## 4 Discussion
The authors, using a sensor (The PressureWire Guidewire) for real-time measurement of IPP inserted into the renal pelvis through the ureter, evaluated changes in intrapelvic pressure during flexible URS in four patients. Following the results reported by [PERSON] et al., the mean IPP during laser lithotripsy using on-demand forced irrigation was 115.3 cmH\({}_{2}\)O. The maximum peak pressure measured during the therapeutic period using forced irrigation was 436 cmH\({}_{2}\)O for UAS 10/12 Fr, while for UAS 12/14 Fr, it reached approximately 340 cmH\({}_{2}\)O [6]. The second study that assessed IPP during flexible URS was one reported in 2023 by [PERSON] et al. Using PressureWire Guidewire, the authors measured changes in IPP in a group of 20 patients undergoing flexible URS. The median IPP during the therapeutic period with the use of forced irrigation on demand reached 61.2 (27.2-149.5) cmH\({}_{2}\)O. The maximum peak pressure measured during the therapeutic period using forced irrigation was 236.6 cmH\({}_{2}\)O for UAS 10/12 Fr, while for UAS 12/14 Fr, it reached 171 cmH\({}_{2}\)O [10]. Printing a 3D model of the pelvicalycesal system on the basis of a RIRS trainer (according to the patent application P.442625) and producing chemically synthesised stones allowed a statistically significant series of RIRS procedures to be performed under almost the same conditions, as well as using PressureWire Guidewire to measure the dynamics of IPP (total of 50 procedures). For the first time, the authors used a 3D model that allowed simulation of the RIRS procedure to assess IPP. The study performed provides a new complement to the knowledge of IPP during flexible URS, and repeatable ex vivo conditions allowed RIRS to be simulated using different holmium laser parameters without any risk to the patient. For the printing of the model, thermoplastic polyurethane (TPU) was chosen after the available materials were analysed because, to the authors' knowledge, it most closely reflects the biostructure in terms of hardness and elasticity. In comparison, one of the new 3D models of the upper urinary tract dedicated to the simulation of the RIRS procedure is the model proposed by [PERSON] et al. The authors introduced the models of the upper urinary tract which were digitally optimised and 3D printed from real-life cases. An additional advantage of the trainer is 3D printed models of urinary stones with two different degrees of hardness. The presented system allows us to perform RIRS simulation, which allows us to learn how to explore the pelvicalycesal system, as well as the performance of lithotripsy [20]. Our model similarly allows us to learn how to navigate a flexible ureteroscope in the pelvicalycesal system, while chemically synthesised stones of different densities enable us to perform laser lithotripsy even more faithfully, which, according to the perception of the surgeons participating in the testing phase of the trainer, practically reflects the real lithotripsy in the patient's body. The lithotripsy of chemically synthesised calculi is not free from the phenomenon of residual lithotripsy formation and the dusty environment in which we work, which fully reflects natural conditions. Another system for RIRS simulation is the model presented by [PERSON] et al. It is also based on a 3D printout of the upper urinary tract, which allows us to learn how to manoeuvre flex-URS and how to mobilise and evacuate natural kidney stones deposited in the pelvicalyceal system using an endoscopic basket. The model in the presented version does not allow one to simulate laser lithotripsy [21]. According to our study, the median IPP measured during the therapeutic period using forced irrigation on demand was 172 cmH\({}_{2}\)O for UAS 10/12 Fr, while for UAS 12/14 Fr, it was 21 cmH\({}_{2}\)O. The maximum peak pressure measured during the therapeutic period using forced irrigation on demand was 461.7 cmH\({}_{2}\)O for UAS 10/12 Fr, while for UAS 12/14 Fr, it reached 106.5 cmH\({}_{2}\)O.