Calculation of Stone Volume and Urinary Stone Staging with Computer Assistance

Stone volume is one of the most important factors in treatment outcome of upper tract urinary stones and should be taken into account as exactly as possible. Therefore, we took 50 urinary stones from an inhouse stone bank and measured each stone's longest axis, the sum of the longest axes in branched stones, and the total area with an image analyzer. Stone volume was determined on the basis of displacement of water. The plot of stone area against stone volume had the least scattering. Using a nonlinear least-squares computer program, the equation volume = 0.6 (area)^1.27 was found to yield the best fit with a coefficient of determination (R²) of 0.97. A computer program was written in BASIC to measure the stone area from a roentgenogram using a sonic digitizing stylus. The program calculates the stone volume and integrates factors similar to those of the first version of the stone staging system PICA (J Urol 1987; 138:253). We suggest that such a computer-assisted staging system be incorporated into studies evaluating treatment of upper tract calculi to achieve better coherence in design and data analysis.