STONE Score Calculator (S.T.O.N.E. nephrolithometry for PCNL)

Understanding the S.T.O.N.E. score calculator — concept and principle

The S.T.O.N.E. nephrolithometry score grades how complex a kidney stone is for percutaneous nephrolithotomy (PCNL), keyhole surgery through the back. Urologists use it to predict the chance of being stone-free after one operation and to counsel patients.

How it is calculated

Score (5–13) = S + T + O + N + E

• S, Size: stone area = length × width (mm²); 0–399 = 1, 400–799 = 2, 800–1,599 = 3, ≥1,600 = 4.
• T, Tract length (skin to stone): ≤100 mm = 1, >100 mm = 2.
• O, Obstruction: none or mild hydronephrosis = 1, moderate or severe = 2.
• N, Number of calyces involved: 1 = 1, 2–3 = 2, full staghorn = 3.
• E, Essence (density): <950 HU = 1, ≥950 HU = 2.

Groups used in validation: low 5–6, moderate 7–8, high 9–13.

The principle

Okhunov and colleagues (2013) chose features from a non-contrast CT that make PCNL harder: larger stones need more fragmentation, a long tract is harder to work through, a dilated system changes access, stones in several calyces need more puncture angles, and dense stones break less easily. Each adds points, and higher totals predict a lower stone-free rate.

Worked example

Default inputs: stone 25 × 20 mm, tract 90 mm, no or mild obstruction, 1 calyx, 1,000 HU.

1. Area = 25 × 20 = 500 mm² → size 2 points.
2. Tract 90 mm → 1 point.
3. Obstruction 1 + calyces 1 + density (1,000 ≥ 950) 2.
4. Total = 2 + 1 + 1 + 1 + 2 = 7 → Moderate (7–8).

How to read the result

Lower scores predict a higher chance of being stone-free after one PCNL; higher scores mean a greater chance of needing extra procedures. The score supports counselling and planning; the operative decision considers the patient's anatomy, health and preferences.

Common mistakes

• Using an ultrasound or plain X-ray instead of CT measurements.
• Measuring tract length at one angle only instead of averaging the planned approaches.
• Using it for stones treated by other methods such as shock wave lithotripsy.

Frequently asked questions

What is a staghorn stone?

A large stone that branches into several calyces, shaped like a deer's antler.

What does Hounsfield unit mean?

A CT measure of density; harder stones usually have higher values.

Is there another score?

Yes, the Guy's stone score is also widely used for PCNL complexity.

Sources: Okhunov Z, Friedlander JI, George AK, et al., "S.T.O.N.E. nephrolithometry: novel surgical classification system for kidney calculi", Urology 2013;81(6):1154–1159