In search of optimal settings for Ho:YAG laser-lithotripsy to maximize the ablation rate, while minimizing the retropulsion

Abstract

Although laser lithotripsy is now the preferred treatment option for urolithiasis due to shorter operation time and a better stone-free rate [1], the optimal laser settings for URS (Ureteroscopic lithotripsy) to enable shorter operating times remain unclear. This study aims to identify optimal laser settings for Ho:YAG laser-lithotripsy to maximize the ablation rate while minimizing the retropulsion, as well as to improve the discharge of fragments via the urinary tract. The net result will be an increase in treatment success and patient satisfaction by ameliorating the stone-free rate. In vitro investigations of Ho:YAG laser-induced stone ablation and retropulsion were performed with a bench top model first introduced by Sroka’s group [2]. A commercial Ho:YAG laser system (Lumenis VersaPulse PowerSuite 100W, Lumenis Ltd., Yokneam, Israel) was used as the laser pulse source, with pulse energy from 0.2 J up to 1.5 J and repetition rate from 5 to 40 Hz. A DOE with two replicate points and two lack-of-fit points was performed on artificial BEGO stones of sample size 14 under reproducible experimental conditions (fiber size: 365 μm, S-LLF365 SureFlex Fiber, Boston Scientific Corporation, San Jose, CA, USA). The best fit to the experimental data was analyzed utilizing the design of experiment software, which can produce the numerical formulas for the response surfaces of ablation rate and retropulsion in terms of laser pulse parameters [3]. The coded numerical formulas for the response surfaces of ablation speed and retropulsion velocity are generated. The coded equation is useful for identifying the relative impact of the factors by comparing their coefficients. Upon examination of the laser ablation of stone phantoms (BEGO), the laser pulse energy is 1.4 times the impact of the frequency, and laser pulse peak power’s impact is the same as the frequency; while for retropulsion, the laser pulse energy is 5.8 times as the impact of the frequency, and laser pulse peak power’s impact is 13 times as the frequency; A series of laser settings for relatively efficient laser lithotripsy were identified in terms of laser pulse energy and peak power. The laser pulse energy or peak power in reference to frequency has a higher impact coefficient to stone retropulsion as compared to stone ablation in Ho:YAG laser-lithotripsy. The most effective way to reduce stone retropulsion during laser lithotripsy is to reduce the laser pulse peak power (which has the highest impact coefficient in the coded response equation).