## 1 Introduction  
Retrograde intrarenal surgery (RIRS) is a standard procedure dedicated to the treatment of kidney stones. The continuous development of endourology allows the use of minimally invasive treatment methods that improve patient safety. In the case of RIRS, regardless of the necessary armamentarium used for the procedure, the most crucial aspect is the balance of physical conditions (intrarenal pressure and temperature) prevailing in the operated space, the pelvicalyceal system of the kidney. Until now, there has been no standardised and universally available method to assess physical conditions in the renal pelvis in real time during flexible ureterorenoscopy [1], hence the need for experimental research to improve endourological techniques, which will lead to more effective surgery and better patient safety. The predominant conditions in the renal pelvis during RIRS are the result of several factors that influence the outcome of laser lithotripsy under optimal visibility conditions. The most important factor includes the balance between irrigation inflow (the IPP-generating irrigation system) and outflow (uretroscope: ureteral access sheath ratio) [2; 3; 4].  
The normal (\"physiological') intranrenal pressure range is 0-20 cmH\({}_{2}\)O [2]. Intracranal backflow (flow from the renal pelvis to the renal parenchyma) and impaired arterial perfusion have been reported at approximately 40 cmH\({}_{2}\)O in both human [3] and animal studies [4]. Pressure above 40 cmH\({}_{2}\)O leads to a pyelovvenous backflow [5]. Previously published studies have shown that during flexible ureterenoscopy, a very high peak IPP of up to 300-400 cmH\({}_{2}\)O can be generated [6; 7]. Therefore, the use of UAS (ureteral access sheath) during RIRS is one of the ways to reduce IPP. The application of UAS allows the safe insertion of endoscopic tools into the upper urinary tract and their easier navigation, as well as the improvement of vision by continuous flow establishment. Based on the cadaver study performed by [PERSON] et al., application of UAS during surgery results in an increase in irrigation flow of 35 to 80% compared to the group without the UAS used [8]. Irrigation pressures transmitted to the renal pelvis are significantly higher during flexible URS without the use of the UAS. [PERSON] et al. showed that, regardless of the type of ureteroscope or the position of the UAS, the irrigation pressures transmitted to the renal pelvis are significantly greater during flexible URS with no utility of the UAS (pressure measured in the renal pelvis through the nephrostomy tube without the use of UAS was more than twice as high compared to measurements with UAS) [9].  
The aim of the study was to evaluate IPP during RIRS procedures on 3D printed models with chemically synthesised stones. The 3D model allows RIRS procedures to be carried out under almost identical conditions, using stones of comparable size and density. The study evaluated the differences in real-time IPP generated during RIRS with two different sizes of UAS (10/12 Fr vs. 12/14 Fr) using different energy settings generated by the holmium:YAG laser. To the best of our knowledge, this is the first study to assess IPP in ex vivo conditions using 3D models created from plasticised thermoplastic polyurethane with chemically synthesised stones.