Reliability and validity of “S.T.O.N.E” nephrolithometry scoring system to predict the stone-free rate after percutaneous nephrolithotomy

OBJECTIVE: The objective was to assess the reliability and validity of “S.T.O.N.E” nephrolithometry scoring system to predict the stone-free rate (SFR) after percutaneous nephrolithotomy (PNL). METHODS: A total of 123 patients with unilateral radiopaque stones ≥2 cm were included in the study. According to S.T.O.N.E score, five parameters available from preoperative computed tomography (CT) without contrast were measured: stone size (S), tract length (T), obstruction (O), number of involved calices (N), and essence of stone (E). The Stone free rates evaluated within one month postoperatively by plain X-ray and/or CT scan without contrast. RESULTS: The mean S.T.O.N.E. score in this study was 7.4 in stone-free (SF) group and 9.3 in residual stone group (P = 0.0001). Patients with SF comprised 82.1% after the first PNL, whereas 17.9% had significant residual stones >4 mm. Postoperative complications were 8%. The most common complications were bleeding requiring transfusion. The size of stone (P = 0.002) and number of calices involved (P = 0.001) had a statistically significant difference between patients with residual stones, other components were not. There was a statistically significant difference between non-SF and SF according to the hospital stay (P = 0.002). CONCLUSION: This score predicted the clearance after PNL. The size of calculi and number of calices involved statistically affected the stone clearance, whereas other S.T.O.N.E scoring parameters were not. There was a statistically significant difference between SF and residual stones groups according to the hospital stay (P = 0.0001).