Stone Breakage Research Articles

Input-output analysis of fluid–structure interaction to optimize stone breakage in burst-wave lithotripsy

Burst-wave lithotripsy is a non-invasive treatment of kidney stones using short pulses of focused ultrasound. In this talk, we present the use of input-output analysis to optimize the design and control of a multi-element array transducer to achieve optimal stone breakage. The analysis is based on a linear fluid-structure coupled system that maps the acoustic forcing from individual elements to the stress in a stone of assumed shape, size, and composition. We determine the optimal frequency and optimal distribution of the phase and amplitude across the aperture by maximizing a cost function that represents the strain energy in stone. The optimal parameters are obtained by applying a state-of-the-art, randomized singular value decomposition (SVD) to the discrete linear operator. The results show that carefully controlling the relative phase and amplitude between elements can increase the strain energy (by 2-3 times in certain cases) compared to a uniform distribution under the same input energy. This suggests that the stone fragmentation can be accelerated or performed with lower energy for safer treatment. The improvement is interpreted and validated by combining high-fidelity simulations with high-speed camera images of crack formation in model stones from in vitro- experiments. [Work supported by NIH, Grant No. P01-DK043881.]

Diminishing comminution of urinary stones with subsequent laser pulses

Ureteroscopic lithotripsy with Holmium:YAG lasers is a common treatment for urinary stones but the physical mechanisms of stone comminution are still under investigation. The dominant mechanisms were reported to be associated with the direct action of light on the stone surface causing the photothermal decomposition of stones and explosive vaporization of interstitial water. The objective of this study was to better understand stone fragmentation by the direct action of light and other potential mechanisms (cavitation) using ultra-high-speed video microscopy. Stone fragmentation was quantitatively assessed using an Open-source library of Computer Vision algorithms (OpenCV) with programs written in Python. Stone fragments were identified by detecting difference between the images and tracked with identification numbers considering cross-section areas and velocities of the fragments. Images of stone comminution were collected using a Shimadzu HPV-X2 camera at 200 000–500 000 fps with dry stones in air and those hydrated in water. Stone fragmentation by the direct action of light was observed to diminish with subsequent laser pulses. The diminishing fragmentation, however, was offset by the action of cavitation bubbles that continued to erode and fragment stone surface, augmenting stone breakage with subsequent laser pulses. [Work supported by the NIDDK of the NIH under Award R43DK129104.]

V05-12 DIMINISHING FRAGMENTATION OF STONES WITH SUBSEQUENT PULSES IN LASER LITHOTRIPSY

INTRODUCTION AND OBJECTIVE:Ureteroscopic laser lithotripsy is a common treatment of urinary stones but the physical mechanisms of stone breakage are still discussed. The dominant mechanism was repo...

Is a Safety Guide Wire Necessary for Transurethral Lithotripsy using Semi-Rigid Ureteroscope? Results from a Prospective Randomized Controlled Trial.

Experts recommend us to keep a safety guidewire during the process of upper urinary tract endoscopy, though there is a lack of high-level evidence to support the efficacy and safety of this opinion. This study conducted to compare the outcome of ureteral stone breakage in the presence or absence of safety guidewire. Patients candidate for endoscopic breakage of ureteral stone using a semi-rigid ureteroscope, were randomly assigned in two groups based on keeping a safety guidewire (group1) or removing the guidewire (group2) before the process of breaking ureteral stone by lithoclast. Demographic factors, history of previous stone treatment, kidney function, stone location, symptoms duration and severity were recorded for each patient. Primary outcomes include success rate of stone treatment and secondary outcomes include number of attempts to enter to ureter, success rate of ureteral entry, success rate of stone achievement, stone migration rate and success rate of ureteral stent insertion. The recorded data were entered to the SPSS software and descriptive statistical analysis including power calculation and non-inferiority design for the primary and secondary outcomes, was performed. P-value less than 0.05 was considered significant. From January 2016 till May 2018, 320 patients were randomized with 160 patients in each arm. Considering the cases who were missed due to follow-up loss, there were 153 patients in group 1 and 147 patients in group 2 at the end of the study. Baseline data were equally distributed in both groups. Based on the initial analysis, the studied variables had no significant difference between two groups; though, according to the subgroup analysis of patients with proximal ureter stones, patients in Group 1 had higher rates of ureteral injury comparing to the patients in Group 2 (p=0.03). According to our findings, keeping the safety guidewire through the process of endoscopic stone breakage (stone size: less than 1.5Cm) seems to add no significant benefit to the procedure outcome, while it increases the ureteral injuries in the proximal ureter stones, but not in mid or distal ureter stones.

An experimental investigation of the response of slender protective structures to rockfall impacts

This article investigates the mechanical response of slender rockfall protection embankments subjected to impacts based on real-scale experiments. More specifically, it deals with rectangular (in cross-section) vertical-sided gabion structures, designed to meet footprint constraints. These three-layered structures, 3 m in width and 4 m in height, are made up of gabion cages filled with different materials, depending on their location in the structure. Real-scale experiments were conducted with impact energies up to about 2000 kJ on two structures differing by the fill material used for their middle layer: ballast or sand–tire mixture. The experiments demonstrate the capacity of these slender structures in resisting high-energy impacts. The response of the structures is also addressed considering data obtained using different measuring techniques and a large number of sensors within the structure. The results are presented and discussed with the aim of highlighting some issues associated with the structure impact response, such as the load lateral diffusion, stone breakage, the contribution of the wire mesh, and the fill material characteristics. In the end, a structure with a middle layer filled with ballast appears more efficient in reducing the structure back face displacement.

High-speed microphotography of stone breakage with Holmium:YAG lasers

To better understand physical mechanisms of stone comminution in Holmium:YAG laser lithotripsy, high-speed microphotography was used to study the breakage of dry stones in air and hydrated stones in water. Surgically retrieved urinary calculi and hydroxyapatite pellets were pulsed with a Holmium:YAG laser at 0.2–1 J and recorded at ∼250 000 fps using a high-speed camera (Shimadzu HPV-X2) mounted to a Nikon microscope. In air, direct light–stone interactions were seen to produce photothermal melting, vaporization, and micro-explosions. The most apparent damage to stones was due to the micro-explosions, which erupt the stone surface producing craters and pits. The micro-explosions, however, were observed to progressively diminish with number of laser shots, typically stalling after several shots. Increasing the laser energy or moving the fiber tip to a new position was seen to restore the occurrence of micro-explosions before stalling a second time. In water, the laser produced cavitation bubbles, the collapse of which continued to fragment, pit, and erode the stone after each laser pulse. This study suggests that cavitation bubbles contribute strongly to stone comminution. Cavitation bubbles may have a particularly important role in sustaining micro-explosions after initial laser shots.To better understand physical mechanisms of stone comminution in Holmium:YAG laser lithotripsy, high-speed microphotography was used to study the breakage of dry stones in air and hydrated stones in water. Surgically retrieved urinary calculi and hydroxyapatite pellets were pulsed with a Holmium:YAG laser at 0.2–1 J and recorded at ∼250 000 fps using a high-speed camera (Shimadzu HPV-X2) mounted to a Nikon microscope. In air, direct light–stone interactions were seen to produce photothermal melting, vaporization, and micro-explosions. The most apparent damage to stones was due to the micro-explosions, which erupt the stone surface producing craters and pits. The micro-explosions, however, were observed to progressively diminish with number of laser shots, typically stalling after several shots. Increasing the laser energy or moving the fiber tip to a new position was seen to restore the occurrence of micro-explosions before stalling a second time. In water, the laser produced cavitation bubbles, the colla...

The role of extracorporeal shock wave lithotripsy in the future of stone management.

The most relevant recent findings on the use of extracorporeal shock wave lithotripsy (ESWL) in adult population to provide an insight of its role in the current and future of stone treatment. Comparing ESWL with other modalities is not in the scope of this review. We conducted a PubMed/Embase search and reviewed recent publications that include relevant information on the development of ESWL. Low-rate shock waves improve stone breakage, ramping energy modalities improve stone fragmentation and have lower incidence of hematoma and kidney injury. Transgluteal approach is suggested to improve stone-free rates for distal ureteral stones in a single session. Proper coupling is the most important technical aspect of the treatment and coupling improvement can be achieved by optical monitorization. Triple D score is a promising tool in proper patient selection, but external validation is needed. Predictive information arising from computed tomography scans has been refined by the variant coefficient of stone density and 3D texture analysis that might improve outcomes in the future. Recent evidence suggests that modifying techniques and protocols, and better patient selection are the current trends for improving ESWL outcomes. EWSL will keep its role as the single noninvasive treatment in stone management with room for outcome improvement in the future.

The Impact of Dust and Confinement on Fragmentation of Kidney Stones by Shockwave Lithotripsy in Tissue Phantoms.

Objective: The goal was to test whether stone composition and kidney phantom configuration affected comminution in extracorporeal shockwave lithotripsy (SWL) laboratory tests. Confinement may enhance the accumulation of dust and associated cavitation bubbles in the fluid surrounding the stone. It is known that high shockwave delivery rates in SWL are less effective because bubbles generated by one shockwave do not have sufficient time to dissolve, thereby shielding the next shockwave. Materials and Methods: Experiments were conducted with a lithotripter coupled to a water bath. The rate of comminution was measured by weighing fragments over 2 mm at 5-minute time points. First, plaster and crystal stones were broken in four phantoms: a nylon wire mesh, an open polyvinyl chloride (PVC) cup, a closed PVC cup, and an anatomical kidney model-the phantoms have decreasing fluid volumes around the stone. Second, the fluid volume in the kidney model was flushed with water at different rates (0, 7, and 86 mL/min) to remove dust. Results: The efficiency of breakage of stones decreases for the dust emitting plaster stones (percentage of breakage in 5 minutes decreased from 92% ± 2% [n = 3] in wire mesh to 19% ± 3% [n = 3] in model calix) with increasing confinement, but not for the calcite crystal stones that produced little dust (percentage of breakage changed from 87% ± 3% [n = 3] in wire mesh to 81% ± 3% [n = 3] in kidney model). Flushing the kidney phantom at the fastest rate improved comminution of smaller plaster stones by 27%. Conclusions: Phantoms restricting dispersion of dust were found to affect stone breakage in SWL and in vitro experiments should replicate kidney environments. The dust around the stone and potential cavitation may shield the stone from shockwaves and reduce efficacy of SWL. Understanding of stone composition and degree of hydronephrosis could be used to adapt patient-specific protocols.

Preliminary Report on Stone Breakage and Lesion Size Produced by a New Extracorporeal Electrohydraulic (Sparker Array) Discharge Device

To determine if an innovative extracorporeal electrohydraulic shock wave (SW) device (sparker array [SPA]) can effectively fracture artificial stones in vitro and in vivo, and if SPA treatment produces a renal lesion in our pig model of lithotripsy injury. Results of these experiments will be used to help evaluate the suitability of this device as a clinical lithotripter. Ultracal-30 artificial stones were placed in a holder at the focus of the SPA and treated with 600 SWs (21.6 kV, 60 shocks/min). Stone fragments were collected, dried, and weighed to determine stone breakage. In vivo stone breakage entailed implanting stones into pigs. These stones were treated with 600 or 1200 SWs and the fragments were collected for analysis. Lesion analysis consisted of treating the left kidney of pigs with 1200 or 2400 SWs and quantitating the hemorrhagic lesion. In vitro, 71% ± 2% of each artificial stone was fractured to <2 mm in size. In vivo stone breakage averaged 63%. Renal injury analysis revealed that only 1 of 7 kidneys showed evidence of hemorrhagic injury in the treated area. The SPA consistently comminuted artificial stones demonstrating its ability to fracture stones like other lithotripters. Also, the SPA caused little to no renal injury at the settings used in this study. These findings suggest further research is warranted to determine the potential of this device as a clinical lithotripter.

Ed Carstensen, advisor and mentor to the shockwave lithotripsy program project group

In the 1980s shockwave lithotripsy emerged as a revolutionary advancement for the treatment of kidney stones. Initial studies with patients showed SWL to be highly effective. The technology was elegant, outcomes exceptionally positive and early tests suggested treatment was safe. As experience with SWL grew, limitations surfaced. A key finding was that SWs have the potential to induce significant trauma to the kidney. Our group convinced the NIH it was time to conduct a rigorous assessment to characterize the adverse effects of SWL and determine the mechanisms of SW action in stone breakage and tissue injury. The NIH Program Project Grant mechanism mandated we establish a panel of external advisors to help guide our work. We needed expertise in physical acoustics, cavitation and animal models of ultrasound exposure. We wanted a leading expert. We were extremely fortunate to land Ed Carstensen. Ed worked with us for nearly 15 years, well into our third renewal cycle. He was a brilliant scientist, a man ded...

Improving environmental and stone factors toward a more realistic in vitro lithotripsy model

To improve in vitro lithotripsy models, we investigated the effects of multiple experimental variables on stone fragmentation. We performed timed burst wave lithotripsy (BWL) and shock wave lithotripsy (SWL) exposures in a water tank with the following variable parameters: water gas content (60, 30, and 15% O2), temperature (20 and 37°C), stone holder degree of enclosure (open wire basket, polyvinyl chloride (PVC) open-ended gel, and similar material anatomically accurate artificial kidney), and stone type (Begostone at 2 mixture ratios, calcite, calcium oxalate monohydrate (COM), and uric acid). At least 3 stones were treated for each condition, with fragmentation defined as percent stone mass <2 mm. Begostone (2:1 powder:water ratio) treated with BWL at 20°C vs. 37°C showed 75±13% vs. 62±6% breakage, respectively. Using the same stone type, gas concentrations of 60, 30, and 15% O2 showed breakage of 23±4%, 55±8%, and 82±16%, respectively. More enclosed kidney phantoms showed decreasing lithotripsy efficacy of 94±11%, 64±21%, and 13±2% breakage in basket, PVC, and anatomic phantoms, respectively. 2:1 Begostone most closely mimicked COM stone breakage. SWL exposures produced similar trends. This work indicates the importance of controlling multiple variables during in vitro lithotripsy experiments. [Work supported by NIH P01 DK043881 and K01 DK104854.]To improve in vitro lithotripsy models, we investigated the effects of multiple experimental variables on stone fragmentation. We performed timed burst wave lithotripsy (BWL) and shock wave lithotripsy (SWL) exposures in a water tank with the following variable parameters: water gas content (60, 30, and 15% O2), temperature (20 and 37°C), stone holder degree of enclosure (open wire basket, polyvinyl chloride (PVC) open-ended gel, and similar material anatomically accurate artificial kidney), and stone type (Begostone at 2 mixture ratios, calcite, calcium oxalate monohydrate (COM), and uric acid). At least 3 stones were treated for each condition, with fragmentation defined as percent stone mass <2 mm. Begostone (2:1 powder:water ratio) treated with BWL at 20°C vs. 37°C showed 75±13% vs. 62±6% breakage, respectively. Using the same stone type, gas concentrations of 60, 30, and 15% O2 showed breakage of 23±4%, 55±8%, and 82±16%, respectively. More enclosed kidney phantoms showed decreasing lithotripsy effic...

Quantification of the shielding of kidney stones by bubble clouds during burst wave lithotripsy

Bubble clouds can shield kidney stones from insonification, and limit stone breakage during burst-wave lithotripsy (BWL), a recently proposed technique that uses focused ultrasound pulses with an amplitude of O(1-10) MPa and frequency of O(0.1) MHz. We use numerical simulations to quantify the magnitude of such shielding. In the simulations, we solve for the radial evolution of Lagrangian bubbles coupled to a compressible fluid using volume-averaging techniques. The resulting equations are discretized on an Eulerian grid. In particular, we quantify the reduction in acoustic energy flux incident on a rigid, plane wall that models the stone surface. We consider a burst wave with an amplitude of 6 MPa and a bubble cloud of diameter O(1) mm. The size distribution of nuclei, the number density of bubbles, and the distance of the cloud from the wall are varied, We show that a cloud containing O(10) bubbles with a diameter of O(10) um can reduce the total energy flux by more than 50%, largely independent of distribution of nuclei. Finally, we compare the simulation results with high-speed images and hydrophone measurements of bubble clouds from companion experiments. [Work supported by NIH 2P01-DK043881.]

Stone hardening and broken stones in Japanese plums (Prunus salicina Lindl.) evaluated by means of computed tomography scans

Stone breakage causes significant financial losses for the South African Japanese plum export industry as affected fruit must be marketed as Class 2 according to OECD (Organisation for Economic Co-operation and Development) standards. While information exists on split pit in peaches no information could be obtained regarding the development and/or possible protocols to reduce broken stones in Japanese plums. The main aim of this study was, therefore, to determine when and why broken stones develop during fruit development in Japanese plums in an effort to find possible ways to reduce the incidence of the defect in Japanese plums produced in South Africa. X-ray micro computed tomography was used to compare the fruit growth characteristics and endocarp density of ‘Laetitia’ and ‘Sapphire’ (which are highly susceptible to stone breakage) and ‘Songold’ (which is less susceptible to stone breakage) plums during the stone hardening phase of fruit development. Similar to what was found in peaches, this study found that endocarp lignification in Japanese plums started at the stylar end of the stone and progressed towards the stem end between 28–63days after full bloom. Lignification was accompanied by an increase in endocarp density. The inner lining of the plum stones started to harden first, after which lignification continued outwardly. Clear differences were observed between plum cultivars and seasons in the incidence of broken stones. In all instances broken stone manifestation coincided with the commencement of rapid fruit growth before the stones were completely hardened. Since the stones were not completely hardened by the time rapid fruit growth started, the stones were not able to resist the strong pulling forces of the growing mesocarp and were pulled apart.

Flexible Ureteroscopy and Laser Lithotripsy Intervention for the Management of Stone Disease

Background: The role of ureteroscopy has dramatically evolved over the past twenty years driven by profound enhancement in various factors and assisting techniques such as the ureteroscope size, deflection capabilities, video-imaging, and in lithotripsy (stone breakage) with the advent of holmium laser, however, the stone size plays a critical role in determining outcomes and operative approach. Aim of the work: we conducted a systematic review of the literature to look at the safety and efficacy of flexible utereroscopy and laser lithotripsy intervention in patients with stone disease; particularly those with stones larger than 2 cm. Methods: A systematic search was performed in the scientific database particularlyMEDLINE (2000–2017), EMBASE (2000– 2017), Cochrane Central Register of Controlled Trials, CINAHL (2000–2017), Google Scholar, and individual urologic journals. Results: The search yielded eight studies involving 392 patients, (390 renal units) were reportedly treated with FURSL. The mean operative time was 80.7 minutes (26-215 min). The mean stone-free rate was 91.2% (77%-96.7%), with an average of 1.6 procedures per patient. The mean stone size was 2.5 cm except for one most recent study which reported stones size less than 0.5 cm. An overall complication rate was 8.1%. Major complications developed in 21 (4.2%) patients and minor complications developed in 19 (3.9%) patients. Conclusion: Flexible Ureteroscopy and Laser Lithotripsy intervention has proven to be not only a less invasive treatment but also a successful with a low complication and stone free rate (SFR) for renal calci larger than 2 cm. FURSL may represent an alternative therapy to standard percutaneous nephrolithotomy (PCNL) with satisfactory efficacy and low morbidity.

Recent advances in lithotripsy technology and treatment strategies: A systematic review update

IntroductionShock wave lithotripsy (SWL) is a well – established treatment option for urolithiasis. The technology of SWL has undergone significant changes in an attempt to better optimize the results while reducing failure rates. There are some important limitations that restrict the use of SWL. In this review, we aim to place these advantages and limitations in perspective, assess the current role of SWL, and discuss recent advances in lithotripsy technology and treatment strategies. MethodsA comprehensive review was conducted to identify studies reporting outcomes on ESWL. We searched for literature (PubMed, Embase, Medline) that focused on the physics of shock waves, theories of stone disintegration, and studies on optimising shock wave application. Relevant articles in English published since 1980 were selected for inclusion. ResultsEfficacy has been shown to vary between lithotripters. To maximize stone fragmentation and reduce failure rates, many factors can be optimized. Factors to consider in proper patient selection include skin – to – stone distance and stone size. Careful attention to the rate of shock wave administration, proper coupling of the treatment head to the patient have important influences on the success of lithotripsy. ConclusionProper selection of patients who are expected to respond well to SWL, as well as attention to the technical aspects of the procedure are the keys to SWL success. Studies aiming to determine the mechanisms of shock wave action in stone breakage have begun to suggest new treatment strategies to improve success rates and safety.

Discrete Element Analysis of Railway Ballast under Cycling Loading

Railway ballast is a highly heterogonous material consisting of a multitude of stone sizes with varying physical characteristics. It is a critical layer of railroad track structures providing drainage, strength, stability and load distribution. Ballast degrades under cyclic train loads and needs to be maintained to prevent rail misalignment, settlement and buckling, which may necessitate lower train speeds to prevent derailments. This paper utilizes the Discrete Element Method (DEM) code PFC2D to model the behaviour of railroad ballast under cyclic loading. It introduces two innovative approaches to capture stone angularity by fitting circular shapes into each stone shape. One of these methods involves a MATLAB® routine and the other an AutoCAD® routine. Inter-particle contact laws in these agglomerations of circular elements reflect the strength of the stones. The advantage of these two techniques over the conventional clumping technique is that they allow modelling stone breakage which is a leading cause of ballast degradation. Previous studies have implemented techniques to simulate the behaviour of a circular particle idealized as a single aggregate under cyclic loads. Three different techniques are presented in this study to better understand the natural behaviour of ballast under loading. In this paper, particle angularity is being introduced by using another method. This numerical study demonstrated the effect of aggregates particle shape on the overall performance of railroad ballast as measured by permanent deformation and degradation of the ballast.

Evaluation of the LithoGold LG-380 Lithotripter:In VitroAcoustic Characterization and Assessment of Renal Injury in the Pig Model

Conduct a laboratory evaluation of a novel low-pressure, broad focal zone electrohydraulic lithotripter (TRT LG-380). Mapping of the acoustic field of the LG-380, along with a Dornier HM3, a Storz Modulith SLX, and a XiXin CS2012 (XX-ES) lithotripter was performed using a fiberoptic hydrophone. A pig model was used to assess renal response to 3000 shockwaves (SW) administered by a multistep power ramping protocol at 60 SW/min, and when animals were treated at the maximum power setting at 120 SW/min. Injury to the kidney was assessed by quantitation of lesion size and routine measures of renal function. SW amplitudes for the LG-380 ranged from (P(+)/P(-)) 7/-1.8 MPa at PL-1 to 21/-4 MPa at PL-11 while focal width measured ~20 mm, wider than the HM3 (8 mm), SLX (2.6 mm), or XX-ES (18 mm). For the LG-380, there was gradual narrowing of the focal width to ~10 mm after 5000 SWs, but this had negligible effect on breakage of model stones, because stones positioned at the periphery of the focal volume (10 mm off-axis) broke nearly as well as stones at the target point. Kidney injury measured less than 0.1% FRV (functional renal volume) for pigs treated using a gradual power ramping protocol at 60 SW/min and when SWs were delivered at maximum power at 120 SW/min. The LG-380 exhibits the acoustic characteristics of a low-pressure, wide focal zone lithotripter and has the broadest focal width of any lithotripter yet reported. Although there was a gradual narrowing of focal width as the electrode aged, the efficiency of stone breakage was not affected. Because injury to the kidney was minimal when treatment followed either the recommended slow SW-rate multistep ramping protocol or when all SWs were delivered at fast SW-rate using maximum power, this appears to be a relatively safe lithotripter.

Size and location of defects at the coupling interface affect lithotripter performance

Study Type--Therapy (case series) Level of Evidence 4. What's known on the subject? and What does the study add? In shock wave lithotripsy air pockets tend to get caught between the therapy head of the lithotripter and the skin of the patient. Defects at the coupling interface hinder the transmission of shock wave energy into the body, reducing the effectiveness of treatment. This in vitro study shows that ineffective coupling not only blocks the transmission of acoustic pulses but also alters the properties of shock waves involved in the mechanisms of stone breakage, with the effect dependent on the size and location of defects at the coupling interface. • To determine how the size and location of coupling defects caught between the therapy head of a lithotripter and the skin of a surrogate patient (i.e. the acoustic window of a test chamber) affect the features of shock waves responsible for stone breakage. • Model defects were placed in the coupling gel between the therapy head of a Dornier Compact-S electromagnetic lithotripter (Dornier MedTech, Kennesaw, GA, USA) and the Mylar (biaxially oriented polyethylene terephthalate) (DuPont Teijin Films, Chester, VA, USA) window of a water-filled coupling test system. • A fibre-optic probe hydrophone was used to measure acoustic pressures and map the lateral dimensions of the focal zone of the lithotripter. • The effect of coupling conditions on stone breakage was assessed using gypsum model stones. • Stone breakage decreased in proportion to the area of the coupling defect; a centrally located defect blocking only 18% of the transmission area reduced stone breakage by an average of almost 30%. • The effect on stone breakage was greater for defects located on-axis and decreased as the defect was moved laterally; an 18% defect located near the periphery of the coupling window (2.0 cm off-axis) reduced stone breakage by only ~15% compared to when coupling was completely unobstructed. • Defects centred within the coupling window acted to narrow the focal width of the lithotripter; an 8.2% defect reduced the focal width ~30% compared to no obstruction (4.4 mm vs 6.5 mm). • Coupling defects located slightly off centre disrupted the symmetry of the acoustic field; an 18% defect positioned 1.0 cm off-axis shifted the focus of maximum positive pressure ~1.0 mm laterally. • Defects on and off-axis imposed a significant reduction in the energy density of shock waves across the focal zone. • In addition to blocking the transmission of shock-wave energy, coupling defects also disrupt the properties of shock waves that play a role in stone breakage, including the focal width of the lithotripter and the symmetry of the acoustic field • The effect is dependent on the size and location of defects, with defects near the centre of the coupling window having the greatest effect. • These data emphasize the importance of eliminating air pockets from the coupling interface, particularly defects located near the centre of the coupling window.

Location of coupling defects influences stone breakage in shock wave lithotripsy.

SWL is the most common treatment for kidney stones. However, compared to ureteroscopy and nephrostolithotomy, SWL is least effective—failing in ∼50% of cases. Since stone breakage is highly effective under controlled conditions, acoustic coupling between the lithotripter and patient may be the weak link. Previous in vitro studies determined that air‐pockets created in routine coupling reduce SW‐amplitude by ∼20% and defects occupying only 2% of coupling area reduced breakage 20%–40%. As a step toward determining if the position of defects influences SW delivery to the target we used styrofoam to selectively block portions of the coupling interface between a DoLi‐50 and the test tank. Stone breakage was ∼three times greater when the entire 13 cm diameter coupling interface was unblocked than when all but the center 6 cm was blocked, consistent with the reduction in surface area. However, the transition was abrupt, with ∼70% of the loss in efficacy occurring upon reduction in the aperture from 7 to 6 cm. Reducing the aperture had a greater effect on P− than P+ (P+/P− no aperture ∼41/−4.3 MPa; 7 cm ∼42/−3.5 MPa; 6 cm ∼37/−2.5 MPa). These initial findings begin to identify a region of the overall coupling interface where defects are likely to be problematic. [Work supported by Grant No. NIH‐DK43881.]

Gas content of the medium surrounding a stone has a significant effect on the efficiency of stone breakage in shock wave lithotripsy.

In shock wave lithotripsy inertial collapse of cavitation bubbles produces daughter microbubbles [Y. A. Pishchalnikov et al., BJU Int. 102, 1681–1686 (2008)]. This will cause proliferation of bubbles only if daughter microbubbles survive until the arrival of the next shock wave (SW). While the time interval between SWs is determined by the firing rate of the lithotripter, the dissolution time of microbubbles depends on concentration of dissolved gas in the water. Thus, the same firing rate may produce different cavitation fields depending on the gas concentration. We assessed how the concentration of dissolved gas in the water tank affected SWs and the efficiency of stone comminution. More numerous bubbles generated in nondegassed water caused a dramatic reduction in the trailing negative‐pressure component of the SWs, while the leading positive‐pressure phase remained almost unchanged. It required 3.5 times more SWs to break stones in nondegassed water than in water degassed to 30% (P<0.001). This suggests that changes in temporal profiles of the SWs associated with cavitation are responsible for reduced efficiency of stone comminution observed in dense cavitation fields, regardless of whether these denser cavitation clouds were generated by using a faster rate or higher gas content. [Work supported by NIH‐DK43881.]