The authors found that the 7.4F flexible ureteroscopes could successfully access the ureter, without need for ureteral dilation, in 99% of cases, while the 8.5F endoscopes were successful in 90-95% of cases, and the 9F endoscopes were successful in only 63% of procedures. It needs to be stated that these are the results obtained in "expert" hands, and that for the rest of us mere mortals, these percentages would likely be proportionately less. Bottom line is that flexible ureteroscopes in the 7-8F range are likely the size of the normal ureteral orifice and tunnel and can enter those structures without the need for dilation in almost all cases; the only caveat is whether the tradeoff of failing access in an additional 5-9% might be worth the possibly increased durability obtained in using an 8F+ flexible ureteroscope. Unfortunately, an analysis of endoscope durability, based on tip and shaft size, has yet to be completed.

BJU Int. 2005 May;95(7):1043-4