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[]. However, patients with unfavorable anatomical factors of the lower calyx have lower success rates in FURS and ESWL[,].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[].Liu 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[].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[].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[].Coskun[] 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.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.
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[2], 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 ≥ 45o, whereas in the subgroup with a IPA < 45o, the success rate of intraoperative stone searching was 79.1% (53/67). 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% (27/67) in the subgroup with an IPA < 45o, with 18.5% patients being transferred to a mini-PCNL (25/135), even after the lower pole stone were displaced using a set of stone baskets. Richards[] noted that an IPA < 45° was associated significantly with the stone-free rate after FURS. 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[] suggested that an acute IPA mainly affected postoperative stone removal rather than intraoperative stone finding and holmium laser lithotripsy. Razvan[] suggested that astone basket can be used to move lower pole stones and effectively improve the efficiency of lithotripsy and prolong the life of reusable flexible ureteroscopes.
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[] 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. Single-use digital flexible ureteroscopes have visibility and maneuverability profiles approaching that of a reusable digital flexible ureteroscope.There is also evidence that single-use flexible ureteroscopes achieve similar clinical outcomes to the more expensive reusable versions[].Another clinical study reported that single-use FURS was an effective and safe alternative to reusable FURS for the management of renal stones, and was associated with a higher SFR than reusable FURS[].
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. A study from Turkey carried out by Goge[] 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. Another study reported that single-use FURS was as effective as reusable FURS for treating lower pole stones, but had shorter operative and intraoperative fluoroscopy times []. 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. It has been reported that a single-use FURS can achieve 297° deflection even when inserted into a 275 µm fiber[8]. 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 ≥ 45o, whereas in the subgroup with an IPA < 45o, 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. 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[].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 seconds. Even when firing for 5 minutes 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[8].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 (reusable FURS, 59.7% vs. single-use FURS, 33.3%) in the subgroup with an IPA < 45°. However, transfer to a mini-PCNL decreased significantly after matching with the stone basket (reusable FURS, 18.5% vs single-use FURS, 14.1%).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 FURS (8/78, 10.3% vs. 31/135, 23.0%, P = 0.021).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.
The study had several limitations as it was a single-center, retrospective study on a relatively small number of patients, which inevitably resulted in selection bias. Further prospective, multi-center, controlled trials are therefore needed to verify the conclusions.In summary, single-use and reusable FURS are alternative methods for treating lower pole stones (2 cm or less). Single-use FURS not only has a high success rate for lithotripsy, short operation time, and high stone clearance rate,but also when the anatomy of the lower calyx is difficult a single-use FURS should be preferable.