This STONE nephrolithometry for kidney stones calculator evaluates the chance of renal calculi free outcome after first procedure based on x-ray findings. The text below the form provides more information on kidney stone recurrence.


Stone size (mm2)


Tract length (mm)


Obstruction (Hydronephrosis)


Number of calyces with calculi


Stone density in HU

How does this STONE nephrolithometry for kidney stones calculator work?

This health tool estimates the recurrence of kidney stones based on clinical radiologic evidence. The five parameters evaluated in the STONE nephrolithometry for kidney stones calculator correspond to the STONE mnemonic:

Stone size (mm2) – meaning the length multiplied by the width in mm of the lesions.

Tract length (mm) – indicating the distance from stone center to skin, measured on CT film at 0°, 45°, and 90°.

Obstruction – also known as hydronephrosis.

Number of calyces with calculi – aggregations in the minor or major calyx.

Essence – meaning stone density in Hounsflied units.

The above are five reproducible variables which can be easily analyzed during preoperative and postoperative noncontrast-enhanced computed tomography.

More recent studies have revealed an increase in kidney stone recurrence and several tools are under study in order to devise new methods of prevention and risk stratification. One of them is the 2014 study published in the Journal of the American Society of Nephrology – ROKS.

Some of the risk factors for recurrent calculi include:

■ A high protein, low fibre diet;

■ Sedentary lifestyle or bed bound;

■ Family history of kidney stones;

Unilateral renal failure;

■ Personal history of urinary infections and kidney stones;

■ Personal history of kidney stones before 25;

■ Personal history of small intestine disease, inflammation or surgery;

■ Under treatment with aspirin, diuretics, antiretroviral therapy, anti acids, calcium supplements.

STONE result interpretation

The STONE observations have been found to have high accuracy in predicting treatment outcome, especially after percutaneous nephrolithotomy, and risk of perioperative complications.

The following table presents the probability of kidney stone recurrence after first successful procedure in correlation with the STONE result:

STONE result Probability of stone free outcome after first procedure
5 94%
6 92%
7 88%
8 83%
9 64%
10 42%
≥11 27%

Percutaneous nephrolithotomy guidelines

This is the medical procedure in which kidney stones are removed by keyhole surgery. PCNL is performed when stones larger than 1.5 cm are localized in the lower pole of the kidney or in the ureter, if the stones have a staghorn shape or if the patient prefers this type of intervention to the other available.

In patients with no other risk factors, the intervention is quite simple and is performed under general anaethesia.

A fine bore tube is passed with the help of a cystoscope through the bladder then the skin over the kidney area in the back is monitored using ultrasound or x-ray to check the location of the stone.

A wire is passed in that location into the kidney and a 1 cm cut allows a rigid tube to be placed. The nephroscope is used to visualize the stone and help the process or extraction. Bigger stones need to be fragmented before extraction and sometimes, several incisions may be necessary.

Post procedure care involves x-rays to check for any residual stone fragments and possibly an x-ray dye test. Recover lasts between 4 to 6 hospital days.

Post procedure complications, beside the possibility of remaining calculi fragments include massive hemorrhage in need for blood transfusion, embolisation of kidney arteries, subsequent damage to spleen or liver and heart conditions.


1) Okhunov Z, Friedlander JI, George AK, Duty BD, Moreira DM, Srinivasan AK, Hillelsohn J, Smith AD, Okeke Z. (2013) S.T.O.N.E. nephrolithometry: novel surgical classification system for kidney calculi. Urology; 81(6):1154-9.

2) Akhavein A, Henriksen C, Syed J, Bird VG. (2015) Prediction of single procedure success rate using S.T.O.N.E. nephrolithometry surgical classification system with strict criteria for surgical outcome. Urology; 85(1):69-73.

3) Kuzgunbay B, Gul U, Turunc T, Egilmez T, Ozkardes H, Yaycioglu O. (2010) Long-term renal function and stone recurrence after percutaneous nephrolithotomy in patients with renal insufficiency. J Endourol; 24(2):305-8.

22 Feb, 2016