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Evaluation of preoperative measurement of stone surface area as a predictor of stone-free status after combined ureteroscopy with holmium laser lithotripsy: a single-center experience.

PURPOSE
To investigate the utility and limitations of stone surface area (SA) as a predictor of stone-free (SF) status after a single semirigid ureteroscopy (URS), with or without a flexible component, for the treatment of patients with urinary stones.


PATIENTS AND METHODS
Cases of patients with urinary stones treated by combined URS with holmium laser lithotripsy at a single institute were retrospectively evaluated. Correlations of possible predictors with SF status were analyzed using a logistic regression model. Two types of SA were measured: "Traced stone surface area" (tSA) and "calculated stone surface area" (cSA).


RESULTS
According to the univariate analysis, the following variables were significantly associated with non-SF status: Stone number (P<0.001), ureteral stone location (P=0.045), presence of renal stones (P<0.001), tSA (P<0.001), cSA (P<0.001), stone volume (P<0.001), and operator experience (P=0.02). According to multivariate analysis, stone volume (P=0.016) was an independent predictor of SF status. The scatter diagrams for tSA and cSA showed strong correlations between these parameters, and Spearman ρ was 0.975.


CONCLUSIONS
Stone volume and SA were highly indicative of stone status after single semirigid URS, with or without a flexible component. The formula for cSA, maximum diameter×width×π×1/4, was demonstrated to accurately represent SA in this study. SA, however, indicated a lower clinical priority and utility as a predictor of stone status than stone volume. The combination of semirigid and flexible URS could access any ureteral stones, including those that semirigid URS alone could not treat. The cutoff points for these predictors of outcome were 110.0 mm(2) for cSA, 125.0 mm(2) for tSA, and 840.0 mm(3) for stone volume.