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.