FURS combined with holmium laser lithotripsy has become an important method for the treatment of upper urinary calculi (stone diameter < 2 cm) due to it being minimally invasive and high efficient [13]. However, patients with unfavorable anatomical factors of the lower calyx have lower success rates in FURS and ESWL [14, 15]. A recent study reported that patients with a high IL or a very acute IPA were more likely to require a second procedure that did not appear to influence the rate of complications and ESWL [16]. Liu et al. compared the outcomes of PCNL, FURS, and ESWL in the treatment for lower pole stones and showed that PCNL and FURS had lower retreatment rates, while PCNL had the longest hospital stay [17].

The surgical complication rate is an important indicator for evaluating the safety of surgery. Some scholars have conducted detailed studies on the complications of PCNL and reported that the total complication rate of postoperative infections, bleeding, blood transfusions, and peripheral organ injury was as high as 15%, a rate significantly higher than that of FURS [18]. In recent years, progress in minimally invasive technology and improvements in surgical instrument research and development have resulted in treatment of kidney stones using mini-PCNL achieving a stone clearance rate comparable to that of standard PCNL, with fewer postoperative complications and shorter hospital stays [19]. Coskun and collagues compared mini-PCNL and RIRS for lower pole stones and demonstrated no meaningful difference in stone-free rates between the two groups, although complications such as the use of fluoroscopy, bleeding, and duration of hospital stay were significantly higher in cases treated with mini-PCNL [20].

Since the European Urological Guidelines in 2015 recommended FURS as the first-line treatment for lower pole stones, this procedure has become the most favored method for removing upper urinary calculi by both doctors and patients due to its less invasive nature and lower risk of intraoperative and postoperative bleeding. In-vitro, it appears that single-use FURSs deflect better than their reusable counterparts, reusable FURSs had better vision characteristics than single-use FURSs. Further in-vivo studies might be necessary to confirm these findings [21]. Professor Jonathan Kan and collagues compared single-use and reusable digital flexible ureteroscopy and found the the URF-V2 group had higher visibility scores than the single-use scopes and higher maneuverability. however there were no differences in operative time, rates of relook flexible ureteroscopes, scope failure or complication rates observed [10].

With the popularization of flexible ureteroscopes, researchers have reported that lower pole stones represent the main challenge for FURS combined with holmium laser lithotripsy. An acute IPA makes it difficult for the lens to reach the stone position, leading to a decrease in the efficiency of lithotripsy [6], or even transferring to a PCNL. In our study, we found that the lens could almost enter the lower calyx of patients in the subgroup with an IPA ≥ 45°, whereas in the subgroup with an IPA < 45°, the success rate of intraoperative stone searching of single-use FURSs was higher reusable FURSs and there is no statistical significance. When the 272 μm holmium laser fiber was inserted, its deflection was affected and it could not enter the lower calyx smoothly. The success rate of intraoperative lithotripsy was only 40.3% in the subgroup with an IPA < 45°, with 18.5% patients being transferred to a mini-PCNL, even after the lower pole stone were displaced using a set of stone baskets. Richards et al. noted that an IPA < 45° was associated significantly with the stone-free rate after FURS [22]. This raises the question of how to maximize the flexible characteristics of flexible ureteroscopes in cases with an adverse anatomical structure of the lower pole stone? Inoue and collagues suggested that an acute IPA mainly affected postoperative stone removal rather than intraoperative stone finding and holmium laser lithotripsy [23].

In recent years, the use of single-use, digital flexible ureteroscopes has been described in published literature that has led to their current use in clinical practice. Leveillee et al. reported a case of a 35-year-old female with a lower pole stone, in who a new disposable digital ureteroscope allowed for extreme lower pole access and the use of a 365 μm holmium laser fiber [24]. Single-use digital flexible ureteroscopes have visibility and maneuverability profiles approaching that of reusable digital flexible ureteroscope. There is also evidence that single-use flexible ureteroscopes achieve similar clinical outcomes to the more expensive reusable versions [10, 25], even a higher SFR than reusable FURS [25].

The terminal deflection ability of reusable flexible ureteroscopes will gradually decrease after repeated use or even following maintenance, especially in patients with a complex anatomical structure of the renal inferior calyceal. Single-use FURS not only overcomes the reduction in deflection ability that occurs after considerable use, but also helps doctors deal with concerns about the high cost of repairing damaged devices, and lets them target the lower calyx in cases with an adverse anatomical structure. Both studies compared single-use and reusable FURS in the treatment of lower pole stones and showed that single-use FURS had better deflection compared with reusable FURS, a characteristic conducive to the treatment of lower pole stones with a heavy load [26, 27]. Our study showed that compared with reusable FURS, a single-use FURS had advantages in surgical time and stone clearance rate, entirely due to their superior deflection ability. In addition, the fact that single-use FURS does not involve the risk of equipment damage, these advantages overcome the disadvantages of reusable FURS in the treatment of lower pole stones. Our data showed that single-use and reusable FURS had a comparable success rate for stone searching. Regarding the success rate of lithotripsy, the efficacy of the two treatments was similar in the subgroup with an IPA ≥ 45°, whereas in the subgroup with an IPA < 45°, single-use FURS had several advantages. This solved the “unattainable” dilemma of FURS combined with holmium laser lithotripsy for patients with a lower pole stone. The experimental results shown that SU-FURS have more advantages in handling the lower pole stones, such as success rate of lithotripsy when IPA < 45° and use of stone basket. In addition, SU-FURS was much lighter than R-FURS in weight, decreased the fatigue of surgeon and shorten the operation time.

On the other hand, placing the laser fiber through the operation hole of the reusable FURS requires the tip to reach a quarter of the endoscope screen to avoid damage to the lens caused by cavitation bubbles produced during laser excitation [28]. However, no deterioration in the quality of the image and illumination was observed when firing the laser at every fiber tip to the working channel position (10 mm to − 2 mm) for 10 s. Even when firing for 5 min at a distance of 0 mm (i.e., fiber tip even to the working channel outlet), no impact on image and illumination quality was observed [12]. This characteristic is a consequence of the unique design of the tip of the single-use FURS (Fig. 1), which ensures accurate lithotripsis in cases with difficult anatomical structures of the lower pole stone. We often encountered a challenge when the fiber was located at the edge of the stone edge and could not effectively break-down the stone because of a problem with the length of the working fiber, resulting in the fiber being out of view when using reusable FURS in clinical practice. However, the length of the working fiber in single-use FURS can be kept shorter to ensure synchronization with the field of vision. In cases with a difficult anatomical structure of the lower calyx or incarcerated stone, we plan to “blind beat” for several seconds to loosen the stone, combined with the use of a set of stone baskets. The critical “few seconds” during surgery could reduce the likelihood of intraoperative transfer of PCNL. This study also showed that there was a high failure rate of lithotripsy in the subgroup with an IPA < 45°. However, transfer to a mini-PCNL decreased significantly after matching with the stone basket. The advantages of single-use FURS in the treatment of the lower pole stone often reduced the use of stone baskets. Our study found that the use of an intraoperative stone basket in single-use FURS was significantly lower than that for reusable FUR. This result confirmed the good deflection of single-use FURS. Razvan and collagues suggested that stone basket can be used to move lower pole stones and effectively improve the efficiency of lithotripsy and prolong the life of reusable flexible ureteroscopes [29]. We reported damage to five lens when using reusable FURS to treat lower pole stones and increased repair costs, while the lower use of stone baskets and zero repair costs for single-use FURS may be reduced economic costs. It is needed that more research about stone economics to prove this. To sum up, the advantage of single-use FURS increased the surgical success rate of lower pole stones and improved the calculi clearance rate,therefore we should choose SU-FURS in the treatment of lower pole stones.

Schematic diagram of tip design of single-use FURS. The single-use flexible ureteroscope has a 30° bevel at the operation hole. On the side of the operation hole, there is a 63o bevel. This characteristic is a consequence of the unique design of the tip of the single-use FURS, and can ensures that the length of the working fiber in single-use FURS can be kept shorter

This study had several limitations as it was a single-center, retrospective study on a relatively small number of patients. Although surgeons were on the same team, SU-FURS appeared later than R-FURS, potentially introducing a bias in technique. Fortunately, Our center was Shanxi center of China Urolithiasis Union, and the source of patients was facing Shanxi Province and surrounding areas. Secondly, as the information of SU-FURS group and R-FURS group were collected through hospital record, potential biases, if any, was unlikely to differ between two groups, meaning it was unlikely to skew the results in either direction in this study. Although we tried to minimize selection bias, the retrospective single-center design could limited the generalizability of our findings.. We will enlarge the sample size of SU-FURS group and R-FURS or ally member of China Urolithiasis Union in further study to verify the experimental results. Further more importantly, prospective, multi-center, controlled trials are needed to verify the conclusions.