Ex Vivo Porcine Kidney Model for Evaluating Flow-Pressure Dynamics and Stone Retrieval in Suction-Assisted Retrograde Intrarenal Surgery

Summary

This protocol establishes an ex vivo porcine kidney platform for controlled, repeatable quantification of intrarenal pressure and stone pull-out time across different irrigation pressures, suction pressures, and ratios of endoscope-sheath diameter.

Abstract

Retrograde intrarenal surgery (RIRS) using a suction ureteral access sheath (UAS) has become an effective approach for renal stone management. However, standardized experimental models capable of simultaneously controlling and evaluating relevant parameters, including irrigation pressure, suction pressure, and the ratio of endoscope-sheath diameter (RESD), remain limited. The aim of this study is to propose an ex vivo porcine kidney model designed to enable controlled and repeatable evaluation of flow–pressure conditions with suction UAS. The protocol utilized fresh cadaveric porcine kidneys placed in a fixed container system, with the suction UAS positioned at the upper pole calyx, and artificial stones mimicking calcium oxalate monohydrate introduced into the collecting system. An automated pressure-control pump system enabled precise regulation of irrigation and suction pressures. Intrarenal pressure (IRP) was continuously recorded for 1 min, and pull-out time (POT) was measured repeatedly under each RESD and pressure setting. The coefficient of variation (CV) and intraclass correlation coefficient (ICC) were calculated to assess measurement variability and reliability. The model demonstrated distinct IRP responses across different RESD and pressure conditions, including values exceeding 40 mmHg. POT measurements varied according to pressure settings, reflecting sensitivity to changes in flow–pressure conditions. CV analysis showed relatively narrow variability across most conditions, while ICC analysis demonstrated moderate reliability for single measurements and high reliability for averaged repeated measurements. Overall, this ex vivo porcine kidney model provides a standardized ex vivo reference for RIRS by presenting objective and reliable repeated measurement data using a suction UAS and a flexible ureteroscope.

Introduction

Retrograde Intrarenal Surgery (RIRS) stands as a primary minimally invasive approach for managing renal stones^1,2,3. A recent technical advancement in RIRS is the development of the suction ureteral access sheath (UAS), which has introduced an adjunctive approach to stone retrieval based on controlled irrigation and suction^4,5,6,7.

The primary challenge in utilizing suction UAS is balancing stone extraction speed with the risk of high intrarenal pressure (IRP)^8,9. IRP exceeding 30–40 mmHg can trigger pyelovenous backflow, potentially leading to systemic infection and sepsis^10,11,12. Factors involved in regulating this balance include irrigation, suction, and the ratio of endoscope-sheath diameter (RESD).

Although numerous clinical studies have investigated suction UAS and IRP^13,14,15, standardized experimental methodologies and validated models enabling the simultaneous control, measurement, and analysis of relevant factors remain limited.

This protocol provides a step-by-step guide for establishing an ex vivo porcine kidney model with acceptable reproducibility, specifically designed to standardize the evaluation of fluid dynamics and stone retrieval in suction-assisted RIRS.

Protocol

This experimental protocol using a cadaveric porcine kidney was deemed exempt from the Institutional Animal Care and Use Committee (IACUC) of Seoul National University Hospital. The reagents and the equipment used are listed in the Table of Materials.

1. Preparation of the porcine kidney model

NOTE: See Figure 1.

1. Obtain fresh kidneys from deceased pigs at an authorized slaughterhouse within 6 h of death. Transport and store the kidneys at room temperature until the experiment.
   NOTE: Cadaveric porcine kidneys should be used within 24 h of death to avoid corruption and further renal perfusion decrease.
2. Select kidneys with a partially remaining ureter to allow stable fixation of the UAS.
   NOTE: The length of the ureter must enable a fixation suture to immobilize the UAS, and the kidneys containing a longer remnant ureter should be preferred.
3. Gently fill the renal collecting system with normal saline using a syringe in a tension-free manner. Determine the calyceal volume when reflux occurs, and saline leakage outside the ureter is observed. Exclude porcine kidneys with a calyceal volume of less than 10 mL.
4. Prepare artificial stones mimicking calcium oxalate monohydrate stones.
5. Crush the stones into fragments of an appropriate size of 2 mm to 2.5 mm using a pestle. Use a digital caliper to select appropriately sized stones.
   NOTE: The stone size must be large enough to prevent direct in-scope suction or spontaneous escape through the gap between the scope and the sheath.
6. Place an artificial stone fragment into the upper pole calyx of the porcine kidney. Use saline to push the stone into the UAS and position it in the collecting system.
   NOTE: Use an endoscope to visually verify that the UAS is consistently fixed in the proper position during the experiment.
7. Place the porcine kidney in a metal box of appropriate size and put the box in a plastic container.
8. Fix the long plastic pipe to the ureter using 3-0 polyglactin sutures to ensure the system remains stable without movement during the experiment.
9. Insert the suction UAS inside the pipe and position the distal tip at the upper pole calyx, at the entrance to the renal pelvis.

2. Experimental system configuration

1. Select an appropriate flexible ureteroscope (outer diameter with 7.5 F, 9.0 F, or 9.5 F) and pair it with a 50 cm suction UAS of a specific size (inner/outer diameter with 10/12 F, 11/13 F, or 12/14 F).
2. Measure the outer diameter of the flexible ureteroscope using a digital caliper at the distal tip and midshaft. Calculate RESD based on measured data (0.68, 0.75, 0.86, or 0.95).
3. Connect the irrigation line from an automated pressure-control pump system to the flexible ureteroscope.
4. Connect the suction line from an automated pressure-control pump system to the suction side port of the UAS.
5. Connect the pressure sensor inside the UAS to an automated pressure-control pump system.
6. Calibrate the pressure sensor to atmospheric pressure without priming, according to the manufacturer’s guidelines.
7. Insert a 200 µm laser fiber into the working channel of the ureteroscope to mimic the real environment of retrograde intrarenal surgery.

3. Intrarenal Pressure (IRP) data collection

1. Put the flexible ureteroscope inside the UAS. Confirm that the endoscope has passed beyond the distal end of the UAS and reached the porcine kidney.
2. Enter flexible endoscopy mode on the automated pump device.
3. Select the specific pressure settings of irrigation and suction.
4. Ensure that other settings are controlled and kept constant. Fix irrigation flow at device’s baseline setting of 200 mL/min.
5. Record IRP values every 5 s for a continuous duration of 1 min under each designated pressure setting.

4. Pull-out Time (POT) measurement

1. Set the irrigation pressure (100 mmHg, 200 mmHg, or 300 mmHg) and the suction pressure (100 mmHg, 200 mmHg, 300 mmHg, or 400 mmHg) on the control unit.
   NOTE: A fixed waiting period of 3 s should be kept after each parameter adjustment to allow the pressure to stabilize before recording any measurements.
2. Visualize the stone fragment using the flexible ureteroscope and bring it to the distal opening of the UAS.
3. Initiate the suction and begin recording the POT exactly when the stone enters the distal tip of the sheath.
   NOTE: When a stone fragment is retrieved through the UAS, it must always be visible in the endoscopic view.
4. Stop the timer when the stone fragment is completely evacuated through the proximal sideport of the UAS. Record the measured POT.
5. Repeat this measurement 10 times for every pressure and RESD setting.
   NOTE: Time measurement should be performed blindly to avoid operator bias.

5. Statistical analysis

1. Input all gathered experimental data into a specialized statistical software package.
2. Present all measured time data as the mean ± standard deviation for each setting.
3. Use Spearman’s rank correlation test to identify the tendency across settings. Check the coefficient of variation and intraclass correlation coefficient for repeatability analysis.
4. Establish the threshold for statistical significance at a p-value of less than 0.05 for all comparative analyses.

Results

The outer diameter of the flexible ureteroscope was measured at both the distal tip and midshaft using a digital caliper. The measured outer diameters were 7.5 F, 9.0 F, and 9.5 F, respectively, consistent with the product catalog. No difference was observed between distal tip and midshaft measurements, confirming that the instrument dimensions were uniform and suitable for reliable RESD calculation (see Figure 2).

IRP was recorded continuously for 1 min at 5 s intervals under predefined pressure settings (see Figure 3). At RESD 0.68 and 0.75, IRP was consistently maintained below 30 mmHg (Figure 3A). At RESD 0.86, IRP showed temporal fluctuations but remained within the borderline range (30–40 mmHg). At RESD 0.95, IRP exceeded 40 mmHg under certain conditions, particularly at higher irrigation pressure (Figure 3B). Subgroup analysis by irrigation pressure showed that higher irrigation settings increased the likelihood of exceeding safety thresholds. These findings show that the model captures dynamic changes in IRP across different pressure conditions.

POT was measured under varying irrigation and suction pressures at RESD 0.75 and 0.86 (Figure 4). Across conditions, POT decreased as irrigation pressure increased, with negative Spearman correlation coefficients observed in both RESD groups. The magnitude of change was smaller at RESD 0.86 compared to RESD 0.75. In some suction settings at RESD 0.86, the correlation did not reach statistical significance. Stone retrieval performance was consistently measurable across different experimental conditions.

The repeatability of POT measurements was evaluated by calculating the coefficient of variation (CV) across 24 experimental conditions (Table 1), defined by RESD (0.75, 0.86), irrigation pressure (100 mmHg, 200 mmHg, and 300 mmHg), and suction pressure (100 mmHg, 200 mmHg, 300 mmHg, and 400 mmHg). The CV ranged from 10.88% to 32.42%, with a mean value of 17.41%. With the exception of one setting (RESD 0.75, irrigation pressure 100 mmHg, and suction pressure 400 mmHg), all CV values were below 25%. Notably, 20 out of 24 settings demonstrated CV values below 20%. The mean CV was lower at RESD 0.86 than at RESD 0.75 (15.71% vs 19.13%). These results showed a consistent range of variability across repeated measurements within the tested conditions.

The intraclass correlation coefficient (ICC) for repeated POT measurements was calculated. The single-measure ICC (ICC 1,1) was 0.55, indicating moderate reliability for an isolated single measurement. In contrast, the average-measure ICC (ICC 1,k) was 0.93, indicating excellent reliability when repeated measurements were averaged. These findings clarify that the high measurement reliability of this experimental system was primarily achieved through the use of averaged repeated measurements rather than relying on a single, one-time measurement.

Figure 1: Experimental setting of the porcine kidney model. (A) Overview of ex vivo porcine kidney model with a ureteral access sheath and flexible ureteroscope inside the kidney. (B) A long plastic pipe fixed to the ureter using 3-0 polyglactin sutures to ensure stable positioning during the experiment. (C) Schematic diagram showing the location of the ureteral access sheath inside the porcine kidney. (D) Flexible ureterscope view with a visible stone fragment when performing the pull-out test. Please click here to view a larger version of this figure.

Figure 2: Outer diameter measurement of flexible ureteroscope. Diameter measurement of each ureteroscope was done using a digital caliper. Measurement was done at both the distal tip and midshaft part to check the difference in diameter according to the ureteroscope site. Please click here to view a larger version of this figure.

Figure 3: Intrarenal pressure (IRP) measurement with different ratios of endoscope-sheath diameter (RESD). IRP measurement was done for 1 min continuously every 5 s. The yellow zone means borderline safety IRP range (30–40 mmHg) to avoid pyelovenous backflow. (A) IRP measurement with RESD 0.68, 0.75, and 0.86. (B) IRP measurement at RESD 0.95 with irrigation pressure of 100 mmHg, 200 mmHg, and 300 mmHg. Please click here to view a larger version of this figure.

Figure 4: Mean pull-out time according to each pressure setting of irrigation and suction. Pull-out time was described by mean ± standard deviation, with Spearman’s rank correlation coefficient(ρ) and p value. (A) Pull-out time at RESD 0.75. (B) Pull-out time at RESD 0.86. Please click here to view a larger version of this figure.

Coefficient of Variation (%)
RESD	Suction pressure (mmHg)	Irrigation pressure (mmHg)
		100	200	300
0.75	100	16.25	17.63	23.81
	200	16.71	19.26	17.98
	300	24.80	19.28	10.88
	400	32.42	15.93	14.61
0.86	100	19.24	12.46	16.78
	200	14.28	12.86	13.06
	300	12.34	19.38	16.13
	400	17.13	22.08	12.74

Table 1: Coefficient of variation of pull-out time according to each irrigation, suction pressure, and RESD setting. Coefficient of Variation (%) was calculated by (Standard deviation/Mean) x 100.

Discussion

This protocol was designed to establish a controlled experimental platform for evaluating flow–pressure conditions during RIRS using a suction UAS. This study provided an ex vivo porcine kidney model capable of systematically controlling irrigation pressure, suction pressure, and RESD settings, while enabling consistent measurement of IRP and POT with statistical validation.

The most critical step in this protocol is to maintain the remnant control settings constant to ensure consistent measurements of the POT and IRP. The position of the UAS was fixed so that the distal tip was always located at the upper pole of the major calyx, at the entrance to the renal pelvis. Previous studies^16,17 were referenced to minimize movement of the kidney and UAS during measurement. The stability of the system was further enhanced using sutures and plastic tubing (Figure 1). Artificial stones used in the pull-out test were manufactured to mimic calcium oxalate monohydrate stones in hardness and physical properties, based on the prior validation established in our previous study¹⁸. The diameter of each endoscope was measured using a digital caliper to confirm consistency with product catalogs (Figure 2). A waiting period of at least 3 s was applied after pressure adjustment to ensure stabilization before measurement.

The experimental system demonstrated distinct IRP responses across different RESD and irrigation conditions, including values exceeding predefined safety thresholds (Figure 3). In addition, POT measurements varied according to pressure settings (Figure 4), reflecting the sensitivity of the system to changes in flow–pressure conditions. In the repeatability analysis, CV remained within a relatively narrow range across most conditions, with increased variability observed under specific settings combining high suction and low irrigation (Table 1). This pattern is associated with insufficient fluid flow under extreme pressure imbalance, leading to unstable measurement conditions¹⁹.

Several troubleshooting steps should be considered during this protocol. If the baseline IRP reading is unstable, checking for reconnecting lines, including the pressure sensor of the UAS and automated pump machine, and confirming the absence of leakage in the irrigation or suction line is needed. For unstable UAS positioning, checking for reconfirmation of ureteral fixation and endoscopic verification of the sheath tip location before measurement is needed. For high POT variability or failure of stone retrieval, reassessing stone size, confirming the 3-s stabilization period after pressure adjustment, and maintaining identical timing procedures across repeated measurements should be checked.

Permissible modifications can be made to the irrigation and suction pressure settings, endoscope settings, kidney container and fixation method, number of repetitions, and statistical software while maintaining consistency. In particular, additional pressure and endoscope settings can be adjusted in further studies to evaluate flow dynamics using this ex vivo model. However, the UAS position, stone size range, stone composition, and automated irrigation pump were kept constant in this experiment, and an additional validation process is required if these factors are changed.

A key strength of this protocol is its ability to provide a controlled experimental environment that enables objective and consistent assessment of flow–pressure dynamics in RIRS. This ex vivo kidney model allows systematic manipulation of irrigation, suction, and RESD parameters while reducing variability associated with anatomical and physiological factors. In particular, its advantages include the use of artificial stones mimicking actual renal stones and the anatomical features of the renal collecting system, which are not reflected in the existing computational fluid dynamics model¹⁷. In addition, the protocol supports repeated measurements under identical conditions, allowing structured evaluation of measurement variability and reliability, which is difficult to achieve in clinical in vivo clinical methodology^5,6. Therefore, this ex vivo model can be used to compare flow-pressure dynamics and device configurations under controlled experimental settings.

This model offers several realistic directions for future application. First, it can serve as a preclinical ex vivo platform to compare different suction UAS designs, ureteroscopes, and sheath sizes, and RESD combinations under controlled pressure settings. Second, it can help evaluate irrigation and suction pressure combinations that balance stone retrieval efficiency with avoidance of excessive IRP. Third, the protocol can be adapted to assess how stone-related variables, such as size, shape, and composition, affect suction-based retrieval performance. Finally, this model can complement clinical in vivo studies by providing controlled, preliminary data regarding device configurations and procedural parameters prior to clinical validation.

Several limitations should be considered. First, the cadaveric porcine kidney does not fully reproduce physiological conditions such as tissue perfusion and compliance. Second, fluid capacity varies depending on calyceal volume, which can influence pressure dynamics. In this study, calyceal volume was assessed using a tension-free saline injection method to exclude kidneys with a capacity less than 10 mL, whereas other studies have measured calyceal volume using 3D Computed tomography^20,21. However, these methods do not fully reflect the compliance or the pressure buffering effect of the in vivo kidney. Third, the distal tip of the UAS was fixed at the upper pole calyx to ensure consistency of measurement. Variations in stone location were not considered in this protocol. Further refinement of the model may improve its applicability to clinical scenarios.

In conclusion, this ex vivo porcine kidney model provides a reproducible and standardized experimental platform for evaluating flow–pressure relationships and stone retrieval performance in suction-assisted RIRS.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Flexible-Y Type Negative Suction Sheath	ZSR System Biomedical Technology Co.	ZSR-UAY-1035 ZSR-UAY-1134 ZSR-UAY-1235	Suction ureteral access sheath
FLEX-XC1	KARL STORZ SE & Co.	091271-06	Flexible ureteroscope
HM30S	HugeMed Medical Technical Development Co.	HM30S	Flexible ureteroscope
i-MIMER-sys	ZSR System Biomedical Technology Co.	ZSR-ISP100	Automated pump
JW Digital Calipers	Jongwon Tooling Co.	404-200	Digital caliper
URUS	Dyne Medical Group Inc.	URUS SU 100R	Flexible ureteroscope