This study was a retrospective and single-center trial and was approved by our institutional review board (No. R03-0204). All patients provided informed consent.
We performed a retrospective review of prospectively collected data for 162 patients who underwent URSL from August 2018 to February 2021. Patients for which flexible ureteroscopes were not used were excluded. All patients underwent general anesthesia. Two endourologists performed URSL, which was standardized as follows. The calculi were approached using a guide wire and semi-rigid ureteroscope (6.5/8.5 Fr; Richard Wolf, Knittlingen, Germany). For proximal ureteral and renal stones, a ureteral access sheath was placed under fluoroscopic guidance, and all calyxes were checked to confirm stones and pelvicalyceal anatomy with channel free. For ureteral stones, the stones were pushed back into the intra-pelvicalyceal system, and all calyxes were checked. Next, the calculi were fragmented with a holmium YAG laser (5–10 Hz and 0.5–1.0 J; VersaPulse 30W; Lumenis, San Jose, CA, USA). First, all calyxes were checked for any stones that needed to be fragmented, and then the calculi were fragmented using a 365 µ laser with high energy. Next, the laser fiber was changed from 365 µ to 200 µ for fragmentation with low energy. After fragmentation with the 200 µ laser, all calyxes were checked for any more stones that needed to be fragmented. If stones could not be fragmented with the 365 µ laser, they were fragmented with the 200 µ laser. All fragments ≥ 2 mm in size were extracted using a 1.5 Fr tipless basket (N-circle; Cook Medical, Bloomington, IN, USA) with a flexible ureteroscope (URF-P5 or P7, Olympus, Tokyo, Japan, or LithoVue, Boston Scientific, Natick, MA, USA). After extraction, all calyxes were checked for residual stones with a 1.5 Fr basket. For mid- and distal ureteral calculi, a semi-rigid ureteroscope was used, and the calculi were fragmented with a holmium YAG laser. All fragments ≥ 2 mm in size were extracted using a 1.5 Fr tipless basket. For stones that migrated to the renal pelvis, the same procedure was followed as for kidney stones. The ureteral access sheath was a Flexor 12/14 F (Cook Medical, Bloomington, IN, USA) or Navigator 11/13 F (Boston Scientific, Natick, MA, USA). No cases required dilation of the ureteral orifice or ureter, and the ureteral stent was left in situ after the procedure.
Retrograde pyelography was performed to check pelvicalyceal anatomy before inserting the ureteral access sheath. An image of the pelvicalyceal anatomy was acquired, and the infundibular length (IL), infundibular width (IW), and infundibulopelvic angle (IPA) were measured according to a previous report by Elbahnasy et al.7. Furthermore, new parameters were measured to evaluate the distance from the lower calyx to the proximal ureter (infundibular distance (ID)) and the angle of the lower calyx (infundibular angle (IDA)) (Fig. 1).
During fURS, all calyxes were checked for residual stones several times with channel free, 1.5 Fr basket, and 200 µ and 365 µ lasers. The methods for reaching the lower calyx were classified into three categories: reachable with active deflection, reachable with passive deflection, and unreachable.
The success rate of reaching the lower calyx was the primary end point. The predictive factors for successfully reaching the lower calyx with active deflection were analyzed. Patient demographics and success rates of reaching the lower calyx were reviewed and compared between three groups using either the Kruskal-Wallis test or χ2 test. Multivariate analysis was performed using a logistic regression model to identify the most significant factors affecting the success of reaching the lower calyx with active deflection. The parameters analyzed included the flexible ureteroscope (disposable (LithoVue) vs reusable (P5 and P7)) and pelvicalyceal anatomical parameters (IPA, IDA, IL, IW, and ID). Furthermore, we calculated the cut-off points of the receiver operating characteristic (ROC) curve, area under the ROC curve (AUROC), and 95% confidence interval (CI) of IDA and IL for successfully reaching the lower calyx with active deflection. Statistical analyses were performed using BellCurve for Excel 3.20 (Social Survey Research Information Co., Ltd., Tokyo, Japan), and the significance level was set at p < 0.05.