Rassweiler J. et al., 2021: In-vitro comparison of two electromagnetic shock-wave generators: low-pressure-wide focus versus high-pressure small focus - the impact on initial stone fragmentation and final stone comminution.

This is one of the most interesting articles regarding SWL that I have seen during the past years. It was early noted that there was a difference in results obtained with the original unmodified HM3-lithotripter and several or all modern late generations of lithotripters. Authorities in the field of stone treatments have claimed that the outstanding results obtained with the HM3 device so far not have been attained [1]. Although personally I am very satisfied with the accomplishments with modern shockwave technology these achievements are based on experience with Storz lithotripters (SLX, SLXF2). Having said that I need to add that some occasional achievements recorded with the Dornier HM3 lithotripter would be, if not impossible, so at least very difficult to attain with the small focus and high-pressure devices that most commonly are used today. This is of course of minor importance, but more remarkable is it that 40 years after introduction of SWL we still do not know which geometry and focus energy that is optimal. One basic question remains: Should we use a large focus with low energy density or a small focus with high energy density (like that in Storz Modulith lithotripters).

Irrespective of what the answer to that question might be, it is important to know that despite complains about the lack of progress in SWL, several technical achievements have been incorporated in modern lithotripters. Thus, it was possible to replace the water tub with a “dry” shock wave transmission and nevertheless maintain acceptable energy levels. The electrohydraulic technology was replaced by electromagnetic sources. It was clearly demonstrated that SWL can be completed with only analgesics and sedatives. However, contrary to what urologists in general believe this kind of pain treatment was possible also with the unmodified Dornier HM3 lithotripter [2].
In this report the authors have compared the in-vitro properties between the Xi lithotripter with large focal volume, low energy density and slow shockwave generation and those obtained with small focal volume and high energy and shockwaves delivered at a frequency of 1.5 Hz.

The authors claim that it now is necessary to consider that in the competition between endoscopic procedures (URS, RIRS and PCNL) and SWL, the latter method seems to come out as a looser. An increasing number of urologists favour endourological methods. Why is this so? There are some factors that might explain this strategy. Endoscopy can result in stone removal usually without re-treatments and this technology certainly satisfies the surgical mind of most urologists, or expressed differently, endourology is considered less boring than SWL.

Is it true that the achievements with the HM3 device is superior to those obtained with modern lithotripters? This question is difficult to answer today because in modern guidelines, and accordingly in the current clinical use of SWL there is an increasingly narrow range of indications for SWL. Whereas stones of 30 mm or more 30 years ago were considered as candidates for SWL, the limits subsequently have decreased to 25, 20 or 10- 15 mm and there are some recent studies that have suggested a limit at 5 mm [3]. The fundamental question that seeks an answer seems to be if there is anything that we have missed during 40 years of development of SWL technology?

One factor that recently has been put forward is that stones with modern laser technology can be turned into dust rather than fragments. The conclusion drawn by the authors of the current report is that if SWL should survive in competition with modern endourology, it is necessary to improve SWL in direction of dusting rather than disintegration. Solutions presented so far have been burst-wave lithotripsy of which there still is very limited clinical experience. Moreover, it was presented at the recent German Urology meeting that high-frequency generation of shockwaves successfully disintegrated stones to ts subsequently should be eliminated with an efficient non-invasive method.

During all years that I spent with SWL, stone disintegration, pain treatment and the need of repeated sessions were not the major clinical problems. The most annoying shortcoming was the presence of residuals at follow-up whether occurring as dust or fragments. In my mind elimination of residuals was the greatest concern encountered in most patients.

References
1. Zehnder P, Roth B, Birkhäuser F, Schneider S, Schmutz R, Thalmann GN, Studer U
A prospective randomised trial comparing the modified HM3 with the MODULITH® SLX-F2 lithotripter. E.Eur Urol. 2011 Apr;59(4):637-44. doi: 10.1016/j.eururo.2011.01.026. Epub 2011 Jan 25.PMID: 21296481 Clinical Trial.
2. Tiselius HG.
Anesthesia-free in situ extracorporeal shock wave lithotripsy of ureteral stones.
J Urol. 1991 Jul;146(1):8-12. doi: 10.1016/s0022-5347(17)37701-7.PMID: 2056608
3. Al-Zubi M, Al Sleibi A, Elayan BM, Al-Issawi SZ, Bani-Hani M, Alsharei A, AlSmadi J,
Abualhaj S, Ibrahim AY.The effect of stone and patient characteristics in predicting extra-corporal shock wave lithotripsy success rate: A cross sectional study. Ann Med Surg (Lond). 2021 Sep 10;70:102829. doi: 10.1016/j.amsu.2021.102829. eCollection 2021 Oct.PMID: 34540217
4. Rassweiler-Seyfried MC, Mayer J, Goldenstedt C, Storz R, Marlhaus E, Heine G, Alken P.
Elektrohydraulische Hochfrequenz ESWL – Evaluation der Niernenschäden. Urologe A 2020; 59; Sippl (Abstract 77)
Hans-Göran Tiselius

In-vitro comparison of two electromagnetic shock-wave generators: low-pressure-wide focus versus high-pressure small focus – the impact on initial stone fragmentation and final stone comminution
Prof. Jens Rassweiler, Dr. Philip Rieker, Dr. Rainer Pecha, Prof. Martin Dressel, and Dr. Marie-Claire Charlotte Rassweiler-Seyfried
https://doi.org/10.1089/end.2021.0416
Comment on Rassweiler 2021
The paper of Rassweiler et al. [1] compares the fragmentation efficiency of a so called wide focus/low pressure (XL) versus a small focus/high pressure (SL) lithotripter with two types of stones (medium hard AST gypsium and hard Bego stones).
The scientific motivation for this study reaches 20 years back to the introduction of the first Chinese lithotripter Xixin, a lithotripter with extraordinary wide focal zone (lateral 15, axial 220mm) and a relatively low pressure of (16-24 MPa). At that time this lithotripter was euphorically welcomed since the data of 5 Chinese hospitals (Eisenmenger et al. [2]) reported superior treatment results (half the shock number required, no pain and anaesthesia, superb stone free rates…)
The early publications of Eisenmenger et al. [2, 3] were impressive and henceforth severed as guideline for an eagerly expected improvement of SWL. Whenever the future of SWL was discussed, the wide focus and low pressure strategy was considered the way to go. This was supported by a new theory of fragmentation by circumferential squeezing developed by Eisenmenger [3]. According to this theory, the focus of a lithotripter has to be big enough to completely cover the stone in order to squeeze circumferentially like a tight belt. Inhomogeneous strain and tensile stress due to the squeezing mechanism were claimed to break the stone in accordance with the theory of binary fragmentation. In consequence a focus as big as suitable to cover a stone is requested.
Surprisingly, to my best knowledge, the previously reported extraordinary good results could not be confirmed by later publications. This is amazing since the authors of the present paper have this Chinese lithotripter (here marked as lithotripter XL) for more than a decade at hand but they not yet published according clinical data.
More than 20 years have passed and the question of the optimal focal size is still under discussion. The authors of the present paper take a new approach to evaluate the advantages of a wide focus/low pressure concept of optimal stone fragmentation.
The result of the study is as follows: The two lithotripter in competition both are effective in stone fragmentation in vitro. An advantage for the wide focus/low pressure version is claimed since the final fragments were judged to be finer and the need to refocus due to spreading of the fragments was minor with an extended focus area. Their instruction: It is up to the manufacturers to bring these promising technologies on the market. Is this really the case?
Regarding the wide focus/low pressure concept the following remarks have to be made:

1. The presented fragmentation results are achieved with the XL lithotripter in its highest operational levels 9.3 and 10,.3 kV (no more energy reserves for difficult stones available)
2. The results for the SL (Siemens) lithotripter are achieved with minimal energy setting (level 3 of 8 levels with ample energy reserves)
3. The first cracks for the AST stones appear after 7(SL) vs 11(XL) and 151(SL) vs 524(XL) pulses for the hard BegoStone. This difference is significant and important to assess progress of fragmentation during medical lithotripsy. It would be uncomfortable not to know before the very end of the treatment whether any fragmentation happens at all.
4. The pulse numbers required in vivo vs in vitro fragmentation usually differ by a factor 10 or more. That means that the treatment time would increase significantly before any effect will occur.
5. After complete comminution to fragments ragmentation and cannot be compared. A previous phase would show similar fragment size and distribution of wide focus/low pressure version. If the theory of binary fragmentation, as claimed in the paper, is applicable in this fragmentation process, fragmentation starts with two big fragments and progresses in a sequence of split fragments.
6. Fragmentation experiments on a flat mesh allow the fragments to spread over the flat area till to the rubber ring. The inner diameter of this ring has a huge impact on the number of pulses requested to final comminution. A wider focus may have some benefit in this case, but stone suspension in a finger cod or in a small net with pores of 2 mm e.g. can mimic clinical conditions of stones in kidneys (pelvis, calices, ureter) in a better way since fragments assemble at the bottom of the device as normal in kidneys.
Looking at the in vitro fragmentation results of the study, the plain data show the equal or mostly superior performance of the small focus/ high pressure SL even in its low energy level (3) mode compared to the wide focus/ low pressure XL in its highest energy mode. The claimed more homogeneous fragmentation of the XL may be real but the reported final comminution of both devices does not support this issue since the end point for both devices was: all fragments iate.
Looking at published data listed

With the exemption of the original paper of Chaussy et al. listed in Table 27.2 (see above), who reached 90% success, the overall efficacy of 70% for the HM3 is reported by the listed sites.
A comparison of the results of the HM3 is listed in the following table 27.3 (see below)

The published data confirm similar or better success rates of small focus/high pressure devices (in particular the STORZ MODULITH) compared to the original HM3.
Newer publications e.g. of Zehnder et al. [5] and Neisius et al. [6] are taken as convincing evidence to prove the superiority of the wide focus/low pressure concept. A closer inspection, however, does not support this issue.
Zehnder et al. [5] report superior results for a modified “hybrid” Dornier HM3 vs. the small focus/high pressure STORZ MODULITH SLX -F2 Lithotripter. Apart from the fact that the inferior results mainly occur with ureteral stones, what may be due to high treatment levels (8-9) which pushes the stone out is location in the ureter what requests frequent repositioning and in increased fluoroscopy times, the modified “hybrid” HM3 cannot be taken as a wide focus/low pressure device.
The modified HM3 “hybrid” lithotripter used for the study in Bern was a unique device with very special features not to mix up with the modified HM3 as known. When Dornier followed the trend for anaesthesia-free-treatment the ellipsoid aperture was increased from 15.0 cm (176 cm2 ) to 17.2 cm (232 cm2) to distribute the shock wave energy over a larger skin surface, and the generator capacity of most modified HM3 lithotripters was reduced to 40 nF. The ‘‘hybrid’’ HM3 lithotripter, however, used for the present trial, combines the wider ellipsoid (Ø 17.2 cm) with the original generator capacity of 80 nF. This modification makes an essential difference. First of all the focal zone is reduced due to the enlarged aperture from 16mm to 7,5mm (Müller [6]), meaning the focused energy is concentrated on a roughly four times smaller focus area which comes close to a small focus device. Secondly, this enlarged ellipsoid generates focus pressures of 37MPa with generator capacity of only 40 nF vs. the unmodified HM3 of only 32 MPa using double generator capacity of 80 nF. The manufacturer did not measure this unique “hybrid” device with the enlarged ellipsoid and double the electric power of the 80nF generator. Obviously, however, the technical data provide significantly higher focal pressures and energies that by far exceed proven limits.
Accordingly, the modified “hybrid” HM3 lithotripter can be considered an extraordinary strong small focus / high-pressure lithotripter (and not a prototype of wide focus / low-pressure lithotripter) which was never marketed elsewhere in the world.
The paper of Neisius et al. [7] reports on a new lens for a commercial electro-magnetic lithotripter (Siemens). The purpose of the lens design was to modify the pressure signal of EM-generator in order to enhance cavitation activity near focus. The authors (Neisius et al.) suppose that cavitation is the essential mechanism required to gain fine-fragmentation of coarse fragment left after initial spallation.
Beyond the desired issue to increase cavitation, the modification of the lens design causes a loss of energy concentration in a wider focus with less pressure, a feature what is again considered favourably by the authors for the wide-focus / low-pressure concept.
The lens modification, however, reduces the fragmentation efficiency considerably. For compensation, the driving power of the generator has to be increased from 13-16kV to 16-19kV, that means that e.g. 45% more energy is needed to reach the same fragmentation efficiency as delivered by the unmodified lens.
It is obvious that increasing energy, fragmentation efficacy increases as well. Turning energy down, pain and possible lesions will also be reduced. As a rule (look at the famous Medicus Paracelsus) effects and side-effects go hand in hand. Accordingly, the modified lens design does not prove superiority of the wide-focus / low-pressure concept.
The ongoing dispute about the best focus dimensions depends on each fragmentation mechanism different individuals prefer.
1 A two-step procedure with coarse fragmentation by e.g. spallation in the first step requires a second step for fine fragmentation by cavitation (Pei Zhong [8]). Accordingly, the modified lens, published by Neisius, is the consequent solution since the modification increases cavitation. In spite of that claimed favourable feature, the fragmentation efficacy is reduced and compensation by increase electrical input is required.
2 If we follow the idea circumferential squeezing, (Eisenmenger), a wide focus is mandatory.
Keeping in mind that solid and brittle material (e.g. kidney stones) predominantly break under tensile and shear forces (compressional strength of solid bodies is roughly 10 times higher than the tensile strength), shock wave pressure is effective if it generates tensile and shear forces in the stone. This happens in favourably manner by internal reflections at the rear surface of the stone. This mechanism of spallation is essential in stone fragmentation.
The squeezing mechanism of Eisenmenger may contribute to fragmentation but not in dominant way, Sapozhnikov O. [9].
There is a new theory for a fragmentation mechanism of shock waves acting on brittle material such as kidney stones and soft tissue, Wess and Mayer [10]. This theory is not developed ”mainly as argument against the advantages of a wide focus” as stated in the paper of Rassweiler. It is a general theory not restricted to small or wide focal configurations. It bases on basic physics laws of momentum and inertia; developed by Isaac Newton in the 17th century.
Shock waves feature a momentum, but the relevance of which was not recognised for more than 40 years. The impact of momentum and its change by reflection is depicted in the following figures.



Fig. 2 Sample stone hanging at 2 filaments in water bath, providing freedom of lateral moving when exposed to single shock wave pulses from right.

Fig. 3 Stone, some microseconds after impact of a shock wave pulse from right. At the focus area material particles and dust are expelled out of the surface and catapulted against the direction of the impinging shock wave from right. The stone is shattered and dust covers the total surface.

Fig. 4 Stone, some milliseconds after impact of the shock wave from right. The stone is pushed by the shock wave and moves with initial velocity of 0.2 m/s.
Shock waves can pass soft tissue e.g. of a human body, without substantial impairment but with generation of significant forces at acoustic interfaces with different impedances (ρc). At the acoustic interface between stone material and soft tissue, the shock wave is reflected and its momentum splits into a reflected and a transmitted part. According Newton’s laws each change of momentum is inherently connected with a force. We could measure the momentum, which was transferred from a single shock wave pulse on gypsum cube of 1cm3 and the weight of app. 1 gram and could calculate the generated forces and the according acceleration. For this exemplary set-up we could measure an extraordinary force of ≈ 370 N and an acceleration of ≈10000 g.

Fig. 5 The effects of a shock wave impinging from right
The effect of a single shock wave pulse is characterised by following steps:
1. Impact on the front surface of stone (in Fig. 3 and 4 from right). Part of the shock wave is reflected without changing its sign. Another part of the shock wave is transmitted into the volume of the stone. The negative pressure from the incoming wave and the negative pressure from the reflected wave superimpose constructively to an increased negative pressure pulse (momentum to the right) and expels small particles and dust out of the surface by tensile forces.
2. The Momentum of the incoming shock wave splits by reflection into the reflected and the transmitted part. This change is inherently connected with a strong force in the direction the incoming shock wave. This force is effective for the time of impact, which is determined by the pulse length of the positive pressure and lasts approximately one microsecond. The force acts against the inertia force of the stone (actio = reactio) and accelerates the stone as displayed in Fig. 4. Since the acceleration takes place in only one microsecond, it is measured in the range of 10000g.
3. The transmitted part of the shock wave propagates to the rear surface where it is reflected changing its sign from positive to a negative amplitude. The reflected part interferes with the transmitted part and generate a strong negative pulse in the moment the negative part of the reflected wave coincides with negative part of the just incoming transmitted wave. This takes place at a small distance of the rear surface. This phenomenon is known as Hopkinson’s effect and responsible for spalling of particle at the rear end of the stone.
This theory of momentum transfer by shock wave reflection describes the mechanism of fragmentation independent of a small or a wide focus. It does not require a minimum size of stones since the basic law F= ma (F= force, m = mass, a = acceleration) is valid for all masses independent of their weight. It covers coarse as well as fine fragmentation and does not require an additional mechanism as claimed by Zhong [8]. The theory was tested in experiments published [10] and can provide quantitative data for force and acceleration values being responsible for fragmentation.
Conclusion: Analysing the relevant literature (see references) in detail, we could not find essential advantages of a wide focus/low pressure concept. Our own experience [10] does not support the theory of Eisenmenger [2] of circumferential squeezing. The claimed advantage of anaesthesia free treatment is presumably due to the minor shock wave energy applied and linked with minor fragmentation efficiency.
A wide focus may reduce the requested precision of stone localisation but it lacks of sufficient power to fracture hard stones in a reasonable time as obvious in the paper under discussion. A smaller focus may be used in an anaesthesia free mode applying low treatment levels and can concentrate the available energy in a small area to crack even hard stones without effecting adjacent tissue considerably.
References:
1. Rassweiler J. et al., 2021: In-vitro comparison of two electromagnetic shock-wave generators: low-pressure-wide focus versus high-pressure small focus - the impact on initial stone
Published Online: 27 Jul 2021 https://doi.org/10.1089/end.2021.0416
2. Eisenmenger W., Du X.X. at al., The first clinical results of „Wide focus and low pressure” ESWL, Ultrasound Med Biol 2002, 28, 769-774
3. Eisenmenger W., The mechanism of stone fragmentation in ESWL. Ultrasound Med Biol 2001, 27, 683 - 693
4. Ogan and Pearl. 2005
Minimally Invasive Urologigcal Surgery
Ed.: Moore, Bischoff, Loening, Docimo
5. Zehnder P et al., A prospective randomised trial comparing the modified HM3 with the MODULITH® SLX-F2 lithotripter. E.Eur Urol. 2011 Apr;59(4):637-44. doi: 10.1016/j.eururo.2011.01.026. Epub 2011 Jan 25.PMID: 21296481 Clinical Trial.
6. Müller M., Dornier-Lithotripter im Vergleich Vermessung der Stosswellenfelder und Fragmentionswirkungen, Comparison of Dornier Lithotripters Measurements of Shock Wave Fields and Fragmentation Effectiveness, Biomedizinische Technik 35 (1990), 250-262
7. Neisius et al., Improving the lens design and performance of a contemporary electromagnetic shock wave lithotripter, March 2014, Proceedings of the National Academy of Sciences 111(13), DOI: 10.1073/pnas.1319203111
8. Zhong P., Delale (Ed.): Bubble Dynamics & Shock Waves, SHOCKWAVES 8, pp. 291–338. DOI: 10.1007/978-3-642-34297-4 10 c Springer-Verlag Berlin Heidelberg 201
9. Sapozhnikov et al., Analysis of stone fracture in lithotripsy, J. Acoust. Soc. Am., Vol. 121, No. 2, February 2007, 1201
10. Wess O.J., Mayer J., Fragmentation of brittle material by shock wave lithotripsy. Momentum transfer and inertia: a novel view on fragmentation mechanisms. Urolithiasis 48, 137–149 (2020). https://doi.org/10.1007/s00240-018-1102-6