Reply to: Letter-to-the-editor: Understanding the Popcorn Effect During Holmium Laser Lithotripsy for Dusting

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^®, Moscow, Russia) and a 30W-MH1-Ho:YAG laser (Rocamed^®, Signes, Provence-Alpes-Côte d'Azur, France), were used with 272 µm-Core-Diameter LF (Sureflex, Boston Scientific^©, San Jose, CA, USA), 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, Bego^©, Lincoln, RI, USA) was used with a 2 s' 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/s vs. 5 mm/s, respectively). With Tm-Fiber, dusting and fragmentation OVDs were similar (5 mm/s). Tm-Fiber ODV was lower than Ho:YAGs in dusting settings (5 mm/s vs. 10 mm/s, 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 ODV^Tm-Fiber is lower compared to ODV^Ho: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.

How much energy do we need to ablate 1 mm3 of stone during Ho:YAG laser lithotripsy? An in vitro study

INTRODUCTION

Holmium:yttrium-aluminium-garnet (Ho:YAG) is currently the gold standard for lithotripsy for the treatment of all known urinary stone types. Stone composition and volume are major determinants of the lithotripsy. This in vitro study evaluated the required energy to ablate 1 mm³ of various stone types with different laser settings using Ho:YAG.

METHODS

272 µm core-diameter laser fibers (Boston Scientific^©) were connected to a 30 Watt MH1 Ho:YAG generator (Rocamed®). An experimental setup consisting of immerged human stones of calcium oxalate monohydrate (COM), uric acid (UA) or cystine (Cys) was used with a single pulse lasing emission (0.6/0.8/1 J), in contact mode. Stones were dried out before three-dimensional scanning to measure ablation volume per pulse (AVP) and required energy to treat 1 mm³ (RE).

RESULTS

All settings considered, ablation volumes per pulse (AVP) for COM were significantly lower than those for UA and Cys (p = 0.002 and p = 0.03, respectively), whereas AVP for Cys was significantly lower than those for UA (p = 0.03). The mean REs at 0.6 J pulse energy (PE) for COM, Cys and UA were 34, 8.5 and 3.2 J, respectively The mean REs at 1 J PE for COM, Cys and UA were 14.7, 6.4 and 2 J, respectively. At 0.6 J PE, RE for COM was more than tenfold and fivefold higher than those for UA and Cys, respectively.

CONCLUSION

This in vitro study shows for the first time a volumetric evaluation of Ho:YAG efficiency by the ablation volume per pulse on human stone samples, according to various pulse energies. The REs for COM, UA and Cys should be considered in clinical practice.