OBJECTIVE: To determine the optimum size of a flexible ureteroscope (FU) to minimize the need for ureteric dilatation. MATERIALS AND METHODS: FUs from several different manufacturers and with different shaft diameters were used for ureteroscopy in adults. An initial attempt was made to pass one of the larger diameter FUs; if this was unsuccessful, the smaller 7.5 F FU was used. The results were reviewed retrospectively. RESULTS: Flexible ureteroscopy was used in 115 consecutive patients at two different institutions. The ability to pass the FUs with no formal dilatation was directly related to the FU diameter. The rate of failing to pass with no formal dilatation was 37% for the 9.0 F, 8.5% for the 8.6 F, 5% for the 8.4 F and 0.9% for the 7.4 F ureteroscope. CONCLUSION: Although other factors influence the overall effectiveness of the FU, the ability to pass FUs is directly related to their outer diameter. When only the ease of introduction is considered, the ideal ureteroscope outer diameter is 7.4 F.
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