Risk assessment of holmium laser induced ureteral stricture: in-vivo experiments using a pig model

To systematically evaluate the safety boundaries of holmium laser application during ureteroscopy by quantifying thermal dose and to explore the detailed repair mechanisms of the ureter and kidneys following injury.

Twelve female piglets were selected. Key variables included holmium laser power, irrigation flow rates (0, 7.5, 15 mL min^-1), and fluid temperatures. Thermal dose was calculated as Cumulative Equivalent Minutes at 43 °C (CEM43). Trauma and repair processes were assessed via microscopic imaging, Masson staining, and immunohistochemistry for inflammatory and fibrosis markers. Statistical analyses were performed using two-way analysis of variance (ANOVA).

At an irrigation flow rate of 15 mL min^-1, the temperature rise was minimal with a negligible thermal dose (CEM43 < 1), ensuring safety even at 30 W. In contrast, compromised irrigation (7.5 mL min^-1) or no-flow conditions resulted in rapid heat accumulation, with CEM43 values reaching extreme levels (>10¹³) at high powers. In the kidneys, the repair process involved a transition from inflammation to fibrosis over time, which was correlated with an M1-to-M2 macrophage polarization. Crucially, anatomical ureteral stricture was observed only when the laser-induced injury involved ≥3/4 of the ureteral circumference.

Sufficient irrigation is critical to maintain thermal safety. While thermal exposure induces fibrotic changes driven by macrophage polarization, mechanical injury extent (≥3/4 circumference) appears to be the dominant predictor for the formation of ureteral stricture. These findings emphasize the importance of maintaining high-flow irrigation and minimizing extensive circumferential damage during surgery.