benefit of an increased shock wave rate has been shown and in vitro studies indicate that slowing the shock wave rate actually improves stone fragmentation. We tested the effect of the shock wave rate on stone comminution in a new animal model. Gypsum model stones were inserted via upper pole percutaneous access into the lower pole calix of the kidneys of female pigs weighing approximately 100 pounds. Shock wave lithotripsy was performed (400 shock waves uninterrupted at 20 kV. and 30 or 120 shock waves per minute) 2 hours later using an unmodified HM3 lithotriptor (Dornier Medical Systems, Marietta, Georgia). After en bloc excision of the urinary tract stone fragments were collected and sieved through 2 mm. mesh. The particles were weighed and surface area was determined. Stones treated at 30 shock waves per minute broke more completely than stones treated at 120 shock waves per minute. The percent of fragments greater than 2 mm. was significantly higher for stones treated at the fast rate of 120 versus the slow rate of 30 shock waves per minute (mean +/- SEM 81% +/- 14% versus 45% +/- 12%, p <0.005). When stone fragmentation was expressed as the percent increase in fragment surface area, significantly greater fragmentation occurred at the slower than at the more rapid rate (327% +/- 63% versus 135% +/- 136%, p <0.02). Slowing the rate of shock wave administration during shock wave lithotripsy significantly improves the efficiency of stone fragmentation in vivo.

In vitro comparison of standard ultrasound and pneumatic lithotrites with a new combination intracorporeal lithotripsy device

Objectives. A new combination intracorporeal lithotripter (Lithoclast Ultra) has been developed that incorporates the beneficial effects of pneumatic lithotripsy (rapid stone fragmentation) and ultrasound lithotripsy (rapid fragment removal). An in vitro study was performed to assess the efficiency of stone fragmentation and clearance of this new combination intracorporeal lithotripter compared with currently available ultrasound and pneumatic units. Methods. Pneumatic and ultrasound lithotrites, along with the combination pneumatic/ultrasound unit, were used through a rigid 27F nephroscope to fragment and remove phantom stones made of BegoForm. The mean fragment removal times and stone fragment sizes for the standard ultrasound and pneumatic devices were compared with the combination unit to determine the completeness and efficiency of stone fragmentation and removal. Results. The average time for stone clearance using the pneumatic and ultrasound devices was 23.8 and 12.9 minutes, respectively. The combination pneumatic/ultrasound unit was significantly more efficient, requiring only 7.4 minutes to completely fragment and clear all stone material ( P <0.002). In addition, the average size of the 15 largest fragments removed was significantly less with the combination device than with the pneumatic and ultrasound lithotrites (1.67 mm versus 9.07 mm and 3.67 mm, respectively, P <0.00001). Conclusions. The combination of pneumatic and ultrasound capabilities in a newly developed lithotrite exhibited a significantly enhanced ability to fragment and clear phantom stones compared with standard ultrasound or pneumatic devices alone. These preliminary studies suggest that this combination pneumatic/ultrasound lithotripter may be an ideal device for the expeditious removal of large-volume renal or bladder calculi. Additional studies are warranted to better assess the capabilities of this new device in treating human stones of various compositions and its safety, as well as the optimal power and frequency settings.

Effect of SW rate on stone fragmentation in vivo

The trend toward ungated SWL means that SW’s are often delivered at a fast rate (≥2 Hz). This is a concern, since rapid SW delivery causes increased renal injury in experimental animals. Also, in vitro studies show that slowing the SW rate (&amp;lt;2 Hz) improves stone fragmentation. In the current study, the effect of SW rate on stone comminution was tested in an in vivo model. Gypsum stones were inserted into both kidneys of 100 lb pigs via percutaneous access. SWL was performed (Dornier HM3, 400 SW’s uninterrupted, 20 kV, 2 Hz or 0.5 Hz), and stone fragments were collected and sieved through 2 mm mesh. The percent of stone weight lost (particles &amp;lt;2 mm), was greater in stones shocked at the slower rate, 55.1±4.9&amp;lt;th&amp;gt;% (n=6), compared to the faster rate, 19.4±5.6&amp;lt;th&amp;gt;% (n=6) [p&amp;lt;0.0008]. These data indicate that slowing the SW rate improves the efficiency of stone fragmentation in vivo. It is observed in vitro that bubbles along the SW axis are more numerous with faster SW rate; this could impose a barrier to transmission of SW energy into the stone. Localization of the site of rate-dependent bubble activity could provide clues to help improve SWL. [Work supported by NIH Grant P01 DK43881.]

Shockwave frequency affects fragmentation in a kidney stone model.

To study the effect of altering shockwave frequency on the efficiency of stone fragmentation using the MFL 5000 spark-gap lithotripter. Standardized solid plaster stones, 12.0 +/- 0.5 mm in diameter, were fragmented at an energy setting of 20 kV. The shockwave frequencies tested were 60, 80, and 117 per minute. Stones were fragmented throughout the entire lifespan of the electrode, from 0 to >100% consumption, at each frequency tested. Electrode pressure output was studied for each frequency. A greater number of shocks was required to fragment the plaster balls at higher frequencies (regression coefficient 1.93; p < 0.003). An inverse relation was found between the number of shocks necessary to break the stones and electrode consumption (regression coefficient -2.16; p < 0.001). The analysis of delivered pressure from the electrode failed to demonstrate a linear relation with frequency (regression coefficient -0.40; p < 0.728) or consumption (regression coefficient -1.11; p < 0.158). The number of shocks required to fragment a stone is influenced in part by the frequency at which the shockwaves are delivered. Increasing the shockwave frequency from 60 to 117 per minute in this study caused a significant rise in the number of shocks required to break the stone. The pressure output of the electrode was similar at the frequencies tested, thus making the difference in stone fragmentation secondary to the mechanism of stone disintegration and not the function of the electrode.

High-speed photographic evaluation of holmium laser.

Little information has been published regarding the relative contributions and exact mechanisms involved in stone fragmentation during holmium laser lithotripsy. High-speed photography has been used to evaluate other intracorporeal lithotripsy devices, and we chose this method to evaluate the holmium laser. High-speed photographic evaluation of cavitation bubbles produced by the four fiber sizes (200, 365, 550, and 1000 microm) showed a moderate degree of correlation (n = 0.71 for 1 J pulses, r = 0.80 for 4 J pulses) between maximal bubble size and fiber diameter. Eliminating the data from the eroded 550 microm fiber strengthened the correlation to r = 0.94 for 1 J pulses and r = 0.99 for 4 J pulses. The importance of keeping fiber tips in good working condition was thereby demonstrated. Evidence of a thermal effect of the laser on stone and chalk was also obtained. In a parallel study, no correlation (r = -0.08) was found between fiber diameter and stone fragmentation efficiency using chalk as an in vitro stone model.

Mechanical impactor employing Nitinol probes to fragment human calculi: fragmentation efficiency with flexible endoscope deflection.

The Browne Pneumatic Impactor (BPI; Browne Medical Systems, Minneapolis, MN) is a new mechanical lithotripter that relies on a pneumatically driven metal probe. It can be employed through both rigid and actively deflectable flexible endoscopes. Prior in vivo studies have shown excellent fragmentation of hard human calculi while employing the BPI with active endoscope deflection of as much as 45 degrees. In this study, we set out to define the efficiency of stone fragmentation with active endoscope deflection and to quantify the extent of retrograde stone migration that is often noted when mechanical lithotripters are employed. Pressure transducer testing of the BPI revealed an inverse relation between increased deflection of the endoscope and transduced voltage. Calculus fragmentation tests showed that as the endoscope was actively deflected to 90 degrees, the BPI still was able to fragment human calcium oxalate monohydrate calculi into extractable fragments with as few as six pulses. The maximum active endoscope deflection was 95 degrees with the 0.020-inch Nitinol probe passed through the working channel. At this deflection, the BPI was able to fragment an 8 mm pure calcium oxalate monohydrate calculus into two fragments after 14 pulses. Retrograde migration was evaluated by employing the BPI in a simulated ureter in a waterbath. The travel distance of a 5-mm 5-g calculus with one pulse averaged 12.2 mm (range 1-44 mm). Smaller, less dense calculi travelled farther. A rebound effect was occasionally noted, and this was associated with an off-center delivery. In conclusion, the BPI is an efficient mechanical lithotrite able to fragment hard human calculi when employed with active endoscope deflection of as much as 95 degrees. Delivered energy decreased with active endoscope deflection, but this change did not prevent fragmentation. Retrograde migration was an important variable more notable with smaller calculi.

Conversion of the Electrohydraulic Electrode to an Electromechanical Stone Impactor: Basic Studies and a Case Report

A 3.3F electrohydraulic electrode (Wolf 2137.23) has been confined within a spring with a metal end cap, irrigated with water and covered with a 0.003-inch metal sheath (outside diameter 5F). The electrohydraulic lithotripsy discharge (Wolf Generator 2137.50) at El causes the metal cap to extend 3mm. at 1,500cm. per second and creates an impact pressure of 600 to 800 bar. Stone fragmentation efficiency of the electromechanical impactor was equivalent to unshielded electrohydraulic lithotripsy (gallstone 2.83mg. per pulse, struvite/apatite 1.41mg. per pulse, cystine 0.41mg. per pulse, uric acid 1.48mg. per pulse and 100% calcium oxalate monohydrate 0.10mg. per pulse).Studies of the discharge of the electromechanical impactor within the pig ureter showed that minimal ureteral submucosal edema and hemorrhage occurred at 300 shocks discharged at a single point, and disruption of the mucosa and partial injury to the muscle layer occurred after 600 shocks given at the site of a pinched pig ureter. Pushing the electromechanical impactor perpendicular to the wall of the pig bladder will create a mechanical perforation within 35 shocks (electrohydraulic lithotripsy within 2 shocks).One patient had excellent fragmentation of a lower ureteral mixed monohydrate and dihydrate stone under direct vision performed with the electromechanical impactor passed via a 9.5F uretero-scope. There was no evidence of mucosal injury with 500 shocks.The electromechanical impactor has been developed to provide a safe and inexpensive method of ureteral stone fragmentation or disimpaction. These studies were performed to establish limits of safety that may allow use of the electromechanical impactor for stone fragmentation in the ureter without the need for ureteroscopy.