Advancements in Shock Wave Lithotripsy: Pushing Boundaries with Innovative Technology and Techniques

Abstract

This chapter explores the significant progress made in shock wave lithotripsy (SWL) for the treatment of urinary tract stones. SWL, a non-invasive treatment modality that uses shock waves to break up stones, is widely employed for urolithiasis treatment. A comprehensive overview of the development of SWL, driven by innovative technology and refined techniques is highlighted. These advancements encompass improvements in lithotripter design, imaging methods, and treatment planning. Notable topics include modifications in shock wave generation, focusing and localization techniques, as well as the clinical application of high-frequency shock waves or ‘burst-SWL’ that may revolutionize treatment outcomes. The impact of these techniques on treatment effectiveness, stone clearance, safety, potential complications, and patient comfort are also discussed. Furthermore, it delves into the challenges and limitations associated with SWL, such as the importance of tailoring treatment protocols to individual patient needs and considering cost-effectiveness in the era of advanced endo-urology.

1. Introduction and historical background

The phenomenon of sound waves being focused has been recognized since ancient times. Greeks utilized this knowledge to construct vaults enabling them to overhear on conversations of imprisoned rivals [1]. For decades, high-energy shockwaves have been acknowledged, encompassing blast effects linked to explosions and the sonic boom resulting in window breakage when aircraft surpass the speed of sound [1].

Established by the German Ministry of Defense in 1969, engineers at Dornier Medical Systems commenced investigating shockwave effects on tissues. Not only could they generate shockwaves, but they also discovered that these waves, when generated in water, could traverse living tissues without causing apparent harm; however, brittle materials in their path tended to fracture. This discovery laid the groundwork for the medical application of shockwaves [2]. After achieving the capability to produce low-energy shockwaves in a reproducible and predictable manner, Dornier lithotripter progressed through several prototypes and eventually in February 1980 resulted in first treatment of a human by shock wave lithotripsy (SWL). This was followed by subsequent production and distribution of Dornier HM3 ™ lithotripter in 1983 and approval by the US Food and Drug Administration (FDA) in 1984 [2]. Since then, this approach has emerged as the primary treatment for the majority of urolithiasis patients due to its minimally invasive nature compared to traditional treatments.

The competitive landscape in stone management has shifted. With advancements in endo-urological procedures and more predictable outcomes using alternative methods such as flexible ureteroscopy (f-URS) and percutaneous nephrolithotomy (PCNL), there’s a trend moving away from SWL, as technical advancements in SWL have not kept pace with endo-urological progress [1]. This shift is also influenced by diverse practice patterns, evolving indications, facility accessibility, improved instruments in urological arsenal, financial considerations, and urologist preferences [1, 3]. While SWL remains a valuable option for certain patients, its limitation has become increasingly evident. Given these challenges, it is imperative to enhance SWL to maintain its relevance and competitiveness. These include not only continued technological advancements in machine itself but also research into refining patient selection criteria and combining SWL with adjuvant therapies. We explored English language literature pertinent to the topic of advancement in SWL, utilizing electronic databases such as PubMed, ScienceDirect, Google Scholar, and Embase for the compilation of this book chapter.

2. Fundamentals of shock wave lithotripsy (SWL) physics

Shockwaves (SW) are specialized sound waves characterized by a sharp positive pressure peak followed by a trailing negative wave. These waves are generated extracorporeally and passed through the body to fragment stones. SWL operates based on the principles of acoustic energy propagation, focusing, and interaction with stones to induce fragmentation. These waves travel unimpeded through substances with similar acoustic impedance. Upon encountering a boundary between substances with different acoustic impedance, such as water and stone, new stress waves are generated and propagated into the stone [3] This leads to stone fragmentation through compression and tensile forces, erosion, shearing, cavitation, and dynamic fatigue.

In cavitation, when SW energy is applied at a focal point, it results in the formation of water vapor bubbles that explosively collapse, generating microjets that erode and fragment the stone (Figure 1).

Figure 1.

Shock wave fragmentation effect on stone.

3. Types of lithotripters

Three types of shockwave generators are available: electrohydraulic (EH), electromagnetic (EM), and piezoelectric (PE) (Figure 2). Depending upon the type, various focusing systems are employed to direct this acoustic energy toward a geometric position typically housing the target i.e. stone of interest. Thus shockwaves generated at the first focal point ‘F1’ are converged at the second focal point ‘F2’. The target area (blast path) is a three-dimensional region at ‘F2’ where shockwaves are concentrated, leading to stone fragmentation [4].

Figure 2.

Different types of shockwave lithotripters. Electrohydraulic (EH), electromagnetic (EM) and Peizoelectric (PE).

3.1 Electrohydraulic (EH) lithotripters

EH lithotripters employ electric discharges to generate shockwaves for stone fragmentation. The original Dornier HM3™ lithotripter utilized this technology, generating shockwaves through an electric spark gap located within a water bath filled with degassed, deionized water. The spark discharge between two electrodes creates a vaporization bubble that expands and collapses, generating a pressure wave [4]. This wave is directed from an ellipsoidal reflector toward the calculus at focal point ‘F2’. Although it offered efficient stone fragmentation and versatility, it has relatively shorter-lasting electrodes with the requirement for general anesthesia. Electroconductive (EC) generators are a variant of EH technology, utilizing highly conductive solutions (carbon-based materials, such as graphite or carbon fiber composites) to reduce spark variation and enhancing shockwave delivery efficiency [5].

3.2 Electromagnetic (EM) lithotripters

These devices employ a high-voltage electromagnetic coil to create a magnetic field, inducing vibration in an adjacent metallic membrane, located within a cylindrical ‘shock tube’ [3, 6]. The resulting shockwave passes through water and is focused at focal point ‘F2’ using an acoustic lens (Dornier™ and Siemens™) or a parabolic reflector (Storz™). These devices offer reliable shockwave delivery and flexibility in energy adjustments, with the advantage of reduced anesthesia needs and precise targeting [4].

3.3 Piezoelectric (PE) lithotripters

Piezoelectric lithotripters employ multiple piezo-ceramic elements to generate shockwaves. Mounted on a hemispherical dish, these elements expand when excited by high-frequency, high-voltage pulses, generating ultrasonic waves which converge toward the geometric focus of hemispherical carrier, forming a high-energy shockwave [7]. PE lithotripters have a wide area of shockwave entry at the skin surface, causing minimal discomfort, but possess a narrow focal point with limited energy at ‘F2’, resulting in poorer comminution efficiency. The newer Wolf Piezolith 3000 ™ employs a double layer of piezo-ceramic elements to address this limitation [3].

3.4 Comparison between different types of lithotripters

Each type of shockwave lithotripter comes with its own set of potential advantages and disadvantages. Electrohydraulic (EH) lithotripters, for instance, offer the advantage of producing high-energy shockwaves, making them highly effective for breaking up harder stones. However, they tend to be bulky and require frequent maintenance. Electromagnetic (EM) generators, on the other hand, demand less maintenance, rendering them suitable for outpatient settings. However, they may lack the energy required for larger or denser stones. Piezoelectric (PE) lithotripters provide precise control over shockwave parameters, enhancing safety, but they are generally less powerful. Most comparative studies have focused on EH and EM lithotripters, assessing their efficacy and safety. In a meta-analysis of published reports for the treatment of renal stones <2 cm, EM lithotripters demonstrated a stone-free rate of 70.1%, a re-treatment rate of 46.26%, and a complication rate of 19.7%. In comparison, EH lithotripters exhibited rates of 51.22, 41.6, and 18.8%, respectively. PE lithotripters, while less successful in terms of stone clearance and associated with a higher re-treatment rate, were found to be safer in terms of complications [6].

4. Advancement in lithotripter design

Advancements in SWL technology have been made to enhance its effectiveness and safety, encompassing shockwave generation, focusing, patient coupling, stone localization (imaging), improving efficiency in stone fragmentation and clearance, and strategies to minimize collateral tissue damage.

The original Dornier HM3 ™ lithotripter featured a cumbersome gantry and water bath. Contemporary lithotripters have evolved into smaller, compact, and portable devices [4]. This has enabled performing lithotripsy in diverse positions such as supine, prone, lateral, inclined, or declined leading to increased patient comfort. Lateral panels incorporate monitors for ultrasound imaging, ECG, and respiratory triggering. Newer lithotripters now use a modular design, separating the fluoroscopy unit from the lithotripter, optimizing storage space, and allowing the fluoroscopy unit’s versatility for other procedures [4].

With the evolution, the operators possess the ability to control various device parameters that influence treatment outcomes, including the lithotripter’s acoustic output, focal volume, coupling to the patient, SW administration parameters (rate, voltage, sequence & ramping), stone localization and anesthetic technique(s).

5. Advancement in stone localization; innovative imaging techniques for enhanced stone targeting

Accurate stone localization holds paramount importance for the success of SWL procedures and can be achieved through fluoroscopy & ultrasonography.

5.1 Fluoroscopy

Fluoroscopy provides operator with a familiar modality, particularly advantageous for effective ureteral stone localization. However, drawbacks of fluoroscopy include exposure to ionizing radiation for patients and operators, its incapability to locate radiolucent stones and space requirements. Advancements in fluoroscopy technology like digital fluoroscopy and low-dose fluoroscopy offer improved image quality with minimized exposure [8]. Additional technological advances include use of collimators, reducing radiation fields, employing dose level settings, and utilizing pulse fluoroscopy for radiation exposure management by adhering to ALARA (As Low As Reasonably Achievable) principle. The latest lithotripter generations mitigate radiation exposure through automated fluoroscopic localization with improved resolution [8].

5.2 Ultrasound

Ultrasonic stone localization offers real-time monitoring without radiation exposure and can effectively identify radiolucent stones and stone fragments as small as 2–3 mm. Recent ultrasonography advancements, such as high-frequency transducers and improved image resolution, bolster stone localization accuracy. Ultrasound’s multi-plane visualization aids in precise targeting and monitoring during shockwave application. Additionally, doppler ultrasound can assess renal blood flow and potential complications such as bleeding and renal hematoma formation. Spectral doppler signal can also provide real-time feedback (hit and miss) information during therapy (Dornier Gemini™) [9].

European Association of Urology (EAU) guidelines suggest both ultrasound and fluoroscopy as valid alternatives to monitor SWL progress [10]. Many new lithotripters combine fluoroscopy and real-time monitoring through ultrasound to reduce imaging radiation exposure. This dual-modality imaging offers enhanced stone localization and treatment precision [8].

As the targeted stones may move out of shockwave focus due to stone or patient movement, shock wave focus/target should be reconfirmed at regular intervals, and continuous ultrasound monitoring or periodic checks every 300 to 500 shocks are recommended [10].

5.3 Virtual tracking system

Stone movement in and out of the focal zone, often due to respiratory and patient movements, impacts fragmentation efficacy. Due to factors such as respiratory rate and excursion length and lower focal width of the SWL machine, the stone may be outside the focal zone during over 50% of SWL sessions [1]. This leads to missed targets and inadvertent renal tissue impacts.

The emergence of virtual tracking systems (VTS) signifies a novel imaging modality that enhances the accuracy and effectiveness of SWL for urolithiasis. Through real-time imaging, tracking technology, and predictive algorithms, VTS achieves precise stone localization and enables targeted shockwave delivery. These systems utilize advanced fluoroscopy and ultrasound-based imaging techniques with sophisticated transducers to track stone position and movement during SWL [1]. Advanced image processing algorithms and built-in systems in piezo-electric lithotripters identify the stone, continuously track its trajectory, and steer the shock waves onto the target [11].

Visio-track (VT) ™ locking system utilizes a handheld probe with infra-red stereo vision to triangulate stone location, adjusting the focal points for enhanced accuracy compensating for respiratory motion, leading to better stone-free rates (79.5% vs. 54.5%, p = 0.001) compared to fluoroscopy [12]. Storz Medical™ has 2 tracking options available, acoustic and optical, with the latter proving superior results. EDAP/TMS Sonolith i-sys™ also employs ultrasound imaging complemented by optical tracking for stone localization [12].

Own et al. [13] introduced a real-time tracking and targeting system where short ultrasonic pulses are delivered to the lithotripter’s focal point, detecting scattered pulses by a receiver transducer from the stone upon target impact. While these systems show promise, they are still undergoing modifications and have not yet gained widespread clinical adoption.

5.4 Evaluating stone breakage with acoustic feedback

Despite the significant improvement in fluoroscopic and ultrasound imaging for precise stone localization over the years, they remain less reliable in determining treatment endpoints [7]. Experienced urologists recognize signs of stone fragmentation through margin softening, density loss, and particle movement, but these can be challenging to assess.

Acoustic feedback systems have emerged to monitor stone fragmentation [14]. These systems employ a broadband receiver to track shockwaves reflected off the stone, including reverberations from acoustic waves transmitted into the stone. As the stone breaks, smaller fragments generate high-frequency signals. In vitro studies show this system can differentiate fragments varying in size by 1–2 mm thereby minimizing excessive shockwave delivery to renal tissues [14].

Another system by Leighton et al. [15] employs an acoustic sensor to detect emissions from each shock at the lithotripter’s focal point. This “listening” device gauges cavitation at the target area, correlating signal quality with stone breakage degree. A computer display indicates if shockwaves hit the stone and whether fragmentation progresses.

5.5 Augmented reality (AR) and virtual reality (VR)

Emerging technologies like augmented reality (AR) and virtual reality (VR) hold potential to transform preoperative planning and intraoperative guidance for shockwave lithotripsy [11]. The integration of AR and robot-assisted ultrasound-guided lithotripsy shows promise for enhancing treatment outcomes and patient care. AR systems overlay virtual information onto real-time ultrasound images, providing augmented visualization and guidance during procedures. Robot-assisted SWL merges robotic precision with real-time ultrasound imaging, enhancing targeting accuracy for shockwave delivery [1].

6. Enhancing treatment strategies and protocols with innovation in technology

To enhance clinical safety and SWL efficiency, extensive research is underway, driven by advancing insights into stone fragmentation mechanisms and tissue injury. The International Alliance of Urolithiasis (IAU) has released comprehensive guidelines for managing urolithiasis, encompassing preoperative evaluation, procedural tips, tricks, and post-procedure follow-up strategies with the aim to provide a clinical framework for urologists engaged in SWL procedures [16].

6.1 Patient and stone-related factors

Despite being non-invasive, SWL encounters limitations and challenges that can influence treatment outcomes. Patient selection significantly influences SWL success rates. Various clinical nomograms aim to identify optimal factors for SWL based on stone-free rates. Predictors encompass clinical parameters like age, sex, body weight, BMI, and CT scan-based factors such as stone location, number, diameter, Hounsfield units (HU), and hydronephrosis presence [16]. One such score is the Triple-D score, utilizing stone diameter, density, and skin-to-stone distance on pre-operative CT scans [17].

Patient-related factors, including obesity (high BMI) and greater skin-to-stone distance, negatively impact SWL success rates [17]. Comparatively, skin-to-stone distance (SSD) outperforms BMI/body weight as a marker of success. Given that most lithotripters possess a 15 cm focal length and adipose tissue attenuates shockwave energy, SSD should not exceed 120 mm [10].

Certain stone compositions, such as brushite, cystine, and calcium oxalate monohydrate, pose challenges to effective fragmentation and may require higher energy level or alternative treatment options. The stone burden is a significant factor that impacts stone free rate (SFR) after SWL. The upper limit for upper tract stones is 20 mm, while for lower pole renal stones, it is 15 mm. Stones with high density (>1000 HU) exhibit resistance to SWL [18]. Stones in anatomically complex sites such as lower calyx stones (with a long infundibulum, narrow infundibular neck, and an acute infundibulopelvic angle), stones in calyceal diverticulum or malformed kidney pose targeting and fragmentation challenges. Lower pole stones treated with SWL tend to have lower clearance rates compared to other collecting system regions. A meta-analysis involving 2927 patients with lower pole stones indicated a decreased stone-free rate (SFR) for SWL (52.9%) compared to PCNL (90%) [19].

6.2 Optimization of SWL parameters

Research has delved into the impact of various shockwave parameters on stone fragmentation, including shockwave energy, frequency, and focal width.

6.2.1 Energy level

The energy level of SW plays a pivotal role in stone fragmentation. Inadequate energy might result in insufficient fragmentation, while excessive energy can lead to tissue injury. Factors such as stone size, composition, and location dictate the suitable energy level.

6.2.2 SW frequency/rate

Ensuring effective stone fragmentation while minimizing harm is essential in SWL. Kidney damage during a single SWL session is dose-dependent on pulse amplitude and shockwave count. The standardized number of shocks per session remains elusive, with a general upper limit of 4000 shocks, subject to energy level used [7].

Lower shockwave doses and slower shockwave rates are recommended to minimize acute and lasting tissue injury. Appropriate shockwave rate can enhance stone fragmentation and limit tissue damage. Dornier HM3 TM, the first clinically used lithotripter, employed a “gated” shockwave rate synchronized with electrocardiogram (ECG), often around 60 to 80 per minute. Subsequent lithotripters shifted to non-gated fixed rates, typically 100–120 per minute, aiming for shorter treatment times [6].

The exact mechanism influencing rate’s impact on SWL efficiency is debated but is different for stone breakage and tissue injury [20]. Cavitation bubbles generated during SWL implode against the stone surface, forming high-speed jets that erode it [2]. At higher rates, subsequent shocks hit existing bubbles, creating less effective bubble clouds that absorb/dissipate energy [20].

Renal tissue damage, including hematoma formation, also ties to cavitation effects. Faster shockwave rates than tissue’s relaxation time induces stress accumulation and vessel rupture, intensifying cavitation’s impact [2]. Numerical models suggest a shockwave threshold for tissue relaxation around 1 Hz, with rates above 60 SW/min causing tissue deformation and injury [2].

Reducing shockwave delivery to 60 per minute from 120 in a randomized controlled trial elevated stone-free rates from 28 to 60% [3]. A meta-analysis comparing 60 vs. 120 shocks per minute confirmed improved success with 60, along with fewer additional procedures and better cost-effectiveness [21].

6.2.3 Focal length and positioning

Proper positioning and focal length of shockwaves are critical for effective stone fragmentation. Focal length determines the distance between the shockwave source and the stone, and it should be adjusted according to the stone’s location within the urinary tract [4]. Precise focusing ensures accurate targeting and concentration of shockwaves on the stones. Aligning stone adjustment with the shockwave’s focal point enhances treatment efficiency and reduces the risk of collateral damage.

6.2.4 Enhancing stone fragment expulsion using adjuvant pharmacotherapy

A key challenge in SWL is the clearance of residual fragments, often necessitating secondary treatments. Residual stone fragments can lead to complications like Steinstrasse, regrowth, infections, renal colic, and persistent discomfort. Innovations aimed at expediting fragment passage encompass various instruments [2].

Studies have demonstrated the utility of pharmacological therapies to promote stone passage and enhance the overall effectiveness of SWL. These treatments involve calcium channel blockers, corticosteroids, nonsteroidal anti-inflammatory drugs (NSAIDs), and alpha-blockers [20]. These modalities effectively reduce the time and pain associated with stone expulsion.

Although treatment protocols remain to be standardized, multiple studies and meta-analyses demonstrate that mechanical percussion, forced diuresis, and body inversion (PDI) with external physical vibration ‘lithecbolc’ (EPVL) are safe and effective methods for assisting the clearance of stone fragments, particularly for lower pole stones [22].

PDI therapy contributes to enhanced stone clearance in three ways: increased urine production for fragment flushing, prone Trendelenburg positioning to exploit gravity, and manual/mechanical flank percussion to dislodge fragments via vibration.

6.2.5 Focused ultrasound therapy

Advancements in focused ultrasound technology for residual fragment clearance are yielding promising outcomes. Transcutaneous focused ultrasound therapy probe is used to generate acoustic streaming forces sufficient to displace stone fragments by several centimeters [23]. Equipped with an ultrasound imaging probe, this therapy probe locates and directs fragments for clearance. This also aids the dispersion of fragment clusters to ascertain effective stone breakup [6].

6.2.6 Timing and treatment repetition

The decision to repeat treatment depends on the stone’s response to the initial session. No prospective study has been conducted to determine the appropriate time interval for repeated shockwave lithotripsy sessions. For kidney stones, the interval between two shockwave sessions should not be less than 1 to 2 weeks. For ureteral stones, an interval of 1 day may suffice. This is attributed to the fact that renal contusions typically improve within 1 to 2 weeks, and there is no evidence that SWL for ureteral stones leads to renal hematoma formation [4].

6.2.7 Improving acoustic coupling

One of the critical parameters to enhance stone fragmentation while minimizing associated SWL side effects is acoustic coupling. The first-generation lithotripter, Dornier HM3™, employed a 1000 L water bath to couple shockwaves to the patient. Water, with an acoustic impedance closely resembling body tissue, efficiently transmits shockwaves into the body, minimizing energy reflection or absorption at the water-skin interface [3].

Subsequent lithotripter generations introduced water cushions with silicon membranes in dry treatment heads. This design improved portability and convenience for both patients and operators. However, these dry treatment heads necessitate coupling media, like gels or oils, to connect the patient to the device. For instance, the use of a water cushion combined with ultrasonic gel replaced the traditional water bath [1, 4]. An exception among current machines is the Storz SLX™ (Storz Medical Switzerland), which employs a partial water bath for shock head coupling. This lithotripter is especially good for obese patients facing issues with coupling efficiency and accurate stone localization [7].

Conventionally, coupling involves applying gel to the treatment head’s cushion and patient’s skin contact area [7]. Enhancing coupling quality involves improved gel application and handling techniques [20]. Effective application includes directly applying gel from a jug to the treatment head’s center, followed by pressing the cushion onto the patient. Further, spreading the gel is facilitated by increasing cushion water inflation pressure [7].

The media’s quality and viscosity significantly impact SWL’s effectiveness. Studies have shown that higher-quality water-soluble lubricating gel with lower viscosity exhibited superior stone fragmentation with the need for lesser number of shock waves [20]. Minimizing trapped air bubbles in the coupling media is crucial, as these bubbles substantially reduce shockwave efficiency. De-coupling and re-coupling during patient repositioning can introduce substantial air pockets in the coupling medium. Studies indicate that a mere 2% of coupling area occupied by air bubbles leads to a 20 to 40% reduction in stone fragmentation efficiency [24].

Direct imaging provides immediate feedback, while some lithotripters equipped with in-line ultrasound probes monitor coupling [3]. Devices featuring video cameras have been developed to ensure optimal coupling by detecting air bubbles in the transmission zone. Tailly et al. [25] have introduced a modification to acoustic coupling by incorporating a camera and LED light in the shockwave head of a Dornier Gemini ™. This modification has been effective in improving coupling quality, leading to a 25% reduction in number of applied shockwaves and energy. This approach outperformed blind coupling in a study involving 336 patients who underwent SWL (SFR: 78.2% for renal and 81.7% for ureteral stones vs. 62.9% for renal and 67.9% for ureteral stones) (p < 0.05) [25].

6.3 Enhancing safety: monitoring and safety measures

While SWL is generally safe, several enhancements have been devised to minimize discomfort, optimize patient compliance, and ensure favorable outcomes. Real-time monitoring of shockwave delivery, patient vital signs, and treatment response has advanced over time. Similarly, adequate pre-hydration, appropriate shockwave parameter selection, maximal shocks per session, and precise focusing are vital [20].

Shockwave delivery monitoring involves tracking shockwave energy, focal point localization, and coupling efficiency. Focal point localization monitoring ensures precise stone targeting, reducing impact on surrounding tissues. Coupling efficiency monitoring evaluates acoustic coupling quality between the shockwave source and patient skin, optimizing energy transmission [25]. Real-time monitoring of patient vital signs and physiological parameters during SWL aids in detecting adverse events and patient discomfort.

6.4 Patient positioning

During SWL, a stable patient position is crucial as patient and respiratory movements can displace stones from the shockwave focus, impairing disintegration rates. Optimal patient positioning is crucial to minimize shockwave travel distance and mitigate interference from skeletal elements such as transverse processes, ribs, sacroiliac bones, and the pelvis [4].

Accessories like neck rolls, knee rolls, wedges, and armrests can stabilize patients. Modern lithotripters offer under-table and over-table positions, enabling supine treatment for all stone locations [16].

6.5 Anesthesia and sedation techniques

Advancements in anesthesia and sedation techniques have significantly improved patient comfort during SWL procedures. By enhancing patient tolerability through analgesia, overall SWL outcomes can be positively influenced.

The initial clinical lithotripter, Dornier HM3™, required general anesthesia for treatment. Subsequent lithotripter generations enable treatment without anesthesia. Newer designs feature wider apertures that distribute the acoustic field over a broader skin area, thereby reducing skin surface pain [2].

Intravenous sedation, often termed conscious sedation, is a well-suited pain management protocol for most SWL patients. This involves administering sedative medications to induce a state of reduced awareness and anxiety during the procedure thus fostering patient cooperation and tolerance during SWL. Commonly used medications include benzodiazepines (e.g., Midazolam) and opioids (e.g., Fentanyl) to achieve conscious sedation.

A meta-analysis of randomized controlled trials assessing opioids, non-steroidal anti-inflammatory drugs (NSAIDs), and simple analgesia (Paracetamol) showed both NSAIDs and opioids provided safe and effective analgesia, with no significant pain score differences [26].

In cases where conscious sedation is not available or complex cases with prolonged procedures are involved, general anesthesia may be necessary. This approach is favored for infants, young children, and extremely anxious patients [2]. General anesthesia also minimizes stone motion by controlling respiratory rate and volume. Studies comparing general anesthesia and conscious sedation for SWL indicated higher success rates (70–87%) in the general anesthesia group compared to 51–55% in sedation group respectively [26].

6.6 Advancement in focal zone and volume

Acoustic output, amplitude, and spatial energy distribution vary between lithotripters [1]. A narrower focal zone dissipates less energy into the stone, particularly if narrower than the stone. Evidence suggests that a wide focal zone benefits stone disintegration through ‘dynamic squeezing’ by accommodating stone motion and increasing shockwave-stone contact [1, 3].

To address limitations of contemporary electromagnetic lithotripters (with relatively narrower focal zones), modifications to the acoustic lens can be implemented (Figure 3). Creating an annular cut on the outer back surface of the acoustic lens generates a second shockwave from the same pulse, leading to “pulse superimposition” in situ [27]. The second wave overlays the first shockwave from the uncut part in the focal area.

Figure 3.

Modification in acoustic lens of EM lithotripter creating ‘pulse superimposition’.

Another enhancement involves an “energy-dependent” focal shift of electromagnetic lithotripters, broadening the focal width by using a smaller lens aperture in the uncut area to reach around 11 mm (49%) at a relevant treatment energy level [3].

6.7 Dual head lithotripters and tandem pulse technology

Ongoing efforts in the field of SWL aim to refine stone fragmentation techniques, specifically enhancing the crucial mechanism of stone breakage - cavitation. Cavitation bubbles play an indispensable role in stone comminution, and an innovative approach involves employing rapid successive shockwaves to forcefully collapse these bubbles against the stone surface [1, 3]. Efforts are directed toward optimizing the efficacy of cavitation bubbles’ action on the stone surface. Emerging as an innovative approach to enhance lithotripsy procedures, dual-head lithotripters offer improved efficiency, precision, and effectiveness. The concept of dual lithotripters suggests that energy distribution between two synchronized or alternating sources can improve breakage by manipulating the acoustic field [6].

Research indicates that dual pulses effectively localize and intensify cavitation damage in laboratory settings [2, 6]. These devices incorporate two independent shockwave generation sources positioned on opposite sides of the patient, resulting in overlapping or converging shockwaves that lead to enhanced stone fragmentation, treatment flexibility, and improved treatment efficiency. Synchronous arrival of two pulses at the focal point results in pressure doubling, leading to enhanced bubble growth and greater focal pit depth.

The advantages over traditional single-source lithotripters are manifold. Dual shockwave generation allows for more focused energy delivery, concentrating energy at the targeted stone, thereby boosting fragmentation efficiency. Additionally, by manipulating the intensity and timing of shockwaves from each source, treatment flexibility is enhanced, accommodating different stone sizes and compositions. Simultaneous application of shockwaves from two sources reduces treatment duration, promoting patient comfort while mitigating the risk of complications [1, 7].

Another modification involving a “reflector insert” integrated with the original Dornier HM3™ reflector creates a second shockwave immediately after the first, reducing the negative tensile component and improving stone fragmentation while minimizing vascular and renal parenchymal injury [6]. Storz Modulith SLX F2™ offers a dual focus system to adjust focal size, and an alternative approach involves modifying the pulse duration from an electromagnetic source to increase the focal size [1, 4]. Wolf Piezolith 3000™ employs a double layer of Piezo-ceramic elements to amplify shockwave energy without enlarging the dish [3, 4]. Although dual-head lithotripters are not yet widely adopted, ongoing refinements and further research could pave the way for their broader application in the future.

Tandem pulse lithotripsy is an active area of investigation, showing potential to enhance stone fragmentation while minimizing associated injury. This technology involves a second shockwave succeeding the first along the same acoustic axis to forcefully collapse bubbles against the stone. There are two ways to achieve this. The first involves integrating an auxiliary lithotripter, specifically a Piezo-electric array, to generate a secondary (trailing) shockwave that follows the trajectory of the initial shockwave along the same acoustic axis [3, 6]. The second method entails incorporating a piezo-electric lithotripter with an additional charging and discharge circuit, resulting in the emission of a second pulse - a tandem shockwave [2, 6].

6.8 Optimizing shockwave sequence and power (energy) ramping

The sequence of shockwave delivery relates to timing, numbers, power levels, and power ramping—gradual energy increase during lithotripsy. Power ramping was introduced to acclimatize patients to shockwaves in anesthesia-free lithotripsy.

Power ramping with a short pause not only enhances SWL stone fragmentation but also mitigates renal tissue injury [6, 28]. Low-energy shockwaves during ramping condition stones, improving subsequent high-energy pulse fragmentation. Low-voltage stress waves fragment stones, while high-energy shockwaves lead to greater cavitary activity and further fragmentation [3]. Energy ramping’s reno-protective effect is well-supported. Pre-treatment with low-energy shockwaves followed by a pause triggers vasoconstriction, reducing bleeding risk in stiffer vessels, and thus safeguarding against renal injury [20, 28].

In 2016, Skuginna et al. [28] randomized patients with renal stones to stepwise voltage ramping or fixed power. Stepwise ramping reduced renal hematoma rates, with 5.6 vs. 13% in fixed power (P = 0.008). Lambert et al. found that ramping protocol SWL led to lower urinary macroglobulin and beta-2 macroglobulin levels, markers of renal injury [29].

Although no standard protocol exists and depends upon operator preference, the preferred approach is to give 100–500 low energy SW and a 3-minute pause before starting the ‘clinical dose’ of SW therapy [20].

A summary of the technological innovations to enhance lithotripsy effectiveness and safety is presented in Table 1.

Strategy	Summary and examples
Modifications and technological innovations in lithotripter	Attenuating cavitation- induced injury; Direct wave suppression Modified reflectors insert
	Enhancing Cavitation-induced stone fragmentation; Dual pulse & Tandem Pulse lithotripsy Dual focus system (Storz Modulith SLX F2) Double layer of PE elements (Wolf Piezolith 3000)
	Enhancing pre-procedure planning; Augmented reality (AR) and Virtual reality (VR)
	Enhancing stone localization and targeting; Combined modality imaging Spectral doppler ultrasound (Dornier Gemini™) Virtual tracking system (Visio-track VT™)
	Advancement in Focal zone and volume; Acoustic lens modification to create ‘pulse superimposition’
	Improving acoustic coupling; Partial water bath (Storz SLX™) Direct imaging with camera and LED (Dornier Gemini™)
	Evaluating stone breakage; Acoustic feedback (Acoustic sensor)- Broadband receiver to track SW reflected off stone
	Enhancing precision and faster stone disintegration; Burst wave lithotripsy (BWL) (Piezoelectric high-frequency soundwave ‘packages’) Electrohydraulic high-frequency SW
Modification of treatment strategies and protocols	Proper positioning of patient; Mitigating interference from skeletal elements. Aligning stone adjustment with SW focal point
	Lowering SW dose and Slowing SW rate; Reducing the dose to 60/min
	Optimizing shockwave sequence; Low voltage pre-treatment (‘priming’ for first 100–500 SW) Power ramping (stepwise increase in voltage) Timing and treatment repetition (1–2 weeks for Kidney stones)
Adjuncts to improve SWL safety and efficacy	Patient selection (Determining SWL success by;); Stone density (HU) Skin-to-stone distance (SSD) Body mass index (BMI) Renal Anatomy (Lower pole, pelvicalyceal angle, infundibular length)
	Adjuvant pharmacotherapy to improve stone expulsion and reduce pain.; NSAIDS Alpha-blockers Calcium channel blockers
	Percussion, Inversion and Diuresis (PID) therapy; Percussion (manual or mechanical) Inversion (45–70°), and Diuresis (500 ml of water or 20 mg furosemide with or without 1–2 L of IV fluid)
	Altering the chemical environment surrounding Urine; Urinary alkalinizer

Table 1.

Advancement in shock wave lithotripsy.

7. Burst waves and high-frequency shockwaves

The introduction of burst shockwave lithotripsy (BWL) or high-frequency SWL is poised to be a transformative development. Developed at University of Washington, BWL stands as an emerging cutting-edge technology and has the potential to resurge interest in non-invasive treatment approaches for urolithiasis. In an in vitro study, Maxwell et al. [30] demonstrated that this technique enables fine stone fragmentation within a brief timeframe, dependent on stone composition and applied frequency.

In contrast to SWL, where high-amplitude (30–100 MPa) acoustic shocks are applied at slow rate (1–2 Hz) to disintegrate the stone, BWL utilizes low-amplitude (< 12 MPa) bursts of ultrasound delivered at higher frequencies (200 Hz) to repeatedly stress the stones until they ultimately fracture (Figure 4). This leads to formation of tiny, uniform-sized fragments separate from the surface of the larger stone body compared to SWL where stones often shatter into larger fragments. These tiny fragments are small enough to pass spontaneously without any additional intervention [31].

Figure 4.

Pressure waveform of SWL and BWL.

This treatment offers numerous benefits when compared to traditional SWL. This modality can be administered using a handheld probe while the patient is awake in an office setting and limiting radiation exposure. It can target and fragment the stones with remarkable precision and can effectively treat wide range of kidney stone types (Figure 5). The reduced formation of cavitation bubbles minimizes tissue injury with lesser pain [31].

Figure 5.

Stone fragmentation in SWL and BWL.

Recently, Harper et al. conducted first feasibility study of BWL to evaluate its effectiveness and safety. They used a device Propulse 1™ and handheld, water-filled, combined imaging and therapy transducer, SC-60 ™, that was coupled with gel to the skin allowing simultaneous visualization and comminution or propulsion of kidney stones. They recruited 19 patients with kidney stones <12 mm of varying composition and location. Within just 10 minutes of treatment, most of the stones either completely or partially disintegrated with minimal damage to the surrounding tissues [32].

A synergistic impact on the effectiveness of stone fragmentation is observed when ultrasonic propulsion (UP) is employed alongside BWL. This phenomenon is attributed to the following mechanism: UP can physically expel fragments that are loosely adhered to the stone, as well as reposition the stone and redistribute stress, ultimately resulting in the formation of new stress fractures within the stones [33].

While BWL holds promise and may find its place in the armamentarium of kidney stone treatments, it currently faces challenges related to technical complexity, limited clinical evidence, cost considerations, and competition with established techniques [31]. The lack of standardized protocols and guidelines for BWL could lead to variances in treatment outcomes, impeding its widespread adoption. Further research, refinement of technology, and the accumulation of clinical data may help address these issues and improve the overall success and acceptance of burst wave lithotripsy in the future.

8. Conclusion and future perspectives

SWL remains the non-invasive surgical technique for the treatment of urinary stones. Continuous exploration of SWL physics and the interaction of shockwaves with stones are anticipated to expand the boundaries of SWL and optimize patient outcomes. The future of SWL is expected to be shaped by evolving lithotripter designs, integration of advanced imaging and tracking technologies, refined treatment protocols, and personalized treatment algorithms.

In the era of minimally invasive endo-urology, the meticulous selection of patients assumes a pivotal role in optimizing patient outcomes. As we acknowledge its limitations and embrace technological advancements, SWL can retain its essential role in the diverse landscape of stone management given its non-invasive nature and suitability for outpatient care. The path forward involves adapting and seamlessly integrating SWL into the broader spectrum of stone management techniques, thereby ensuring that patients receive the most suitable and effective treatment tailored to their specific conditions.

Acknowledgments

The author graciously acknowledges contribution of Engineer Syed Muhammad Shakir Fateh, Manager (Protection and Automation) Siemens Pakistan Engineering Company Limited for technical support.