Impact of Renal Access Angle and Speed of Nephroscope Retrieval Movements on the Vortex Effect

Abstract

To analyze the influence of different renal access angles (AAs) and nephroscope retrieval speeds on the efficacy of the vortex effect (VE) in mini-percutaneous nephrolithotomy (mini-PCNL). This study aimed to understand the poorly understood physical components of the VE. A Pexiglas™ (KUS®) model was built based on the dimensions of a 15/16F mini-PCNL set (Karl Storz). The flow rate was continuous via an automatic pump and calibrated to achieve hydrodynamic equivalence to the real equipment. One experiment consisted of manually retrieving all 30stone phantoms (3mm diameter) utilizing only the VE. Cumulative time to retrieve all stones was measured. An accelerometer recorded instant speeds of the nephroscope every 0.08 seconds (s), and 3 experiments were performed at each angle (0°, 45°, and 90°). A logistic regression model was built utilizing maximum speeds and access angles to predict the effectiveness of the VE. Mean cumulative time for complete stone retrieval was 28.1seconds at 0° vs 116.5seconds at 45° vs 101.4seconds at 90° (P<.01). We noted significantly higher speeds at 0° compared to 45° and 90° (P<.01); however, differences in average and maximum speed between 45° and 90° were not statistically significant (P=.21 and P=.25, respectively). The regression model demonstrated a negative association between increasing maximum speed and VE's effectiveness (OR 0.547, CI 95% 0.350-0.855, P<.01). When controlling for maximum speed, the 0° angle had significantly higher chances of achieving at least a partially effective VE. Increasing the renal access angle or nephroscope extraction speed negatively impacts the effectiveness of the VE. This significantly increased procedure time in the laboratory model, suggesting that the VE is less effective at higher sheath angles.