Kidney Stone Calculator for Percutaneous Nephrolithotomy

S.T.O.N.E. nephrolithometry is a structured system for describing renal stone complexity before percutaneous nephrolithotomy, often abbreviated PCNL. Its name represents five computed tomography findings: stone size, tract length, obstruction, number of involved calyces, and stone essence or density. Each feature receives points, and the total ranges from 5 to 13. Higher totals generally describe more complex anatomy and have been associated with lower stone-free rates and more demanding procedures. Stone burden is measured as area in square millimeters, commonly by multiplying the largest stone dimensions on CT. Skin-to-stone tract length is the distance in millimeters from the skin to the target stone along a plausible access path. Obstruction describes hydronephrosis, while the calyx item reflects how broadly the stones occupy the collecting system. Stone density is measured in Hounsfield units and provides a rough indication of hardness. Accurate imaging technique and consistent measurement are essential because a value near a category threshold can change the score. The score supports standardized communication, research, counseling, and procedural planning. It does not choose a treatment by itself. A urologist also considers stone location and shape, renal anatomy, infection, anticoagulation, kidney function, prior operations, body habitus, available equipment, and patient goals. A higher score does not mean PCNL cannot succeed, and a lower score does not guarantee complete clearance or freedom from complications. This calculator uses conventional S.T.O.N.E. categories: increasing burden contributes one to four points; tract length over 100 millimeters contributes an additional point; obstruction contributes one or two points; calyceal involvement contributes one to three points; and density over 950 HU contributes an additional point. The simplified numeric obstruction entry should be mapped from the formal radiology interpretation. Measurements must come from diagnostic imaging reviewed by a trained professional. This educational result is not a substitute for radiology review, specialist assessment, informed consent, or local surgical protocols. Fever, chills, severe flank pain, vomiting, or signs of urinary obstruction can require urgent clinical evaluation.