This study protocol was approved by the Ethics Committee of the Xiangya Hospital, Central South University. Four hundred forty patients (10 patients underwent two lithotripsy procedures) were treated with reusable digital FURS (between January 2018 and February 2020) and 151 patients (nine patients underwent two lithotripsy procedures) were treated with single-use digital FURS (between March 2020 and September 2020) for upper urinary calculi and their charts were retrospectively reviewed. All procedures were performed by experienced surgeons at our medical center and the course of the surgery is described in the Surgical Technique (the single-use digital FURS as shown in Figure 1). The inclusion criteria were as follows: (1) patients age ≥18 years old; and (2) patients were treated with reusable digital FURS and single-use digital FURS for upper urinary calculi. According to the following exclusion criteria: (1) patients age <18 years old; (2) patients undergoing bilateral procedures or simultaneously combined with other surgery; (3) patients with special situations such as pregnancy, duplicate ureteral deformity and horseshoe kidney, 408 patients and 142 patients were enrolled in single-use FURS and reusable FURS group respectively for treatment outcomes analysis. Subsequently, through 1:1 propensity-score matching analysis based on age, sex, body mass index (BMI), American Society of Anesthesiologists classification (ASA), stone hardness, stone burden, stone location, ureteric stent implanted preoperatively, positive preoperative urine culture, solitary kidney stone, procedural laterality, history of ipsilateral urolithiasis surgery, and degree of hydronephrosis, ultimately, 238 patients (119:119) in the two groups were compared in terms of treatment outcomes. All procedures (450 procedures with reusable FURS and 160 procedures with single-use FURS) were reviewed for costs analysis.

All patients underwent an abdominal non-contrast computed tomography (CT) scan preoperatively to evaluate the stone size, position, and hardness (measured in Hounsfield units, HU). Plain film of kidney-ureter-bladder (KUB) was performed to evaluate stone-free status at 1 day and 1 month postoperatively, and CT will be performed again only when patients need a secondary lithotrity. The demographic variables, operation time, length of hospital stay, postoperative complications, and other clinical data were collected through our electronic medical record system. The urinary microbial culture was performed in all patients 1 week before surgery. Any patient with a positive culture was given sensitive antimicrobial therapy preoperatively based on antibiotic sensitivity tests and well-controlled urinary tract infections were confirmed by urinary cultures before surgical intervention. Patients with negative urine culture received intravenous antimicrobial (Cefuroxime) prophylaxis 30 min before the anesthetic.

Patients were placed in the lithotomy position after general anesthesia. Under the guidance of 4F ureteral catheters, a 9.8-F semirigid ureteroscope (URS) (Karl Storz, Germany) was placed into the ureter to detect whether there was stenosis or abnormality and to dilate the ureter to facilitate the placement of a ureteral access sheath (UAS). Subsequently, a Zebra guidewire was inserted into the ureter through the URS. Then, the URS was removed and a 12/14-Fr Flexor UAS (Cook Urology, 45 cm for male, 35 cm for female) was advanced into renal pelvis directed by the guidewire (If UAS implantation failed, double J tubes were implanted in the first stage, and the second procedure was performed 2 weeks later). Subsequently, the 8.6-F single-use digital FURS ZebraScope^TM (Figure 1) or 9.9-F reusable digital FURS (URF-V; Olympus, Tokyo, Japan) was placed into the pelvis through the UAS. Lithotripsy was performed using holmium:yttrium-aluminum-garnet laser (Ho: YAG) with a 200-μm fiber at an output power of 50–60 W and a frequency level of 15–24 Hz. The rubble fragments were recovered using a 2.4F zero-tip Nitinol stone basket (Cook Medical, Bloomington, IN, USA). After repeated examination of the collection system, it was confirmed that the stones were completely broken and removed. The operation ended with the placement of a double J stent in the ureter for drainage for 1 month.

The extent of hydronephrosis was assessed according to the Society of Fetal Urology grading system (11, 12). Postoperative complications were evaluated according to the Clavien-Dindo classification system (13). Septic shock was defined according to the third international consensus (14). Stone volume was calculated using the following formula (0.785 × length_max × width_max) according to CROES (15), and the burden of multiple stones was calculated as the sum of the volume of all stones. Postoperative stone-free status was defined as the absence of stone fragment > 3 mm on KUB. According to the size and location of residual stones, two experienced professors comprehensively evaluated whether retreatment was needed (CT will be performed only when patients need secondary lithotripsy). In addition, medical images of all patients were independently read by a radiologist and a urologist to measure the calculi burden as determined by CT and to evaluate calculi-free status as determined by KUB after surgery. The clinical outcomes of patients who received their first treatment with FURS lithotripsy during this treatment period were evaluated.

As this was a retrospective study, we were unable to balance the preoperative characteristics in the cost analysis. We performed a crude cost analysis for all procedures undergoing FURS lithotripsy during this study period. All costs were presented in dollars ($) (One dollar is ~6.541yuan). Two reusable FURS were available in our institution which were purchased at market price in 2016 and 2017 respectively. Due to those devices were not new at the time of the study, and we could not count the number of procedures performed before the study. The original purchase costs of the two sets of reusable equipment were modeled as residual value by annual depreciation rate (Approximately $275220; 1800000yuan). Between January 2018 and February 2020, the reusable FURS conducted six repairs at a cost of ~$183480 (1200000yuan). Extrapolating from the data provided by the Disinfection supply center in our hospital, reprocessing costs were ~$80 (523yuan) per procedure, which included the costs of inspection, pre-cleaning, decontamination, assembly, and sterilization. Purchasing prices of disinfection equipment have been left out in our study. The personnel cost was about $40 (262yuan) per procedure based on the hourly wage of the central disinfection technician combined with the average approximate time to reprocess FURS. According to the present local market price, the cost of single-use FURS was about $1529 (10000yuan) per procedure. The total costs were estimated based on the following equations which are similar to the provided by Martin et al. (9).

The cost per procedure was the total costs divided by the number of procedures. Assuming that the maintenance cost per procedure is roughly constant over a long period (Excluding the possible increase in the number of repairs due to aging of FURS), from the above two equations, we can also get a formula that can help the institution to calculate the number of operations performed when the total costs of the two devices reach the equilibrium point.

Chi-square test or Fisher's exact test was used to analyze the proportion of categorical variables; Student's t-test was used to analyze numerical variables with normal distribution. A two-sided P-value less than or equal to 0.05 was considered statistically significant. The logistic regression model was used to calculate the propensity score of each research object for 1:1 propensity-score matching analysis. Statistical analysis and 1:1 propensity-score matching analysis were performed using the Statistical Package for the Social Sciences 22.0 (SPSS for Windows, Chicago, IL, USA).

Preoperative clinical data of the two groups for treatment outcomes analysis (408 vs. 142) are shown in Table 1. After 1:1 propensity-score matching analysis, baseline characteristics of those patients were evenly distributed in two groups (Table 1).

The treatment outcomes with two surgical devices are shown in Table 2. There was no significant difference in the mean operative time between the two groups (60.43 ± 22.76 vs. 61.61 ± 19.36 min, P = 0.666). The mean length of hospital stay in the single-use FURS group was significantly shorter than that in the reusable FURS group (6.86 ± 1.82 days vs. 7.42 ± 2.06 days, P = 0.026), but there was no significant difference in postoperative length of hospital stay between the two groups (2.64 ± 1.32 vs. 2.81 ± 1.55 days, P = 0.368). The average decrease of hemoglobin (Hb) (P = 0.224) and hematocrit (Hct) (P = 0.345) was also no significant difference between the two groups.

The two groups experienced similar rates of overall postoperative complications (10.1% vs. 11.8 %, P = 0.678). The single-use group was associated with a higher incidence of grade I complication (7.5% vs. 5.0%, P = 0.424) than the reusable group, but it had no statistical difference. Urosepsis requiring only additional antibiotics was the main grade II complication and occurred no statistically different incidence rates in the two groups (2.5% vs. 3.4%, P = 1.0). Only one patient in the reusable FURS group developed steinstrasse after discharge and underwent surgery (Grade III). Septic shock (Grade IV) was observed in 2 (1.7%) and 1 (0.8%) patients in the reusable and single-use groups, respectively (P = 1.00). There was also no significant difference in moderate to severe infection-related complications (4.2% vs. 4.2%, P = 1.00).

Initial SFR of the reusable FURS group and single-use FURS groups were 75.6% and 78.2% (P = 0.645). There was also no significant difference in final SFR between the two groups (82.4% vs. 84.9%, P = 0.599). And there were 10 patients (8.4%) in the reusable FURS group and 9 patients (7.6%) in the single-use FURS group who required repeated surgery to remove residual stones (P = 0.811).

The costs of reusable FURS or single-use FURS per procedure are shown in Table 3. Between January 2018 and February 2020, the repair cost per procedure is about $408 (2668yuan) for reusable FURS. After the original purchasing costs, the average cost per reusable FURS was ~$528 (2453yuan). When taking into account original purchasing costs, we should consider the impact of procedure volume on the final cost per procedure, which will decrease with the increase of procedure volume. The cost per single-use FURS was ~$1529 (10000yuan). According to our formula, the break-even point between the two alternatives appears to be 275 procedures in our institution. Total costs or cost per procedure of single-use FURS would exceed the reusable FURS after 275 procedures as shown in Figure 2.