Factors Associated with Microdamage to Single-Use Flexible Ureteroscope: Prospective Ex Vivo Post-Use Analysis

This prospective ex vivo study investigated microdamage to single-use exible ureteroscopes (fURS) after ureteroscopy and endoscopic combined intrarenal surgery (ECIRS). The performance of 30 WiScope devices (OTU Medical, San Jose, CA, USA) was examined immediately after use, dividing them into three equal groups: ureteroscopy and ECIRS in the prone and supine positions. The overall scope microdamage assessment included the scope deection, bending radius, resolution, and water ow rate. Additionally, we analyzed the association between scope status and surgical parameters. The deection, bending radius, and resolution remained similarly above the thresholds in all groups. However, the water ow rate was below the threshold in seven scopes (70%) in the ureteroscopy group and none in the ECIRS groups (P = 0.001). Univariate and multivariable logistic regression analyses demonstrated that basket wire catheter use was associated with an increased risk for overall scope microdamage (odds ratio [OR], 22.70; P = 0.006 and OR, 22.40; P = 0.019, respectively). Stone size, total laser energy, and surgical position were not associated with a risk for scope microdamage. In conclusion, ureteroscopy was more closely associated with scope damage than ECIRS, and basket wire catheter use seemed to inict more damage to the fURS.


Introduction
The exible ureteroscopes (fURS) technology has been developing over the recent decades [1] . As a result, fURS is widely used worldwide as rst-line endoscopic management for renal or ureteral stones [2] . Its development has contributed to fewer invasive surgeries, resulting in a shorter surgery time, higher stonefree rate, and shorter hospitalization [3] .
Reusable fURS (re-fURS) is known for its signi cant initial purchase and maintenance costs, including cleaning and sterilization [4] . Although the reduced scope diameter during fURS development improved operability, it became more delicate, and the repair costs have increased [5] . Scope damage requiring repair occurs after about 9-12 procedures, and the scope needs frequent repairs after the rst damage has occurred [6] .
Some single-use fURS (su-fURS) types have been introduced and are widely used for endoscopic management. These incur no maintenance or repair costs and provide consistent performance during surgery [4,7] . Hennessey et al. suggested that su-fURS should be used instead of re-fURS for cases that pose a high risk of scope damage, such as lower pole and staghorn stones. However, the costeffectiveness of using su-fURS or re-URS remains debated [8][9][10] . To extend the life of re-fURS, we should focus on its durability and strive to suppress the costs of repair or scope replacement.
Microdamage to re-fURS that occurs during every surgery is believed to accumulate, resulting in the need for repair or poor scope performance. Understanding the microdamage caused during each surgery could help prevent major damages and reduce costs. This study evaluated the microdamage caused to su-fURS after ureteroscopy and endoscopic combined intrarenal surgery (ECIRS).

Results
Patient characteristics are summarized in Table 1. The three groups were similar with respect to sex, age, body mass index (BMI), and stone location. The median stone size was larger, and the median average stone radiodensity was higher in the supine and prone ECIRS groups than in the ureteroscopy group (P < 0.001 and P = 0.018, respectively). The three groups had similar stone-free rates, total surgery times, ureteroscopy usage times, and total laser energy used (P = 0.754, 0.402, 0.717, and 0.383, respectively). A basket wire catheter was used in all patients in the ureteroscopy group and none in the ECIRS groups (P < 0.001). Findings in the scope performance evaluation after its use are shown in Table 2 and Supplementary Tables S1-S3. De ection failure was observed in three scopes (30%) in the ureteroscopy group and one each (10%) in the supine and prone ECIRS groups (P = 0.574). As shown in Supplementary Table S1, two scopes in the ureteroscopy group (20%) and one in the supine ECIRS group (10%) could not control the up and down bending of the de ection section because the de ection mechanism was severely damaged, described in the table as not applicable. Failure to reach the threshold bending radius was also observed in these three scopes (Supplementary Table S2). Failure to reach the resolution threshold was not observed in any of the scopes (Supplementary Table S3). As shown in Supplementary Table S4, a decrease in the water ow rate was observed in seven of the ureteroscopy group scopes (70%) and none in the two ECIRS groups (P = 0.001). Insu cient resolution 0 (0%) 0 (0%) 0 (0%) 1 Decreased water ow 7 (70%) 0 (0%) 1 (10%) 0.001 De ection and minimum bending radius were assessed in both up and down directions.
The logistic regression analysis results are shown in Table 3. Univariate and multivariate analyses revealed that basket wire catheter use was associated with an increased risk of overall scope damage (odds ratio [OR], 22.70, P = 0.006 and OR, 22.40, P = 0.019, respectively). Stone size, total laser energy, and surgical position were not associated with a risk for scope microdamage.

Discussion
The nancial burden on urolithiasis management is substantially increased by the costs of re-fURS maintenance and repair [12] . This study investigated the microdamage caused to su-fURS during ureteroscopy and ECIRS surgeries. Our results showed that ureteroscopy was more closely associated with scope damage than ECIRS, as was basket wire catheter use. In contrast, stone size, total laser energy, and surgical position were poorly associated with scope microdamage. These ndings could help optimize the urolithiasis treatment by selecting the appropriate fURS for each procedure. Moreover, they suggest that we should be careful with possible scope damage when removing fragments with a basket wire catheter.
Our study demonstrated that ureteroscopy tended to in ict slightly more microdamage to the scope de ection than ECIRS. Hosny et al. reported that the fURS de ection tip was one of the scope's most fragile parts [13] . Excessive stress on the de ection mechanism decreases the de ection angle [14] .
Applying excessive force to bend the scope tip in the pelvis or careless processing of the scope through the access sheath could damages the de ection mechanism [15,16] . We initially hypothesized that the fURS was more likely to be damaged during ECIRS than ureteroscopy because of the larger stones in the former. However, ureteroscopy seems to cause more damage to the scope than ECIRS. This might be because frequent insertions of the laser ber, basket wire catheter, and fURS into the access sheath were necessary to collect stone fragments during ureteroscopy. In contrast, stone fragments were collected by retrograde irrigation through the fURS in ECIRS. Proper access sheath use during ureteroscopy is essential to reduce scope damage [17] . Some scopes, including WiScope (OTU Medical, San Jose, CA, USA) used in this study, cannot be automatically straightened when the articulation lever is released. The highly damaged up and down de ection mechanism observed in two of the scopes used for ureteroscopy in this study occurred because the scopes were removed through the access sheath without straightening. These scope types must be consciously straightened during insertion or removal.
WiScope (OTU Medical), a digital fURS, has a better image quality than ber optic fURS but no additional bene t in scope durability and surgical performance [18,19] . The resolution failure after their use has not been investigated before, although studies comparing the resolution between scopes before surgical use are available (e.g., su-fURS vs. re-fURS) [20] . Our study demonstrated that a single surgical use did not cause signi cant damage to scope resolution, regardless of the operation type.
We alternately inserted the catheter and the laser ber during stone fragment collection with a basket wire catheter. Seto et al. reported that repeated insertions of these accessories cause damage to the fURS working channel, resulting in decreased water ow rate [21] . They also indicated that scope de ection with a basket wire catheter in the channel does not cause signi cant damage to the scope despite using a de ection angle of over 120°. However, de ection of scopes with 200 µm holmium laser bers in the channel could cause visible damage to the channel when the de ection angle is over 60°. Therefore, the scope must be straightened when inserting or removing the laser bers. Moreover, su-fURS might be better than re-fURS for ureteroscopy because alternate insertion of a basket wire catheter and the laser ber is needed, particularly in cases with large or impacted stones that could damage the scope.
We found no association between the total laser energy and microdamage to fURS. Thermal laser damage to fURS is common, frequently occurring approximately 3-4 mm from the scope tip [5] . It is essential to advance the laser ber tip to one-quarter of the screen (3 mm or more from the scope tip) during fragmentation to avoid thermal damage [22] . This safe distance could reduce the damage caused by the plasma bubbles generated by the laser ber tip, even when high-energy settings are used. We always attempt to maintain a safe distance during stone fragmentation, which may explain the weak association between the damage and total laser energy.
The limitations of the current study include the relatively small number of scopes assessed, which might have resulted in an underpowered study. Additionally, we investigated the fURS microdamage using only one scope type; therefore, the results might not apply to other fURS types, such as re-fURS and su-fURS other than WiScope (OTU Medical). Moreover, surgeries were performed by different surgeons, which might have affected the results. Despite these limitations, the use of su-fURS in this study allowed a unique evaluation of the scope status immediately after its surgical use. Our data could contribute to the reduction of damage caused to re-fURS, resulting in the extension of its life and reducing the costs. Furthermore, our study supports the choice of fURS (su-fURS or re-fURS) in ureteroscopy and ECIRS.

Conclusions
We investigated the microdamage caused during surgery to su-fURS. Ureteroscopy was more closely associated with scope damage than ECIRS. Basket wire catheter use was associated with scope damage, while the stone size, total laser energy, and patient position were not. These results help better understand the microdamage caused during each surgery, which could help prevent major damages and reduce costs.

Patients
We recorded the patient sex, age, BMI, stone location, stone size (mm 3 ), and average stone radiodensity preoperatively. Moreover, we evaluated surgical parameters, including total surgical time, ureteroscope usage time, total laser energy, stone-free rate, and use of a basket-wire catheter. Stone-free status was de ned as no residual stones or stones smaller than 4 mm in diameter, as determined by plain abdominal radiography three months postoperatively. Patients with a single kidney, urinary diversion, age < 20 years, or medical history of ureteroscopy or ECIRS were excluded from this study.

Study design
The Institutional Review Board of Nagoya City University Hospital approved this ex vivo study before it started (60-19-0044). The study followed the tenets of the Declaration of Helsinki. All patients provided informed consent to participate in the study.
The study design is summarized in Fig. 1. We assessed the performance of 30 WiScope devices (OTU Medical) immediately after use. The scopes were divided into three equal groups: ureteroscopy and ECIRS in the prone and supine positions. Ureteroscopy was performed in patients with proximal ureteral stones < 10 mm and kidney stone < 20 mm in diameter, and ECIRS for patients with larger proximal ureteral stones (> 10 mm) and kidney stones (> 20 mm). Patients with lower pole stone 10-20mm were excluded from this study. The surgical position was randomly determined when treated by ECIRS. The scopes were sent to the laboratory at OTU Medical after use to evaluate their de ection, bending radius, resolution, and water ow rate.

Surgical techniques
All patients were treated under general anesthesia. A 0.035-inch guidewire was inserted through the ureteral ori ce followed by a 10/12-Fr or 12/14-Fr ureteral access sheath. In the ureteroscopy group, retrograde fragmentation was performed using a 272-μm holmium YAG laser (Cyber Ho, Quanta System, Milan, Italy), and the fragments were removed using a basket wire catheter (NCircle, Cook Medical, Bloomington, IN, USA). In the ECIRS groups, percutaneous access was established using a 16/17.5-Fr miniature percutaneous nephrolithotomy tract (Karl Storz, Tuttlingen, Germany). Two urologists simultaneously fragmented the stones, one by antegrade fragmentation using LithoClast lithotripsy (Electro Medical Systems S.A., Nyon, Switzerland) with a 12-Fr mini-nephroscope (Karl Storz), and the other by retrograde fragmentation using a holmium YAG laser with fURS. The fragments were washed through the nephrostomy sheath using retrograde irrigation.