Overview

Electrohydraulic lithotripsy (EHL) was the first method for intracorporeal lithotripsy utilized in clinical applications. The technology, first invented by Yutkin in 1955 at the University of Kiev, was used for upper tract stones during an open nephrolithotomy in 1975 [3, 4]. Since that time, EHL probes have been miniaturized for use in rigid and flexible ureteroscopes for treatment of ureteral stones [5, 6]. Despite its effectiveness and low cost, it is rarely used because of the high risk of ureteral injury. Yet, its low cost still makes EHL an option in certain situations.

Technology

The mechanism for EHL fragmentation involves discharge of electrical energy in a liquid medium. The probe is composed of two electrodes of different polarities; a current passes between the electrodes and the energy release forms a plasma channel and vaporization of nearby water. This vaporization creates a shock wave and cavitation bubble, whose collapse creates secondary shock waves or high‐speed microjets, all of which cause stone fragmentation.

Advantages/disadvantages

The advantage of EHL is the low cost of components. The EHL generator and probes are currently less expensive than other intracorporeal lithotripters, with one or two probes used per procedure depending on stone composition [7, 8]. Additionally, modern EHL probes are small and flexible, allowing for treatment of stones through all types of ureteroscopes.

The main disadvantage of EHL is high risk of intraoperative injury. Initial experience of EHL reported high complication rates, as the lithotripsy was performed with large probes (9 Fr) and without endoscopic guidance [4, 9]. With improvement in endoscopic visualization, decrease in probe size (to 1.6 Fr), and improved control of energy output, EHL has become safer, although the complication rate remains high, between 6 and 17.6% [6, 10–12]. Complications of EHL range from ureteral perforation and hemorrhage to mucosal erosion, inflammation, and ureteral stricture.

Studies have been performed to understand the mechanisms by which EHL causes ureteral injury. In addition to direct injury to the ureteral wall from firing of the probe onto mucosa, expansion of the cavitation bubble may induce a burst effect on the ureter [11]. With energy of more than 1300 mJ, the cavitation bubble can reach 1.5 cm in size; thus, decreasing the energy can reduce capacity for injury. One in vitro study demonstrated that 24 pulses to the ureteral wall were sufficient for perforation from 0.5 mm, highlighting the care needed while using EHL [13].

The stone fragmentation and clearance rates are not significantly higher for EHL than for newer lithotripters. In addition to fragment retropulsion, which appears more common than with other methods, EHL produces more and larger fragments, reducing stone clearance [14]. EHL is less effective with harder and larger stones (>1.5 cm) and the stone‐free rates range widely from 67 to 98% [7].

Technical tips

Given its risks, EHL should be performed under careful attention and with the lowest power settings that can produce fragmentation. Smaller probes (1.6–1.9 Fr) should be used to decrease perforation risk and to improve irrigation. Prior to use of the lithotripter, insulation layers of the probe should be examined to ensure they are intact. The distal tip of the probe should be placed 2–5 mm from the end of the ureteroscope to reduce damage to the lens, while the probe should be kept 1 mm from the stone to improve shock‐wave interaction [15].

Visualization of the stone must be maintained, along with adequate irrigation to distend the ureter away from the probe, to reduce risk of injury. Lithotripsy should begin with low voltage (50–60 V) and single pulses with a slow increase in generator output until the fragments are small enough for basket removal. After 60 seconds of firing, the probe should be examined for loss of insulation, as a new probe may be required. Care must be taken to avoid firing upon any safety wire or retrieval device, as these are damaged by EHL energy.

Overview

Similar to EHL, ballistic and pneumatic lithotripsy is an older method of fragmentation. These lithotripters come in many sizes and types of energy production and all necessitate a straight probe during use. The rigid probes decrease the practicality of this technology for upper tract stones requiring flexible ureteroscopy. Still, ballistic and pneumatic lithotripsy remains safe and effective when used in the proper setting.

Technology

Ballistic and pneumatic lithotrites pass energy from a projectile to the probe (0.8–3.0 mm) and then directly to the stone with a “jackhammer” effect. When fired repeatedly (on the order of 12 Hz with automatic lithotripters), this production of 2–30 atm at the stone surface causes cracking and fragmentation.

Production of this ballistic energy varies between lithotripters, with use of large‐scale compressed air (Lithoclast, Electromedical Systems/Boston Scientific), handheld high‐pressure CO₂ cartridges (StoneBreaker, LMA Urology/Cook Medical), or electromagnetic acceleration of a magnetic core (LithoBreaker electrokinetic probe, Electromedical Systems). In each case, the probe is held in contact with the stone to transmit the ballistic energy. This method requires energy transfer through a rigid probe; bowing of the device reduces power transmission, limiting the utility of this method to unflexed endoscopes.

Advantages/disadvantages

Ballistic lithotripsy is a low‐cost option. Disposable costs are minimal, as probes have a long life span, unlike laser and EHL [10]. While free‐standing pneumatic lithotripters require more expensive consoles compared to EHL, there is minimal maintenance and no disposable costs. The handheld pneumatic lithotripter is less expensive and the disposable CO₂ cartridges, which deliver approximately 80 shocks each, add minimal cost to the device.

Ballistic and pneumatic lithotripsy, utilizing direct mechanical energy transfer, are safe compared to other technologies. In contrast to EHL, ultrasonic lithotripsy, and holmium laser, ballistic lithotripsy causes less injury and has lower risk of ureteral perforation [16]. After 6 minutes of direct application of pneumatic lithotripsy to a ureteral wall in vitro, no perforation was seen [13]. Additionally, thermal injury is unlikely as minimal heat is produced. Compared to the relatively slow comminution of laser fragmentation (which is discussed later), ballistic lithotripsy has a more rapid stone clearance time [17, 18].

Despite its safety and low cost, ballistic lithotripsy has slightly lower success than other methods, likely secondary to its inability to fragment stones when torqued and the high risk of stone retropulsion. Stone‐free rates after ballistic or pneumatic lithotripsy range from 70 to >90%, although the lower rates occur in settings of probe deflection and retropulsion [19–21]. Through a flexible ureteroscope, deflection of more than 24° will significantly reduce energy transmission; distal semirigid ureteroscopy allows for decreased need for flexion and decreased power loss [22]. Thus ballistic lithotripsy is very effective through a semirigid ureteroscope for mid/distal ureteral stones.

The other disadvantage of ballistic lithotripsy is its retropulsion rate, which has been reported in 2–17% of ureteroscopic treatments. Due to this high rate of retropulsion, especially given the rigid lithotripter, migration into the proximal ureter or kidney can render stone fragments untreatable with this method alone. Many tools have been developed to prevent stone migration, including stone‐trapping devices (such as stone baskets, gel polymeric plugs, and ureteral balloons) and suction devices to be used alongside the lithotripter [23, 24]. These methods reduce the risk of retropulsion and improve fragmentation efficiency and stone‐free rates, although careful use of the lithotripter may preclude the need for such techniques [25, 26].

Technical tips

As with all lithotripters, endoscopic visualization of the stone is a prerequisite for treatment. The stone should be pinned in place for fragmentation and to decrease retropulsion, whether held against urothelium or using a trapping mechanism. Bending of the endoscope or probe should be avoided in order to maintain power during lithotripsy. The probe must be in contact with the stone and it is recommended to work from the periphery to separate fragments small enough for spontaneous passage. Given the minimal flexibility of this device, if larger fragments migrate proximally, chasing and treating such pieces becomes problematic.

Overview

Ultrasonic lithotripsy was described by Mulvaney in 1953, with the first ureteral applications in the 1970s and 1980s [27, 28]. This method uses ultrasonic vibrations to fragment calculi; many ultrasonic lithotripter probes include a suction channel to allow simultaneous fragment removal. Similar to ballistic lithotripsy, this requires a rigid device, reducing the utility of ultrasonic lithotripsy during ureteroscopy. Smaller probes (2.5 and 4.5 Fr), with or without suction capabilities, can be used through a semirigid ureteroscope with success and safety.

Overview

Nanosecond electropulse lithotripsy (NEPL) is the newest method of intracorporeal lithotripsy, described in 2007 [36]. It utilizes similar technology to EHL, although due to different characteristics of energy discharge it has a better safety profile. Early data show similar ease of access and flexibility compared to laser/EHL, along with excellent fragmentation, making this a promising new method for lithotripsy.

Technology

While EHL and NEPL probes contain two distal electrodes across which current is applied, the NEPL probe differs due to the voltage and time over which energy is released. In contrast to EHL discharge time of hundreds of microseconds at low voltages, NEPL releases higher voltages in less than 50 nanoseconds. This method of energy release, when the probe is touching a non‐conducting material such as urinary calculi, produces spontaneous conduction and direct electrical breakdown of the target. In the absence of a solid target, the probe performs similarly to EHL and releases energy into the surrounding fluid, creating a shock wave and cavitation bubble. The focus of electrical energy into the target (instead of the surrounding fluid) is what provides the proposed safety of NEPL.

Advantages/disadvantages

NEPL has become the focus of recent studies given the potential for improvement over previous technology. In vitro studies have shown improved energy and time efficiency compared to EHL and laser lithotripsy [37–39]. Stone‐free rates are comparable to other methods, with 91–100% success depending on the stone location (mid ureter with better success than upper ureter) [36]

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