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Using an experimental system that mimics stone fragmentation in the renal pelvis, the role of stress waves and cavitation in stone comminution in shock-wave lithotripsy (SWL) is noted. Spherical plaster-of-Paris stone phantoms (10mm Dia) were exposed to 25, 50, 100, 300 and 500 shocks at the beam focus of a Dornier HM-3 lithotripter. 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. Similar observations were confirmed using kidney stones with a primary composition of calcium oxalate monohydrate. 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
The presence of kidney stone in the kidney causes discomfort to patients. Hence, removal of such stones in an effective and non-harmful way is important. The management of kidney stone has been revolutionized by the development of lithotripters.
A urinary calculus is a crystalline structure. If it is bombarded with shock wave of sufficient energy it will disintegrate into fragments. Disintegration of renal calculi in a lithotripter field is the consequence of dynamic fracture of stone materials due to the mechanical stresses produced either directly by the incident Lithotripter Shock Wave (LSW) or indirectly by the collapse of cavitation bubbles.
In Shock-Wave Lithotripsy (SWL), two basic mechanisms of stone fragmentation have been well-documented; namely, spalling at the posterior surface and at internal crystalline-matrix interfaces of a stone due to reflected tensile waves and cavitation erosion at the anterior surface of a stone due to violent collapse of bubbles.
Here the role of stress waves and cavitation in stone comminution is noted by experimenting stone fragmentation using plaster-of-paris stone phantoms exposed to different range of shocks at the beam focus of a Dornier HM-3 lithotriptor.
In the Dornier lithotripter, shock waves are generated outside of the body and transmitted
through water and the outer tissues of the body to the stone in the kidney or upper ureter. Immersion of the patient in a water bath allows the shock wave to pass from the generator (an electrode) to the patient without either damaging tissue or damping the wave, since water and tissue have similar acoustic impedance properties. The waterâ„¢s temperature, gas content, and conductivity are controlled by a treatment system in the lithotripter.
FUNCTIONING OF LITHOTRIPTOR
The shock waves are generated by an underwater spark from an electrode located at the first geometric focus of a semi-ellipsoidal reflector. The stone is positioned at the second focus of the reflector, which is the point of highest energy density. A two-dimensional radiographic scanning system, using two X-ray units, and a patient positioning system ensure proper location of the stone. A large pressure zone is created as the shock wave passes from tissue or urine into the stone. This pressure exceeds the strength of the stone material and causes its destruction. Repeated shock wave applications result in the fragmentation of a stone into small pieces (2 mm or less), which normally are passed spontaneously out of the body in the urine.
MECHANISM OF LITHOTRIPTOR SHOCK WAVE
When a Lithotriptor Shock Wave impinges on a stone surface, at least two types of stress waves (longitudinal and transverse waves) will be generated at the wave entry site and propagate into the stone material. Upon reaching the posterior surface of the stone, the leading longitudinal stress wave, which has a pressure profile similar to the LSW in water, will be partially reflected and undergo a phase inversion due to the decrease in acoustic impedance from the stone to surrounding tissue/ urine. The reflected tensile wave, propagating back toward the shock-wave source, will subsequently superimpose with the remaining tensile component of the incident longitudinal stress wave, producing a strong tensile stress near the posterior surface of the stone. Because most kidney stones are brittle materials, they fail much more easily under tension than under compression. In general, spalling damage refers to stone fragmentation initiated and facilitated by the reflected tensile waves.
In addition to wave superposition, strong tensile stresses can also be produced by the focusing and crossing of wave fronts near the posterior surface of the stone. Using photo elastic imaging technique and ray-tracing analysis, these basic features of stress wave propagation, evolution and resultant damage in stone phantoms of different size and geometries have been confirmed. Furthermore, it was found that the transmitted transverse (or shear) wave could facilitate the extension and propagation of micro cracks initiated by the reflected tensile waves.
Theoretically, if the stone structure is weak (i.e., with numerous pre-existing flaws or fissures) the stone may also fail readily under the influence of the transmitted compressive and tensile waves
In comparison to stress-wave-induced fragmentation, cavitation damage in SWL is characterized by surface erosion with numerous minute pittings, produced either by the secondary shock waves or micro jet impingement due to the violent collapse of bubbles near a stone surface. The fragments produced by cavitation damage are usually small, due to the highly localized stresses generated by a collapsing bubble. This finding is in great contrast to the large fragments produced by spalling mechanism.
Furthermore, when cavitation was significantly inhibited either by increasing the viscosity of the medium or by excessive overpressure, the fragmentation of gallstones by LSWs was found to be greatly reduced. Considering that about 2000 shocks are generally employed for the successful comminution of kidney stones in patients, a better understanding of the contribution of stress waves and cavitation to stone comminution in SWL could be helpful to improve the treatment efficiency of SWL.
In this work, a series of experiments were carried out using a phantom system that mimics in vivo stone comminution in the renal pelvis. Emphasis is placed on delineating the contribution of various working mechanisms to the overall success of stone comminution, and on understanding of the progressive development of stone comminution in SWL in relation to the contributing mechanisms. It was found that both stress waves and cavitation play critical roles in the comminution of kidney stones; they act synergistically, rather than independently, to ensure an effective and successful fragmentation of renal calculi in SWL.
EXPERIMENTING LITHOTRIPSY BY MIMIC STONE FRAGMENTATION
MATERIALS AND METHODS
In this study, we used a Dornier HM-3 lithotripter with a truncated ellipsoidal reflector (semi major axis a _ 138 mm, semi minor axis b _ 77.5 mm, and half-focal length c _ 114 mm). The lithotripter was operated at the typical clinical setting of 20 kV and 1-Hz pulse repetition rate.
Spherical stone phantoms (D =10 mm) were made of plaster-of-Paris (powder/water ratio =2:1 by weight), using a specially designed mold. The acoustical properties of the plaster-of-Pairs stone phantom are comparable to those of magnesium ammonium phosphate hydrogen (or struvite) stones. In addition, six pairs of kidney stones of different compositions, surgically removed from patients, were used for comparison of stone comminution in different fluid media. Each pair of the stones as selected based on their similarity in composition, size, color and weight Figure 1, with their primary chemical composition determined to be calcium oxalate monohydrate (COM), the most commonly observed crystalline material in kidney stones.
FIG. 1 Dimension and chemical compositions of the kidney stones
To mimic stone comminution in the renal pelvis, a recently developed phantom system was used Figure 2 shows the experimental setup. The stone sample was placed in a plastic holder with disposable finger cot at the end. The holder was filled with either degassed water (a cavitation-supportive medium) or freshly poured caster oil (a cavitation-inhibitive medium) to delineate the contribution of stress waves and cavitation to stone comminution. Using this setup, the finger cot and the test fluid inside the sample holder can be replaced easily following each test. The stone holder was immersed in an acrylic testing chamber filled with caster oil and with a slab of 25-mm thick tissue-mimicking phantom placed at the bottom to simulate tissue attenuation on the incident LSWs.
FIG. 2 Schematic diagram of the experimental setup for mimicking
stone comminution in the renal pelvis
The stone phantoms were randomly divided into two populations for shock-wave treatments either in degassed water or in castor oil. To determine the dose dependency of stone comminution, samples in each population were further subdivided into six groups (n = 6 in each group) and exposed to 25, 50, 100, 200, 300 and 500 shocks, respectively. For kidney stones, the comparison was made at 200 shocks only. Before the experiment, the weight of each sample in the dry state was measured.
Prior to shock-wave treatment, each stone sample was immersed in the prospective test fluid (degassed water or caster oil) for at least 20 min until no visible bubbles could be seen at the stone surface. Based on our experience, prolonged immersion of the sample beyond 20 minutes does not change the fragmentation results significantly. Alignment of the stone phantom to F2 was aided by a pointer and verified by the fluoroscopic imaging system of the HM-3 lithotripter.
Following the shock-wave treatment, all the fragments in the finger cot were carefully removed and spread into a layer on a dry paper. For samples treated in castor oil, facial tissue was used gently to absorb any excessive oil left on the specimen surface. After drying in air for 48 hours, the fragments were collected and their size distribution was determined by sequential sieving. Briefly, the fragments were filtered through a set of four sieves which were placed vertically in a rack with descending order of pore size from 4.0 mm to 0.5 mm. The fragments were then removed from each sieve and weighed.
EXPERIMENTAL SETUP FOR CAVITATION
To identify the contribution of cavitation alone to stone comminution in SWL, another set of experiments was. This laboratory lithotripter is equipped with a transparent water tank, so that high-speed shadowgraphs can be recorded to visualize the transient shock wave-stone interaction). To eliminate the contribution of stress waves, the stone phantom was placed in the focal plane, but with its center shifted transversely by a 13-mm distance from the shockwave axis to position the stone just outside the lithotripter beam focus (sees Figure 7b). Cavitation bubbles, however, were still produced at F2 in water and collapsed near the lateral surface of the stone. In the fragmentation tests, the stone sample was placed inside a thin-wire net affixed to an inverted U-shape holder, and exposed to LSWs in degassed water without tissue-mimicking materials. For comparison, another group of stone phantoms were treated at F2.
Dose-dependency of stone comminution in degassed water
In degassed water, stone phantoms break up progressively as the number of shocks increases (Fig. 3). The size distribution of the fragments at different numbers of shocks is shown in Fig. 4. Initially, after 25 to 50 shocks, the stone was broken into several large pieces (4 mm) that account for more than 90% of the fragments by weight. As the shock number increased, more medium (2â€œ4 mm) and small (2 mm) fragments were produced, indicating a progressive comminution of the initial large fragments. After 200 shocks, the large fragments were reduced to 28% of the total weight, the medium fragments reached a maximum of 36% and the small fragments contributed to the remaining 36%. As the shock wave exposure continued, the percentage of small fragments increased further and the percentage of large and medium fragments decreased.
Figure 4 shows the dose-dependency in stone comminution using the 2-mm criterion for passable fragments, based on the clinical observation that stone fragments less than 2 mm can be discharged spontaneously following SWL. Initially (50 shocks), stone comminution increases slowly with the shock number. Between 50 and 300 shocks, there is a rapid, linear increase in fragmentation and, after 300 shocks, the comminution rate slows down. One important reason for this reduced rate of stone comminution at a large number of shocks is the scattering of LSWs by residual small fragments. During the experiments, it was observed that small fragments tend to settle down at the bottom of the finger cot. When a significant portion (50%) of the stone was fragmented, a layer of small fragments could be accumulated underneath the residual large pieces, which would significantly attenuate the ensuing LSWs; thus, decreasing subsequent stone comminution efficiency. Similar effects of residual fragments on the comminution of ureteral stones have been reported previously
THE CONTRIBUTION OF STRESS WAVES
To determine the contribution of stress waves, stone phantoms were immersed in castor oil to suppress cavitation. Under this circumstance, stone phantoms were fragmented primarily by the stress waves produced by the incident. The result, shown in Fig. 5, demonstrates that, although stone phantoms were disintegrated into multiple pieces in castor oil, the fragments remained fairly sizable even after 500 shocks. Quantitatively, large and medium fragments were produced after 25 shocks accounting for 75% and 20% of the stone weight, respectively (Fig. 6). With continued shock-wave exposure, the large fragments were reduced to medium-size pieces, which, however, were not further comminuted into small fragments. For example, after 500 shocks, only 1.5% of the stone mass was reduced to less than 1 mm and no fragments less than 0.5 mm were produced (Fig. 6). This is in great contrast with the progressive comminution of stones into fine fragments in degassed water (Figs. 3 and 4). Using the 2-mm criterion, it is clear that if stone comminution in SWL were produced by the stress waves alone, most fragments would not pass spontaneously following the treatment (Fig. 6). From 25 to 500 shocks, the percentage of passable fragments increases only slightly from 2% to 11%. Clearly, under the influence of stress waves alone, there is an apparent limitation on the smallest fragments that can be produced.
FIG. 3 Photographs of fragments of plaster-of-Paris stone phantoms
treated in water after exposure to 25â€œ500 shocks at 20 kV
using an HM-3 lithotripter.
FIG. 4 Dose-dependent size distribution of the fragments of
Plaster-of-Paris stone phantoms in water after shock-wave
treatment in an HM-3 lithotripter at 20 kV.
THE CONTRIBUTION OF CAVITATION
To isolate the contribution of cavitation alone to stone comminution in SWL, a plaster-of-Paris stone phantom was placed off-axis transversely from F2 by 13 mm. As shown in Fig. 7b, the incident LSW propagated through F2 in water, sweeping by the lateral surface of the stone. With this arrangement, stress waves produced inside the stone could be minimized. Yet, cavitation bubbles were still generated by the incident LSW around F2. The bubbles first expand to a maximum size in about 200 _s and then collapsed violently near the lateral surface of the stone, emitting secondary shock waves (see circular rings at 600 _s in Fig. 7b). Some bubbles were also seen to aggregate on the lateral surface of the stone, and their subsequent collapse could be asymmetric with resultant formation of micro jets impinging toward the stone. Overall, the bubble dynamics in the off-axis arrangement are similar to those generated when the stone is placed at F2 (Fig. 7a), except that, in the latter case, the bubbles collapse primarily near the proximal surface of the stone facing the incident LSW (frames at 500 _s and 680 _s in Fig. 7a). When the stone phantom was placed off-axis, only 3% of the stone mass was fragmented to less than 2 mm after 30 shocks at 24 kV in the experimental HM-3 lithotripter (Fig. 8). In comparison, when placed directly at F2, 27% of the stone mass was disintegrated into passable fragments. It should be noted that these results could not be compared directly with the ones (see Fig. 4) obtained using the renal-pelvis-mimicking phantom system because of the significant differences in the experimental setup. Macroscopically, damage to the stones placed off-axis was primarily surface erosion produced by the collapse of cavitation bubbles without any bulk disintegration of the stones, whereas stones placed at F2 were fragmented into pieces of different sizes. These results suggest that, with cavitation alone, although damage to the stone (primarily surface erosion) can be produced, the comminution efficiency is significantly reduced from that produced by the combination of stress waves and cavitation, which may also include contribution from the interaction between LSWs and small bubbles stabilized on large residual fragments.
FIG. 5 Photographs of fragments of plaster-of-Paris stone phantoms
treated in castor oil after exposure to 25â€œ500 shocks at 20 kV using an HM-3 lithotripter
FIG. 6 Dose-dependent size distribution of the fragments of
Plaster-of-Paris stone phantoms in castor oil after shock-wave
treatment in an HM-3 lithotripter at 20 kV.
STONE FRAGMENTATION AT BEAM FOCUS AND 13MM OFF- AXIS
FIG. 8 Comparisons of stone fragmentation at the beam focus of a laboratory HM-3 lithotripter and at a 13-mm transverse distance from the beam focus. The lithotripter was operated at 24 kV.
FIG. 7 Representative high-speed image sequences of shockwave,
cavitation bubbles-stone interaction produced by a laboratory
HM-3 lithotripter in water at 24 kV. The plaster-of-
Paris stone phantom (10 mm in diameter) was placed (a) at F2,
(b) At a 13-mm transverse distance from F2. The number above
each image frame indicates the time delay in _s after the spark
discharge of the lithotripter electrode.
EFFECT ON KIDNEY STONES
Paired kidney stones of similar composition, size, shape and weight were exposed to 200 shocks at 20 Kv in the HM-3 lithotripter either in degassed water or in castor oil. The results, shown graphically in Fig. 9 and quantitatively in Fig. 10, revealed that, in water, most stones were comminuted into passable pieces, whereas, in castor oil, most fragments remained large in size. For example, in water, no fragments were larger than 4 mm and the residual weight for 2â€œ 4 mm, 1â€œ2 mm, and _ 1 mm fragments were 11%, 37% and 52%, respectively. In contrast, in caster oil, the fragments _ 4 mm and 2â€œ 4 mm were 51% and 28%, respectively (Fig. 10). Using the 2-mm criterion, about 89% of fragments from the kidney stones in water could be passed spontaneously after 200 shocks, whereas, in caster oil, the passable fragments were only 22% (Fig. 10b). Although these results cannot be compared directly with that from plaster-of-Paris stone phantoms (see Figs. 3â€œ 6), the overall trend is consistent. Altogether, these experimental findings confirm that, with stress waves alone, kidney stones will be primarily disintegrated into large, impassable pieces.
FIG 9 Original Kidney Stones After Treated By 200 Shocks
FIG. 10a Size distribution of the kidney stone fragments after
being treated by 200 shocks produced by a HM-3 lithotripter at
20 kV, either in water or in castor oil.
FIG. 10b Size distribution of the kidney stone fragments after
being treated by 200 shocks produced by a HM-3 lithotripter at
20 kV, either in water or in castor oil.
The fragmentation of kidney stones in SWL is the consequence of dynamic fracture of stone materials in response to the mechanical stresses produced either by LSW or cavitation. Kidney stones, like most crystalline materials, have preexisting flaws or micro cracks randomly distributed at the crystalline-matrix interface or at the grain boundary of the crystalline materials. Under the mechanical stresses imposed by the LSWs, these pre-existing micro cracks may extend if the accumulated stress-intensity factor at the crack tip exceeds a threshold value, also known as the fracture toughness of the material.
The observation that stone phantoms and kidney stones immersed in castor oil cannot be fragmented into passable pieces (see Figs. 5 and 9b) indicates that there is a size limitation on the fragments produced by stress waves alone in SWL. Among various proposed mechanisms, this finding is most consistent with the spalling mechanism.
ANALYSIS OF TRASMITTED LITHOTRIPTOR SHOCK WAVE
A simple analysis of wave reflection and superposition at the stone boundary may provide some critical insights to this problem. Let us consider the reflection of a transmitted longitudinal wave at the posterior surface of a kidney stone, a critical process involved in the production of spalling damage. Because of the decrease in acoustic impedance from stone material to surrounding tissue or fluid, the leading compressive component of the wave will be inverted in phase, generating a reflected tensile wave. This reflected tensile wave, propagating back into the stone material, will first superimpose with the remaining portion of the compressive component of the incident wave, resulting in a mutual reduction of their respective amplitudes. Subsequently, as the reflected tensile wave propagates further into the stone and superimposes with the trailing tensile component of the incident wave, a strong tensile stress will be produced at some distance from the posterior surface of the stone. This is the reason why, in cylindrical stone phantoms (LSW propagating along the axis of the cylinder), spalling damage always occurs at a distance from the posterior surface of the stone.
When the size of the residual fragments becomes less than this minimal distance, there will be destructive superposition of the stress waves reverberating inside the fragment. Consequently, the net stress imposed on the stone material will be significantly reduced. If the corresponding stress-intensity factor at the tip of preexisting micro cracks in the fragment falls below the fracture toughness of the stone material, subsequent shock-wave exposure will not cause the micro cracks to extend and, therefore, no further disintegration of the fragment will occur.
Based on this analysis, if only stress waves contribute to stone comminution in SWL, most renal calculi will not be comminuted to fragments small enough (_ 2 mm) for spontaneous discharge, as demonstrated by the stone comminution results in castor oil (see Figs. 5 and 9b). On the other hand, when cavitation is the only contributory force for stone comminution, the result- ant fragmentation efficiency is very low, compared with that produced by the combination of stress waves and cavitation (see Fig. 8).
EFFECT OF CAVITATION
Cavitation-induced damage is primarily surface erosion and it does not penetrate much into the bulk of the stone material. Despite this, cavitation damage may weaken the surface structure of large residual fragments (Fig. 11), making them much more susceptible to the impact of subsequent LSWs. With the progression of shock-wave exposure, liquid may enter into the bulk of the stone material through crevices produced on the surface The expansion of bubbles inside the stone by ensuing shock waves may generate large tensile stress at the crevice root, leading to crack expansion.
The analogy to SWL-induced tissue injury is the tensile rupture of small blood vessels due to large intraluminal bubble expansion as the number of fragments increases, the total surface area of the fragments will increase rapidly, which would favor the progression of cavitation-facilitated damage. Although the contribution of each individual cavitation bubble to the overall stone comminution is small, the cumulative effect from numerous bubbles generated during the course of SWL treatment could be significant. It is conceivable that, although stress wave-induced fracture, such as spalling and squeezing damage, is responsible for the initial fragmentation of the stone, cavitation is necessary to produce fine, passable fragments that are most critical for the success of clinical SWL treatment.
COMBINED EFFECT OF CAVITATION AND STRESS WAVES
All together, it appears that, although individually both stress waves and cavitation have limitations in producing satisfactory or effective stone comminution, when combined they work synergistically to produce efficient and successful stone fragmentation. As shown in Figs. 3 and 4, although only a few hundred shocks are needed to break up the stone into distributed fragments, clinically it usually requires a few thousand shocks completely to reduce the size of the fragments to below 2 mm. Therefore, strategies to alleviate the scattering of LSWs by small fragments surrounding large residual stone pieces should be explored in future investigations to improve the treatment efficiency of SWL.
FIG. 11. Photograph and SEM pictures of large fragments of
Plaster-of-Paris stone phantoms produced by 100 shocks in a
HM-3 lithotripter at 20 kV. Comparison of surface damage
of the fragments produced in water (right) with numerous
pittings observed on the surface and in castor oil (left) without
Stress waves and cavitation are both found to be important for the success of stone comminution in SWL. Although stress waves are initially important for breaking up kidney stones into distributed pieces, their effectiveness is hindered when the size of residual fragments becomes less than half of the compressive wavelength in the stone material, due to destructive superposition of reverberating waves inside the residual fragments.
Cavitation, on the other hand, although working at a much slower rate of stone comminution, can significantly weaken the structure of the stone surface, making it much more susceptible to the impact of ensuing LSWs and associated bombardments of cavitation bubbles. Therefore, stress waves and cavitation work synergistically, rather than independently, to produce effective and successful disintegration of renal calculi in SWL. Optimal utilization of the stress waves and cavitation in SWL may help to improve treatment efficiency and reduce adverse tissue injury.
Modern mankind saw the advancements of studies in the field of stone comminution which lead to the development of Extracorporeal Shock Wave Lithotriptors (ESWL). Modern ESWL machines do not have a water bath; the fluid is confined to the path the shock waves must follow to reach the kidney. The shocks may be generated by the discharge of an array of piezoelectric cells and they may be aimed by ultrasonography rather than X-Ray imaging. The devices also differ in the strength the disruptive force that they can develop. Less powerful machines are less effective in breaking stones and several treatment sessions may be necessary to achieve clearance of a calculus
Each ESWL treatment may use from less than 1,000 to more than 2,500 shocks. The shocks are synchronized with the patientâ„¢s heart rhythm, as monitored by an electrocardiogram, and are delivered during the contraction of the heart, when it is not responsive to electrical stimuli. This arrangement avoids the complications, experienced in the early clinical trials, of triggering arrhythmias of the heart.
There is currently great interest in the long term outcome of patients treated by ESWL. Certainly some stones recur, especially if small fragments remain after treatment. Long term renal damage now seems unlikely.
FIG 12 Modern lithotripters (Litho Tron, HMT) for the treatment of kidney and ureteral
stones with extracorporeal shock wave.
Songlin Zhu, Franklin H. Cocks, Glenn M. Preminger and Pei Zhong, The Role of Stress Waves And Cavitation In Stone Comminution In Shock Wave Lithotripsy , Journal of Ultrasound In Medicine & Biology, Vol. 28, No. 5, pp. 661-671, 2002.
S K Shrivastava and Kailash, Stone Fragmentation By Ultrasound , Journal of Bulletin of Material Science, Vol. 27, No. 4, August 2004, pp. 383-385.
Bailey And Love, Short Practice For Surgery
J. A. Showstack, E.J Perez-Stable and E. Sawitz, Extracorporeal Shock Wave Lithotripsy: Clinical Application and Medicare Physician Paymerit , Journal of Office of Technological Assessment, Washington, DC, August 1, 1995.