Abstract
Background
According to our hypothesis, a classification system that grades stone impaction may better predict surgical success and complications. Therefore, we developed a new classification system, and in this study, we aimed to evaluate the effect of stone impaction and its degree on success and complications of ureteroscopic lithotripsy (URS-L), as well as to investigate preoperative factors predicting stone impaction and its degree.
Methods
Data of patients over the age of 18 years who underwent URS-L for a single ureteral stone were prospectively collected. First, stone impaction was determined by the primary surgeon using a Likert scale. Subsequently, non-impacted and impacted stones were further divided into two subgroups and classified according to the degree of stone impaction between Grades 1 and 4. Patient characteristics, preoperative non-contrast computed tomography (CT) findings, laboratory values, and intraoperative and postoperative outcomes were analyzed. Stone-free status was defined as the absence of residual stones > 2 mm on non-contrast CT at postoperative month 1.
Results
The study included a total of 211 patients. Operative time, ureteral injury, degree of ureteral injury, failure (nephrostomy placement), hospital stay, and stone-free rates differed significantly between impacted and non-impacted stones. When evaluated by degree of impaction, significant differences were identified in operative time, presence of pyuria, ureteral injury, degree of ureteral injury, failure, hospital stay, and stone-free status. Colic severity, time from pain onset to surgery, location (upper ureter), stone volume, peri-calculus ureteral wall thickness (PUWT), and presence of severe hydronephrosis were found to be predictive factors of stone impaction, while female gender, stone location (upper ureter), and PUWT were independent predictors of Grade 4 stones.
Conclusions
This study showed that stone impaction and the new classification system are associated with surgical outcomes. Not all non-impacted or impacted stones are the same, and grading according to the interaction of the stone with the ureteral wall can better predict surgical outcomes.
Keywords: Ureteroscopic lithotripsy, Impaction, Ureteral kinking, Stone-free status, Ureteral injury
Introduction
When ureteral stones remain in the same position for a prolonged period, they may lead to chronic inflammation in the surrounding ureteral mucosa, resulting in ureteral edema, polyps, and stone fixation. Such stones are termed impacted ureteral stones [1]. The ureteroscopic treatment of impacted ureteral stones continues to pose challenges for urologists. In a prospective, observational, and multicenter study by the Clinical Research Office of the Endourological Society, impacted ureteral stones were reported to be associated with lower stone-free rates and higher intraoperative complications compared to non-impacted ureteral stones [2].
Determining stone impaction before treatment has several important advantages. First, it allows for better patient counseling regarding the need for ureteral stenting, the type of surgery, likelihood of success, need for additional treatments, and risk of complications. Second, it helps determine the optimal treatment strategy. For instance, the efficacy of shock wave lithotripsy for impacted ureteral stones is considered to be limited due to ureteral edema, polyps, and stone fixation [3]. Furthermore, delays in surgical treatment caused by failed shock wave lithotripsy sessions may exacerbate stone impaction and negatively affect surgical outcomes. An alternative for treating impacted upper ureteral stones is an antegrade approach via percutaneous access, and studies have demonstrated that this approach is both effective and safe [4, 5]. Third, preoperative identification of stone impaction allows patients to be informed in advance about the antegrade approach and obtain necessary consent.
Numerous studies have investigated the variables that predict ureteral stone impaction. However, due to methodological discrepancies and varying focus among these studies, the findings remain inconsistent. Moreover, the definition of impacted stones differs considerably across the literature [6–9]. These definitions range from the inability to advance a guide wire during the initial attempt [6] to surgeon-determined assessments using a Likert scale [7, 8], lack of distal contrast passage on intravenous urography or computed tomography (CT) urography, or confirmation that the stone (and/or symptoms) has been present in the same location for more than one month [9]. Such heterogeneous definitions encompass a wide spectrum of ureteral stones, from those mildly adherent to the ureteral wall and impeding guide wire passage to those associated with edema or polyps, stones embedded within the ureteral wall, and even those difficult to access due to ureteral kinking. According to our hypothesis, a classification system that grades stone impaction may better predict surgical outcomes and complications and assist in determining the optimal treatment strategy. Therefore, we developed a new classification system to categorize the changes in the ureteral wall caused by stones. In this study, we aimed to evaluate the effect of stone impaction and its degree on success and complications of ureteroscopic lithotripsy (URS-L), as well as to investigate the preoperative factors predicting stone impaction and its degree.
Materials and methods
Study Population
After local ethics committee approval (BAEK 2024/08-154), this prospective, observational study was conducted at the Urology Clinic of University of Health Sciences, Erzurum City Hospital, a high-volume center located in Turkey, an endemic region for urolithiasis. The study was carried out in accordance with the Declaration of Helsinki, and informed consent was obtained from all patients included in the study. Patients over the age of 18 years who underwent URS-L for a single ureteral stone between August 2024 and March 2025 were included. Patients with multiple stones, preoperative double-J (DJ) stent or nephrostomy, those who underwent emergency surgery due to refractory renal colic, patients with more than 10 days between non-contrast CT and surgery, and those with tight ureter preventing engagement of the ureteroscope from the orifice were excluded. In addition, patients with ureterovesical junction stones were excluded due to the difficulty in measuring the transverse ureteral diameter.
Initially, stone impaction was determined by the primary surgeon using a Likert scale, as described by Tran et al. [7]. Subsequently, non-impacted and impacted stones were further divided into two subgroups and classified according to the degree of stone impaction from Grade 1 to 4:
Grade 1: Completely mobile stones that do not cause edema around the stone (non-impacted).
Grade 2: Stones stuck in ureter, causing mild edema in the stone area, but can be displaced with mild irrigation (non-impacted).
Grade 3: Stones stuck in ureter, causing marked edema/polyps in the stone area, not displaceable with irrigation (impacted).
Grade 4: Stones stuck in ureter, causing marked edema/polyps in the stone area, not displaceable with irrigation, and causing ureteral kinking (impacted).
Data Collection
The parameters examined included preoperative characteristics (age, gender, body mass index, hypertension, diabetes mellitus, history of stone treatment, positive preoperative urine culture, colic severity, and time from pain onset to surgery), stone-related features (stone location [upper/lower ureter], volume, density, peri-calculus ureteral wall thickness [PUWT], presence of severe hydronephrosis, and renal parenchymal thickness), laboratory values, intraoperative data (operative time, presence of pyuria, ureteral injury, degree of ureteral injury, and failure), stone-free status, length of hospital stay, and postoperative complications (transient hematuria and fever).
The colic severity was assessed using the Visual Analog Scale (0: no pain – 10: worst pain ever experienced). In asymptomatic patients, the time from pain onset to surgery was calculated based on the earliest imaging date when the ureteral stone was detected. Stone length and width were measured in the axial plane, and height was measured in the coronal plane. Stone volume was calculated using the following formula: length × width × height × π × 0.167. Stone density was measured in the axial plane by drawing a region of interest and recorded in Hounsfield unit (HU). PUWT was calculated in the coronal plane at the stone level by subtracting the transverse diameter of the stone from the widest transverse ureteral diameter using maximum magnification (Fig. 1). Hydronephrosis was graded according to the Society for Fetal Urology grading system [10], and Grade 3–4 hydronephrosis was defined as severe hydronephrosis. Renal parenchymal thickness was measured at the thickest point in the axial plane. All non-contrast CT measurements were performed by an independent radiologist (D.Ö.K.) blinded to patient characteristics.
Fig. 1.
Measurement of peri-calculus ureteral wall thickness (PUWT): (a) Visualization of the ureteral stone in the coronal plane, (b) Measurements performed at maximum magnification (PUWT = transverse diameter of the ureter at the level of the stone (blue) – transverse diameter of the stone (red))
All patients underwent preoperative urinalysis, urine culture, complete blood count, and creatinine testing. Neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, and systemic immune-inflammation index (SIII) were calculated. SIII was derived using the formula neutrophil × platelet/lymphocyte [11]. Patients with positive preoperative urine culture were treated according to antibiotic susceptibility tests and operated on only after sterile cultures were obtained. Prophylactic antibiotics were administered to all patients at the time of anesthesia induction.
Surgical technique
All procedures were performed under general anesthesia in the lithotomy position. The degree of stone impaction was determined during initial ureteroscopy (6-7.5-Fr, ultrathin semi-rigid ureteroscope, Richard Wolf, Germany). A guide wire (0.035-inch Sensor™, Boston Scientific, USA) was inserted for safety if advancement was possible. Stones were fragmented using a 272 μm Ho: YAG laser (Litho, Quanta System, Italy). When necessary, flexible URS-L (9.12-Fr, RP-U-C12 single-use flexible ureteroscope, Redpine, China) was performed using a ureteral access sheath (9.5–11.5-Fr, Indovasive, India). In cases of pyuria, upon relieving the obstruction, the procedure was terminated with placement of a DJ stent (4.7-Fr, Indovasive, India). DJ stenting was performed at the discretion of the surgeon. Operative time was defined as the duration from the beginning of ureteroscopy to placement of the DJ stent. Ureteral injury was assessed prior to DJ stent placement by examining the ureteral wall at the stone site and was graded according to the Traxer classification [12]. In cases where URS-L failed, percutaneous nephrostomy was inserted. Stone-free status was defined as the absence of residual stones > 2 mm on non-contrast CT at postoperative month 1.
Study objectives
The primary objective of the study was to evaluate the effect of stone impaction and its degree on surgical success and complications. The secondary objective was to identify preoperative parameters predicting stone impaction and its degree.
Statistical analysis
Statistical data analyses were performed using IBM SPSS Statistics for Windows version 25 (IBM Corp., Armonk, NY, USA). Data were presented as mean ± standard deviation for normally distributed continuous variables, median (interquartile range) for non-normally distributed continuous variables and frequency (%) for categorical variables. Distribution of continuous variables was assessed using the Shapiro-Wilk test. Homogeneity of group variances was assessed with the Levene test. The independent samples t-test for normally distributed variables and Mann-Whitney U test for non-normally distributed variables were used for comparing two continuous variables. The Kruskal-Wallis test was used for comparing three or more continuous variables. Pearson’s chi-square test or Fisher’s exact test was used to compare categorical variables.
Binary logistic regression analysis was performed using Jamovi version 2.6.25 (The jamovi project (2024). [Computer Software]. Retrieved from https://www.jamovi.org.). Univariate and multivariate binary logistic regression analyses were used to determine independent risk factors associated with stone impaction (Grade 3–4) or Grade 4 stones. To correct for confounding bias, only variables that were found significant in the univariate logistic regression analysis were included in the multivariate logistic regression analysis. Two variables were considered collinear if their variance inflation factor (VIF) was greater than 3. Receiver operating characteristic (ROC) analysis was performed to evaluate the performance of independent factors using the trial version of MedCalc (version 23.2.8). A p-value of less than 0.05 was considered statistically significant.
Results
The study included a total of 211 patients, of whom 65 (30.8%) had impacted ureteral stones and 146 (69.2%) had non-impacted ureteral stones. According to the degrees of impaction, 82 (38.9%), 64 (30.3%), 48 (22.7%), and 17 (8.1%) patients had Grade 1, 2, 3, and 4 stones, respectively. The stone-free rate was 78.7%. Nephrostomy was performed in four patients (1.9%) due to failure. When evaluating preoperative variables, the colic severity (p < 0.001), time from pain onset to surgery (p < 0.001), stone location (p < 0.001), stone volume (p < 0.001), PUWT (p < 0.001), presence of severe hydronephrosis (p < 0.001), and renal parenchymal thickness (p = 0.009) were found to differ significantly between impacted and non-impacted stones (Table 1).
Table 1.
Comparison of patient characteristics, non-contrast CT findings, and laboratory results of patients with impacted and non-impacted stones
| Variables | All patients | Non-impacted group | Impacted group | p-value |
|---|---|---|---|---|
| (n = 211) | (n = 146) | (n = 65) | ||
| Age, years, median (IQR) | 43 (22.5) | 42 (21.8) | 43 (25) | 0.925m |
| Gender, n (%) | 0.086* | |||
| Male | 153 (72.5) | 111 (76) | 42 (64.6) | |
| Female | 58 (27.5) | 35 (24) | 23 (35.4) | |
| Body mass index (kg/m2), mean ± SD | 27.4 ± 4.5 | 27.6 ± 4.6 | 26.8 ± 4.2 | 0.214t |
| Hypertension, n (%) | 28 (13.3) | 18 (12.3) | 10 (15.4) | 0.546* |
| Diabetes mellitus, n (%) | 14 (6.6) | 11 (7.5) | 3 (4.6) | 0.557f |
| History of stone treatment, n (%) | 46 (21.8) | 33 (22.6) | 13 (20) | 0.672* |
| Shock Wave Lithotripsy | 12 (5.7) | 9 (6.2) | 3 (4.6) | 0.759f |
| Ureteroscopic Lithotripsy | 37 (17.5) | 27 (18.5) | 10 (15.4) | 0.584* |
| PNL/open kidney surgery | 5 (2.4) | 4 (2.7) | 1 (1.5) | 1.000f |
| Preoperative positive urine culture, n (%) | 10 (4.7) | 4 (2.7) | 6 (9.2) | 0.072f |
| Colic severity (VAS score), median (IQR) | 9 (3) | 10 (2) | 7 (4) | < 0.001m |
| Time from pain onset to surgery (days), median (IQR) | 15 (33) | 11.5 (24) | 40 (71) | < 0.001m |
| Time from CT to surgery (days), median (IQR) | 3 (5) | 3 (5) | 4 (5) | 0.173m |
| Stone location, n (%) | < 0.001* | |||
| Upper ureter | 86 (40.8) | 48 (32.9) | 38 (58.5) | |
| Lower ureter | 125 (59.2) | 98 (67.1) | 27 (41.5) | |
| Stone volume (mm3), median (IQR) | 110.1 (175.7) | 90 (123) | 183.5 (361.8) | < 0.001m |
| Stone density (HU) median (IQR) | 1,036 (390) | 1,034.5 (409) | 1,052 (293) | 0.435m |
| PUWT (mm), median (IQR) | 3 (3) | 2 (3) | 4 (2) | < 0.001m |
| Presence of severe hydronephrosis, n (%) | 39 (18.5) | 12 (8.2) | 27 (41.5) | < 0.001f |
| Renal parenchymal thickness (mm), median (IQR) | 25 (6) | 25 (5) | 24 (7) | 0.009m |
| Perirenal stranding, n (%) | 50 (23.7) | 37 (25.3) | 13 (20) | 0.399* |
| Opacity, n (%) | 147 (69.7) | 96 (65.8) | 51 (78.5) | 0.064* |
| Creatinine (mg/dL) median (IQR) | 1.01 (0.42) | 1.01 (0.41) | 0.94 (0.43) | 0.450m |
| White blood cell count (109/L), median (IQR) | 9.5 (4.3) | 9.8 (4.7) | 9.1 (3.6) | 0.452m |
| Neutrophil count (109/L), median (IQR) | 6.53 (4.29) | 6.84 (4.26) | 5.81 (4.52) | 0.196m |
| Lymphocyte count (109/L), mean ± SD | 2.31 ± 0.83 | 2.29 ± 0.82 | 2.35 ± 0.85 | 0.662t |
| Platelet (109/L), median (IQR) | 264 (90) | 269.5 (100) | 256 (88) | 0.974m |
| Monocyte count (109/L), median (IQR) | 0.68 (0.34) | 0.69 (0.36) | 0.67 (0.31) | 0.191m |
| SIII, median (IQR) | 724.4 (688.7) | 731.8 (649.9) | 618 (669.4) | 0.151m |
| Neutrophil-to-lymphocyte ratio, median (IQR) | 2.7 (2.4) | 2.8 (2.1) | 2.3 (2.8) | 0.163m |
| Platelet-to-lymphocyte ratio, median (IQR) | 118.5 (59.9) | 118.5 (64.3) | 118.5 (58.4) | 0.585m |
IQR Interquartile range, SD Standard deviation, PNL Percutaneous nephrolithotomy, VAS Visual Analog Scale, CT Computed tomography, HU Hounsfield unit, PUWT Peri-calculus ureteral wall thickness, SIII Systemic immune-inflammation index
*Pearson’s chi-square test, fFisher’s exact test, tIndependent-samples t-test, mMann-Whitney U test
In terms of intraoperative and postoperative variables, operative time (p < 0.001), ureteral injury (p < 0.001), degree of ureteral injury (p < 0.001), failure (nephrostomy placement) (p = 0.008), hospital stay (p = 0.005), and stone-free status (p < 0.001) also showed significant differences between impacted and non-impacted stones. Upon assessing intraoperative and postoperative variables based on degree of impaction, it was determined that operative time (p < 0.001), presence of pyuria (p = 0.004), ureteral injury (p < 0.001), degree of ureteral injury (p < 0.001), failure (nephrostomy placement) (p < 0.001), hospital stay (p = 0.003), and stone-free status (p < 0.001) were all significantly different (Table 2).
Table 2.
Comparison of intraoperative and postoperative parameters according to stone impaction and degree of impaction
| Variables | Non-impacted group |
Impacted group |
p-value | Degree of impaction | p-value | |||
|---|---|---|---|---|---|---|---|---|
| Grade 1 | Grade 2 | Grade 3 | Grade 4 | |||||
| (n = 146) | (n = 65) | (n = 82) | (n = 64) | (n = 48) | (n = 17) | |||
| Operative time, (min), median (IQR) | 15 (9.5) | 25 (27) | < 0.001m | 15 (6)a | 15 (10)a | 25 (15)a, b | 50 (52.5)b | < 0.001+ |
| Pus discharge, n (%) | 8 (5.5) | 4 (6.2) | 1.000f | 0 (0.0)a | 8 (12.5)b | 3 (6.3)a, b | 1 (5.9)a, b | 0.004f |
| Double-J stenting, n (%) | 141 (96.6) | 65 (100) | 0.327f | 77 (93.9) | 64 (100.0) | 48 (100.0) | 17 (100.0) | 0.071f |
| Ureteral injury, n (%) | 30 (20.5) | 51 (78.5) | < 0.001* | 15 (18.3)a | 15 (23.4)a | 36 (75.0)b | 15 (88.2)b | < 0.001* |
| Degree of ureteral injury, n (%) | < 0.001f | < 0.001f | ||||||
| Grade 0 | 116 (79.5) | 14 (21.5) | 67 (81.7)a | 49 (76.6)a | 12 (25)b | 2 (11.8)b | ||
| Grade 1 | 28 (19.2) | 33 (50.8) | 15 (18.3)a | 13 (20.3)a | 26 (54.2)b | 7 (41.2)a, b | ||
| Grade 2 | 1 (0.7) | 15 (23.1) | 0 (0.0)a | 1 (1.6)a | 9 (18.8)b | 6 (35.3)b | ||
| Grade 3 (perforation) | 1 (0.7) | 3 (4.6) | 0 (0.0)a | 1 (1.6)a, b | 1 (2.1)a, b | 2 (11.8)b | ||
| Treatment failure (nephrostomy placement), n (%) | 0 (0) | 4 (6.2) | 0.008f | 0 (0.0)a | 0 (0.0)a | 0 (0.0)a | 4 (23.5)b | < 0.001f |
| Transient hematuria, n (%) | 26 (17.8) | 17 (26.2) | 0.165* | 16 (19.5) | 10 (15.6) | 13 (27.1) | 4 (23.5) | 0.493f |
| Fever, n (%) | 6 (4.1) | 6 (9.2) | 0.195f | 3 (3.7) | 3 (4.7) | 5 (10.4) | 1 (5.9) | 0.390f |
| Hospital stay (days), median (IQR) | 1 (0) | 1 (0) | 0.005m | 1 (0)a | 1 (0)a | 1 (0)a, b | 1 (1)b | 0.003+ |
| Stone-free status, n (%) | 126 (86.3) | 40 (61.5) | < 0.001* | 73 (89)a | 53 (82.8)a | 34 (70.8)a, b | 6 (35.3)b | < 0.001f |
IQR Interquartile range
*Pearson’s chi-square test, fFisher’s exact test, tIndependent-samples t-test, mMann-Whitney U test, +Kruskal-Wallis test
Failure (nephrostomy placement) was observed exclusively in patients with Grade 4 stones. The median operative time increased from 25 min in Grade 3 stones to 50 min in Grade 4 stones. The stone-free rate declined to 70.8% in Grade 3 stones and further to 35.3% in Grade 4 stones. Severe ureteral injury (Grade 2–3) was significantly higher in Grade 3 (20.8%) and Grade 4 (47.1%) stones compared to Grade 1 (0%) and Grade 2 (3.1%) stones. In addition, the rate of Grade 3 ureteral injury (perforation) was notably higher in Grade 4 stones (11.8%) (Table 2).
To identify predictive factors for stone impaction (Grade 3–4 stones) and Grade 4 stones, univariate logistic regression analysis was initially conducted (Table 3), followed by multivariate logistic regression analysis using significant variables (Table 4). In multivariate logistic regression analysis, colic severity (p = 0.024), time from pain onset to surgery (p = 0.005), stone location (upper ureter) (p = 0.002), stone volume (p = 0.044), PUWT (p = 0.002), and presence of severe hydronephrosis (p = 0.013) were determined to be independent predictors of stone impaction. Independent predictors for Grade 4 stones were identified as female gender (p = 0.020), stone location (upper ureter) (p = 0.006), and PUWT (p = 0.014).
Table 3.
Univariate logistic regression analysis for predicting stone impaction and grade 4 stones
| Variables | Stone impaction (Grade 3–4) | Grade 4 Stones | p-value | |
|---|---|---|---|---|
| OR (95% CI) | p-value | OR (95% CI) | ||
| Age | 1.00 (0.979–1.02) | 0.976 | 1.023 (0.988–1.059) | 0.205 |
| Gender (female) | 1.737 (0.921–3.276) | 0.088 | 3.329 (1.218–9.103) | 0.019 |
| Body mass index | 0.959 (0.897–1.02) | 0.214 | 0.943 (0.841–1.06) | 0.315 |
| Hypertension | 1.293 (0.561–2.98) | 0.547 | 2.18 (0.657–7.232) | 0.203 |
| Diabetes mellitus | 0.594 (0.160–2.204) | 0.436 | 0.87 (0.107–7.086) | 0.897 |
| History of stone treatment | 0.856 (0.416–1.76) | 0.673 | 0.753 (0.207–2.74) | 0.666 |
| Preoperative positive urine culture | 3.61 (0.983–13.261) | 0.053 | 3.10 (0.604–15.923) | 0.175 |
| Colic severity | 0.729 (0.642–0.828) | < 0.001 | 0.772 (0.662–0.899) | < 0.001 |
| Time from pain onset to surgery | 1.016 (1.008–1.024) | < 0.001 | 1.007 (1.003–1.012) | < 0.001 |
| Stone location (upper ureter) | 2.873 (1.574–5.247) | < 0.001 | 12.993 (2.888–58.447) | < 0.001 |
| Stone volume | 1.004 (1.002–1.006) | < 0.001 | 1.002 (1.001–1.003) | < 0.001 |
| Stone density | 1.000 (0.999–1.002) | 0.355 | 0.999 (0.998–1.001) | 0.482 |
| PUWT | 1.391 (1.189–1.626) | < 0.001 | 1.423 (1.17–1.731) | < 0.001 |
| Presence of severe hydronephrosis | 7.934 (3.675–17.128) | < 0.001 | 8.128 (2.863–23.074) | < 0.001 |
| Renal parenchymal thickness | 0.90 (0.839–0.966) | 0.003 | 0.874 (0.779–0.98) | 0.022 |
| Perirenal stranding | 0.736 (0.361–1.503) | 0.400 | 0.67 (0.185–2.433) | 0.543 |
| Opacity | 1.897 (0.958–3.757) | 0.066 | 2.14 (0.593–7.722) | 0.245 |
| Creatinine | 0.877 (0.372–2.07) | 0.765 | 0.154 (0.021–1.11) | 0.064 |
| White blood cell count | 0.957 (0.868–1.05) | 0.372 | 0.934 (0.79–1.104) | 0.423 |
| Neutrophil count | 0.946 (0.852–1.05) | 0.294 | 0.952 (0.796–1.139) | 0.592 |
| Lymphocyte count | 1.083 (0.760–1.542) | 0.660 | 0.782 (0.419–1.457) | 0.438 |
| Platelet count | 1.001 (0.996–1.00) | 0.775 | 1.001 (0.993–1.008) | 0.862 |
| Monocyte count | 0.447 (0.148–1.35) | 0.153 | 0.262 (0.034–2.009) | 0.198 |
| SIII | 1.00 (1.00–1.00) | 0.592 | 1.00 (1.00–1.001) | 0.615 |
| Neutrophil-to-lymphocyte ratio | 0.956 (0.861–1.063) | 0.409 | 1.013 (0.881–1.163) | 0.860 |
| Platelet-to-lymphocyte ratio | 1.00 (0.995–1.004) | 0.873 | 1.002 (0.996–1.008) | 0.523 |
OR Odds ratio, CI Confidence interval, PUWT Peri-calculus ureteral wall thickness, SIII Systemic immune-inflammation index
Table 4.
Multivariate logistic regression analysis for predicting stone impaction and grade 4 impaction based on univariate results
| Variables | Stone impaction (Grade 3–4) | Grade 4 Stones | p-value | |
|---|---|---|---|---|
| OR (95% CI) | p-value | OR (95% CI) | ||
| Gender (female) | --- | --- | 6.333 (1.338–29.978) | 0.020 |
| Colic severity | 0.833 (0.71–0.977) | 0.024 | 0.793 (0.605–1.04) | 0.094 |
| Time from pain onset to surgery | 1.014 (1.004–1.024) | 0.005 | 1.002 (0.996–1.007) | 0.595 |
| Stone location (upper ureter) | 3.47 (1.557–7.732) | 0.002 | 11.339 (1.992–64.532) | 0.006 |
| Stone volume | 1.002 (1.00–1.005) | 0.044 | 1.001 (0.999–1.003) | 0.259 |
| PUWT | 1.357 (1.118–1.646) | 0.002 | 1.51 (1.088–2.095) | 0.014 |
| Presence of severe hydronephrosis | 3.662 (1.314–10.206) | 0.013 | 5.296 (0.875–32.062) | 0.070 |
| Renal parenchymal thickness | 1.021 (0.922–1.131) | 0.684 | 1.137 (0.935–1.383) | 0.199 |
OR Odds ratio, CI Confidence interval, PUWT Peri-calculus ureteral wall thickness
The results of ROC analysis of independent factors predicting stone impaction and grade 4 stones are shown in Fig. 2. Cut-off values for predicting stone impaction were determined as ≤ 7 (sensitivity 56.9%, specificity 82.9%) for colic severity, ≥ 25 days (sensitivity 64.6%, specificity 73.3%) for time from pain onset to surgery, > 125.9 mm3 (sensitivity 63.1%, specificity 65.8%) for stone volume and ≥ 4 mm (sensitivity 60%, specificity 71.2%) for PUWT, while PUWT ≥ 5 mm was the cut-off with 59% sensitivity and 80% specificity for predicting Grade 4 stones.
Fig. 2.
Receiver operating characteristic (ROC) curves of (a) colic severity (area under the curve [AUC]: 0.715), time from pain onset to surgery (AUC: 0.764), stone location (upper ureter) (AUC: 0.628), stone volume (AUC: 0.667), peri-calculus ureteral wall thickness (PUWT) (AUC: 0.682), and presence of severe hydronephrosis (AUC: 0.667) for predicting stone impaction, (b) female gender (AUC: 0.638), stone location (upper ureter) (AUC: 0.758), PUWT (AUC: 0.726), and combination of three factors (AUC: 0.703) for predicting Grade 4 stones
Discussion
The prospective literature supports the association of stone impaction with increased operative time and intraoperative complications [2, 13]. However, the findings regarding stone-free rates remain inconsistent. While Legemate et al. reported significantly decreased stone-free rates in impacted stones [2], Hamamoto et al. stated that increasing degrees of edema, polyp, and mucosa-stone adherence did not affect stone-free rates, and only Grade 3 distal ureteric tightness (inability to advance the ureteroscope) significantly reduced stone-free rates [13]. In our study, we also determined that stone impaction was associated with increased operative time, increased ureteral injury, higher grades of ureteral injury, increased failure rates, prolonged hospital stay, and decreased stone-free rates. Additionally, to the best of our knowledge, we conducted the most comprehensive prospective study encompassing preoperative patient characteristics, non-contrast CT parameters, and laboratory values. Reduced colic severity, prolonged time from pain onset to surgery, upper ureteral stone location, increased stone volume, elevated PUWT, presence of severe hydronephrosis, and reduced renal parenchymal thickness were all found to be associated with stone impaction. Furthermore, we evaluated, for the first time in literature, whether laboratory values were associated with stone impaction. We hypothesized that due to increased inflammation in impacted stones, inflammatory markers in laboratory findings might differ; however, no significant difference in any laboratory parameter was found between patients with impacted and non-impacted stones.
In previous studies, impacted and non-impacted stones have been sharply divided based on varying definitions. However, we observe various degrees of changes in the ureteral mucosa caused by ureteral stones. Particularly, due to inflammation induced by stone impaction, the stone and the adherent ureteral wall may become fixed to the periureteral tissues and potentially cause ureteral kinking during ureterorenoscopy. In our view, ureteral kinking is a condition that significantly complicates the case, increases the risk of complications and failure, and has not been extensively studied in literature. To investigate whether all impacted and non-impacted stones are the same, we developed a practical classification system based on our clinical experience, which also includes ureteral kinking. Our focus was to evaluate how effectively this classification could predict operative time, ureteral injury, failure, hospital stay, and stone-free rates. As a result, we observed that as the degree of impaction increased, so did operative time, ureteral injury, severity of ureteral injury, and hospital stay, while the stone-free rate decreased. However, these changes were minimal between Grade 1 and 2 stones, whereas Grade 4 stones differed significantly from the others. Nephrostomy placement due to failure was observed only in Grade 4 stones. Therefore, while stone impaction is a factor that complicates the operation, ureteral kinking appears to have a dramatic effect on surgical success. Our study demonstrated that not all impacted stones yielded the same results, and that classification might better predict surgical outcomes.
In literature, the endoscopic findings of ureteral stones have been investigated in only a limited number of studies. In a study evaluating the endoscopic features of impacted ureteral stones, Mugiya et al. found that 30.9% of patients had villous inflammatory fibrous polyps, 17% had non-villous, edematous hemispheric lesions narrowing the lumen, and 52.1% had no lesion at all. They identified a correlation between ureteral lesions and stones that had been impacted for more than one year. However, they did not assess the impact of these findings on surgical success [14]. In another study, Yoshida et al. reported that ureteral wall thickness ≥ 3.49 mm was more frequently associated with ureteral edema, polyps, white lesions suspicious of ischemic mucosa, and stone fixation, but they also did not clearly evaluate the impact of endoscopic findings on surgical success [15]. Hamamoto et al. classified ureteral stones based on endoscopic findings (SMART classification). However, they evaluated the impact of each endoscopic finding (edema, polyps, mucosa-stone adhesion, and distal ureteric tension) separately on surgical success and complications [13]. We consider that edema, polyp formation, and mucosa-stone adhesion are pathologies resulting from stone impaction and should be assessed collectively.
Studies investigating the predictors of stone impaction have identified various factors such as female gender, American Society of Anesthesiologists score > 1, prior stone treatment, positive preoperative urine culture, and larger stones [2]; HU values measured over and under the stone, degree of hydronephrosis, and ureteral wall thickness [16]; older age, prior stone treatment, degree of hydronephrosis, and ureteral wall thickness [6]; younger age, stone location (mid-ureter), and ureteral wall thickness [15]; HU value measured under the stone [7], peri-calculus ureteral wall surface area [8]; and ureteral wall thickness and hydronephrosis area [17]. The results have shown variability, likely due to differences in evaluated parameters and methodologies. In our study, we determined that colic severity, time from pain onset to surgery, stone location (upper ureter), stone volume, PUWT, and presence of severe hydronephrosis were independent predictors of stone impaction (Grade 3–4 stones). Female gender, stone location (upper ureter), and PUWT were identified as independent predictors of Grade 4 stones.
The effect of age on stone impaction remains unclear in literature. Both older [6] and younger [15] ages have been reported as predictors. In our study, we found that age was not a predictive factor. Female gender was associated with stone impaction only in the study by Legemate et al. [2]. We observed that gender did not influence stone impaction; however, interestingly, Grade 4 stones were more common in women, suggesting that stone impaction may be associated with more adverse outcomes in female patients. Ureteral mobility may be the reason why kinking is more common during ureteroscopy for impacted stones in women. Contrary to the literature findings [2, 6], prior stone treatment was not a predictive factor in our study. Although we found a higher rate of positive preoperative urine cultures in patients with impacted stones compared to those without, the difference was not significant, unlike the findings by Legemate et al. [2]. Yoshida et al. reported that mid-ureteral stones were associated with stone impaction, which they attributed to anatomical narrowing at the iliac crossover [15]. In contrast, we determined that upper ureteral stones were significantly associated with both stone impaction and Grade 4 stones. Consistent with the literature, we also determined that stone volume [2] and hydronephrosis [6, 16] were associated with stone impaction. However, stone volume and severe hydronephrosis were not predictors of Grade 4 stones. Notably, our study is the first in the literature to identify an inverse correlation between colic severity and stone impaction. This may be related to delayed presentation in patients experiencing less severe colic pain.
Normal ureteral wall thickness is approximately 1 mm. Inflammation, fibrosis, hypertrophy, edema, and polyps associated with stone impaction may increase ureteral wall thickness in the affected area [15]. Among all parameters investigated in the literature, ureteral wall thickness appears to be the most consistent predictor of stone impaction [6, 8, 15–17]. Yoshida et al. proposed a ureteral wall thickness of ≥ 3.49 mm as the cut-off value for predicting stone impaction [15]. In contrast, Seitz et al. reported that none of the secondary CT findings (renal enlargement, perinephric stranding, ureteral dilation, periureteral edema, and ureteral rim sign) were associated with stone impaction or stone-free rates and that only proximal stone location and endoscopically verified impaction were predictive of stone-free rates [18]. In our study, we determined that PUWT was an independent predictor of both stone impaction and Grade 4 stones. A PUWT of ≥ 4 mm predicted stone impaction with 60% sensitivity and 71.2% specificity, while a PUWT of ≥ 5 mm predicted Grade 4 stones with 59% sensitivity and 80% specificity.
Our study has certain limitations. PUWT measurements were performed by a single radiologist, and interobserver agreement was not assessed. Although stone impaction, degree of impaction, and degree of ureteral injury were all determined using strict criteria, these decisions relied on the primary surgeon and introduced potential selection bias. Furthermore, the number of impacted stones is also relatively small. In particular, the presence of only 17 cases of Grade 4 stones may have limited the generalizability of the multivariate analysis findings. A major strength of our study is that it evaluated the impact of categorizing stones according to their interaction with the ureteral wall on surgical success and complications for the first time. Additionally, to the best of our knowledge, this study is the first to highlight the significant unfavorable effect of ureteral kinking on surgical success and ureteral injury. Nevertheless, our proposed classification system requires external validation and may be modified as further studies and evidence emerge.
Conclusion
This study showed that stone impaction and the new classification system are associated with surgical outcomes. Not all non-impacted or impacted stones are the same, and grading according to the interaction of the stone with the ureteral wall can better predict surgical outcomes. In particular, Grade 4 stones differed significantly from the others. Nephrostomy placement due to failure was observed only in Grade 4 stones. Colic severity, time from pain onset to surgery, stone location (upper ureter), stone volume, PUWT, and the presence of severe hydronephrosis were independent preoperative predictors of stone impaction. Independent preoperative predictors of Grade 4 stones were female gender, stone location (upper ureter), and PUWT. Patients with preoperative risk factors for stone impaction and/or Grade 4 stones should be informed in more detail about unfavorable surgical outcomes. However, due to the low number of Grade 4 stones, multivariate analysis findings should be confirmed by multi-center studies.
Acknowledgements
None.
Clinical Trial Number
Not applicable.
Abbreviations
- URS-L
Ureteroscopic lithotripsy
- CT
Computed tomography
- PUWT
Peri-calculus ureteral wall thickness
- DJ
Double-J
- HU
Hounsfield unit
- SIII
Systemic immune-inflammation index
- ROC
Receiver operating characteristic
- VIF
Variance inflation factor
Authors’ contributions
EŞ, DÖK, FA and EG designed the study protocol. EŞ, MY and MÇA collected the data. DÖK performed the computed tomography measurements. EŞ and BE did the statistical analysis. EŞ and MSA wrote the main manuscript. FA and EG revised it critically for intellectual content. All authors read and approved the final manuscript.
Funding
This research did not receive any specific grant from public, commercial, or not-for-profit sectors’ funding agencies.
Data availability
The data collected in this research are available from the corresponding author on a reasonable request.
Declarations
Ethics approval and consent to participate
The study was approved by the Ethical Committee of the Erzurum Faculty of Medicine (BAEK 2024/08-154). Informed consent was obtained from all patients participating in the study.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
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Data Availability Statement
The data collected in this research are available from the corresponding author on a reasonable request.