Objective assessment of turbidity during laser lithotripsy: standardization of a dedicated in vitro experimental model.

PURPOSE: Endoscopic visibility is critical for the safety and efficacy of retrograde intrarenal surgery (RIRS), but turbidity caused by stone dust during laser lithotripsy is still mainly assessed subjectively. We developed an optical turbidimeter and aimed to standardize an in vitro model for objective turbidity measurement. METHODS: A custom optical turbidimeter was used in an in vitro lithotripsy model with Begostone Plus® particles under continuous saline irrigation. Sensor distance, powder height, particle size, water column height, ambient lighting, and prolonged laser activation were sequentially evaluated using a pulsed Thulium:YAG laser (p-Tm:YAG). Mean turbidity, peak signal, ON/OFF variation, area under the curve, post-activation recovery, and coefficient of variation (CV) were analysed. RESULTS: Control experiments confirmed stable baseline measurements in clear saline, with no significant ON/OFF variation during laser activation. Sensor distance strongly influenced signal detection, with 1 cm providing the best balance between sensitivity, stability, and reproducibility. A powder height of 1 cm produced a more stable signal than 0.5 cm, while higher amounts (≥ 1.5 cm) caused sensor occlusion. Fine particles ≤ 200 μm generated significantly deeper turbidity peaks than larger fragments (- 2.07 ± 0.49 vs. - 1.24 ± 0.51; p = 0.022), indicating greater detection sensitivity. Water column height did not significantly affect turbidity measurements. Measurements performed under endoscopic illumination alone showed improved peak signal detection compared with ambient light (p = 0.003). During prolonged laser activation, turbidity increased significantly during firing (p = 0.032) and decreased after cessation, but baseline levels were not fully restored, indicating persistent particle suspension under these experimental conditions. CONCLUSION: This study establishes standardized experimental conditions for objective turbidity measurement with stable signal acquisition during laser lithotripsy. The proposed model provides a methodological foundation for future comparative in vitro studies by measuring particle-induced optical attenuation rather than surgeon-perceived visibility. Further validation against endoscopic image quality and visual performance is required before optical attenuation can be interpreted as a surrogate of intraoperative visibility.