Multiple procedural modalities for treating nephrolithiasis are available to patients with the ultimate choice dependent on stone size, location, and patient factors. For over 30 years, the holmium: yttrium-aluminum-garnet (Ho:YAG) laser has been the gold standard for laser lithotripsy due to its efficiency in stone fragmentation irrespective of stone composition and its enviable safety profile. Different energies, pulse rates, pulse durations, and work settings are utilized based on stone density and surgeon intent for fragmentation versus dusting.

Basic physical principles underlie the effectiveness of the Ho:YAG laser, which made a revolution over two decades ago. Energy is transmitted through a flexible quartz fiber that varies in size from 150-1,000 micrometers making it amenable to flexible ureteroscopy. Its wavelength of 2,100 nm is effectively absorbed by water conferring a safety profile which is further bolstered by its 0.4mm depth of penetration. The photothermal mechanism by which it exerts force requires direct stone contact leading to stone vaporization. As a pulsed laser, holmium emits a cyclic burst of light wavelength with a corresponding omission. The average of the laser's pulses accounts for the power output. Contemporary advancements have led to utilization of higher power by taking advantage of the pulse technology. At present pulse duration can be lengthened from 350 microseconds to 1500 microseconds and utilized to reduce stone retropulsion. The latest evolution of the technology includes the aptly named "Moses Technology" (now in version 2.0) whereby a short vapor bubble precedes a longer duration, high energy pulse that can conserve energy by means of travel through the vapor bubble. This technology has been key in reducing stone retropulsion leading to improved ablation rates. Data has suggested that the Moses technology improves patient outcomes by reducing procedural duration and reducing fragmentation time.

A newer alternative to the Ho:YAG laser is the thulium fiber laser (TFL) lithotripter, introduced in 2018. It can provide greater power output from a smaller fiber core due to the diode construction with silica chemically doped with thulium ions. This is achieved by using a 50-150 micrometer optical laser fiber core rather than mirrors to focus light energy as in the Ho:YAG. Small form factor renders the fiber more adaptable in acute angle applications. The low power consumption of the TFL arises from fan cooling and plug efficiency permitting increased pulse repetition rates up to 2,000 - 2,200 Hz with pulse durations from 0.2 to 12 milliseconds. Due to these improvements, the TFL can both fragment and dust with moderated heat generation. Moreover, the TFL is compatible with variable operating room configurations based on its standard electrical wall socket (110V) making it more accessible. Similar to the Ho:YAG laser its wavelength varies between 1,908 and 1,940 nm bestowing a higher absorption coefficient in water which grants a similar safety profile owing to higher energy density focused at the tip of the laser fiber. In summation, the TFL has comparable results for stone ablation and low retropulsion with lower power consumption and less retropulsion leading to better dusting.

Recent studies have compared specific features of each laser to standardize best practices albeit with no clear winner. Pulse profiles differ with each laser. The Ho:YAG pulse has an acute spike in voltage with a dramatic drop within 150 microseconds whereas the TFL pulse profile has an acute upsurge that is sustained for the duration of the pulse. This inherent property within the TFL allows greater dialed in variability. With regard to energy absorption the TFL has been shown to be 4-5 times more effective thus correlating with lower pulsed energies for stone ablation in the in vitro setting. Lower pulsed energies in conjunction with increased pulse durations allow for less retropulsion with better dusting. The TFL has demonstrable improvements in stone free rates by more readily creating fine dust than can the Ho:YAG and the Ho:YAG-MOSES in in vivo settings. Not to be undone, the Ho:YAG laser has statistically higher ablation rates at slow speeds of 500 mm/min when using approximately identical energies and frequencies. Data suggests that higher ablation rates are more readily obtained at high-powered settings (24W) albeit with increased risk for thermal damage. Moreover, in vivo models suggest increased risk for urothelial tissue injury when using TFL in the absence or ureteral access sheaths owing to increased temperature within the collecting system. At best, current in vivo and in vitro studies lack certain corollary to clinical scenarios however they do suggest an equivalency for tissue temperature damage by either laser modality via high-power low-irrigation settings. When going head-to-head the Ho:YAG has shown superiority with denser calcium phosphate stones. TFL may confer greater stone clearance but has also been found to increase the rate of infectious complications. Other studies have concluded no significant clinical time advantage, stone free rate, complication, or patient related stone quality of life outcomes. Therefore as both lasers confer safety with effectiveness, surgeon preference and institutional preference may dictate technological approach.

Studies have begun to compare TFL to high power holmium with Moses technology for ureteroscopy and laser lithotripsy (URSLL), however, data is lacking as to the superior laser technology for medium or large stones approached antegrade in the mini percutaneous nephrolithotomy (PCNL) setting.

This is a single institution, single surgeon, randomized controlled trial that evaluates the holmium laser vs. the thulium laser for medium-to-large stones in the mini Percutaneous Nephrolithotomy setting. Patients will be screened and consented for participation in this study, and will be randomized to be treated using either the holmium or thulium laser for their procedure. This study aims to evaluate the efficiency of both FDA-approved lasers for this procedure to ascertain whether one of them has any advantage over the other. Procedural information will be evaluated as well as up to 90 day outcomes for the participants of this trial.