Abstract
Background:
Generally, there are many methods for the treatment of urinary stones, of which percutaneous nephrolithotomy (PCNL) is a minimally invasive and highly effective method, and now become the first-line management for urinary stones, especially in the cases of complex stones and staghorne calculi. Accurate assessment of stone location, stone morphology, degree of hydronephrosis as well as urinary system abnormalities is extremely important in the percutaneous nephrolithotomy strategy.
Objective:
The aim of this study was to evaluate the S.T.O.N.E score as well as other factors that influenced the effectiveness of PCNL.
Methods:
Descriptive study on 71 patients with kidney stones, who underwent multi-slice CT scan of the urinary system before PCNL and then PCNL at Hanoi Medical University Hospital from July 2022 to July 2023. All patients received the informed consent and agreed to participate in the study. The factors included the stone area, the track length (from the skin surface to the stone central), the degree of urinary tract dilatation, the number of involved calyces, the density of stone, the renal parenchyma thickness, the ureteral wall thickness and fat infiltration measured on MSCT non-contrast phase. These factors were used to predict the effectiveness of PCNL including the stone clearance rate (SCR) and the operation time.
Results:
The mean age of the patient group was 53.8±12.3. The male/female ratio was 1.54. There was a significant difference (p<0.05) between the following factors and the operation time: the stone area (<400, 400-799, 800-1599 and >1600 mm2), the degree of urinary tract dilatation (no or might and moderate or severe dilatation), the number of involved calyces (≤ 2, 3 and staghorne calculi), the renal parenchyma thickness (<18 mm and ≥18mm). In contrast, there were no significant differences between the following factors and the surgery time (p>0.05): the track length (<100 and ≥100 mm), and the stone density (<950 and ≥950 HU). Regarding the S.T.O.N.E score (included five factors: Size, Track length, Obstruction, Number of involved calyces, and Evaluation of stone density), there was a strong correlation between S.T.O.N.E score and the surgery time (p<0.001, r=0.94), and the SCR (p=0.001, r=–0.97).
Conclusion:
The evaluation of these factors played an important role in the prediction of the effectiveness of PCNL.
Keywords: S.T.O.N.E score, percutaneous nephrolithotomy, renal stones
1. BACKGROUND
Urolithiasis is a fairly common disease in the world especially in Southeast Asia, accounting for about 5-19% of the population (1). Multi-slice computer tomography (MSCT) of the urinary system is a highly accurate method for diagnosing urolithiasis and increasingly applied in clinical practice recently. Moreover, MSCT also allows assessment of kidney function, parenchymal thickness, renal pelvis dilatation as well as perinephric fatty infiltration. At the same time, MSCT is also very effective in assessing complications of urinary stones, follow-up, and evaluating nephrolithotomy results.
Generally, there are many methods for the treatment of urinary stones, of which percutaneous nephrolithotomy (PCNL) is a minimally invasive and highly effective method, and now become the first-line management for urinary stones, especially in the cases of complex stones and staghorne calculi (2–5). Accurate assessment of stone location, stone morphology, degree of hydronephrosis as well as urinary system abnormalities is extremely important in the percutaneous nephrolithotomy strategy. Therefore, multi-detector CT plays an important role in preoperative patient assessment, thereby reducing nephrolithotomy time, increasing treatment effectiveness, and minimizing complications and hospital stays for patients.
A number of scoring systems have been proposed for the purpose of preoperative stone diagnosis, including the S.T.O.N.E. nephrolithometry score. This score published in 2013 by Okhunov Z et al. is used to quantify the complexity of kidney stones and patient prognosis before nephrolithotomy (4). This scale includes 5 factors established from non-contrast computed tomography (CT) images including stone size, tract length, obstruction, number of involved calyces, and essence of stone density. Currently, there are not many studies in the literature evaluating the correlation between the S.T.O.N.E score and the effectiveness of PCNL.
2. OBJECTIVE
The aim of this study was to evaluate the S.T.O.N.E score as well as other factors that influenced the effectiveness of PCNL.
3. MATERIAL AND METHODS
Study population
A descriptive study on 71 patients with kidney stones, who underwent multi-slice CT scan of the urinary system before PCNL and then PCNL at Hanoi Medical University Hospital from July 2022 to July 2023. This study was reviewed and approved be the Institutional Review Board (Ref: 4907/QD-DHYHN dated on 19 October 2023). Our retrospective study was performed in compliance with the principles outlined in the Declaration of Helsinki. Written informed consent was obtained from all patients. The requirement for informed consent from patients was waived by the Institutional Review Board of Hanoi Medical University due to the retrospective study, which analyzed anonymized imaging data.
MSCT technique
The patient had a CT scan of the urinary system using a 16-slice CT scan from Philips and a 128-slice CT scan from GE Healthcare with a slice thickness of 0.625 mm. The field of view (FOV): 32-40 cm, from the diaphragm to the end of the pubic symphysis. The imaging protocol includes 4 phases: Pre-contrast phase, 2) Arterial phase (30 seconds after contrast injection), 3) Parenchymal phase (60 seconds after contrast injection), 4) The excretory phase (depending on kidney function with stones, minimum 5 minutes). A water-soluble contrast agent was used. The injection dose is 1-1.5ml/kg body weight, minimum injection speed is 2.5ml/sec.
Image analysis
CT results were interpreted according to current radiological criteria by a radiologist with more than 10 years of experience in a blinded manner with no knowledge of the clinical findings or previous imaging findings. The following imaging characteristics were established using non-contrast CT results:
1. Stone characteristics
- Stone burden (mm2) according to the formula of Tiselius et al. (6)
S=π/4 x (1)x(2)
in which (1) measures the maximum length of the stone and (2) measure the maximum width of the stone on the non-contrast CT in the coronal plane. In case there are many stones, we sum the total area of all the stones.
The number of involved calyces divided into 3 groups (≤ 2, 3 and staghorne calculi)
The tract length: distance from stone to skin surface: ( <100mm, ≥ 100mm)
Degree of hydronephrosis: Divided into 4 degrees according to the classification of SFU (Society of Fetal Urology) from 1 to 4
The renal parenchyma thickness (<18 mm and ≥18mm)
The stone density (<950 and ≥950 HU).
2. Calculate the S.T.O.N.E nephrolithometry score scale 4).
The S.T.O.N.E scale includes 5 factors established from non-contrast CT images including 1) Stone size is determined by combining the measures of length and width in square millimeters, with scores from 1 to 4 according to a calculated area of 1- 399 mm2, 400-799mm2, 800-1599 mm2 and ≥ 1600 mm2, respectively. 2) Tract length: the skin-to-stone distance is defined as the mean vertical distance from the center of the stone to the skin measured on a supine non-contrast CT scan, with scores 1 and 2 corresponding to ≤ 100 mm and > 100 mm. 3) The obstruction is hydronephrosis on CT scan (scores 1 corresponding to no or grade 1 hydronephrosis, and scores 2 corresponding to grade 2 or grade 3 hydronephrosis. 4) Number of involved calyces, score of 1 if one or two kidney calyces have stones, and score of 2 if three kidney calyces have stones and if it is staghorn calculi, it is recorded as 3 points (highest score). 5) The stone essence: stone density measured on preoperative CT scan. Stones with density ≥ 950 HU are 2 points and < 950 HU are 1 point.
Figure 1. A 72-year-old male patient presented with right staghorne calculi. Non-contrast CT scan in coronal plane show right renal stone with these following characteristics. Stone burden: 1747mm2. The number of involved calyces: staghorne calculi. Distance from stone to skin surface: 95mm. Degree of hydronephrosis: 3; The stone density 1367 HU; The renal parenchyma thickness 20mm; S.T.O.N.E score 12. The PCNL time was 140 minutes and patients had the residual stones after the first PCNL procedure.
The percutaneous nephrolithotomy procedure technique
The patient then underwent PCNL by the urological surgeon's team at the Department of Urology, Hanoi Medical University Hospital. PCNL was done by holmium laser lithotripsy machine using Accutech; model: Acu – H2H, wattage 90W. The PCNL technique: a ureteral catheter was placed. Then, under ultrasound guidance, surgeons chose the appropriate puncture location and direction (usually into the middle calyx) for the percutaneous renal access and placed Amplatz (creation of multiple tracts if necessary to remove all stones); using laser energy to remove stones with the pressure maintained at 10-20 kPa. Finally, surgeons placed an antegrade JJ ureteral catheter and drained the kidney. The operation time was defined from ureteral catheter placement until the placement of JJ and renal drain. X-ray test 1 day after surgery and CT scan before patient discharge from the hopital and JJ catheter removed to assess stone-free or residual stones (stone – free defined as no stone fragments of ≥ 4mm) and evaluate the stone clearance rate (SCR).
Statistical analysis
Data were analyzed using SPSS 20.0 (Statistical Package for Social Sciences version 20.0, SPSS Inc., Armonk, NY, USA) to determine correlations between CT characteristics, S.T.O.N.E score, and PCNL results (operation time, SCR). Qualitative variables calculate the percentage (%); while quantitative variables calculate average and standard deviation, min-max values. Categorical variables were compared using the Chi-square test or Fisher’s exact test. The average values were compared using the T-Test (or Mann-Whitney). Calculate the Pearson correlation coefficient (r) between the S.T.O.N.E scale with the stone operation time and stone clearance rate. The difference was considered statistically significant if p-values < 0.05.
4. RESULTS
General characteristics of the patient in the study
Average age was 53.8± 12.3 years. The lowest age was 31 years old, and the highest age was 73 years old. The majority of patients were concentrated in the age group of 40-79 years old, accounting for 81.7%, of which the largest number was the age group 40-59 years old, accounting for 43.7%, followed by the group of patients 60-79 years old accounting for 38%, the lowest was the group <40 years old, accounting for 18.3%. Male patients account for 60.6%, female patients account for 39.4%, male/female ratio is 1.54. Factors predicting the effectiveness of PCNL (Table 1 and 2)
Table 1. Patient demographics, radiology characteristics and the operation time.
| Variable | Number (%) | Operation time | P-value |
|---|---|---|---|
| N | 71 (100) | 67.5±32.8 | - |
| Age (years) | 53.8± 12.3 | - | |
| Gender - Male - Female |
43 (60.6) 28 (39.4) |
- | |
| The stone burden (mm2 ) - <400 - 400-799 - 800-1599 - >1600 |
21 (29.6) 30 (42.3) 14 (19.7) 6 (8.5) |
50,8±12.6 62.7±25.0 83.3±42.4 112.5±41.4 |
0.003 |
| The degree of hydronephrosis - No/Might - Moderate/Severe |
45 (63.4) 26 (36.6) |
59.6±26.7 81.8±38.4 |
0.006 |
| The number of involved calyces - 1-2 caculi - 3 caculi - Staghorne calculi |
38 (53.5) 10 (14.1) 23 (32.4) |
54.3±16.6 64.5±26.5 90.4±42.7 |
0,003 |
| The renal parenchyma thickness (mm) - <18 - ≥18 |
39 (54.9) 32 (45.1) |
80.0±38.12 57.18±23.7 |
0,005 |
| The track length (mm) - <100 - ≥100 |
62 (87.3) 9 (12.7) |
67.3± 33.5 68.1± 29.4 |
0,950 |
| The stone density (HU) - <950 - ≥950 |
9 (12.7) 62 (87.3) |
58.7±28.1 68.7±32.8 |
0.4 |
Table 2. Correlation between S.T.O.N.E score and the operation time (OR), the stone clearance rate (SCR.
| S.T.O.N.E score | N | OR | P-value | SCR (%) | P-value | |
|---|---|---|---|---|---|---|
| Stone-free | Residual stone | |||||
| 6 | 17 | 44.7±13.3 | 0.000 | 17 (100) | 0 | 0.001 |
| 7 | 16 | 55.6±13.9 | 15 (93.8) | 1(6.2) | ||
| 8 | 13 | 63.5±17.2 | 11 (84.6) | 2 (15.4) | ||
| 9 | 8 | 65.6±27.8 | 6 (75) | 2 (25) | ||
| 10 | 9 | 97.3±43.4 | 5(55.6) | 4(44.4) | ||
| 11 | 5 | 98.0±31.9 | 3 (60) | 2(40) | ||
| 12 | 3 | 141.7±22.5 | 2 (66.7) | 1(33.3) | ||
| N | 71 | 67.5±32.8 | 56(78.9) | 15(21.1) | ||
There was a strong correlation between S.T.O.N.E score and the operation time (p<0.001, r=0.94), and the SCR (p=0.001, r=-0.97)
5. DISCUSSION
The average age of the patient in our study was 53.8±12.3; similar to the results of the previous studies. Lai WH et al. with the study of 1,000 percutaneous nephrolithotomy patients found that the average age of the patients was 54.4 ± 12.6 years old (7). In our study, the most common age group was 40-59 years old, accounting for 43.7%. The majority of patients were of working age, similar to the study of Liu Y et al. (1). Research by Liu Y et al. also suggests that the urolithiasis incidence increased with age, peaked in the age group of 30-60 years, and decreased afterward.
In our study, 43 male patients accounted for 60.6%; 28 female patients accounted for 39.4%; the male/female ratio was 1.54; which was similar to the research of Liu Y as well as Bartoletti R et al. (8). Research by Liu Y et al shows that males had a higher risk of urinary stones than women with a ratio ranging from 1.3 to 5. Effects of diet (alcohol, coffee, eating a lot of meat), hormones testosterone, and prostate hypertrophy are thought to be the causes of increased risk of stone formation in males.
Preoperative prediction of success and complication rates of PCNL has attracted the attention of urosurgeon in recent years. Parameters on CT scans are extremely important to provide surgeons with the necessary information in surgical planning as well as predicting the possibility of nephrolithotomy’s success. Many studies in the literature have shown that stone characteristics affect the success rate of PCNL such as stone density and area (9-12).
Our study shows that the stone burden, the degree of hydronephrosis, the number of involved calyces as well as parenchymal thickness are meaningful features in the preoperative prognosis of PCNL. In our study, the average PCNL operation time was 67.5±32.8 minutes, the shortest was 20 minutes, and the longest was 180 minutes. Short operation time reduces complications of surgery and anesthesia as well as patient charge and helps patients recover more quickly.
Regarding the stone burden, currently, there is no consensus on the assessment of the stone burden. In our study, we applied the formula of Tiselius, similar to some other authors (6). Turna et al. studied 234 patients with PCNL and also applied the same stone burden calculation method, resulting in the majority of stones with an area <500mm2 and from 500-1000mm2 (13). Our research shows that stones mainly have an area of 400-799 mm2 (42.3%), stones with an area of <400 mm2 accounting for 29.6%, and 799-1599 mm2 accounting for 19.7%. Stones ≥ 1600 mm2 account for 8.5%. The average stone surface area in our study was 755.4 ± 630.4. Turna et al.’s study also showed that the mean PCNL operative time as well as the need for multiple tracts rose with increasing stone burden (p<0.05), therefore increasing the surgery complication rate. Our study results also showed that the difference in the average PCNL time between different stone area groups was statistically significant with p=0.003, similar to Kumar et al.'s study (3).
The degree of hydronephrosis is also a significant preoperative prognostic feature. Shaker et al.'s study showed that mild hydronephrosis accounted for 17%, moderate hydronephrosis accounted for 63%, and severe hydronephrosis accounted for 20% (10). Our study also showed that mild and moderate hydronephrosis accounts for the higher rate and there was a strong correlation between the degree of hydronephrosis and PCNL time with p < 0.05. In general, the more dilated the renal pelvis system is, the more favorable it is for the calyces puncture process. However, when the kidneys are too dilated, the stones will not be fixed well, making it difficult to remove stone fragments. Furthermore, when the kidney is too dilated and the renal parenchyma is too thin, Amplazt can easily slip off and affect the surgical process.
In our study, the average kidney parenchymal thickness was 17.8±5.0mm. The results of Tepeler et al. 's study also showed similar results with an average parenchymal thickness of 17.33±5.32mm (14). The average operation time for the kidney parenchymal thickness group ≥18mm and <18mm is 57.18±23.7 minutes and 80.0±38.12 minutes, respectively. The average PCNL time is related to the renal parenchymal thickness with p<0.05. Research by Karalar et al. (2016) with 120 PCNL patients showed that the average surgical time for the group with kidney parenchymal thickness >14.5mm and kidney parenchymal thickness <14.5mm was 117.6 ± 6.2 minutes and 125.7±7.1 minutes, p>0.05 (11). Stone length and density have no prognostic significance. Karalar et al.'s study divided into two groups with density <1250HU (67 patients) and group >1250HU (53 patients) (11). The difference in the average operation time between these two groups is not statistically significant with p>0.05. Anastasiadis et al. studied PCNL patients divided into two groups, stone density <1000HU (n=562), stone density >1000HU (n= 449), the average surgery time was 88.6 ± 41.6 minutes and 87.4 ± 49.5 minutes (p = 0.565) (9). Our study shows that the average operation time between groups with stone density <950HU and ≥950HU are 58.7±28.1 minutes and 68.7±32.8 minutes, respectively, p=0.4 > 0.05. Our research results are similar to the studies of the authors mentioned above. It is also shown that the average surgical time of the studies in the group with high density >1000HU is higher than the group with low density <1000HU.
A number of scoring systems have been proposed for the purpose of preoperative stone diagnosis such as the Guy stone score (2011) (12). De la Rosette et al. (2011) classified kidney stones based on stone load and found a significant correlation between stone load and surgery time (2). Smith et al. (2013) introduced the CROES kidney stone measurement chart based on a multivariate analysis of 2806 cases of PCNL at 96 centers around the world (15). The S.T.O.N.E scoring system reported by Okhunov Z et al. (2013) is based on the 5 most important characteristics of kidney stones related to PCNL results, including size (S), tract length (T), obstruction (O), number (N) and density of stones (E) (4). This scale is more appropriate than the Guy scale because it includes stone area, which is the main predictor of PCNL success rate.
Some retrospective studies and prospective have used the S.T.O.N.E score to predict the success rate of PCNL. Labadie et al. (2015) in a retrospective comparative study showed that the S.T.O.N.E score had a strong correlation with the stone-free rate (p = 0.004; AUC 0.67) (16). The stone-free rate in our study was 78.9% (Table 2), lower than the study results of Kumar et al. (2018) (86.3%) (3). Our research also shows a close correlation between S.T.O.N.E score and operation time as well as stone clearance rate (Figure 1 and 2). Thus, using a score to evaluate the complexity of stones before surgery is very useful for predicting the preoperating success rate and complication rate. The best scoring scale needs to be easy to apply, easy to replicate across centers and should correlate with procedural success and complication rates. The S.T.O.N.E score meets this criterion even though it is based solely on CT scans, which are expensive and associated with higher radiation exposure.
Figure 1. Correlation between S.T.O.N.E score and the operation time of PCNL.
Figure 2. Correlation between S.T.O.N.E score and Stone clearance rate (SCR).
The limitations of the study are that the sample size is small and it is a single-center study. Therefore, in the future, it is necessary to conduct large-scale multicenter prospective studies to more clearly determine the role of this scale in predicting the success rate of PCNL
6. CONCLUSION
Our study on 71 patients showed that the evaluation of the stone area, the degree of hydronephrosis, the number of involved calyces, the renal parenchyma thickness, and especially the S.T.O.N.E score played an important role in the prediction of the effectiveness of PCNL
Ethical statement:
The study was approved by an institutional ethics committee (Ref: 4907/QD-DHYHN dated on 19 October 2023). Written informed consent was obtained from all patients.
Declaration of consent:
None
Author's contribution:
Each author was included in all phases of preparation of this article. Final proofreading was made by the first and last author (T.Q.H and H.D.A) before printing.
Conflict of interest:
The authors declare no conflict of interests.
Financial support and sponsorship:
The authors received no specific funding for the work described in this article.
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