We evaluated a series of currently available flexible ureteroscopes at our institution: the Flex-Xc and Flex-X2s (Karl Storz SE & Co. KG, Tuttlingen, Germany), URF-P7 and URF-V3 (Olympus, Center Valley, PA, USA), Uscope 7.5F PU3033A, Uscope 9.2F PU3022A (Zhuhai Pusen Medical Technology Co. Ltd. Guangdong, China), as well as the WiScope (OTU Medical Inc, CA, USA). To reflect real-world operating room situations, the single-use scopes (Pusen 7.5F, Pusen 9.2F and OTU WiScope) were brand new scopes from sealed packages. Reusable ureteroscopes (Storz and Olympus scopes) had all been rinsed and decontaminated after use in the operating theater, with no record of the number of previous interventions.

For the Storz Flex-X2s, the Power LED 175 light source (unit used < 100 h) was used with a corresponding 230 cm/3.5 mm fiber-optic cable, together with the IMAGE1 S HX-P HDTV 1-Chip pendular camera (Karl Storz SE & Co. KG, Tuttlingen, Germany). For the Olympus URF-P7 and URF-V3, the VISERA elite CLV-S190 light source (Xenon short-arc lamp used < 100 h) was used with a WA03310A 300 cm/4.3 mm fiber-optic light cable, as well as the CH S190 08 LB camera head (Olympus, Center Valley, PA, USA). Fiber-optic cables were entirely new.

A color spectrometer housing the Vishay VEML 6040 color sensor (RGBW200, ELV Elektronik AG, Leer, Germany) was used for lux (lx) measurements in saline, as previously described [13].

A 3D-printed pink obstructive kidney calyx model was used to hold the ureteroscopes at a fixed distance of 20 mm from the center of the opposite concave surface of the kidney calyx model in a dark room (Fig. 1). The model consisted of a closed spherical cavity replicating a human kidney calyx and included an obstructive crescent and a background illuminance measurement opening located at a 45°angle relatively to the axis of the scope. (Fig. 1a–c). The position was chosen to simulate a common occurrence in ureteroscopy with the target pathology being beyond the partially obstructing obstacle in the background (Fig. 1d). Pink was chosen for the kidney calyx model to replicate human urothelial mucosa. The ureteroscope was maintained in a straight position, with the center of the scope view aligned to the center of the opposite concave pink surface. The size of the target field and distance from the light sensor were chosen with reference to dimensions of models constructed on data from endocasts [15] used in studies testing scopes in the setting of laser lithotripsy [16, 17], to reflect in vivo settings. The obstructive crescent was shaped to mimic the calyceal neck when entering a kidney calyx.

Experimental setup: obstructive kidney calyx model. a 3D visualization of the obstructive kidney calyx model from a side view. b Front view of the 3D-printed pink model. The large opening would fit a black rubber cylinder to hold the ureteroscope during experiments. To display the obstructive crescent within, the black rubber cylinder was removed in this photograph for the sake of better visualization. c Still image of the endoscopic view with the obstructive crescent situation at 6 o’clock. The spectrometer measurement opening is labeled with an asterisk (*). d Still image of an in vivo endoscopic view illustrating a typical clinical scenario where the light source of the endoscope was partially obstructed by the calyceal neck when trying to reach the target stone fragments (*) with the laser fiber (blue). The model of the current study was inspired from this clinical scenario

All measurements were performed in saline to replicate the usual conditions found in clinical routine during ureteroscopy, in view of a previous study showing fundamental differences of ureteroscope illumination between air and saline [13].

For each obstructive situation, the sensor of the color spectrometer was placed at the 45° opening and measurements were repeated 5 times. The model was then turned in the axis of the scope to obtain measurements in four situations representing endoscopic calyx views with obstruction at 12, 3, 6, and 9 o’clock. For each repeated measurement, the ureteroscope was withdrawn and reinserted in the kidney calyx model.

Whenever the ureteroscope setup allowed for light brightness adjustment, measurements were repeated at light brightness settings of 50% and 100%. For the Storz Flex-Xc, this was found available to be adjusted only via the menu buttons on the scope handle. For the Storz Flex-X2s, in addition to brightness settings adjustable on the light stack, buttons on the camera head also allowed for separate brightness adjustment. Based on our findings with the Storz Flex-Xc, brightness setting for the Storz Flex-X2s in the camera head was kept at 50% and brightness settings only adjusted on the light source unit. For all other scopes, the brightness setting was adjusted on the light source unit only. Since light brightness can be set either to automatic or manual mode on the Olympus light source as well as with the Storz camera head, all measurements were performed using the manual mode to ensure consistent measurements throughout all experiments.

Overall background illuminance was defined as mean from all 4 obstructive situations for a given scope. Background illuminance for a given obstructive situation was expressed as mean value of 5 repeated measurements. An analysis was performed for all scopes comparing inter-scope overall background illuminance, and inter-scope background illuminance for respective obstructive clock situations using one-way ANOVA with Tukey post hoc comparisons.

To quantify background illuminance variability for each scope, the obstructive clock situations for each scope were considered and reported as percentage relative to the highest in each scope (3 percentages for each scope). The highest relative percentage for each scope was defined as the intra-scope background illuminance variability, which reflects the range of background illuminance variance. Analysis with Student’s T test was carried out for all background illuminance variability values comparing scopes based on their tip design characteristics, as reported in a previous study [13]: 1 vs 2 light sources, position of working channel, and transparent vs non-transparent tips.

Intra-scope analysis for background illuminance comparing each obstructive situation was performed with one-way ANOVA.

For all tests, a two-sided p value < 0.05 was considered statistically significant. All statistical tests were performed with GraphPad Prism 10.0.2 (GraphPad Software, La Jolla CA, USA).