Impact of flow rate and ratio of endoscope-sheath diameter on stone removal in flexible ureteroscopic lithotripsy: in vitro and CFD analyses insights.

Abstract

Purpose: To develop a theoretical framework for stone removal in flexible ureteroscopic lithotripsy (FURL) using a soft-tip negative pressure ureteral access sheath (SNP-UAS), we examined how flow rate (FR) and ratio of endoscope-sheath diameter (RESD) affect maximum pull-back speed (max-PBS) through in vitro experiments and computational fluid dynamics (CFD) analyses. Furthermore, we sought to initially clarify the principles behind stone movement in FURL.

Methods: Two sub-studies examined the effects of FR and RESD on PBS during SNP-UAS-assisted FURL while focusing on the principles of stone movement. The first sub-study involved in vitro experiments that referenced the male urinary tract, using an appropriately curved SNP-UAS with a 0.30 cm ball as a stone model. Initially, the minimum FR required for effective max-PBS in in vitro experiments was observed under a commonly used RESD (0.73) and FR between 20.00 and 50.00 ml/min. Subsequently, variations in max-PBS were analyzed across three RESD values (0.73, 0.79, and 0.87) and available FR levels (50.00, 70.00, and 90.00 ml/min). The second sub-study involved CFD analyses utilizing a simplified model, with an SNP-UAS oriented vertically and a circle of 0.30 cm diameter as stone model. Initially, circle's movement was examined at a PBS of 0.00 cm/s, RESD of 0.73, and FR from 20.00 to 100.00 ml/min, with preliminary movement principles explained. Next, circle's movement was assessed at a PBS of 5.00 cm/s, FR of 50.00 ml/min, and RESD values of 0.73, 0.79, and 0.87.

Results: In the first sub-study, only the 50.00 ml/min FR group achieved the max-PBS, and max-PBS increased with higher FR and larger RESD. In the second sub-study, at FR of 50.00 ml/min and above, the circle ascended faster towards the water inlet as FR increased. During the circle's accelerated and uniform movement phase, the pressure surrounding it balanced, but vortices persisted and changed. At FR 50.00 ml/min and 5.00 cm/s PBS, higher RESD led to quicker and further circle ascent.

Conclusion: In vitro experiments and CFD analyses reveal that higher RESD and FR enhance PBS during FURL using SNP-UAS, aiding individual stone removal. The stone movement is directly driven by surrounding vortices, not pressure differences.