New Devices for Kidney Stone Management

Russell S. Terry; Patrick S. Whelan; Michael E. Lipkin


Curr Opin Urol. 2020;30(2):144-148. 

In This Article

New Innovations in Laser Lithotripsy

The holmium:yttrium-aluminum-garnet (Ho:YAG) laser has been the gold-standard clinical laser for performing laser lithotripsy of urinary stones since the 1990s. The reasons for Ho:YAG predominance over alternative contemporary laser modalities are myriad and include: improved thermal characteristics compared with the older continuous-wave ruby, Nd:YAG, and CO2 lasers,[4] production of more optimally sized (smaller) stone fragments relative to pulsed-dye lasers,[5] effective ablation of stones of any composition,[6] the ability to be coupled to small flexible glass fibers,[7] and usefulness for soft-tissue applications due to the strong water absorption peak at its operating wavelength of 2100 nm and resulting optical penetration depth of 400 μm.[8,9] Since the introduction of Ho:YAG into the urology marketplace decades ago, the most notable new developments to the delivery systems have been ever-increasing power and pulse frequency capabilities as well as the ability to vary pulse width.[10] The ability to achieve very high pulse frequencies, up to 80 Hz, with the highest power lasers has contributed to the development of new ablation techniques such as 'dusting,'[11] and these high-frequency, low-pulse energy settings have been shown to significantly enhance procedural efficiency and total time spent lasering compared with older generation lasers with lower power capabilities.[12] Furthermore, the introduction of variable pulse width settings into the majority of modern lasers allows the urologist to fine-tune lithotripsy effects even further.[13] Longer pulse widths have been shown to produce less retropulsion compared with short pulse widths, and this feature allows for even greater enhancement to procedural efficiency when dusting at high-frequency settings[14] (Table 1).

The most recent iterative development in Ho:YAG laser lithotripsy has been the addition of pulse modulation. In contrast to variable pulse width, which maintains the standard pulse shape with generally symmetric energy distribution while either shortening or lengthening the time period (usually on the order of 350–1100 μs) over which the pulse energy is delivered, pulse modulation techniques are capable of varying the profile of the laser pulse in such a way as to introduce significant asymmetry into the amount of energy which is delivered over the course of a single pulse. A general theme with these techniques is to use an initial energy peak to create a bubble within the surrounding fluid medium through which the subsequent pulse energy can travel relatively unimpeded.[15] The idea is that this initial bubble phenomenon, called the Moses effect,[16,17] allows more energy to be delivered to the target since the laser energy does not get absorbed by water as it travels through the bubble. Indeed, benchtop measurements of stone ablation rates and crater characteristics varied by pulse type have shown that Moses modes produce more stone ablation than standard short and long pulse modes.[18] Purported secondary benefits of the technique include decreased retropulsion as the collapse of the vapor bubble pulls the stone back toward the laser tip. The first pulse modulation technique to enter the market was Moses Technology, which was developed by Lumenis and introduced in 2017. Subsequent market entrants which claim to offer retropulsion-minimizing pulse characteristics include Quanta System's Vapor Tunnel technology and Dornier MedTech's Advanced Mode.

Beyond modifications of the existing Ho:YAG modality, the newest laser lithotripsy technology to emerge is the thulium fiber laser (TFL). This technology is fundamentally different from conventional Ho:YAG systems in several ways which are relevant to the practicing urologist.[19] First, the TFL is not pumped by a classic flashlamp system, and therefore it does not require the heavy and expensive internal water-cooling systems that have become associated with the higher wattage Ho:YAG systems.[20] Instead, the TFL is pumped by a diode laser source and the gain medium is a long, internal thulium-doped silica fiber which then can couple separately to the familiar low-hydroxyl silica fibers which are currently the standard for Ho:YAG. Moreover, as a result of this coupling mechanism and a more focused spatial beam profile, smaller diameter fibers are a possibility, and the use of fibers as small as 50 μm has been reported.[8,21] An additional important difference is the 1940 nm operating wavelength of TFL, which more closely matches an absorption peak in water compared with the Ho:YAG 2100 nm operating wavelength. This results in significantly higher water absorption by the TFL and may translate to safety and ablation efficiency improvements in stone and/or soft tissue applications. As with any emerging technology, there is limited published clinical or safety data regarding the TFL at present,[22–25] and it is not yet available for widespread clinical use in the United States – although that may be changing soon.[26]