The European Association of Urology (EAU) and Chinese Association of Urology (CAU) guidelines on the management of staghorn calculi recommend PNL as a first-line treatment for intrarenal calculi larger than 2 cm and ESWL for renal calculi smaller than 2 cm [2, 3]. Choosing a proper puncture position and a right puncture direction is the key point for PCNL. For most renal stones, the puncture position is usually beneath the twelfth rib or in the eleventh intercostal space from the axillary line to the scapular line, but for upper calyceal stones, it is appropriate to choose the tenth or eleventh intercostal space as the puncture position to shorten the distance between the puncture point and the target renal calyx. It may reduce the possibility of renal parenchyma avulsion due to the movement of the endoscope during surgery. However, it is time consuming to build an operation channel with a higher incidence of intraoperative complications, such as haemorrhage, perforation and urinary sepsis, which are major concerns in clinical practice [5, 6].
With the advent of new generation flexible ureteroscopes with greater deflection and control, FURSL is increasingly used as a primary modality in the management of renal calculi and even for kidney stones > 2 cm with minimal complications and a higher success rate . Pieras et al.  compared the efficiency of flexible ureteroscopy and PCNL for treating kidney stones between 2 and 3 cm in a prospective matched study and demonstrated that the SFR was 76% and 87%, respectively, without a significant difference (p = 0.1). The complications were 27% and 29%, respectively (p = 0.4). However, the ureteroscopy group had shorter surgical times, shorter hospital stays and shorter convalescence but higher readmission rates than the nephrolithotomy group. Akman et al.  reported that the SFR after one session was 73.5% and 91.2% for RIRS and PCNL, respectively (p = 0.05), while the SFR in the RIRS group improved to 88.2% after a secondary procedure, which was not significantly different from PCNL.
A meta-analysis reviewed the literature for renal stones > 2 cm managed by ureteroscopy and holmium lasertripsy from 1990 to 2011 and showed that FURSL has a 95.7% stone-free rate with stones 2–3 cm and 84.6% in those > 3 cm, drawing the conclusion that FURSL can successfully treat patients with stones > 2 cm with a high stone-free rate and a low complication rate . In the present study, FURSL was used to deal with upper pole calyceal calculi larger than 2 cm, and the initial SFR was 91.9%, which was in accordance with previous studies [1, 10]. The distinction in SFR may be explicable by the characteristics of the two procedures. The higher laser power and quick clearance across the percutaneous tract allow PNL to achieve stronger lithotripsy effectiveness and higher stone removal rates than FURSL. However, the time required for position change and placement of the ureter catheter accounted for a significant portion of PCNL's duration, making it longer than FURSL in our study.
It was reported that the overall incidence of complications of FURSL was 10.1%, while the incidence of severe complications was 0% ~ 5.3% . Baş et al.  analysed 1359 cases of upper ureteral calculi and renal calculi treated with FURSL and found that the overall incidence of complications during operation was 5.9%, and the overall incidence of complications after operation was 7.3%. There were no severe complications in our present study, and the incidence of postoperative complications was 13.5%, which was much lower than that of PCNL. The RIRS group also had lower overall complication rates and shorter hospitalization times than the PCNL group. The risk of PCNL could not be disregarded while its character of high SFR. The surgery was associated with a higher incidence of complications, including bleeding, severe infection, and renal parenchymal damage. In the current study, intraoperative and postoperative complication rates were greater in the PNL group, particularly hemorrhage. This is also reflected in the drop levels of hemoglobin.
Despite its inferior SFR, FURSL has become a viable option for the treatment of the majority of kidney stones and its complication rates remain low. Nevertheless, rare fatal events, such as septic shock, may occur, especially in complex cases with a history of urinary tract infections . Therefore, there are some tips to FURSL to improve the SFR of the procedure for upper pole calyceal calculi and maintain the safety of the operation. First, we recommend the Trendelenburg position for upper calyceal calculi to keep the stones as well as stone fragments thereafter in the upper calyx to avoid stone shifting. A UAS is also recommended for all FURSL procedures. It may ensure the circulation of flushing water, allow for optimal visualization, minimize resistance to the rotation of the ureteroscope and facilitate extraction of stone fragments. As an additional benefit, the sheath protects the ureter from repeated insertion and removal of the flexible ureteroscope . However, it is difficult to place UAS in approximately 10% of patients due to ureteral stenosis . For those patients, F6/7.5 ureteroscopic examination is recommended to check and dilate the ureter. If the stone burden is large or the patient cannot endure a long operation time, FURSL can be treated by several stages or combined with PCNL.
Second, in order to protect the ureteroscope, a high-power laser was not recommended, and a laser fibre too close to the lens was also not allowed. The flexible ureteroscope should be pulled back into the UAS when the holmium laser is being placed, adjusting the curvature of the ureteroscope and the angle of the optical fibre to bring the fibre close to the stones. During the operation, we used a low-frequency and high-energy strategy to break stones into fragments and then use a low-energy and high-frequency strategy for stone pulverization. Thus, during this series, we used a 365-μm holmium laser to break stones, and after extracting the larger fragments, the 200-μm holmium laser was then used for stone pulverization. Third, if the calculi are difficult to pulverize, a stone basket can be used to remove calculi over 2 mm to prevent steinstrasse and shorten the time of extraction. Finally, for patients with preoperative urinary infection, prophylactic antibiotics should be used, and percutaneous nephrostomy should be performed when necessary, which could be helpful to reduce intrarenal pressure and control the infection. Sustained low intra-pelvic pressure and application of UAS to shorten the operation time were conducive to reducing the incidence of postoperative infection .
FURSL offers several potential advantages in the management of large upper calyceal calculi. Using the natural route to the kidney reduces the degree of trauma and the risk of severe blood loss and possible irreversible loss of renal parenchyma; hence, it has low rates of severe haemorrhage, pleural injury and other complications. Furthermore, FURSL can save time and resources by avoiding the repositioning of the patient and the need to establish percutaneous access for prone PCNL [14, 15].
This research has certain drawbacks. This is a retrospective analysis conducted by a single institution. Our findings are based on a relatively limited number of samples. This strategy will also need to be examined with greater patient populations across multiple institutions.