Dynamic Pulse Modulation During Ho:YAG Laser Lithotripsy: When to change from Magneto to Virtual Basket? Insights from a Multicenter Study

Technical Summary: Dynamic Pulse Modulation During Ho:YAG Laser Lithotripsy

Problem Statement
The field of endourological lithotripsy has evolved from simple fragmentation to sophisticated dusting strategies, driven by the need to balance ablation efficiency with the minimization of stone retropulsion. While the Thulium Fiber Laser (TFL) has gained prominence for its low peak power and favorable dusting characteristics, the Holmium:YAG (Ho:YAG) laser remains the gold standard due to its versatility. Recent innovations in Ho:YAG pulse modulation, specifically Magneto (M-Ho:YAG) and Virtual Basket (VB), aim to replicate TFL-like low-retropulsion dusting while maintaining Ho:YAG versatility. M-Ho:YAG utilizes extended pulse durations and reduced peak power (~500 W) to stabilize stones, whereas VB employs a dual-pulse strategy to create transient cavitation for improved dusting and reduced migration.

However, a critical gap exists in understanding the intraoperative dynamics of these technologies. Current evidence largely relies on static comparisons between modes. In practice, lithotripsy is a dynamic process where stone morphology and behavior evolve. It remains poorly defined which specific preoperative or intraoperative factors necessitate a surgeon to transition from the low-peak-power M-Ho:YAG mode to the potentially more aggressive VB mode during a single procedure. This study addresses the need to identify predictors for such intraoperative transitions to optimize procedural efficiency.

Methodology
This was a multicenter, observational, real-world study analyzing data from 400 consecutive patients undergoing Retrograde Intrarenal Surgery (RIRS) for kidney stone disease between January 2025 and March 2026. The procedures were performed across five international high-volume centers using the Cyber Ho™ 150W platform.

• Study Design: All procedures initiated lithotripsy using the M-Ho:YAG pulse modulation as the default strategy.
• Intervention Criteria: Surgeons were permitted to switch intraoperatively to Virtual Basket (VB) mode at their discretion if they perceived suboptimal dusting efficiency, excessive energy consumption, prolonged laser time, or inadequate fragmentation.
• Grouping: Patients were categorized into two groups:
  1. M-Ho:YAG-only: Patients treated entirely with Magneto mode ($n=301$).
  2. M-Ho:YAG + VB: Patients requiring a transition to VB mode ($n=99$).
• Data Collection: Variables included demographics, stone characteristics (volume calculated via ellipsoid formula, density in Hounsfield Units [HU], location, multiplicity), anatomical factors (lower pole location, infundibulopelvic anatomy), and procedural outcomes (laser time, ablation rate, energy efficiency, retropulsion, Stone-Free Rate [SFR] at 30 days).
• Statistical Analysis: Correlations between variables were assessed using Spearman's rank correlation coefficient ($r$). A $p$-value $< 0.05$ was considered statistically significant.

Key Contributions and Results
The study identified specific clinical and anatomical predictors associated with the need to switch from M-Ho:YAG to VB mode.

1. Stone Volume: A strong positive correlation was found between stone volume and the need for pulse modulation switching ($r=0.69$). Larger stone burdens were significantly more likely to require the transition to VB.
2. Stone Density: Higher stone density (measured in HU) was associated with an increased likelihood of switching. The data suggests that the low peak-power settings of M-Ho:YAG may exhibit reduced fragmentation efficiency in harder, high-density calculi.
3. Anatomical Location: The lower pole was the most frequent anatomical location among patients requiring a modulation change. This suggests that gravity-dependent locations, combined with stone burden, may necessitate more aggressive pulse profiles to maintain clearance.
4. Procedural Efficiency:
   • Ablation Rate: The M-Ho:YAG + VB group demonstrated a higher mean ablation rate ($1.31 \text{ mm}^3/\text{s}$) compared to the M-Ho:YAG-only group ($1.12 \text{ mm}^3/\text{s}$).
   • Energy Delivery: The M-Ho:YAG-only group delivered significantly more total energy ($71.87 \text{ kJ}$) compared to the transition group ($46.98 \text{ kJ}$), though laser-on times were comparable between groups.
   • Safety: The study reported a favorable safety profile with no major complications (Clavien-Dindo $\ge$ III). Minor complications (Grade I-II) were rare ($<1\%$) and included minor bleeding and ureteral injuries. Postoperative renal function (eGFR) remained stable, and the 30-day Stone-Free Rate (defined as no fragments $>2 \text{ mm}$) was 98.5%.

Significance and Claims
The paper posits that lithotripsy should not be viewed as a static procedure governed by a single laser setting, but rather as a dynamic process requiring adaptive strategies. The primary significance of these findings is the identification of large stone burden, lower pole location, and high stone density as the principal predictors for the intraoperative transition from M-Ho:YAG to VB.

The authors claim that these results support a dynamic pulse modulation strategy tailored to specific stone characteristics. By recognizing when low-peak-power modes (M-Ho:YAG) may be insufficient for hard or large stones in difficult anatomical locations, surgeons can proactively or reactively switch to VB to optimize ablation efficiency and procedural performance. The study concludes that while further prospective, randomized studies are warranted, current evidence suggests that flexibility in pulse modulation selection, rather than reliance on a single mode, enhances the efficacy of high-power Ho:YAG laser lithotripsy.