What are the predictors of residual stone after ureteroscopy for urolithiasis?

The management of urolithiasis has evolved over time to minimally invasive endoscopic laser lithotripsy. Technological advancements such as the use of holmium laser and flexible ureteroscope have led to the establishment of ureteroscopy (URS) for intrarenal stones (1-4). Moreover, the use of equipment such as basket catheter and anti-retropulsive device has improved the outcomes of URS (5-7). Recently, along with improvements in laser and surgical instruments, the number of URS procedures for urolithiasis has increased significantly. There are available nomograms and scoring systems for predicting surgical outcomes. Furthermore, urologists have recently used nomograms and scoring systems to predict the success of treatment and the outcomes of different urological conditions (8,9).

The stone-free rate (SFR) of patients with upper ureteral stone is lower than that of patients with middle or distal ureteral stone (10). Wu et al. identified easily accessible risk factors associated with residual stones after URS for upper ureteral stones, and established a simple and reliable predictive model (11). They showed that stone length and shape, type of treatment modality, and distance of stones to the ureteropelvic junction (UPJ) were used in a simple and reliable predictive model. The stone was classified based on shape (quasi-circular and oval). The types of treatment modality were semirigid URS alone, semirigid URS with an anti-retropulsion device, and flexible URS. Moreover, the stones were also divided based on their distances from the UPJ, which were as follows: ≤30, 31–90, and >90 mm. Quasi-circular stone was defined as a stone with a difference of ≤2 mm between the stone length and width. The oval stone was defined as any other stones. The evaluation of stone shape is also a novel point. Stone shape may be correlated with stone retropulsion. In relation to this, it may have an impact on achieving stone-free status. Compared with other predictive models, the novel model is advantageous as it is simple to use. Further, it focuses on four parameters only, which can be observed via kidney, ureter, and bladder (KUB). Hence, further measurement or transformation is not required.

As shown in previous studies, some nomograms and scoring systems have been used to predict the success rate of URS (12-16). Moreover, it is important to note the targeted stones (renal, ureteral, or both) and the treatment method (rigid or flexible URS). Imamura et al. developed a nomogram for predicting the outcomes of rigid URS for ureteral stones (12). Stone length and location, number of stones, and the presence of pyuria were significant independent factors correlated with SFR. Meanwhile, single stone, stone size, distal position, and absence of hydronephrosis, which are significant predictive factors of stone-free status, were used in another nomogram for predicting the outcome of rigid URS for ureteral stones (13). Resold et al. developed a scoring system for assessing the outcome of flexible URS for renal stones, ureteral stones, and the results showed that stone size, stone location, stone number, renal malformation, and lower pole infundibulopelvic angle significantly affected SFR (14). Ito et al. developed a nomogram for investigating the outcome of flexible URS for renal stones (15). This study revealed that stone volume, presence of lower pole stone, operator with an experience of >50 flexible URS, stone number, and presence of hydronephrosis were stone-free status. Hori et al. developed a nomogram for evaluating the outcomes of flexible URS for ureteral and renal stones (16). This study showed that stone length, stone occupied lesion, and Hounsfield unit were significant predictive factors of stone-free status.

Flexible URS is important for the treatment of upper ureteral stones in some cases. A flexible ureteroscope is useful for upper ureteral stones if the ureter is tortuous due to hydronephrosis or if the rigid ureteroscope does not provide a clear view of the upper ureteral stones. In such cases, the upper ureteral stone must be pushed back from the narrow ureter to the renal pelvis, where there is more space. Furthermore, the stone should be fragmented in the renal pelvis and then extracted to improve SFR (5,6).

The outcomes of URS are challenging to compare due to various reasons. First, there is no uniform definition for stone-free status, stone size (no fragmentation, <2 mm, or <4 mm), timing of post-surgical evaluation (2 weeks or 1 month), and assessment instrument (computed tomography or KUB study). Second, there are differences in terms of surgical methods (rigid or flexible URS) and surgical equipment (laser device or basket catheters). In addition, the presence of a preoperative ureteral stent and impacted stone has a significant impact on surgical outcomes (17). As patient background, a relatively large number of patients in some nomograms had a ureteral stent inserted before the URS procedure (18). Also, there is no uniform definition for impacted stone. All nomograms have their own characteristics. However, there is no such thing as a perfect nomogram. Nomograms are created at different times of the year, and the periods of the cases used to create the nomograms differ as well. The outcomes will vary based on the time period considered because endoscopes and lasers are evolving every year.

Rather than comparing the superiority or inferiority of nomograms, it is more important to use them as a reference when selecting the appropriate surgical procedure (flexible URS or shock wave lithotripsy or percutaneous nephrolithotomy) (19,20). In addition, the use of nomograms and scoring systems could allow surgeons to select a staged procedure rather than a single URS procedure. Additionally, predicting not only the SFR but also the complications of URS using nomograms and scoring systems is important. In the future, the effect of using nomograms and surgical prediction models on treatment selection should be considered.

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