Feasibility and safety of 6.3 Fr vs. 7.5 Fr digital disposable ureteroscopes in retrograde intrarenal surgery: a prospective randomised trial

Abstract

Introduction

Flexible ureterorenoscopy has become one of the cornerstones of minimally invasive treatment for nephrolithiasis. One of the most critical variables in scope design is the outer diameter, improving accessibility. Recently introduced 6.3 Fr ultra-slim digital ureteroscopes may offer clinical advantages over bigger scopes, but there is no data on their performance.

Objectives

This study aimed to prospectively compare the feasibility, safety, durability, and procedural outcomes of RIRS performed using 6.3 Fr and 7.5 Fr digital disposable ureteroscopes from the same manufacturer, evaluating intraoperative parameters, image quality, access rates, and early postoperative results.

Material and methods

Thirty adult patients with renal stones < 1.5 cm or cumulative volume < 850 mm³ were enrolled in a single-centre, randomised, prospective trial. All procedures were performed by a single experienced urologist using CE-certified digital disposable ureteroscopes. Lithotripsy was conducted with a high-power holmium laser and a 272-micron fiber. Subjective evaluations of manoeuvrability and image quality were recorded using 5-point Likert scales. Operator workload was assessed with NASA Task Load Index. In-vitro deflection range and image quality were measured before and after clinical use. Postoperative outcomes, complications, and intraoperative stone-free rates were recorded.

Results

Baseline characteristics were comparable between groups not showing major differences. Ureteral access sheaths were used in 47% of cases. Endoscope insertion into the kidney was successful in all patients. Lithotripsy was complete in 87% of cases using the 6.3 Fr scope and 73% with the 7.5 Fr scope. No statistically significant differences were found in image quality or manoeuvrability scores. Both devices allowed access to the lower pole with and without a working channel instrument. No intraoperative device failures occurred, and postoperative deflection loss was minor and infrequent. One patient required prolonged hospitalisation due to infection while all remaining patients were discharged within 24 h. No ureteral injuries or complications exceeding Clavien-Dindo grade II were observed.

Conclusions

The results of this study indicate that the 6.3 Fr ultra-slim ureteroscope is a feasible and safe alternative to the bigger 7.5 Fr model, with no compromise in visualisation, manoeuvrability, or device integrity. Given its comparable performance and potential for reduced ureteral trauma, the ultra-slim scope may serve as a valuable tool in modern endourology.

Introduction

Retrograde intrarenal surgery (RIRS) has revolutionised the management of nephrolithiasis, offering in some cases a minimally invasive alternative to percutaneous nephrolithotomy (PCNL) and more efficient option to shockwave lithotripsy (SWL). The continuous advancements in ureterorenoscopes have enabled improved manoeuvrability and visualization, both contributing to better surgical outcomes. However, the outer diameter of the scope remains a critical factor influencing both procedural safety and success, particularly in cases requiring access through narrow ureters or for patients at higher risk of ureteral trauma.

Recent developments have introduced ultra-slim ureteroscopes, with the outer diameter smaller than 7 Fr. This innovation raises important clinical questions regarding its durability and feasibility in routine RIRS procedures, especially in comparison to the bigger endoscopes. While a smaller caliber ureteroscope theoretically minimises ureteral trauma and facilitates access in challenging anatomy, concerns remain regarding potential compromises in deflection strength, both longitudinal and rotational stiffness, visibility, and tools compatibility. Additionally, whether the smaller scope translates to improved patient outcomes, such as reduced postoperative discomfort, lower risk of ureteral injury, and shorter hospital stays, requires further investigation.

Given the growing trend toward miniaturisation in endourology, understanding the impact of scope diameter on procedural success and patient safety is paramount. This study aims to evaluate the feasibility, safety, and short-term clinical outcomes of RIRS performed with the 6.3 Fr ultra-slim ureteroscope and compare it with the 7.5 Fr ureteroscope from the same manufacturer.

Materials and methods

This study utilised and assessed the clinical utility of two digital disposable flexible ureteroscopes manufactured by HugeMed 6,3 Fr (HU30M) and 7,5 Fr (HU30S). These are CE-certified and widely available on the European market. Nominal technical data of both scopes are shown in Table 1.

Table 1.

Technical specifications of the devices as provided by the manufacturer

Size and serial number	6,3 Fr (HU30M)	7,5 Fr (HU30S)
Distal width (Fr)	6.0 Fr	7,5 Fr
Shaft width (Fr)	6,3 Fr in bending parts	7,5 Fr
Working channel (Fr)	3,6 Fr	3,6 Fr
Length (mm)	650 mm	650 mm
Light source	2 integrated LEDs	2 integrated LEDs
Active deflection	280°	280°
Resolution	CMOS 160 K	CMOS 160 K
Total weight	280 g	240 g
Cable length	2500 mm	2500 mm

LED light-emitting diode, CMOS complementary metal-oxide-semiconductor

This was a prospective, randomised feasibility study with participant accrual based on a pre-generated random list. Each patient provided informed consent prior to participation, and the study was approved by the ethical board (KB 610/2024). Data from 30 RIRS procedures for nephrolithiasis, conducted between January and March 2025, were collected and analysed. The study was conducted at a tertiary university urology department, which serves as a referral centre for urolithiasis treatment.

Individuals aged over 18, with either a single renal stone not exceeding 1.5 cm in longest diameter or multiple stones with a total volume below 850 mm³, were included in this study. Patients with evident congenital or acquired urinary tract abnormalities (horseshoe kidney, duplicated collecting system, ureteropelvic junction obstruction, ureteral stricture, transplanted kidney) and those with symptomatic infection were considered ineligible. The preoperative stone assessment was based on computed tomography (CT) in all cases. Patients were admitted only with a reliable urine culture obtained no more than four weeks prior to the procedure. Pre-stenting was left to the discretion of the endourologist in each case, based on the individual clinical circumstances.

All procedures were conducted under spinal anaesthesia by a single experienced urologist (WK). Lithotripsy was performed using a high-power holmium laser. The settings depended on the clinical circumstances, but the pulverisation parameters with a 272-micron fiber were most commonly utilised (energy of 0.5–0.8 J, frequency 10–20 Hz, long pulse, or pulse modulation). The completeness of lithotripsy was assessed intraoperatively and defined as the absence of stone fragments larger than twice the laser fiber’s diameter.

Image quality was assessed subjectively at the beginning of procedure after kidney drainage, with the irrigation valve fully open (only gravitation irrigation was used). The evaluation was performed both with an empty working channel and with a 272-micron fiber inserted and the mean score was given by the operating urologist. A 5-point Likert scale was used, with 1 representing the worst and 5 the best possible image quality, considering factors such as resolution, blurriness, colour, brightness adjustments.

​​Manoeuvrability was scored as a general impression after the procedure. During each RIRS attempt was made to enter the lower calyx with and without the laser fiber in the working channel. ​​Manoeuvrability was also scored with a 5-point Likert scale. Additionally, each procedure conducted was evaluated postoperatively with the NASA Task Load Index (NASA TLX) questionnaire.

Before the study onset, in vitro analysis of scopes was performed, including measurement of the deflection range of new 6,3Fr and 7,5Fr scopes that were empty, with 272 micrometres fibers and with 1,5 Fr basket (Fig. 4). Also, every scope was re-checked after the RIRS procedure for possible loss in deflection ability. In-vitro image quality was assessed with 6,3Fr and 7,5Fr scopes in water, 0,9% saline and betadine solution (Fig. 2).

Fig. 4.

In vitro deflection comparison of tested devices: 6.3 Fr endoscope before and after the procedure (A, B), and 7.5 Fr before and after the procedure (C, D)

Fig. 2.

In-vitro image quality assessment in 0.9% saline and betadine: A 6.3 Fr in saline, B 6.3 Fr in betadine, C 7.5 Fr in saline, D 7.5 Fr in betadine

Statistics

Statistical analyses were conducted using SPSS 27.0.1.0. Depending on variable distribution, continuous variables were reported as means ± standard deviations (SD) or medians and interquartile ranges (IQR). Categorical variables were expressed as counts with percentages (%). Group comparisons were conducted with unpaired t test, Mann-Whitney U or chi square test as appropriate. The statistical significance was set at a two-sided p value ≤ 0.05. The study volume was limited by the availability of the endoscopes.

Results

In-vitro scopes assessment

All evaluated endoscopes functioned properly upon unpacking. Figure 1 illustrates the comparison of two scopes, along with basic measurements. Figure 2 presents the device’s image quality assessed solely in in-vitro settings with a certified colour grid. Furthermore, Fig. 3 presents the perioperative image quality before the lithotripsy for both scopes.

Fig. 1.

Side-by-side comparison of evaluated endoscopes A, D; measurements for 6.3 Fr device B, C; measurements for 7.5 Fr E, F; dimensions shown at the tip and mid-shaft (from left to right). (B, C – 1.89 mm, E – 2.29 mm, F – 2.40 mm)

Fig. 3.

Images taken intraoperatively with both devices, A and B 6.3 Fr scope, C and D 7.5 Fr scope

For endoscopes with empty working channels, the deflection range was symmetrical within the measurement error margin for all devices, reaching between 270 and 285 degrees. The deflection with the 272-micron laser fiber and 1,5 Fr Dormia basket was slightly reduced, particularly for the smaller-caliber device (~ 20^O reduction). Nevertheless, the flection decline did not exceed 20 degrees in any case. Lastly, the deflection range was measured after conducting the procedure and compared with initial readings. The postoperative deflection was reduced significantly in three cases (two 6.3 Fr endoscopes and one 7.5 Fr endoscope) B of Fig. 4. Each of these procedures were conducted in the lower calyx for the maximum allowed time in our centre (~ 60 min).

RIRS procedures

Thirty patients underwent the procedure. Median patient age was 50.0 years (range 19–83). The median maximum stone diameter was 9 mm (range: 6–15 mm), while the median stone volume was 394mm3 (range 97-1403). Detailed clinical characteristics of enrolled individuals are presented in Table 2.

Table 2.

Detailed clinical characteristics of included individuals

Parameter	All Patients (n = 30)	6,3 Fr Group	7,5 Fr Group	p-value
Median Age (IQR)	50.0 (37.0-61.75)	49.0 (35.5–65.5)	51.0 (40.0-54.5)	0.835 (1)
Sex, Male/Female (%)	18/12 (60.0%/40.0%)	6/9 (40.0%/60.0%)	12/3 (80.0%/20.0%)	0.062 (2)
Affected Kidney, Left/Right (%)	20/10 (66.7%/33.3%)	11/4 (73.3%/26.7%)	9/6 (60.0%/40.0%)	0.698 (2)
Stone Type (Solitary/Multiple)	15/15 (50.0%/50.0%)	7/8 (46.7%/53.3%)	8/7 (53.3%/46.7%)	1.000 (2)
Median HU (IQR)	1064.5 (977.0-1223.75)	1117.0 (1049.5-1300.5)	1026.0 (891.5-1088.5)	0.051 (1)
Solitary Stone Position				0.472 (2)
Renal pelvis (%)	5 (16.7%)	1 (6.7%)	4 (26.7%)
Upper calyx (%)	3 (10.0%)	2 (13.3%)	1 (6.7%)
Medial calyx (%)	3 (10.0%)	1 (6.7%)	2 (13.3%)
Lower calyx (%)	4 (13.3%)	3 (20.0%)	1 (6.7%)
Median biggest stone size, mm (IQR)	9.0 (7.0–11.0)	10.0 (7.0–12.0)	9.0 (7.5–10.0)	0.414 (1)
Median stone volume, mm³ (IQR)	364.5 (181.8-695.4)	500.0 (181.8-915.8)	364.5 (218.8–515.0)	0.429 (1)
DJ Presenting, Yes/No (%)	17/13 (56.7%/43.3%)	10/5 (66.7%/33.3%)	7/8 (46.7%/53.3%)	0.461 (2)
Preoperative urine Culture, positive/negative (%)	7/23 (23.3%/76.7%)	5/10 (33.3%/66.7%)	2/13 (13.3%/86.7%)	0.387 (2)
UAS used/not used (%)	14/16 (46.7%/53.3%)	8/7 (53.3%/46.7%)	6/9 (40%/60%)	0.714 (2)
Median operation Time, min (IQR)	60.0 (45.75–68.25)	59.5 (43.5–65.5)	64.5 (57.0-68.5)	0.061 (2)
Post-Op DJ Stent, yes/no (%)	24/6 (80%/20%)	11/4 (73.3%/26.7%)	9/6 (60%/40%)	0.438 (2)
Median hospitalization Time, hours (IQR)	27.0 (24.0–28.0)	26.0 (23.5–33.5)	27.0 (24.5–27.0)	0.230 (1)
Stone clearance		26 (87%)	22 (73%)	0.33 (2)

(1) Mann-Whitney U Test (2) chi-square test

HU hounsfield units, IQR interquartile range, post-op post operative, UAS ureteral access sheath

Almost half procedures (47%) were conducted utilising ureteral access sheaths (UAS). ClearPetra 10/12Fr sheaths, measuring 36 cm for ladies and 46 cm for men, were used. The image quality and manoeuvrability assessment along with NASA TLX scores are presented in Table 3. For the 5-point Likert scale assessments no scores below 3 were recorded. Moreover, there was no statistically significant difference observed between the 6.3 Fr and 7.5 Fr groups. Not a single endoscope malfunctioned during the procedure which would have required tool replacement.

Table 3.

Results of the image quality and manoeuvrability assessments scored on a 5-point likert scale along with between-group comparisons of NASA-TLX scores

Score	5		4		3
Image quality
6,3 Fr	11 (73%)		3 (20%)		1 (7%)
7,5 Fr	10 (67%)		4 (27%)		1 (7%)
Manoeuvrability
6,3 Fr	9 (60%)		4 (27%)*		2 (13%)
7,5 Fr	11 (73%)		1 (7%)*		3 (20%)
NASA-TLX	Mental Demand	Physical Demand	Temporal Demand	Performance	Effort	Frustration
6,3 Fr (mean)	7.0	6.2	7.0	7.8	7.8	8.2
7,5 Fr (mean)	7.2	7.2	7.8	7.0	7.2	7.6
p value (t-test)	0.845	0.154	0.861	0.500	0.721	0.798

An asterisk indicates a statistically significant difference

The collecting system access was successful in every case. However, in three non-pre-stented patients, it was not possible to introduce the UAS. Nevertheless, the operator proceeded without it. Among the 30 patients enrolled, 6 were pre-stented, with an equal distribution of 3 patients in each group.

The procedures were considered complete in terms of stone clearance in 87% of 6,3 Fr and 73% in 7,5 Fr group (p value = 0.33). In three patients the procedure was terminated prematurely because of mucosal bleeding precluding safe continuation. One individual presented with intraoperative shivers after 15 min suggestive of septic complication. In two cases, the procedure was terminated because the time limit (90 min) was reached. During endoscope removal, neither low- nor high-grade ureteral injury was recorded.

No > 2 clavien-dindo complications were recorded. All but one patient were discharged the next morning. In the patient who experienced shivering, postoperative laboratory tests revealed infection, and he was hospitalised for two more days for intravenous antibiotic therapy (7.5 Fr group). In another patient postoperative episode of 37,6 degrees fever was noted, which responded well to NSAIDs (6.3 Fr group). Because of negative inflammatory markers, the antibiotic was not administered. Two patients experienced evident haematuria without clots postoperatively before discharge (6.3 and 7.5 group).

Discussion

The complex anatomy of the upper urinary tract makes endoscopic access to the kidney challenging. For many years, the standard treatment for ureteral and renal stones consisted of invasive open surgeries, which carried a high risk of complications and prolonged recovery times. However, technological advancements have progressively reduced the diameter of endoscopic instruments, improving accessibility and safety, thereby shifting the standard of care towards endourology. Numerous studies have demonstrated that the smaller the instrument, the easier the access to the kidney, and the lower risk of ureteral injury and long-term complications. Hudson et al. evaluated the influence of shaft diameter on the successful advancement of endoscope through non dilated ureters in a cohort of 115 patients [1]. Their findings indicated that for safe access, the optimal outer diameter of the ureteroscope should be ≤ 7.4 Fr.

The current generation of flexible digital ureterorenoscopes, with an outer diameter ranging between 7.5 and 8,5 Fr, allows for safe and effective surgery in most patients, particularly those who have undergone preoperative ureteral stenting [2]. ​​Geavlete et al. published their first experience with 7,5 Fr Pusen digital scopes [3]. Agrawal et al. published a comparison of two 7,5 and 9 Fr scopes also form Pusen, showing that the vision, deflection, and manoeuvrability do not differ between the two scopes [4]. However, in patients with anatomical abnormalities, a history of multiple procedures, or in small children, the bigger “adult-sized” endoscope may be too large. Therefore, the development of ultra-slim ureterorenoscopes represents a promising advancement, potentially expanding the indications for retrograde intrarenal surgery and improving outcomes in these challenging patient populations.

This is the first study aimed at evaluating the feasibility, safety, and short-term clinical outcomes of retrograde intrarenal surgery performed with the 6.3 Fr ultra-slim ureteroscope while comparing it with the 7.5 Fr ureteroscope from the same manufacturer. Our findings demonstrate that the ultra-slim ureteroscope is as effective as the bigger scopes, with no compromise in image quality, manoeuvrability, or durability. These results support the feasibility of the 6.3 Fr scope as a reliable and safe option for RIRS.

A common misconception regarding slim ureteroscopes is that a smaller outer diameter equals a smaller working channel, potentially limiting irrigation flow and instrument functionality. However, the vast majority of the available scopes, including endoscopes included in this study, feature an identical working channel diameter of 3.6 Fr, ensuring that irrigation dynamics, laser fiber compatibility, and accessory use remain unaffected [5].

The quality of vision was comparable in in-vitro assessment in the two scopes (Fig. 3). During surgery, however, reliably distinguishing differences in image quality proves to be challenging. Nevertheless, the subjective evaluation by the operating surgeon in this study indicated that both types of ureteroscopes provided very similar - if not identical - visual quality. Based on the available marketing information released by the manufacturer, one can assume that both tools share the same video system (Table 1). It is important to note, however, that neither of the endoscopes is equipped with image enhancement systems for mucosal lesion detection, which limits their utility in diagnostic imaging of subtle urothelial irregularities.

Previous studies have raised concerns about laser-induced image interference, which may vary depending on the type of endoscope used [6]. In our analysis, both ureteroscopes exhibited horizontal, migrating interference bands during laser activation, which partially distorted the image. However, it was not possible to determine intraoperatively which specific laser parameter was primarily responsible for the artifact. Importantly, the intensity of visual interference was comparable between the two scopes.

A key concern with smaller caliber ureteroscopes is potential reduction in deflection capabilities. However, our study showed that the 6.3 Fr device provided equivalent pre- and postoperative deflection. When equipped with 272 laser fiber, the angle of deflection was slightly worse, especially for the smaller scope, but still no less than 250 degrees. Moreover, it was possible to enter the lower calyx in every case, with and without the laser fiber. These results are similar to those from Agrawal et al., comparing two different sizes of ureterorenoscopes from Pusen [7]. They showed that the scope deflection was comparable and lower pole access was achieved in all cases [7]. Lastly, there were no differences in terms of subjective impression of manoeuvrability in this study. Also, there was no need to replace the scope in any of the procedures because of navigational problems.

The miniaturisation raises concerns regarding structural durability, particularly in the context of single use ureteroscopes, which often demonstrate inferior mechanical performance compared to their reusable counterparts. As demonstrated by Juliebø-Jones et al. in a recent study, a substantial proportion of disposable scopes exhibit damage – including primary malfunctions – notably, many of these issues do not appear to result from operator error [8]. In the present study, all devices functioned properly upon initial connection, and none required replacement due to critical technical failure. While a minor reduction in deflection angle was observed postoperatively compared to baseline measurements, such changes were infrequent and likely not clinically significant. The procedural success rates were high, with a majority of cases achieving stone clearance without the need for auxiliary interventions.

Nevertheless, this study is not free from limitations that ought to be disclosed. The population is small, homogenous, and highly selected, including mainly low stone burden cases. Therefore, the results might not translate to the general population. Especially, in cases with larger stones or otherwise difficult, the durability of slim scope might be an issue. Second, the study lacks a longer follow-up, concerning long-term sequelae, such as ureteral stricture. However, each ureter was thoroughly assessed at the conclusion of the procedure, and no injuries were observed in any case. While our study did not directly compare postoperative pain scores or long-term risks, the necessary tools are already in place to facilitate such comparisons [9]. Nevertheless, one can assume the reduced outer diameter may potentially translate into lower ureteral injury rates and decreased need for preoperative ureteral prestenting.

Conclusion

The results of this study indicate that the 6.3 Fr ultra-slim ureteroscope is a feasible and safe alternative to the bigger 7.5 Fr model, with no compromise in visualisation, manoeuvrability, or device integrity. Given its comparable performance and potential for reduced ureteral trauma, the ultra-slim scope may serve as a valuable tool in modern endourology.

Author contributions

Conceptualization: WK; Methodology: WK, WT, TO; Formal analysis and investigation: WK, BM, TS; Writing - original draft preparation: WT, ŁN, JC, JŁ, KM; Writing - review and editing: WK; Supervision: WK,

Funding

No funds, grants, or other support was received.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Conflict of interest

The authors declare no competing interests. The authors have no relevant financial or non-financial interests to disclose.

Ethical approval

This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Local Ethics Board of Wroclaw Medical University (No. KB 610/2024).

Informed consent

Informed consent was obtained from all individual participants included in the study.