An artificial kidney model was created by using Urovac evacuator™ (Boston Scientific, Marlborough, MA, USA; capacity: 340 ml, length: 14 cm, maximum diameter: 7.3 cm, thickness: 3.2 mm) as a renal pelvis calyx based on previous studies [12, 13]. The nephrostomy sheath was connected to the Urovac evacuator™ so that it was 70° above the horizontal plane, which was designed to represent puncture of the lower calyx in a prone position in a human body. A silicon connector was attached so that the nephrostomy sheath could be replaced. A ureteral access sheath (UAS) was installed at a site that resembled the renal pelvic ureteral junction, and it was made possible to replace it with a silicone connector. An arterial tonometry line (Edward Phillips, USA) was attached to the head side of Urovac evacuator™, and the renal pelvic pressure was measured. The position of the transducer was adjusted to the height of the base of the Urovac evacuator™, and zero correction was performed in any of the measurements.

Experimental setup. (a) Nephrostomy, UAS, and f-URS placed in the artificial kidney model. The nephrostomy sheath was connected to the Urovac evacuator™ so that it was 70°above the horizontal plane, representing puncture of the lower calyx in a prone position in a human body. The UAS was installed at a site that resembled the renal pelvic ureteral junction. An arterial tonometry line was attached to the head side of it. (b) Artificial kidney model details. For nephrostomy and UAS, a silicone connector is used for the connection to the Urovac evacuator™, making it possible to change the sheath size. (c) Schematic representation of all equipment used in the experiment. UROMAT E.A.S.I.™ pump was used for irrigation from the nephroscope to achieve a steady continuous irrigation pressure

We used UROMAT E.A.S.I.™ pump (Karl Storz, Tuttlingen, Germany) for irrigation from the nephroscope to achieve a steady continuous irrigation pressure. It was used to adjust the pressure in four steps: 40 mmHg, 80 mmHg, 120 mmHg and 160 mmHg.

A single-use digital flexible ureteroscope (f-URS) PU3033A (PUSEN Medical, Zhuhai, China) was inserted through a UAS until reaching the renal pelvis. Two types of UAS were used: 10/12 Fr and 12/14 Fr Flexor (Cook Medical, Bloomington, IN, USA). When using the f-URS, the tip was placed in the renal pelvis through the UAS and irrigation pressure was set at 80 cmH2O. When not in use, the measurement was performed with the UAS open.

For the nephroscope, we used minimally-invasive PCNL (MIP) systems™ (Karl Storz). Four types of MIP systems™ were used, and three types of nephroscope were used according to the sheath system of the outer cylinder: MIP-L (shaft diameter 25/26 Fr) uses 19.5 Fr nephroscope, MIP-M (shaft diameter 16.5/17.5 Fr) uses 12 Fr, MIP-S (shaft diameter 11/12 Fr) and MIP-XS (shaft diameter 8.5/9.5 Fr) uses 7.5 Fr. The working channel was empty.

The length of each sheath and nephroscope was the same, and the insertion position of the nephroscope was aligned in all cases. The tip of the nephroscope was inserted until it protruded 2 cm from the sheath. Irrigation was performed by connecting a UROMAT E.A.S.I. pump™ (Karl Storz) to the nephroscope.

A combination of the MIP system™, UAS and f-URS, adjustment of irrigation pressure from the nephroscope, and multiple combinations were performed. In all experiments, the working channels of both nephroscope and f-URS were empty. All tests were repeated six times for each combination. Test combinations of MIP system™, UAS, f-URS and irrigation pressure are shown in Fig. 2. Zeroing was performed for each test, and after 30 s or more had passed from the start of measurement, it was confirmed that a plateau had occurred, and that value was measured and the average was calculated.

Test combinations. Multiple tests were performed under various combinations of different MIP system™, UAS, f-URS and irrigation pressures. We performed each combination six times

Results were presented as the mean with standard deviation (SD). The Student’s t-test was used for repeated measures in a single group. All trials used JMP PRO version 16.0.0 (JMP, Cary, NC, USA) and p-values < 0.05 were considered significant.