Several modalities are available for the treatment of large upper ureteral stones [12,13,14,15,16,17]. Although controversial, SWL and URS have been recommended by the AUA and EAU guidelines as the first choice for proximal large ureteral stones, whereas LUL has been used as one of the options in the management of upper urinary tract stones [5, 6]. With regard to the dimension of large proximal ureteral stones, it seems that there is no clear definition. In a recent meta-analysis, Torricelli et al. analyzed six randomized controlled trials for large upper ureteral stones. These six studies used different inclusion criteria in terms of stone size: two studies used 10 mm [13, 16], whereas the other three used 12 mm , 15 mm , and 20 mm . In our study, we analyzed patients with stones exceeding 15 mm in size. The main advantage of LUL is the high probability of removing impacted stones in one session without additional procedures, whereas SWL and ureteroscopic approaches are characterized by higher risks of remnant stones, stone-free failure (especially in cases of large stones), impacted stones, and hard stones.
Although many surgeons prefer to insert double-J stents after LUL, there is controversy about whether ureteral stenting is necessary. Hammady et al.  performed a randomized controlled study and concluded that LUL without stent insertion is safe, cost-effective, and relatively quick. In addition, LUL without stenting does not require auxiliary procedures for removing the stent afterwards. On the other hand, Karami et al.  reported that placing a stent during LUL does not increase the operation time and may play an important role in preventing urinary leakage. In our study, there were no patients with flank pain or increased postoperative drainage after LUL with double-J stent placement.
The procedures used to insert the double-J stent intracorporeally after LUL are challenging and time consuming for inexperienced surgeons . Therefore, various methods have been described to accomplish this successfully. In one of the most commonly used techniques, a retrograde double-J stent is placed beneath the stone using cystoscopy before LUL and advanced after stone removal [19,20,21,22,23,24].
Chen et al.  evaluated the feasibility of ureteroscope-assisted ureteral double-J stenting after LUL and found that it was a simple and safe alternative method of correct stent placement. However, retrograde cystoscopic or ureteroscopic stenting requires additional position changes. Moreover, cystoscopic retrograde stenting is associated with risks due to advancing the suture site without direct visualization. Alongside these retrograde cystoscopic or ureteroscopic stenting techniques, several intracorporeal stenting techniques have previously been described [25,26,27,28]. However, these techniques require fluoroscopic confirmation of correct placement after closing the ureter. It is difficult to place the film between the patient’s body and the operation table to take an intraoperative KUB image when the patient is draped. Additionally, it takes substantial time to obtain KUB radiography results. Moreover, upon recognition using KUB radiography and correction of a malpositioned double-J stent, another KUB image would be necessary to confirm correct positioning. Therefore, readjustment of the stent using intraoperative KUB radiography for guidance may be technically difficult and increase operative time, potentially leading to stent failure. In this regard, our technique does not require position changes or fluoroscopic confirmation. Furthermore, the additional time required for flexible cystoscopy was <2 min if the stent was correctly placed in the bladder. Even when the stent was not adequately placed in the bladder, the mean additional time for readjustment was only 4 min. Flexible cystoscopy is an especially suitable method for male patients who might experience moderate pain if ureteroscopic removal of the stent is needed after LUL because of its upward malpositioning.
Surgeon and use of flexible cystoscopy were significant predicting factors for upward malpositioning. To our knowledge, no previous studies related to malpositioning of double-J stents have addressed this question. It makes intuitive sense that the rate of malpositioning differs according to the operator. Experience of the surgeon is important in preventing malpositioning. However, all our surgeons had several cases of upward malpositioning. Additionally, the use of flexible cystoscopy resulted in a 100% success rate, and malpositioning was corrected with a p-value of 0.008 in our study.
We recognize several limitations in this study. The first is that the data were collected from four different surgeons with different surgical experience without randomization. The differences in the number of performed LUL among the surgeons may indicate different levels of experience, resulting in variations in operative time, blood loss, and complication rates.
The second limitation is a lack of precise definition of upward malpositioning of the double-J stent. In fact, criteria for deciding whether the double-J stent is accurately positioned have not been defined in any previous study. Additionally, in this retrospective study, methods for the removal of the double-J stent after LUL were not described in the medical records in many cases. Therefore, we defined upward malpositioning as placement of the double-J stent such that its tip is straight instead of being curled on postoperative KUB radiography. This strict definition may have contributed to the high rate of malpositioning of double-J stents in this study. This rate would be lower if malpositioning was defined to occur if the double-J stent had to be removed with an ureteroscope after LUL. In fact, flexible cystoscopy through the urethral route determined that the double-J stent was malpositioned in 10 of 50 cases (20%), which supports our assertion that the malpositioning rate in many cases is actually higher.
The third limitation of our study is the small sample size. Thus, future studies with a larger number of patients are needed.
Despite these limitations, our results show that flexible cystoscopy may be a useful method for confirming the placement of ureteral stents in the bladder and reducing malpositioning rates in male patients. The use of flexible cystoscopy after intracorporeal double-J stenting following LUL is an effective, quick, simple, and safe method that does not require position changes.