Retrospective analysis of flexible ureteroscopic lithotripsy using end-fire,flexible negative-pressure ureteral access sheath for the management of 2–3-cm kidney stones

Abstract

Objective

To compare the efficacy and safety of flexible ureteroscopic lithotripsy using end-fire flexible negative-pressure ureteral access sheath (F-UAS) versus traditional ureteral access sheath (T-UAS) for 2–3-cm kidney stones.

Methods

This retrospective case–control study included 136 patients with kidney stones treated with flexible ureteroscopic lithotripsy using F-UAS at Shenzhen Second People’s Hospital from November 2022 to November 2023 (F-UAS group) and 132 patients treated with flexible ureteroscopic lithotripsy using T-UAS from November 2021 to October 2022 (T-UAS group). Preoperative assessments included imaging studies (kidney, ureter, and bladder X-ray; noncontrast computed tomography; and renal ultrasound) and laboratory tests (complete blood count, biochemical tests, coagulation function, and urine culture). The stone clearance rate, operation time, intraoperative blood loss, and complications were compared between the two groups.

Results

Baseline characteristics of the two groups were balanced. The F-UAS group showed significantly higher stone clearance rate (91.9% vs. 81.8%, p = 0.014), shorter operation time (51.0 ± 13.9 vs. 59.8 ± 18.2 min, p < 0.001), and lower postoperative antibiotic use (2.9% vs. 6.8%, p = 0.027) than the T-UAS group. The intraoperative blood loss and length of hospital stay did not differ significantly between the two groups.

Conclusions

Flexible ureteroscopic lithotripsy using F-UAS improves stone clearance, reduces infection-related complications, and shortens operation time compared with that using T-UAS in the treatment of 2–3-cm kidney stones. Further prospective studies are warranted to confirm these findings.

Introduction

Kidney stones represent a global urological disease that has a significant negative impact on patients’ health and quality of life. They exhibit a high recurrence rate and unpredictable occurrence patterns; therefore, identifying effective and long-lasting treatment methods for kidney stones is an important goal in the field of medicine. ¹ Since 2000, minimally invasive treatment methods have gradually become more widespread in urology, with flexible ureteroscopic lithotripsy (FURS) emerging as the first-line treatment for kidney stones <2 cm in diameter. However, percutaneous nephrolithotripsy remains the gold standard for treating high-burden stones (2–3 cm in diameter). With ongoing advancements in endoscopic techniques and laser equipment, flexible ureteroscopy is increasingly being used for high-burden stones (2–3 cm) because this method can reduce surgical trauma and enhance patient comfort and recovery. ² Despite the advantages of FURS, such as high stone clearance rates and low postoperative pain, it is associated with potential surgical risks and challenges. The efficiency of stone removal during the procedure, operation time, intraoperative blood loss, postoperative recovery, and incidence of complications remain focal points of concern for both doctors and patients. ³ Optimizing these factors may not only help improve the success rate of surgeries but also significantly enhance patients’ postoperative recovery and quality of life. Therefore, exploring more advanced and advantageous instruments to improve the overall effectiveness of FURS has become an important direction in current research. ⁴

The use of ureteral access sheath (UAS) is an important method for effectively managing and regulating renal pressure during FURS. The UAS not only facilitates smooth entry of the endoscope into the kidney but also continuously drains fragments and flushing fluid during the operation, reducing renal pressure and minimizing the risk of infection.^5–7 Traditional ureteral access sheaths (T-UASs) have been used for several years in clinical practice, resulting in the accumulation of rich experience and data. However, there are still some limitations in terms of operational flexibility, stone removal efficacy, and complication control. This is particularly evident for patients with medium-to-large kidney stones, where issues such as prolonged surgical times, low stone removal rates, high intraoperative blood loss, and postoperative complications remain unresolved.

In recent years, UASs with flexible tips and integrated negative-pressure suction capabilities have gradually emerged in the clinical landscape, offering a potentially transformative technical approach for FURS. ⁸ Compared with traditional rigid or semi-rigid sheaths, these novel devices are not simply single modifications of existing structures but rather collaborative designs at the level of device dynamics and microfluidics. An adjustable flexible tip allows terminal angle adaptation in complex calyces or unusual anatomies, thereby significantly improving accessibility and device stability in lower pole calyces or difficult-to-reach locations. The accompanying continuous negative-pressure suction module accelerates the immediate evacuation of small fragments, improves intraoperative visualization, and reduces fragment retention and secondary migration within the collecting system by establishing a controlled local pressure gradient at the stone fragmentation site. The combination of these two design elements not only optimizes fragment dynamics and stone contact efficiency but may also fundamentally reduce the retrograde flow of microorganisms or endotoxins resulting from irrigation pressure differences, thereby simultaneously improving both stone clearance rate and infection control.^9–11 Although several in vitro simulation studies and small-scale clinical series have initially suggested the aforementioned advantages, their evidence was often limited by inadequate sample size, endpoint heterogeneity, and inconsistent follow-up, particularly when faced with the clinical challenge of treating medium-to-high burden stones with diameters of 2–3 cm. Data on systemic comparisons between flexible negative-pressure suction sheaths (F-UAS) and T-UASs remain scarce and insufficient to support widespread adoption.

Therefore, we conducted a retrospective case study to systematically evaluate the effectiveness and safety of a single-use electronic ureteroscope combined with an end-fire F-UAS and a T-UAS in the treatment of kidney stones measuring 2–3 cm in diameter. This study provides a detailed comparison of the optimal performance of these two instruments, aiming to offer practical data and experience to help clinicians make more scientifically sound decisions in treating patients with kidney stones, thereby optimizing FURS surgical protocols, improving treatment outcomes, and enhancing patient safety. We believe that these new instruments will find broader application in urological practice, further advancing the technology for treating kidney stones.

Materials and methods

This single-center retrospective case–control study involved patients with kidney stones who had undergone F-UAS treatment at our hospital between November 2022 and November 2023. The T-UAS group consisted of consecutive cases treated with T-UAS at our hospital from November 2021 to October 2022 (before the introduction of F-UAS) who met the following inclusion and exclusion criteria (Figure 1). The inclusion criteria were age ≥18 years, preoperative imaging–confirmed kidney stones with a maximum diameter of 2–3 cm, and no severe renal dysfunction before surgery. The exclusion criteria included prior urological surgery, pregnancy or lactation, severe cardiopulmonary disease, or other systemic conditions that made them ineligible for endoscopic surgery. Finally, 136 eligible patients were enrolled in the F-UAS group and 132 in the T-UAS group. This study was conducted and reported in accordance with the ethical principles outlined in the 1975 Declaration of Helsinki and its 2024 revision as well as the Strengthening the Reporting of Observational studies in Epidemiology (STROBE) guidelines. ¹²

Figure 1.

Treatment flow of patients with kidney stones. After excluding surgical contraindications, patients were divided into two groups and subjected to ureteroscopic lithotripsy: Group 1 was the T-UAS group, and Group 2 was the F-UAS group. F-UAS: flexible negative-pressure ureteral access sheath; T-UAS: traditional negative-pressure ureteral access sheath.

Imaging assessment and stone size measurement

All patients underwent multiple preoperative imaging assessments, including kidneys, ureters, and bladder (KUB) radiography; noncontrast abdominal computed tomography (CT); and urinary tract ultrasound, for comprehensive evaluation of stone location and burden. The maximum stone diameter was determined using noncontrast CT. All CT images were independently measured for longest diameter (unit: mm) by two radiologists, each with ≥5 years of urinary imaging experience, who were blinded to group assignment. If the difference between the two measurements was >2 mm, a third senior radiologist reviewed the images, and the median value was recorded as the final measurement. For multiple stones, the diameter of the largest single aggregate was recorded as the primary stratification variable, and the overall stone burden (sum of the maximum diameters of each stone) was also reported. The image-viewing software and window width/level settings used for measurements were standardized and unified within the hospital.

Preoperative and perioperative assessments

Routine preoperative tests included complete blood count, blood chemistry, coagulation function, and urine culture to exclude active infection and assess surgical tolerance. For patients with positive urine cultures, targeted antibiotics were administered preoperatively and intraoperatively according to the department’s infection-control protocol; patients with negative cultures received prophylactic antibiotics per routine. Comprehensive anesthesia evaluation and cardiopulmonary assessment were also performed preoperatively to mitigate perioperative risk. All surgeries were performed by the same experienced surgeon leading the team. Intraoperative treatment–related data (including imaging, operative records, and instruments used) were uploaded to and archived in the hospital database with patient consent at the time of surgery for quality control and research purposes.

Data protection

This retrospective data analysis did not involve public disclosure of any patient images/photographs or personally identifiable information that could directly identify patients; the study procedures did not violate patient privacy. All data used for the study were deidentified (via removal of names, identification numbers, hospitalization numbers, and other direct identifiers) according to institutional protocols before extraction and analysis, and the results were reported only in an aggregated form. Patients provided written informed consent at the time of surgery, which included consent for de-identified clinical data to be used for research. This retrospective study was filed with and overseen by the Ethics Committee of the First Affiliated Hospital of Shenzhen University for supervision and guidance.

Surgical techniques

T-UAS

After successfully inducing general anesthesia, the patients were placed in the lithotomy position. Under the guidance of a zebra guidewire (INNOVEX, Shanghai, China), an 8/9.8-Fr ureteroscope (Wolf, Schleswig-Holstein, Germany) was inserted to observe the affected ureter for any twists or strictures, advancing the scope to the ureteropelvic junction. After leaving the zebra guidewire in place, the ureteroscope was withdrawn, and a 12-Fr T-UAS (Well Lead Medical Co. Ltd.) was inserted under the guidance of the wire. Subsequently, a 9.2-Fr single-use digital flexible ureteroscope (Scivita Medical, China) was advanced through the scope sheath into the renal pelvis, where the collecting system was carefully examined. After assessing the stones, a 200-μm holmium laser fiber was inserted, and fragmentation was performed using the holmium laser under a perfusion and suction system (power: 20–30 W) in a fragmentation mode. After a thorough assessment of the renal pelvis, when all calyces showed no significant residual stones, the zebra guidewire was placed, and a 4.8-Fr ureteral stent was inserted and retained.

F-UAS

The anesthesia methods used during the surgical procedure and the management of ipsilateral kidney stones in the F-UAS group were consistent with those used in the T-UAS group. During the fragmentation process, the ureteral flexible endoscope and the terminally adjustable F-UAS were brought close to the stones and actively suctioned to improve visual clarity (Figure 2).

Figure 2.

F-UAS stone retrieval technique. F-UAS: flexible negative-pressure ureteral access sheath.

On postoperative day 1 and at 1 month after the surgery, all patients underwent renal and ureteral imaging, along with KUB radiography or CT, for evaluation of their postoperative status. The final assessment, conducted 1 month postoperatively, was utilized to determine the stone-free rate; if imaging revealed residual stones or fragments with a maximum diameter of ≤2 mm, they were deemed clinically insignificant residual fragments, and the patient was classified as stone-free. Patients who developed ureteral strictures postoperatively or whose imaging indicated the need for further surgery received appropriate management within 1 month after the surgery. All patients completed a minimum of 3 months of postoperative follow-up; routine 3-month reexaminations employed ultrasound and/or KUB and/or CT to assess midterm outcomes.

All intraoperative and postoperative complications were prospectively identified from the electronic medical records and retrospectively reviewed and graded according to the Clavien–Dindo classification. Complication assessment was conducted by the study team’s clinical review panel, which included two urology attending physicians and one research nurse; the assessment was based on each patient’s clinical presentation, documented therapeutic interventions (administration of additional antibiotics, requirement for imaging-guided or surgical re-intervention, or prolongation of hospitalization), and relevant ancillary test results (complete blood count, urine culture, and imaging). Discrepant assessments were resolved through panel discussions and, when necessary, adjudicated by a senior supervising physician. Most adverse events in this cohort were of low grade (Clavien–Dindo I–II); therefore, the specific clinical manifestations and management strategies for each group are detailed to aid clinical interpretation.

Statistical analyses

Baseline characteristics and postoperative data were obtained from the hospital’s electronic medical records. Preoperative stone status and postoperative stone-free status were independently assessed by one urology specialist and one imaging specialist (assessors were blinded to group allocation). Categorical variables were presented as counts or percentages and were compared using the chi-square test or Fisher’s exact test, as appropriate. Continuous variables conforming to a normal distribution were presented as mean ± standard deviation and compared using independent samples t-test; non-normally distributed continuous data were compared using the Wilcoxon rank-sum test. Statistical analyses were performed using the Statistical Package for Social Sciences (SPSS) software, version 21.0. All tests were two-sided, and p < 0.05 was considered statistically significant.

Results

Among the 268 patients included in this study, 132 underwent FURS using T-UAS, while 136 underwent FURS using F-UAS. Ultimately, there were no significant differences in baseline characteristics between the two groups (Table 1), including sex (p = 0.614), age (p = 0.718), body mass index (BMI) (p = 0.138), stone size (p = 0.287), stone location (p = 0.389), and preoperative health status (hypertension (p = 0.258) and diabetes (p = 0.722)).

Table 1.

Comparison of preoperative baseline data between the two groups.

Group	Number of cases	Sex		Age (years)	BMI (kg/m2)	Stone diameter (cm)	Stone location		Comorbidities
		Male	Female				Renal pelvis	Renal calyx	Hypertension	Diabetes
T-UAS	132	72	60	48.3 ± 9.2	23.5 ± 2.3	2.68 ± 0.24	112	20	28	11
F-UAS	136	70	66	48.7 ± 8.9	23.7 ± 2.1	2.71 ± 0.22	110	26	25	13
p-value		0.614		0.718	0.138	0.287	0.389		0.258	0.722

BMI: body mass index; T-UAS: traditional ureteral access sheath; F-UAS: flexible negative-pressure ureteral access sheath.

All patients underwent KUB imaging on postoperative days 1 and 30, with CT evaluations conducted for any ambiguous suspected residual stones. In terms of operation time, the average procedure time in the F-UAS group was 51.0 ± 13.9 min, while that in the T-UAS group was 59.8 ± 18.2 min (p = 0.000). Additionally, the stone clearance rate in the F-UAS group was significantly higher than that in the T-UAS group (91.9% vs. 81.8%; p = 0.014).

None of the patients in either group experienced serious complications during the surgery. Compared with the T-UAS group, the overall complication rate in the F-UAS group was significantly lower. In the F-UAS group, the incidence of infectious complications, including fever, and the frequency of additional antibiotic use were significantly reduced, with a statistically significant difference in postoperative antibiotic use (2.9% vs. 6.8%; p = 0.027). The incidence of persistent hematuria was 2.2% in the F-UAS group and 1.5% in the T-UAS group (p = 0.383). The average length of postoperative hospital stay was 1.15 ± 1.21 days in the F-UAS group and 1.41 ± 1.32 days in the T-UAS group (p = 0.094). Table 2 summarizes the intraoperative and postoperative outcomes.

Table 2.

Perioperative indicators in the two groups of patients.

Indicator	T-UAS group	F-UAS group	(95% CI)	t/χ2	p-value
Stone clearance rate% (n)	81.8% (108/132)	91.9% (125/136)	10.1% (2.3% to 17.9%)	6.011	0.014
Surgery duration (min)mean ± SD	59.8 ± 18.2	51.0 ± 13.9	−8.8 (−12.6 to −5.0)	4.457	0.000
Decrease in Hb level on postoperative day 1 (g/dL)mean ± SD	0.35 ± 0.15	0.38 ± 0.19	0.03 (−0.02 to 0.08)	−1.432	0.153
Fever (body temperature ≥38.1°C)% (n)	8.3% (11/132)	4.5% (6/136)	−3.8% (−9.7% to 2.1%)	11.173	0.188
Additional antibiotics% (n)	6.8% (9/132)	2.9% (4/136)	−3.9% (−7.4% to 0.4%)	4.867	0.027
Persistent hematuria % (n)	1.5% (4/132)	2.2% (7/136)	2.1% (−1.6% to 5.8%)	0.763	0.383
Postoperative hospital stay (days)mean ± SD	1.41 ± 1.32	1.15 ± 1.21	−0.26 (−0.55 to 0.03)	1.682	0.094

T-UAS: traditional ureteral access sheath; F-UAS: flexible negative-pressure ureteral access sheath; CI: confidence interval.

All complications listed in the table are based on electronic medical records and were re-evaluated by the study team according to the Clavien–Dindo classification; most of them were mild-to-moderate (Clavien–Dindo I–II).

“Additional antibiotics” refers to systemic therapeutic antibiotics administered postoperatively beyond routine perioperative prophylaxis for suspected or confirmed infections (e.g. postoperative fever with body temperature ≥38.0°C, positive urine culture, or the occurrence/worsening of urinary symptoms).

Discussion

FURS is a minimally invasive procedure widely used for the treatment of kidney stones. With advancements in technology and the introduction of new surgical instruments, FURS has gained widespread recognition globally. The European Association of Urology recommends FURS as the preferred method for removing kidney stones with a diameter ≤2 cm and regards it as an alternative for patients in whom percutaneous nephrolithotomy is contraindicated. ¹³ However, despite the increasing popularity of FURS, high intrarenal pressure during the procedure can lead to the entry of bacteria and endotoxins into the bloodstream, thereby increasing the risk of postoperative infections, particularly for patients with a history of recurrent urinary tract infections. Therefore, reducing intrarenal pressure during the procedure has become an effective strategy. The use of UAS during FURS can significantly prevent increases in intrarenal pressure as well as accelerate perfusion and drainage, thus reducing the incidence of infectious complications. ¹³

In recent years, the improved negative-pressure UAS has been validated for use in the treatment of urinary stones. With the gradual expansion of indications for FURS, this technology has also shown potential in treating kidney stones >2 cm in diameter. ¹⁴ However, there are currently insufficient comparative data on the use of F-UAS or T-UAS with single-use flexible ureteroscopes in the treatment of kidney stones ≥2 cm in diameter. To address this gap, this retrospective case–control study was conducted.

When comparing the differences between traditional and new types of UASs, the significant differences in design and structure are key factors. T-UASs are usually made from rigid materials, providing a stable operational channel; however, their rigid structure increases the potential risk of damage to the renal pelvis and ureter to some extent. ¹⁵ In contrast, the new F-UAS features structural innovations that significantly reduce friction and pressure on the inner wall of the ureter by using softer materials, thereby minimizing damage during the surgical process. Additionally, the new flexible F-UAS can flexibly enter the renal pelvis under the physician’s guidance, allowing effective removal of stones at specific locations. Importantly, the F-UAS can continuously aspirate small stone particles and flushing fluid during the lithotripsy process, which helps maintain a lower renal pressure within the renal pelvis.

In this study, we focused on analyzing the differences in the effectiveness of T-UAS and the novel F-UAS in the treatment of unilateral kidney stones. The results indicated that F-UAS offered significant advantages with regard to multiple surgical indicators. The proportion of patients with postoperative stone-free status was notably higher in the F-UAS group than in the T-UAS group (91.9% vs. 81.8%), especially when addressing stones in the lower renal calyx. With the assistance of the F-UAS, the single-use electronic ureteroscope could enter the target renal calyx, maintaining a clear surgical field at all times. Small stone fragments could be extracted through the gaps in the access sheath, while larger stone fragments could be removed via intermittent suction. This significantly improved the postoperative stone clearance rate. These findings are consistent with those reported by Liu et al., who stated that negative pressure suction not only enhanced stone removal efficiency but also effectively reduced the occurrence of postoperative abnormal symptoms. ¹⁶ Moreover, the use of F-UAS significantly reduced the reliance on stone retrieval baskets during surgery and decreased the number of repetitive operations performed by the physician, thereby reducing the overall operation time. This indirectly lowered the incidence of complications and anesthetic-related risks. In a large-scale clinical trial conducted by Chen et al., approximately 54.7% of patients did not use stone retrieval baskets. ¹⁷ This is consistent with our findings, highlighting the contribution of optimizing surgical processes to improved resource allocation.

In terms of postoperative complications, the F-UAS group had lower rates of infection and ureteral injury. Several factors may explain this result. Existing studies have shown that high renal pressure is a risk factor for postoperative infections. ¹⁸ In our study, the postoperative fever rate in the F-UAS group was lower than that in the T-UAS group, which may be attributed to the continuous intraoperative suction mechanism in the F-UAS group that helped maintain low intrarenal pressure. Additionally, the rigid structure of the T-UAS necessitates special caution during surgery to prevent injuries caused by excessive manipulation. The design of the F-UAS tip offers greater adaptability and safety margins, reducing the mechanical stress on the ureteral wall during the procedure, thereby lowering the risk of postoperative inflammation and damage. ¹⁹ This feature not only expands the indications for ureteral flexible endoscopy but also provides better treatment options for complex kidney stone cases. Furthermore, although there was no significant statistical difference in the average length of hospital stay between the two groups, postoperative follow-up showed that patients in the F-UAS group reported less postoperative discomfort and exhibited faster recovery, resulting in a significant improvement in their quality of life. Although the novel access sheath demonstrated numerous advantages, there are several challenges that limit its widespread adoption. First, this was a single-center retrospective cohort study; therefore, inherent selection and information biases associated with the retrospective design may affect causal inference. Although the sample size was sufficient to allow the detection of several short-term outcome differences, the patient population and treatment processes at a single center may limit the generalizability of these findings to other clinical settings. Second, the study did not include a cost-effectiveness analysis; therefore, it cannot provide quantitative conclusions about the economic feasibility of F-UAS in different health systems. Third, the learning curve of the surgeons or teams was not quantitatively assessed; this is a crucial gap as the learning curve may affect operation time and complication rates. Given these limitations, we recommend that large, multicenter, prospective randomized controlled trials with extended follow-up, cost-effectiveness analysis, and learning-curve evaluations be conducted in the future to validate and refine these findings and assess their wider applicability.

Conclusion

This study demonstrates that for kidney stones measuring 2–3 cm in diameter, FURS using F-UAS offers significant advantages over FURS using T-UAS; it markedly increases the postoperative stone-free rate, reduces the operation time, and minimizes postoperative interventions related to infection. This combination enhances intraoperative stone-clearance efficiency, suction control, and visualization, providing a pathophysiological and procedural rationale that improves both short- and mid-term outcomes. Given that this study employed a single-center, retrospective design, multicenter, prospective randomized controlled trials are necessary to verify the long-term efficacy, safety, and cost-effectiveness of this combination as well as to further evaluate benefits in different patient subgroups and optimize indications, thereby providing robust evidence for broader clinical adoption of the technique.

Supplemental Material

sj-pdf-1-imr-10.1177_03000605251381472 - Supplemental material for Retrospective analysis of flexible ureteroscopic lithotripsy using end-fire, flexible negative-pressure ureteral access sheath for the management of 2–3-cm kidney stones

Supplemental material, sj-pdf-1-imr-10.1177_03000605251381472 for Retrospective analysis of flexible ureteroscopic lithotripsy using end-fire, flexible negative-pressure ureteral access sheath for the management of 2–3-cm kidney stones by Chenglong Wu, Cong Yin, Jieqing Chen, Qiaolin Mei, Yujia Sun, Hongbing Mei and Xinhui Liao in Journal of International Medical Research