Shaft stiffness heterogeneity in single-use flexible ureteroscopes: implications for maneuverability and robotic applications.

PURPOSE: Shaft-level mechanical behavior of single-use flexible ureteroscopes (su-fURS) remains poorly defined despite its relevance to maneuverability and force transmission during retrograde intrarenal surgery (RIRS). This study aimed to quantify axial and bending stiffness across six contemporary su-fURS and to evaluate whether nominal shaft diameter predicts mechanical performance.

METHODS

Standardized benchtop testing was performed under controlled laboratory conditions. Axial stiffness was measured using a vertical tension setup, while effective bending stiffness was assessed using a horizontal large-deflection configuration. For each device model, three units were tested and measurements were performed in triplicate to assess technical repeatability. Associations between stiffness values and device specifications were explored using linear regression analysis.

RESULTS

Substantial between-model variability was observed in the tested units. Axial stiffness ranged from 4338 ± 434 to 12,996 ± 3643 N/m, while effective bending stiffness ranged from 22.54 ± 2.12 to 41.30 ± 0.74 N/m. Devices with smaller nominal diameters did not consistently demonstrate lower stiffness values, suggesting that internal shaft architecture may play a more important role in determining mechanical behavior than external geometry.

CONCLUSION

Contemporary su-fURS exhibit significant heterogeneity in axial and bending stiffness, independent of nominal diameter. Shaft stiffness may be substantially influenced by internal structural design and may influence maneuverability during ureteral insertion, passive lower pole deflection, and unsupported ureteroscopy. Quantitative mechanical characterization may support device selection and provide foundational data for robotic RIRS force modeling.