Abstract

To compare the Guy’s, S.T.O.N.E, and CROES nomograms for predicting stone-free status in patients who underwent percutaneous nephrolithotomy for renal stones.

The data of 1114 patients who underwent percutaneous nephrolithotomy for renal calculi between 11/2008 and 08/2018 in our clinic were retrospectively reviewed. Various parameters evaluated by preoperative computed tomography and the scoring systems of the patients and postoperative stone-free status were compared.

Out of 1000 patients who met the study criteria. Gender, body mass index, and stone density were not statistically different between the group with residual stones and the stone-free group. However, stone size, number of renal accesses, duration of fluoroscopy usage, duration of operation, number of stones, and complication rate were significantly higher in the group with residual stones than in the stone-free group. A statistically significant correlation was found between the postoperative stone-free rate and scoring systems. The applicability and preoperative prediction ability of all three systems were evaluated by receiver operating characteristic analysis. The area under the curve (AUC) was detected in the Guy’s, CROES, and S.T.O.N.E scoring system (AUC: 0.642, 0.665, 0.592 respectively).

In this study, where the perioperative and postoperative results of 1000 patients were evaluated, we found that all three scoring systems could predict the stone-free rate. “We believe that the use of these scoring systems before surgery can guide surgeons.”

What’s known on the subject? and What does the study add?

Some scoring systems are known to predict stone-free percutaneous nephrolithotomy. All three of the Guy’s stone score, S.T.O.N.E. and CROES nomograms predicted stone-free percutaneous nephrolithotomy.

Introduction

There are many invasive and non-invasive methods in the current treatment of urinary system stone disease. Because of advances in endoscopic approaches together with developing medical technology, open surgery is required in a very small proportion of urinary system stones. Today, minimally invasive treatments are mostly used in urinary system stones (1, 2). In the 1950s, with the use of percutaneous needle aspiration treatment and antegrade pyelography in kidneys with hydronephrosis, percutaneous interventions on the kidney became widespread (3, 4). Percutaneous nephrolithotomy (PNL) for the treatment of kidney stones was first described by Fernström and Johansson in 1976 (5).

Despite advances in surgical techniques and technology, there is an increase in the number of complications in parallel with the number of surgeries (6, 7). There have been ongoing studies for years on preoperative variables that can predict PNL success and complication rates. However, there is no preoperative variable that can be accepted as a standard yet (8-10).

Until recently, there were no useful scoring systems that could predict success and complication rates in PNL. Guy’s stone score (GSS), stone size, tract length, obstruction, number of involved calices, andessence/density (S.T.O.N.E), and the Clinical Research Office of the Endourological Society (CROES) nomograms have been recommended as preoperative assessment tools since 2011 (11-13). It has been shown in these studies that nomograms inform the surgeon about the stone-free rate and complication ratio. In addition, different studies have yielded results showing nomograms’ use in daily practice. We conducted this study to contribute to the literature because it is one of the studies with the largest patient population in the literature and is a tertiary reference hospital.

In this study, we aimed to investigate the predictive ability and superiority of GSS, S.T.O.N.E, and CROES nomograms in a retrospective extensive patient group.

Materials and Methods

This study was conducted after the Ethics Committee approval of University of Health Sciences Turkiye, İzmir Tepecik Education and Research Hospital (IRB no: 2019/9-13). Informed consent was obtained from all patients. A total of 1114 patients who underwent PNL in our clinic between November 2008 and August 2018 were screened. Patients younger than 18 years of age with a solitary kidney or horseshoe kidney anomaly and who underwent endoscopic combined retrograde intratrenal surgery (eCIRS) were excluded from the study. The remaining 991 patients were included in the study.

A detailed anamnesis was taken from all patients before surgery, and physical was examined. An informed consent form was obtained from the patients before the procedure. Complete blood count, biochemical tests (kidney and liver function tests and electrolyte levels), bleeding parameters, and urine culture were evaluated. The medications of the patients using anti-aggregant or anticoagulant were discontinued in consultation with the relevant units, and appropriate treatment (low molecular weight heparin) was initiated when necessary. All patients were evaluated with a kidney-ureter-bladder X-ray and non-contrast enhanced computed tomography (NCCT) preoperatively. These protocols were routinely applied to all stone patients. All stone protocol NCCT were evaluated according to the S.T.O.N.E, GSS, and CROES nomograms. In the GSS, the location of the stone, number of stones, presence of partial or complete staghorn stones, and presence of anomalies in the kidney anatomy were evaluated and scored between 1 and 4 (Figure 1). In the CROES nomogram, six parameters including stone burden, stone location, previous intervention history, presence of staghorn stone, number of stones, and number of annual cases were evaluated (Figure 2). In the S.T.O.N.E nomogram, five parameters including the size of the stone (size), the distance of the stone from the skin (tract of length), the presence of hydronephrosis (obstruction), the number of calices in which the stone is located (number of calices), and the density of the stone (essence) were evaluated (Figure 3).

All procedures were performed under general anesthesia. A 6F open-ended ureteral catheter was inserted into the ureter in the lithotomy position. The patients were then placed in the prone position on a table compatible with the C-arm. Using fluoroscopy, the appropriate calyx was accessed with the triangulation technique using an 18G percutaneous access needle. A single-step Amplatz dilation technique was used. A 26-F nephroscope (Karl Storz GmbH, Tuttlingen, Germany) was used through the 30F Amplatz sheath, and the stones were broken with the help of a pneumatic and/or ultrasonic lithotripter. The residual stone status was evaluated by perioperative fluoroscopy. After the operation, a nephrostomy tube was placed and the procedure was terminated.

Operation time, number of percutaneous accesses, fluoroscopy time, intraoperative and postoperative complications, duration of nephrostomy catheterization, and length of hospital stay were evaluated. Residual stone status was assessed by perioperative fluoroscopy and NCCT 1 month after surgery. After PNL, residual fragments of 4 mm that did not cause obstruction or infection were considered “stone-free”. Patients were evaluated according to three nomograms. Stone-free were defined as group 1, and residual stone was defined as group 2.

Statistical Analysis

The SPSS 15.0 package program (SPSS for Windows, 15.0, SPSS Inc., Chicago, Illinois, USA) was used for statistical analysis. Independent sample t-test, chi-square test, receiver operating characteristic (ROC) curve test, and correlation analysis were used to analyze the data. A p<0.05 was considered statistically significant.

Results

In our study, the patients’ mean age was 49+13.2 (standard) years. 59% of the patients were male. The mean body mass index (BMI) was 28.3 (23-38). Hypertension, hyperthyroidism, and diabetes mellitus were detected in 16%, 11%, and 4% patients, respectively.

Perioperative and postoperative data of the patients and postoperative stone-free status are shown in Table 1. While the mean stone size of the patients before the operation in the stone-free group was 265 mm², it was calculated as 563 mm² in the patients with residual stones. The stone size was found to be statistically higher in patients with residual stones (p<0.001). According to the number of stones, a statistically significant difference was found between the groups (p<0.001). The mean stone density was not statistically significant difference between groups (Table 1).

The overall stone-free rate of the patients was 81.6%. When stone-free rates were evaluated according to GSS grades, the stone-free rate was 89.6% in grade 1, 77.2% in grade 2, 72.5% in grade 3, and 71.4% in grade 4. While the stone-free rate was 84.8% in the group with S.T.O.N.E score of 5-7, it was 74.4% in the group with 8-10 and 60.9% in the group with 11-13. In the CROES scoring system, stone-free rates were 92.7% in the 276-340 score, 89.7% in the 211-275 score, 76.6% in the 146-210 score, and 71.3% in the 80-145 score (Table 2).

Scoring systems and the relative risk of residual calculi were evaluated as risk groups.

The GSS was determined as grade 1 low, grade 2 moderate, grade 3 high, and grade 4 very high-risk groups. With the S.T.O.N.E nephrolitometry score, 5-7 were determined as low risk, 8-10 as medium risk, and 11-13 as high risk. The CROES nephrolitometry score was also divided into four groups according to the risks. It was determined as low risk between 276 and 340, medium risk between 211-275, high risk approximately 146-210 and very high-risk group between 80 and 145 (Table 3). All Three scoring systems were found to be significant on logistic regression analysis. The odds ratio and confidence index are shown in Table 3.

All stone nomograms gave statistically significant results in predicting stone-free status. GSS p<0.001 [95% confidence interval (CI) 0.598-0.686, area under the curve (AUC): 0.642], S.T.O.N.E p<0.001 (95% CI 0.544-0.640, AUC: 0.592), CROES p<0.001 (95% CI 0.622-0.708, AUC: 0.665). A comparison of scoring systems for stone-free status is shown in Table 4. Simultaneously, the predictive capabilities of the scoring systems were compared by calculating the area under the curve with ROC analysis (Figure 4). The optimal cut-off score values, AUC, sensitivity, and specificity score are shown in Table 4.

Considering the complications, fever was observed in 73 patients, bleeding or urinary leakage not exceeding 12 h in 75 patients, pneumothorax requiring intervention in 5 patients, arteriovenous fistula in 4 patients, and colon injury in 18 patients. The overall complication rate was 17%. The complication rates according to Clavien classification are given in Table 1.

Discussion

Since its definition, PNL has become the first-line treatment option for large, complex, and staghorn stones. The main goal for treating kidney stones is to provide maximum benefit with minimal harm. Although there are many scoring systems predicting stone-free status after PNL, there is no standard method that is widely accepted.

Among these scoring systems, GSS, S.T.O.N.E, CROES, Seoul National University Renal Stone Complexity and acute angle, complicated calyx and stone size are frequently discussed in the literature. In recent years, the importance of systematic and standard reporting of results after various endourological surgeries, including PNL, has been emphasized (14-16). GSS, CROES, and S.T.O.N.E are the nomograms most frequently used recently.

Many factors have been defined that predict the stone-free rate after PNL, such as stone burden, stone type, number and location of the stone, HU, BMI, stone skin distance, and abnormal kidney anatomy. However, a single parameter that predicts stone-free status has not yet been identified. Therefore, nomograms in which many factors are combined have been developed by the authors (15-17). Numerous studies have attempted to validate these scoring systems. However, most studies are retrospective, and the stone size definition of residual stone, method, and timing of imaging after PNL are different. This has made it impossible for scoring systems to be accepted as a standard (18-25).

Although the parameters evaluated by the scoring systems are different from each other, all of them can help the surgeon in predicting stone-free status and operation-related complications. There are several important differences between the aforementioned scoring systems. While S.T.O.N.E is based entirely on data from preoperative NCCT, GSS and CROES also include patient variables. Because the definition of full staghorn or partial staghorn is not clear and subjective, its effects on scoring systems are suspicious. Some authors classify staghorn stones as follows: “borderline”, stones filling the pelvis and calyx; “partial”, stones filling the pelvis and extending into the two calyces; “complete”, stones filling ≥80% of the pelvicalyceal system, extending to the pelvis and all major calyces; and “huge”, stones filling the entire pelvicalyceal system accompanied by dilatation (26, 27). Since the number of calyx filled by the stone is one of the parameters in the S.T.O.N.E scoring system, it can evaluate the staghorn stone status more objectively than other systems.

Spina bifida or spinal injury history of the patient, stone burden, and calyceal anatomy were evaluated using GSS (15). Studies have reported that GSS is important for predicting stone-free status. In addition, the stone-free rate was found to be associated with an increase in GSS grade (GSS grade 1 81%, GSS grade 2 74.2%, GSS grade 3 35%, GSS grade 4 29%). However, no significant correlation was found between GSS and postoperative complications.

The GSS symptom score has some limitations. The use of different imaging methods other than CT in the evaluation of stone-free status can be considered as a weakness of the scoring system because the sensitivity of imaging methods in showing residual stones is not the same. In our study, all patients were evaluated with NCCT postoperatively, and GSS could again predict stone-free status significantly.

In the S.T.O.N.E. scoring system, 4-5 were defined as low, 6-8 as medium and 9-11 as high-risk groups, and it was shown that the success of the operation decreased as the S.T.O.N.E score increased. In the study, the S.T.O.N.E score was the best predictor of stone-free status, whereas the second most effective factor was found to be stone size. Complication rates were given in the study, and it was determined that only stone size was associated with complications. The small number of complications prevented detailed analysis of the complications (28). In our study, it was observed that the success rate decreased with the increase in the number of scores. In the S.T.O.N.E scoring system, calyx stones and pelvis stones affect the scoring system equally, with a score of “1”. However, the stone-free rates of pelvic and calyx stones are not the same in the literature (29).

In addition to stone parameters and patient characteristics, the experience of the clinic was evaluated using the CROES nomogram (30). When the stone volume calculation in the CROES study is used as maximum length maximum width x π x 0.25, it is valid for round and oval stones, but unfortunately it is not possible to give the same exact result for large, complex, staghorn stones. Similar to stone burden, HU is not applicable to large/staghorn stones. The reason for this is that the density is different in the periphery and center of the stone because of its lamellar structure.

Stone scoring systems are also used in solitary kidney stone disease. The authors concluded that stone burden was associated with SFR and complication rate. Moreover, the CROES score was the only independent factor associated with SFR status in their study. Also, this study has shown correlation with the literature (31).

Unlike the GSS, the CROES nomogram is difficult and time consuming for clinical applicability. However, in our study, it was shown that the AUC in CROES is better than the GSS and S.T.O.N.E scores (Table 4).

Our study was retrospective. In addition, evaluation of scoring systems by a single urologist and performing surgeries by different surgeons are limitations of our study. However, the evaluation of a larger patient population than that of many studies in the literature can be shown as the strength of the study.

Conclusion

It was determined in our study with a large patient population that the three scoring systems could predict stone-free rates significantly. Because of the weak correlation with the complication rates, a clear opinion on this issue could not be declared yet. However, it is possible to say that all three methods are suitable and usable for preoperative planning in PNL. For scoring systems to be widely used in daily practice, there is a need for large prospective studies and elimination of their weaknesses.