Introduction

The goal of treatment selection in renal stones is to achieve the highest success rate with minimal complications and morbidity. Treatment options for kidney stones include extracorporeal shock wave lithotripsy (ESWL), ureterorenoscopy, percutaneous nephrolithotomy (PNL), and open or laparoscopic surgery. The main parameter in the selection of renal stone treatment is the stone size. In European and American urology guidelines, PNL is recommended as the first choice for the removal of kidney stones larger than 2 cm (1). PNL is almost completely replaced by open surgery for treating kidney stones bigger than 2 cm due to its advantages of being minimally invasive, cost-effective, short operative time, short hospital stay, lower complication rate, and high success rate. Although PNL is more invasive than other minimally invasive methods currently available, the trend in kidney stone surgery with advanced, more sophisticated equipment thanks to advancing technology is to use PNL more and more. In a recent meta-analysis by Chung et al. (2), the highest success and stone-free rate was achieved with PNL in kidney stones larger than 2 cm compared to other treatment modalities. Failure can lead to complications, increased additional treatment, and economic burden for the patients and the health system. Therefore, reliable prognostic parameters are needed to optimize the choice of treatment in patients, to plan the operation, to provide information about the operation and its results, and to evaluate the findings. In the current literature, the stone-free rate in PNL varies between 71% and 100% (3). This wide range in the success rates is due to the evaluation of stones of different sizes in studies and the different parameters and methods used in the assessment of success. Many factors such as stone size, density, complexity, anatomical variations, patient-related factors [age, body mass index (BMI), and comorbidities], as well as parameters such as entry point, access number, duration of operation, complication rates, and surgeon experience, are the main factors investigated regarding the success rates (4-6). Plain radiography, nephrography, or non-contrast abdominal computed tomography (NCCT) have been reported as diagnostic tools for detect stone -free rate after PNL in the current literature. In our study, the success rates following PNL were evaluated by CT.

In this retrospective study, we evaluated the stone-free rates and outcomes of patients who underwent PNL treatment in our clinic. We also analyzed factors related to the patient, stone, surgery, and several factors that may influence the success of PNL, with the aim of demonstrating which factors have a significant impact on the success rate. Identification and evaluation of these prognostic factors may play a prominent role in increasing the success of PNL.

Materials and Methods

The data of 508 patients who underwent PNL over a total of 5 years at our institute were analyzed retrospectively in terms of success rate. The demographic characteristics of the patients, accompanying comorbidities, stone characteristics, kidney characteristics, information on operation and hospitalization are shown in Table 1. Of the patients, 300 (59%) were male and 208 (41%) were female. The mean age was 42.89±16.57 years. In our study, various factors affecting the success rates of PNL operations were analyzed. These factors included patient’s demographics, comorbidities, kidney features, stone characteristics, and operative information. The patients were evaluated with complete blood count, serum urea, creatinine, bleeding and coagulation profile, complete urinalysis, and urine culture before the operation. Detailed information about the operation was given to the patients, and informed consent forms were obtained from all patients. The patients were evaluated preoperatively with intravenous pyelography or non-contrast whole abdominal spiral tomography. The size of the stone is calculated by multiplying the maximum diameter by the diameter of the stone cut vertically with the help of a ruler in mm². Complex renal stones are defined as calculi greater than 3.0 cm in diameter with multiple renal stones, staghorn stones, abnormally positioned renal calculi, or horseshoe kidney calculi in a single kidney.

All patients were evaluated with NCCT scan for residual stones in the postoperative period. Post-PNL follow-up controls were performed at the 1^st, 3^rd, and 6^th months. In the evaluation of success, the control results obtained in the first month, which is the period without any additional treatment, were taken as criteria. According to the postoperative period CT scan results, it was classified as “stone-free (SF)”, “Clinically insignificant residual fragments (CIRFs)” and “failed (the presence of residual stones)”. Asymptomatic, less than 4 mm, non-obstructing, and non-infected stones were evaluated as CIRF (7). Patients with SF and CIRF were considered successful cases.

Surgical Technique

All the operations were performed in the prone position. The pelvicalyceal system was visualized by administering radio-opaque material through the ureteral catheter under C-arm fluoroscopy. Under fluoroscopy, the relevant calyx was entered using 18 Gouge diamond-tipped percutaneous access needles (18G Percutaneous Access Needle, Boston Scientific). The tract was dilated through the guidewire using an 8F Amplatz dilator, 8F Teflon catheter, and 18F, 22F, and 30F Amplatz dilators over an 8F Teflon catheter, respectively. The access sheath (20-30F) (Amplatz sheat, Boston Scientific) was advanced up to the kidney and entered the collecting system. Under saline irrigation, the pelvicalyceal system was observed through the access sheath with a 26-F nephroscope. Stones detected in the pelvicalyceal system were fragmented with a pneumatic lithotripter (Vibrolith, Elmed).

Statistical Analysis

The analysis of the data was performed using the “SPSS for Windows 11.5” package program (185). Descriptive statistics were expressed as mean ± standard deviation or median (minimum-maximum) for continuous and discrete variables, and categorical variables as number of cases and percentile. The significance of the difference between the groups in terms of means was analyzed with the “Student’s t-test” and the significance of the difference in terms of median values ​​was investigated with the “Mann-Whitney U test”. Categorical variables were evaluated with Pearson’s “chi-square” or Fisher’s “Exact Probability test”. The significance of the linear relationship between continuous variables was analyzed by Spearman’s “Correlation test”. The combined effects of risk factors that had a significant effect on success and complications because of univariate statistical analyses or that were thought to have a significant effect in multivariate analyses were investigated with “Multiple Retroactively Eliminated Logistic Regression analysis”. Odds ratio, 95% confidence interval, Wald statistics, and significance levels for each risk factor were calculated. Similarly, Multiple Linear Regression analysis was performed to determine the risk factors that had the most significant effect on the operation and dilatation times. The regression coefficient, standardized regression coefficient, 95% confidence interval, and significance levels for each risk factor were calculated. Variables found to be p<0.25 because of univariate analyses were included in the multiple regression models as candidate risk factors. Because the operation and dilatation times were not normally distributed, logarithmic transformation values ​​were used in multiple linear regression analyses. The results were considered statistically significant for p<0.05.

Results

The mean stone burden was 9.78±13.5 cm² (4-78). Sixty-one percent of the stones were within the range of 3-15 cm². While simple stones constituted 53,15% (n=270) of the cases, the rate of complex stones was 46,85% (n=238). Mean operation time, fluoroscopy time, and length of hospital stay were respectively 102.25±38.64 minutes (28-270), 9.53±6.1 minutes (3-15), and 3.43±1.11 days (1-8). In the first month-evaluation, success was achieved in 427 of 508 patients who underwent PNL. The success rate was calculated as 84%. After additional treatments (58 ESWL, 4 repeat PNL, 11 ureteroscopy) were applied to 73 of 81 patients who were evaluated as a failure, the success rate increased to 86% in the postoperative third month and to 88% in the sixth month.

In the evaluation of first-month success rates, no difference was observed between the success and failure groups in terms of age, gender, socioeconomic status, and family history of stone disease. Furthermore, morbidities such as high BMI, hypertension, diabetes mellitus, hyperlipidemia, and heart disease had no effect on success. It was observed that the success rate decreased statistically in cases with accompanying macroscopic hematuria. The stone-free rate found in patients with a solitary kidney or congenital renal anomaly was statistically similar to patients without these pathologies. The history of previous kidney surgery and ESWL had no impact on the success rate. The success rate was found to be similar in patients with various degrees of hydronephrosis (Table 2).

While the success rate was similar for stones with a stone volume of less than 3 cm² and those between 3-15 cm², it was observed that the success rate decreased statistically significantly in stones larger than 15 cm². Although the success rate was 90.3% for simple stones, it was 76.9% for complex stones. The success rate between simple and complex stones was statistically significant (p=0.038). The lowest success rate among complex stones was observed in complete staghorn stones. Although the calyx entry point did not have a significant effect on success, it was noted that the success rate decreased significantly in stones requiring multiple attempts for entry.

It was also seen that the structure of the stone had an impact on the success. The lowest success rate was observed in cystine stones (59.3%) (p=0.010). The success rate was also found to be lower in infection stones compared with other types of stones (62.2%) (p=0.014). It was observed that intraoperative complications and long operation time significantly affected the success rate (p=0.022, p=0.030). The success rate was 74.8% in our first 254 cases, and it reached 93% in the last 254 cases. It was found that increased surgical experience was a factor that had a statistically significant effect on success (p=0.036). Combined effects of risk factors were also evaluated. According to the results of the retrospective logistic regression analysis, surgeon’s experience, stone structure, complexity of the stone, and stone size were found to be independent factors affecting the success rate.

Complications associated with PNL were classified as major and minor complications. Major complications developed in 8.3% of the patients who underwent PNL operations, whereas minor complications occurred in 21.3%. As a major complication, bleeding requiring transfusion was encountered and 5.5% (n=28) of the cases. Three patients required conversion to open surgery due to bleeding and four patients due to access failure. Collecting system damage occurred in three patients, which required Double J insertion as an additional treatment. Hemothorax occurred in two patients which were managed with thoracal drainage. An arteriovenous fistula was observed in two patients and treated with superselective embolization.

Discussion

PNL provides the highest stone-free rate for treating kidney stones and with the effect of technological devices developed recently, has become the treatment of choice for complex and large kidney stones (3). The main goal of PNL is to achieve stone-free status with minimal morbidity with a high success rate. In a recent meta-analysis, PNL in the surgical treatment of kidney stones was shown to have the highest success and stone-free rate compared with other treatment options such as ESWL and retrograde intrarenal surgery (2). PNL has a near 100% success rate for treating non-staghorn stones, and up to 85% in staghorn stones (8). In our study, a total success rate of 84% was observed at 1-month follow-up in 508 patients undergoing PNL. In the current literature, different imaging techniques and success criteria’s have been defined in the assessment of PNL success (9). In our study, success rates were assessed using NCCT because commonly used imaging modalities, such as plain abdominal radiography (KUB film) or ultrasonography were reported as deficient for the diagnosis of small residual fragments (10). The question of whether small stones that are not clinically important (i.e., do not cause obstruction, pain, and infection in the urinary system) overshadow the success of the operation has been enquired and the concept of clinically insignificant residual fragments (CIRFs) has been put forward. In this concept, the stone size was limited to 4 mm, and it was concluded that stones below this size had no effect on the success of the operation as it has been determined that 85% of stones of this size will pass without causing clinically symptomatic pain (11). We also evaluated stones below 4 mm as a successful group. Many factors shown in the literature to influence the success of PNL have been evaluated in various studies (5). We evaluated the impact of several factors on the success rate of PNL. A prospective study by Olbert et al. (12) showed that age and gender had no effect on success rates. We also found that age, gender, socioeconomic status, and family history has got no effect on the PNL success. No studies have investigated the impact of socioeconomic status and family history on PNL outcomes. In our study, similar to demographic characteristics, comorbidities such as hypertension, diabetes, hyperlipidemia, KVH, and BMI did not significantly affect surgical outcomes. Alyami et al. (13) found no difference on stone -free rate based on BMI. In a study of 430 cases conducted by Tefekli et al. (14), patients with metabolic syndrome and its components such as hypertension, diabetes, hyperlipidemia, and obesity were evaluated, and similar success rates were reported. We also did not demonstrate a significant effect of higher serum urea, creatinine levels, or presence of a solitary kidney and congenital renal anomaly on success rates. The global percutaneous nephrolithotomy study group showed that renal congenital abnormalities do not significantly impact treatment outcomes (15). It has also been also shown that PNL can be performed with similar success and complication rates in patients with solitary kidneys (16). In our study, we observed that previous surgery and a history of ESWL had no significant effect on the success rate. Patients with a history of open stone surgery reported as have a lower chance of stone-free rate in the current literature (4). Basiri et al. (17) stated that an open surgical history on the same side did not affect the success or complication rates in patients treated with PNL. Yuruk et al. (18) reported that similar success and complication rates could be achieved with PNL after failed ESWL, but it was usually more difficult with a prolonged operative time and fluoroscopic screening time. In our study, the success rate was significantly lower in cases with accompanying macroscopic hematuria. There was no statistically significant difference in success rates between patients with a higher degree of hydronephrosis and those with a lower degree of hydronephrosis. In hydronephrotic systems with stones, the entrance is easy, but the rate of failure increases because the stones can escape to other calyces (19). We found that stone size, complexity, and structure were the most important predictors of stone-free rate after PNL. Complex stones accounted for almost half (46.85%) in our cases. Our results are consistent with those of Knox et al. (20), who revealed that stone sizeand number, are associated with reduced stone clearance. Bagrodia et al. (21) reported that stone size was the only independent predictor of treatment success. Yamasaki (22) reported that the size of the stone, the location, and the presence of multiple stones were prognostic factors for stone recurrence. Notably, in contrast to these observations, the European Association of Urology guidelines on urolithiasis discuss that the treatment success of PCNL is hardly affected by stone size (23). In our study, it was determined that the success rate in cysteine and infection stones was lower than other stone compositions. A reason for the low success rate of infected stones may be that these tend to be large and even coralliform. The best of our knowledge, there is no study evaluating the impact of stone composition on the success of PNL in the current literature. The success rate of multifocal access is expected to be lower as the stones-requiring multifocal intervention are larger and more complex. In this study, a higher number of accesses was performed in the failed group, confirming that a higher number of accesses was associated with less favorable outcomes. We demonstrated that higher intraoperative complication rates, prolonged dilation, and operative time reduced the success rate of PNL. The experience of the surgeon and learning curve are also important factors affecting the overall surgical success. In our series, the first 204 cases had a success rate of 73.8, whereas the latter 204 cases had a success rate of 93%. In line with our results, Tanriverdi et al. (23) reported a success rate of 73% for the first 60 cases and 85% after 60 cases. Although PNL is a significant treatment option with a high success rate, it should not be forgotten that serious complications can occur, even to the point of being life-threatening. The rate of major complications such as urosepsis, bleeding necessitating intervention, pleural injury, and colonic injury was reported to be between 0.2% and 4.7% fin a large study with 5803 cases (24). In our study, the rate of major complications was 8.3%. Bleeding was the most common complication. The rate of renal hemorrhage requiring transfusion and 5.5%. Transfusion rates ranging up to 20% have been documented with an overall of 7% demonstrated in a systematic review (25).

Study Limitations

Our study has some limitations that need to be considered while evaluating its findings. First, it is a retrospective study that can be affected by all potential weaknesses stemming from its retrospective design. Second, it has a relatively low number of cases and short evaluation time of success compared to clinical trials with high number of cases in the current literature.

Conclusion

PNL surgery is an effective and minimally invasive method that requires surgical experience and can be safely applied to stone treatment with adequate equipment and experience, at least as successfully as open surgery. After reviewing our experience with PNL, we showed several clinical and stone factors related to the success rate after PNL. Stone size, complexity, and composition, surgeon’s experience were the independent predictors of stone-free status after PNL.

Ethics

Ethics Committee Approval: The study protocol was approved by Health Sciences University Dışkapı Yıldırım Beyazıt Training and Research Hospital Clinical Research Ethics Committee (project no: 24/07, date: 18.04.2011).

Informed Consent: Informed consent forms were obtained from all patients.

Peer-review: Externally and internally peer-reviewed.

Authorship Contributions

Surgical and Medical Practices: S.Y., M.T., F.D., Concept: S.Y., M.T., F.D., Design: S.Y., M.T., F.D., Data Collection or Processing: S.Y., M.T., F.D., Analysis or Interpretation: S.Y., M.T., F.D., Literature Search: S.Y., M.T., F.D., Writing: S.Y., M.T., F.D.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declare that they have no relevant financial.