In Vitro evaluation of laser-stone interaction

Laser lithotripsy is the main surgical treatment for urinary stones, using Holmium: Yttrium-Aluminium-Garnet (Ho: YAG) and Thulium Fiber Laser (TFL) lasers. Despite technological advances, the stone fragmentation determinants remain unclear. A better understanding would optimize laser lithotripsy (LL) outcomes (efficacy and safety). We aimed to investigate, in vitro, laser-stone interactions. TFL and Ho: YAG lasers were tested using 270 μm fibers on hard synthetic stone samples under four experimental conditions (dry, wet, immersed in contact and at 2 mm distance). A single pulse (0.5-1 J) was delivered. Crater dimensions, carbonization, and temperature profiles were assessed using optical microscopy and thermocouples. Crater's maximum radius increased with pulse energy regardless to laser source, pulse mode and experimental condition. Crater's max depth was reported for immersed HSS, LF in contact. A peripheral carbonization halo was systematically reported with TFL, regardless to laser settings. The maximum contact temperature (T^maxcontact) increased with the pulse energy regardless to laser source, settings or experimental conditions. No temperature above 60 °C underwater were reported. TFL (High Peak Power) showed significantly higher crater's maximum radius and depth than Ho: YAG (Long Pulse) for immersed HSS. TFL exhibited higher T^maxcontact than Ho: YAG, significant only at 0.5 J dry HSS in air (101.3vs27.96 °C, p = 0.001) and at 0.5 J immersed HSS, LF in contact (49,6 vs. 29.73 °C, p = 0.04). These findings suggest that thermal effects play a major role in laser-stone interaction, although additional mechanisms as photomechanical may contribute. Further studies, including numerical modeling, are required to better characterize these interactions and optimize clinical outcomes.