A thermal model on thulium fiber laser lithotripsy of kidney stone

The paper presents a thermal model for pulsed thulium fiber laser lithotripsy using the finite element method. We simulated the temperature and vaporization rate of kidney stones (calcium oxide monohydrate). When the laser energy increased from 35 mJ to 50 mJ, the vaporization rate increased from 131 µg/s to 1118 µg/s. Additionally, we investigated the effect of the pulse repetition rate on the vaporization rate. The vaporization rate increased significantly from 21 μg/s to 131 μg/s for 35 mJ when the pulse repetition rate increased from 10 Hz to 100 Hz. Our simulated results are in excellent agreement with the experimental results available in the literature. Further, the simulation showed that the water-filled micropores of kidney stones achieved a temperature of 5000 K while its remaining part gained 2600 K. This large variation is due to the strong absorption of radiation in water. The pressure of the superheated vapor inside the micropores is 2.6 times higher than the mechanical strength of kidney stones, which leads to a microexplosion of stone. This study also revealed that the thermal stress would play a crucial role in the fragmentation of kidney stones. The retropulsion effect is a well-known lithotripsy issue for urologists. We estimated the retropulsion distance and found it to be negligible under the laser settings used in the present study.