We investigated preoperative factors to predict stone impaction that was defined by intraoperative uteroscopic findings. High UWT, high grade of hydronephrosis and high HAB ratio were each independent preoperative factors that predicted stone impaction. Additionally, the higher the number of these factors, the greater the risk of impacted stones.
In the case of impacted stones, the stone remains in the same position in the ureter, causing chronic inflammation of the mucosa, fibrosis of the interstitium, and thickening of the uroepithelium, resulting in mucosal edema, polyps, and stone adherence [1, 12]. When URSL is performed, these changes in the ureteral wall often make the operative procedures difficult because of the inability to maintain clear visualization and the need to remove stones from ureteral mucosa. In a previous study, operation time was longer, SFR was lower, and intraoperative complications rate was higher in the patients with impacted stones than in those without impacted stones (all P<0.001) . Similarly, operation time was found to be longer and the rate of ureteral injury was higher in the impacted group than in the non-impacted group (all P<0.01).
Several definitions of impacted stones have been reported, but no single definition has yet been established. The first definition is the failed initial attempt to pass a guidewire or catheter through the stone . Although this definition has been the most cited, it is difficult to evaluate whether a guidewire or catheter could be passed through the stone retrospectively, and it cannot be assessed in patients undergoing SWL without ureteroscopic procedures. The second definition is when stones stagnate without moving in the ureter for more than two months [5, 13]. In a previous study, the period from first diagnosis or onset of symptoms to the date of treatment was calculated, but it is difficult to assess the exact start date. The third definition is when the ureter distal to the stone is not contrasted on computed tomographic urography (CTU) or intravenous urography (IVU) . However, CTU and IVU may not be possible in all patients because of renal dysfunction or allergies . Previous definitions thus have issues and do not always accurately reflect the actual impacted stones that are difficult to treat. In the present study, we defined impacted stones based on ureteroscopic findings in order to accurately extract ureteral stones that are difficult to treat with URSL.
UWT, ureteral wall thickness at the stone site, was first reported as a predictive factor of SWL success . It was hypothesized that UWT would be increased in impacted stones because of the formation of mucosal edema and polyps at the stone site, and they reported that high UWT was an independent factor predicting stone impaction (odds ratio [OR]: 5.43, P<0.01, optimal cut off value: 3.49 mm) . Similarly, in the present study, UWT was an independent predictor of stone impaction, and the optimal cut off value was also close (OR:1.20, P<0.01, optimal cut off value: 3.98 mm). UWV (volume of the ureteral wall at the stone site) was previously reported to be a better predictive factor of treatment success for SWL than UWT , but no previous reports have evaluated UWV in URSL. In the present study, UWV was not a significant factor in predicting stone impaction (OR:1.00, P=0.06).
CT attenuation of the ureter above and below ureteral stones, was shown in another recent study to be a novel predictor of stone impaction (‘HU above’; OR:0.92, P=0.000, ‘HU below’; OR:1.13, P=0.000) . ‘HU below’ (CT attenuation of the ureter below ureteral stones) was reported to be significantly higher, while ‘HU above’ (CT attenuation of the ureter above ureteral stones) was reported to be lower in impacted stones. The cause of these results has been unclear. However, ‘HU above’ tended to have liquid density because of hydronephrosis, while ‘HU below’ tended to have tissue density because of ureteral mucosal edema and inflammatory changes. We therefore focused on HAB ratio, the ratio of ‘HU above’ and ‘HU below’. Ozbir et al. reported that the area under curve (AUC) of the ratio of ‘HU above’ and ‘HU below’ was equivalent to that of UWT in terms of predicting stone impaction . In the present study, HAB ratio was an independent predictor of stone impaction (OR:1.47, P<0.01, optimal cut off value: 3.00).
The grade of hydronephrosis is expected to be high in patients with impacted stones because of poor urinary passage. Hydronephrosis was previously reported to be an independent predictor of stone impaction , which was consistent with the results of this study.
Figure 3 shows the association between the rate of the patients with impacted stones and the number of predictors (UWT ≥ 3.98 mm, grade of hydronephrosis ≥ 3 and HAB ratio ≥ 3.00). When the number of risk factors was 0, 1, 2, 3, the rate of the patients with impacted stones was 4.4%, 22.2%, 43.9%, 76.9%, respectively. These results suggest that preoperative factors might be used to predict stone impaction.
This study has a small number of limitations. First, it was retrospective and used a comparatively small cohort. Second, CT slice thickness was not unified. Consequently, there might be some small errors in HAB ratio because ‘HU above’ and ‘HU below’ were measured in the one slice proximal and distal to the stone. Third, all URSL were not performed by the same single surgeon, although all procedures were performed or observed by expert surgeons. Despite these limitations, UWT, HAB ratio and grade of hydronephrosis were shown to be novel preoperative predictors of stone impaction in which definition was based on ureteroscopic findings. We believe that this information will aid the selection of the treatment of impacted stones.