Laser Fiber Displacement Velocity during Tm-Fiber and Ho:YAG Laser Lithotripsy: Introducing the Concept of Optimal Displacement Velocity

Background: Endocorporeal laser lithotripsy (EL) during flexible ureteroscopy (URS-f) often uses “dusting” settings with “painting” technique. The displacement velocity of the laser fiber (LF) at the stone surface remains unknown and could improve EL’s ablation rates. This in vitro study aimed to define the optimal displacement velocity (ODV) for both holmium:yttrium-aluminium-garnet (Ho:YAG) and thulium fiber laser (Tm-Fiber). Methods: A 50W-TFL (IRE Polus®, Russia) and a 30W-MH1-Ho:YAG laser (Rocamed®), were used with 272µm-Core-Diameter LF (Sureflex, Boston Scientific©), comparing three TFL modes, “fine dusting” (FD:0.05–0.15 J/100–600 Hz); “dusting” (D:0.5 J/30–60 Hz); “fragmentation” (Fr:1 J/15–30 Hz) and two Ho:YAG modes (D:0.5 J/20 Hz, Fr:1 J/15 Hz). An experimental setup consisting of immerged cubes of calcium oxalate monohydrate (COM) stone phantoms (Begostone Plus, Begoã) was used with a 2 seconds’ laser operation time. LF were in contact with the stones, static or with a displacement of 5, 10 or 20 mm. Experiments were repeated four times. Stones were dried and µ-scanned. Ablation volumes (mm3) were measured by 3D-segmentation. Results: ODV was higher in dusting compared to fragmentation mode during Ho:YAG lithotripsy (10 mm/sec vs. 5 mm/sec, respectively). With Tm-Fiber, dusting and fragmentation OVDs were similar (5 mm/sec). Tm-Fiber ODV was lower than Ho:YAGs in dusting settings (5 mm/s vs. 10 mm/sec, respectively). Without LF displacement, ablation volumes were at least two-fold higher with Tm-Fiber compared to Ho:YAG. Despite the LF-DV, we report a 1.5 to 5-fold higher ablation volume with Tm-Fiber compared to Ho:YAG. Conclusions: In dusting mode, the ODVTm-Fiber is lower compared to ODVHo:YAG, translating to a potential easier Tm-Fiber utilization for “painting” dusting technique. The ODV determinants remain unknown. Dynamic ablation volumes are higher to static ones, regardless of the laser source, settings or LF displacement velocity.