Flexible ureterorenoscopy vs. mini percutaneous nephrolithotomy for kidney stones in chronic kidney damage: a prospective study

Introduction

Managing kidney stones involves several treatment modalities, including percutaneous nephrolithotomy (PCNL), flexible ureteroscopy (f-URS), and extracorporeal shock wave lithotripsy. Treatment selection is based on stone size, location, kidney anatomy, and patient characteristics. PCNL is the gold standard for stones >2 cm, achieving stone-free rates of up to 96%.1 For stones measuring 1–3 cm, f-URS demonstrates comparable outcomes despite requiring multiple sessions and routine postoperative ureteral stent placement.2 While PCNL offers higher stone-free rates in a single session, it is more invasive.

Each treatment option poses a risk of kidney damage via various mechanisms. PCNL and f-URS require continuous saline irrigation, which increases intrarenal pressure beyond the normal range of 5–10 cmH2O. Although some studies suggest a safe upper limit of 40 cmH2O for intraoperative pressure, higher pressures elevate the risk of complications.3

Ureteral access sheaths (UASs) optimize irrigation by enhancing visualization while maintaining safe intrarenal pressures and fluid temperatures. UASs also contribute to shorter operative times and higher stone-free rates. However, they may increase complication risks through direct organ injury or ureteral ischemia.4

Pressures exceeding 40 cmH2O are associated with postoperative complications such as bleeding, perirenal fluid collection, and sepsis.5 Sepsis is often attributed to pyelovenous reflux, wherein bacteria are transported from the urinary collecting system into the bloodstream. A pig model study revealed significant saline backflow and renal parenchymal scarring at pressures exceeding 30 cmH2O, while no scarring occurred at lower pressures.3

Additional PCNL-associated complications include renal bleeding, colonic or pleural injury, and postoperative pulmonary issues.6,7 The procedure involves puncturing and dilating the renal parenchyma, which risks damage to adjacent structures such as the colon, lungs, or major vessels. Meticulous anesthetic and surgical management during PCNL are essential to minimize complications.

Chronic kidney disease (CKD) is a clinical syndrome characterized by irreversible kidney damage and progressive decline in kidney function. Patients with CKD undergoing surgery face an increased risk of 30-day mortality and complications such as stroke, myocardial infarction, pneumonia, septic shock, deep vein thrombosis, and postoperative bleeding.8 Therefore, treating patients with CKD having kidney stones requires heightened caution. Hence, this study aimed to compare the effects of f-URS and mini percutaneous nephrolithotomy (m-PCNL) on long-term renal outcomes in patients with CKD who had kidney stones.

Material and Methods

Study design and participants

Written informed consent was obtained from all participants, and ethical approval was granted by the Haseki Training and Research Hospital Ethics Committee (approval date/No.: 01.03.2023/88-2022). Patients with renal stones measuring between 1 and 4 cm in maximum diameter and a confirmed diagnosis of CKD were included in the study. All participants were required to be suitable candidates for surgery and able to provide written informed consent. Patients were recruited from the Urology Clinic of the University of Health Sciences, Haseki Training and Research Hospital, during the study period between January 2023 and June 2023. This relatively broad size range was selected to reflect real-world clinical practice in CKD patients, where stones of varying sizes within this range may require intervention. The decision also allowed sufficient recruitment in this high-risk patient group, in whom treatment options may be limited. Patients under 18 years of age, those with a history of kidney transplantation, renal or skeletal anomalies, or pregnancy were excluded.

The primary endpoint was the change in renal function at 6 months postoperatively, assessed by glomerular filtration rate (GFR), serum creatinine levels, and CKD stage progression. A sample-size calculation was performed using previously published data on renal functional outcomes after endourological interventions in patients with CKD, which reported between-group differences in postoperative estimated glomerular filtration rate (eGFR) in the range of 4–6 mL/min with standard deviations of approximately 6–7 mL/min.9,10 Assuming a clinically relevant difference of 5 mL/min in eGFR at 6 months between groups, a standard deviation of 6–7 mL/min, a two-sided α of 0.05, and 80% power, we calculated that at least 50 evaluable patients were required. A total of 102 patients were initially considered, of whom 93 met the inclusion criteria. Thirteen patients declined randomization, resulting in 80 participants. Patients were randomized to the f-URS and m-PCNL groups, with 40 patients each using a sealed envelope system. Of the 80 randomized patients, sixteen in the f-URS group and four m-PCNL group were excluded due to noncompliance with follow-up protocols. The main reasons for loss to follow-up were patient preference to continue care at another institution, inability to attend scheduled visits due to comorbid conditions, and withdrawal of consent. Finally, the study was conducted as f-URS (n = 24) and m-PCNL (n = 36) (Figure 1). Importantly, baseline characteristics of the initially randomized cohort (n = 80) were comparable to those of the final study population (n = 60), suggesting that attrition did not significantly alter group demographics.

FIGURE 1. Study flow

CKD is diagnosed when a patient exhibits a GFR below 60 mL/min/1.73 m2 for 3 months or longer or when structural kidney damage is evident despite a GFR above 60 mL/min/1.73 m2.11 CKD is classified into five stages based on GFR; Stage 1: Kidney damage with normal GFR (>90 mL/min), Stage 2: Mild reduction in GFR (60–89 mL/min), Stage 3a: Moderate reduction in GFR (45–59 mL/min), Stage 3b: Moderate reduction in GFR (30–44 mL/min), Stage 4: Severe reduction in GFR (15–29 mL/min), Stage 5: Renal failure (GFR < 15 mL/min).

Stone-free status (SFR) was defined as the absence of any residual stone fragments on non-contrast abdominopelvic computed tomography (CT) at 1 month postoperatively. In line with recent literature, including the systematic review by Çavdar et al. residual fragments of any size were considered clinically relevant in CKD patients due to their potential to promote infection, obstruction, and further renal function deterioration.12 Therefore, the presence of any residual fragment, regardless of size, was classified as not stone-free. A 6-month follow-up period was deemed appropriate when reviewing studies on kidney stone patients with chronic kidney disease.9

The primary endpoint of the study was the change in renal function at 6 months postoperatively, assessed by serum creatinine levels, eGFR, and CKD stage progression.

Intervention and procedure

All patients received comprehensive preoperative information about the procedures, and informed consent was obtained before surgery. Each surgery was performed under general anesthesia by the same experienced endourologist team. In both procedures, irrigation was delivered by gravity only (saline bags elevated to 80–100 cm above the operating table), without any pressure bag systems.

Mini percutaneous nephrolithotomy (m-PCNL)

Patients were initially placed in the lithotomy position, where a guidewire and a 5-Fr ureteral catheter were inserted under fluoroscopic guidance, followed by the placement of a Foley catheter. The patients were then repositioned to the prone position for renal access. The renal puncture was performed using an 18-gauge needle, followed by serial dilation up to 21 Fr. Stones were fragmented using a 16-Fr Nephroscope (Hangzhou HAWK Optical Electronic Instuments, Hangzhou, China) holmium: yttrium-aluminum-garnet (Ho:YAG) laser lithotripter and subsequently removed. Ureteral and/or nephrostomy tubes were placed at the surgeon’s direction. Foley catheters were typically removed the morning after the procedure.

Flexible ureterorenoscopy (f-URS)

A disposable digital 8.5-Fr f-URS (Zhuhai Pusen Medical Technology, Zhuhai, China) was used for the procedure. Patients were placed in the lithotomy position under general anesthesia. A 9.5-Fr rigid URS (AMNOTEC, Neuhausen ob Eck, International Medical GmbH, Germany) was used to guide the insertion of a guidewire (Boston Scientific Corporation, Natick, MA, USA) into the ureter. Passive dilation was performed after advancing the scope to the ureteropelvic junction (UPJ).

In all f-URS cases, an 11–13 Fr ureteral access sheath (UAS) was used (45 cm for males, 35 cm for females). The UAS was inserted under fluoroscopic guidance following passive ureteral dilation with a semi-rigid ureteroscope. No ureteral trauma or insertion failure occurred. UAS use allowed continuous outflow, reduced intrarenal pressure, and improved visualization during lithotripsy. Stones were fragmented using a Ho:YAG laser with a 200-μm probe. After stone fragmentation, a 4.8-Fr, 26-cm ureteral stent was placed. Postoperative Foley catheters were removed 6 h after surgery. No patient required additional treatment sessions for residual stones.

Statistical analysis

Data analysis was performed using SPSS software version 26 (IBM Corp., Armonk, NY, USA). Continuous variables were compared using independent samples t-tests, while categorical variables were analyzed using chi-square tests. Descriptive statistics were expressed as median (IQR) for continuous variables and as frequency (%) for categorical variables. Statistical significance was set at p < 0.05. An analysis of covariance (ANCOVA) was performed to compare postoperative eGFR and creatinine values between groups, adjusting for their respective baseline values.

A post hoc subgroup analysis was also conducted for patients with stones measuring 10–20 mm to explore potential size-related differences; as this analysis was not pre-specified, its findings should be interpreted as exploratory. For the 10–20 mm subgroup, Bonferroni correction was applied to account for multiple comparisons, and adjusted p-values were reported.

All primary analyses were performed per protocol and included only patients with complete 6-month renal function data, because imputing missing eGFR or creatinine values was deemed unreliable in this relatively small cohort. Baseline characteristics of the randomized population (n = 80) and the final analytic sample (n = 60) were comparable, which reduces the potential impact of attrition bias. An intention-to-treat analysis was not performed because 6-month renal function values were missing for several participants, and imputing these data in a relatively small sample was considered unreliable; therefore, a per-protocol approach was adopted.

Results

Baseline patients and stone characteristics, including stone size, location, opacity, side, and history of prior surgeries, were comparable between the two groups (p > 0.05, Table 1). Most stones in both groups were located in the pelvis and were radiopaque (80.6% in m-PCNL vs. 70.8% in f-URS).

The f-URS group demonstrated significantly better outcomes in hemoglobin reduction, operative duration, fluoroscopy exposure, and hospital stay (p = 0.002, p < 0.001, p < 0.001, and p < 0.001, respectively). Stone-free and overall complication rates were similar between the groups (Table 2). In the m-PCNL group, postoperative fever was observed in six patients, with two requiring blood transfusions. Two patients required ureteral catheter repositioning due to stent migration. In the f-URS group, postoperative fever developed in two patients, while one patient experienced pulmonary edema, which resolved with diuretic therapy. Additionally, one patient’s ureteral stent was replaced under anesthesia due to migration.

Preoperative eGFR and creatinine values were comparable between the groups (median [IQR] of eGFR: 48.0 [32.3, 50.8] mL/min for m-PCNL vs. 50.5 [40.0, 56.3] mL/min for f-URS, p = 0.158; median [IQR] of creatinine: 1.5 [1.3, 2.3] mg/dL vs. 1.4 [1.3, 1.6] mg/dL, p = 0.123). At the 6-month follow-up, the f-URS group had significantly higher eGFR and lower creatinine values compared to the m-PCNL group, indicating better renal function at that time point (median [IQR] of eGFR: (44.0 [28.0–53.0] mL/min vs. 51.5 [40.8–57.0] mL/min, p = 0.042); median [IQR] of creatinine: 1.6 [1.3, 1.9] mg/dL vs. 1.4 [1.3, 1.5] mg/dL, p = 0.031).

However, when the absolute change in eGFR from baseline to 6 months (ΔeGFR, median [IQR]) was analyzed, no statistically significant difference was observed between the groups (m-PCNL: 2.0 [−8.8, 6.0] mL/min vs. f-URS: 1.0 [−3.0, 3.8] mL/min, p = 0.516). Similarly, Δcreatinine values (median [IQR]) did not differ significantly (−0.1 [−0.2, 0.3] mg/dL vs. 0.1 [−0.1, 0.1] mg/dL, p = 0.189) (Table 3).

CKD stage progression differed markedly between the groups. Two patients (5.5%) showed improvement, while 11 (30.5%) patients experienced stage progression and 23 (63.8%) remained stable in the m-PCNL group (Figure 2). In contrast, 9 (35.5%) patients in the f-URS group showed improvement, only 1 (4.1%) experienced progression, and 14 (58.3%) remained stable (Figure 3).

FIGURE 2. Change in preoperative and postoperative 6th month renal failure stages in patients who underwent percutaneous nephrolithotomy

FIGURE 3. Change in preoperative and postoperative 6th month renal failure stages in patients who underwent flexible ureterorenoscopy

In a post hoc exploratory subgroup analysis of patients with 10–20 mm stones (m-PCNL n = 16; f-URS n = 14), SFRs were similar between the groups (100% vs. 92.8%, p = 0.74), but perioperative and functional outcomes were significantly better in the f-URS group. Operative time was shorter (median [IQR]: 72.00 [70.25, 73.75] min vs. 41.50 [39.25, 43.75] min, p = 0.004), hemoglobin drop was lower (median [IQR]: 1.20 [1.12, 1.30] g/dL vs. 0.60 [0.50, 0.67] g/dL, p < 0.001), and hospital stay was significantly reduced (median [IQR]: 61 [48, 69] h vs. 24 [21, 26] h, p < 0.001). In terms of renal function, f-URS led to a modest improvement (median [IQR] of ΔeGFR: 1.15 [0.83, 1.73] mL/min) compared to a decline in the m-PCNL group (median [IQR]: −3.85 [−4.07, −3.52] mL/min, p < 0.001). CKD stage progression was observed in none of the patients in the f-URS group (0/14), whereas 4 of 16 patients (25%) in the m-PCNL group experienced worsening of CKD stage (p = 0.041) (Table 4). Adjusted p-values using Bonferroni correction for multiple comparisons are presented in Table 4.

Discussion

Advancements in endourology have increased the efficacy of m-PCNL and f-URS in achieving high stone-free rates. A retrospective study by Ergin et al. involving 90 patients reported a mean stone size of 13.9 ± 2.9 mm and found no significant difference in SFR between m-PCNL and f-URS groups (p > 0.05).13 In a meta-analysis, PCNL achieved higher stone-free rates than f-URS, particularly in subgroup analyses of stones ≥2 cm.14 In our study, the stone-free rates were 86.1% for m-PCNL and 79.2% for f-URS. Although literature reports favor m-PCNL for stones >2 cm, our findings did not reveal a statistically significant difference. The definition of SFR in our study was intentionally strict, considering any residual fragment as treatment failure. This approach was based on recent evidence highlighting the clinical significance of even small residual fragments in CKD patients, who may be more susceptible to infection, obstruction, and progressive renal impairment. Çavdar et al. emphasized that residual fragments, regardless of size, should not be underestimated in high-risk populations.12 In our cohort, 5 patients in the m-PCNL group and 5 patients in the f-URS group had small residual fragments (<4 mm), all of whom were classified as not stone-free and were managed with surveillance, or medical therapy.

The patient positioning and kidney access required for PCNL typically result in longer operation times than f-URS.14 Fluoroscopy is employed during f-URS to position access sheaths and identify non-visible stones, while PCNL utilizes fluoroscopy at various stages, including kidney access and stone evaluation. Aligning with these findings, our results indicated that f-URS was advantageous regarding operation and fluoroscopy times (p < 0.001). This discrepancy may be partially attributed to the limited use of ultrasonography during percutaneous access in our clinic.

Percutaneous nephrolithotomy is inherently associated with a higher risk of bleeding. A retrospective study by Akbulut et al., which included 94 patients, demonstrated a significantly lower postoperative hemoglobin drop in the f-URS group than in the m-PCNL group (0.39 g/dL vs. 1.15 g/dL, p < 0.001).15 Similarly, our study revealed a greater hemoglobin decrease in the PCNL group (p = 0.002), contributing to longer postoperative hospital stays (p < 0.001). Acute blood loss adversely affects renal function, potentially explaining the observed deterioration in eGFR and creatinine levels 6 months postoperatively.16

Regarding postoperative complications, Wan et al. reported no significant differences between f-URS and m-PCNL groups.17 Consistent with this, our study found no statistically significant variation in complication rates between the two groups (p = 0.746).

Several previous studies have demonstrated perioperative advantages of f-URS over m-PCNL for stones around 2 cm; however, most excluded patients with impaired renal function, limiting their applicability to CKD populations. In contrast, CKD-specific evidence is largely derived from retrospective cohorts. Consistent findings have been reported across other retrospective series; however, interpretation remains limited by selection bias, heterogeneity in surgical techniques, and incomplete biochemical follow-up9,10,18 By providing prospective randomized data with standardized follow-up, the present study strengths existing evidence by directly comparing functional outcomes between f-URS and m-PCNL in CKD patients. Nevertheless, potential cofounders such as stone composition, baseline hydronephrosis, and chronic parenchymal changes may also influence postoperative renal function and were not fully captured in prior studies nor in the current analysis.19,20 These factors should be considered when interpreting renal functional outcomes and warrant further investigation in larger, CKD-specific trials.

In parallel with the existing literature on PCNL in patients with compromised renal function, our findings also indicate that postoperative renal functional trajectories vary within this population. In the study by Patel et al., among 60 patients with chronic kidney damage who underwent percutaneous nephrolithotomy, 6-month follow-up revealed improvement in GFR values in 45 patients (75%) and deterioration in 15 patients (25%). Seven of these patients showed improvement in their CKD stages, while 53 were stable.9 In our cohort of 36 PCNL patients, only two patients’ CKD stages had improved, while 23 remained stable and 11 experienced a decline. This is concorded with an overall improvement in renal function, as seen in the reported literature.18,21

Conversely, f-URS has demonstrated more favorable outcomes in renal function. A study investigating f-URS surgery in 163 patients reported significant deterioration in kidney function in 4.9% of cases and significant improvement in 14.1%.22 Similarly, our findings showed kidney function improvement in 35.5% (n = 9) of patients, stability in 58.3% (n = 14), and decline in only 4.1% (n = 1). These results align with existing literature, highlighting the renal preservation advantage of f-URS.

At the 6-month follow-up, absolute eGFR values were higher in the f-URS group (p = 0.042), while the m-PCNL group showed a slight decrease. However, this difference was not statistically significant when the ΔeGFR from baseline was analyzed. The apparent discrepancy may be explained by the categorical nature of CKD staging, where even small fluctuations in eGFR can cross threshold values and result in apparent stage progression or improvement. This limitation should be considered when interpreting CKD stage outcomes, as stage shifts may overestimate the true clinical impact of renal function changes.23 When viewed in this context, the lower CKD stage progression observed in the f-URS group may indicate a potential advantage, but this should be interpreted cautiously given the absence of significant differences in functional change from baseline. These observations are consistent with those of Reeves et al., who demonstrated the safety of f-URS in CKD patients, particularly regarding renal function preservation10 Importantly, our study adds value by focusing specifically on CKD patients, where even small differences in renal preservation may be clinically meaningful.

Our subgroup analysis of 10–20 mm stones further refine this perspective. This size range—often regarded as a therapeutic grey zone—poses a clinical decision-making challenge, as both f-URS and m-PCNL are considered viable.24,25 In this subset, f-URS demonstrated superior functional preservation and perioperative safety, lending support to a more conservative, kidney-sparing strategy for appropriately selected CKD patients. Given the small sample size (n = 30) and the post hoc nature of this group analysis, these findings should be interpreted cautiously and regarded as hypothesis-generating rather than conclusive, as the statistically significant differences may partly reflect and increased risk of type 1 error.26 Taken together, these findings highlight the need for individualized treatment plans and underscore the importance of larger, stratified studies to confirm the optimal approach for CKD patients with urolithiasis.

The main strength of our study lies in its prospective randomized design. However, the relatively small sample size may have limited the statistical power and reduced the ability to perform robust subgroup or multivariate analyses. Although baseline characteristics did not differ significantly between groups, numerical trends in comorbidities such as diabetes and hypertension could still have influenced the outcomes. The inclusion of stones measuring 1–4 cm was intentional to reflect real-world clinical practice in CKD patients and to ensure adequate recruitment, but it inevitably introduced heterogeneity in stone burden and procedural complexity. The dropout rate differed between the groups, with a greater proportion of exclusions occurring in the f-URS arm. Although the baseline characteristics of the initially randomized 80 patients were comparable to those of the 60 patients who completed the study, suggesting no major demographic imbalance, this differential attrition may still have introduced bias. The subgroup analysis of patients with 10–20 mm stones was performed post hoc and included a limited number of patients, so its results should be interpreted as exploratory. Additionally, no multivariable adjustments were applied to control for potential confounders. Future studies with larger cohorts should include stratified analyses and multivariable models to further validate these findings.

Conclusions

In this prospective randomized study of CKD patients with 1–4 cm renal stones, f-URS and m-PCNL achieved comparable SFRs. At 6 months, absolute eGFR and creatinine values favored f-URS, although ΔeGFR and Δcreatinine did not differ significantly between groups. CKD stage progression was more frequent after m-PCNL, whereas improvement or stability was more common with f-URS. Taken together, these results suggest that f-URS may provide advantages in perioperative safety and renal function preservation, but larger cohorts and long-term follow-up are needed to confirm these findings. Given the high-risk nature of this population, treatment selection should be individualized, balancing stone burden, comorbidities, and procedural risks.

Acknowledgement

None.

Funding Statement

None.

Author Contributions

Nazım Furkan Guünay: Conceptualization (lead); writing—original draft (lead); formal analysis (lead); writing—review and editing (equal). Mücahit Gelmiş, Çağlar Dizdaroğlu: Software (lead); writing—review and editing (equal). Ufuk Çağlar, Abdullah Esmeray: Methodology (lead); writing—review and editing (equal). Faruk Özgör, Ömer Sarılar: Conceptualization (supporting); writing—original draft (supporting); writing—review and editing (equal). All authors reviewed the results and approved the final version of the manuscript.

Availability of Data and Materials

Data are available from the corresponding author upon reasonable request.

Ethics Approval

This study was approved by the Haseki Training and Research Hospital Ethics Committee (date/No: 01.03.2023/88-2022) and was performed in accordance with the Declaration of Helsinki.

Informed Consent

Written informed consent was obtained from all individual participants prior to enrollment.

Conflicts of Interest

The authors declare no conflicts of interest to report regarding the present study.

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