Exploring the influence of laser parameters on retropulsion and ablation rate in thulium fiber laser lithotripsy: an in vitro study

This study aims to explore the influence of different laser parameters on stone retropulsion and ablation rate during thulium fiber laser (TFL) lithotripsy, and to evaluate its dusting effect.

An experimental setup was designed to simultaneously measure the displacement and mass loss of stones. Ex vivo lithotripsy experiments were performed using a self-designed TFL system with average power ranging from 10 to 30 W, frequency from 10 to 1000 Hz and pulse energy from 25 to 3000 mJ. Stone retropulsion and ablation rates were systematically evaluated across various parameter configurations. Additionally, the size distribution of particles generated during lithotripsy was comprehensively assessed.

A higher average power resulted in a more intense stone retropulsion and a higher ablation rate. With a constant average power, both decreased as the frequency increased and the pulse energy decreased. The trend of mass loss was similar to that of the ablation rate. When the frequency was fixed, an increase in pulse energy significantly enhanced the retropulsion, whereas an increase in frequency at fixed pulse energy had a smaller impact on retropulsion. When the frequency reached 250 Hz and the pulse energy was reduced to 120 mJ, the stone retropulsion essentially disappeared. The maximum ablation rate (112.247 mg/min) was achieved at 10 Hz × 3000 mJ. For average powers fixed at 20 W and 10 W, the optimal parameters for the highest ablation rates were identified as 20 Hz × 1000 mJ and 20 Hz × 500 mJ, resulting in ablation rates of 65.451 mg/min and 40.742 mg/min, respectively. Under several representative configurations, all generated particles had diameters less than 600 μm.

Compared with frequency, pulse energy has a greater impact on both retropulsion and ablation rate. For non-fixed stones, increasing the pulse energy enhances the ablation rate but also intensifies the retropulsion effect, which can partially offset the increase in ablation rate. The TFL exhibits an excellent dusting effect. A conservative setting of 10 W, frequency below 50 Hz and pulse energy above 200 mJ can be referenced for clinical application.