Dear Editor:
We have read the article written by Zorba and colleagues that is about CT-based determination of ureteral stone volume (SV) and its predictive effect on spontaneous passage (SP) with great interest. For the first time the authors evaluated the factors effective on SP of ureteral stones less than 1 cm in diameter using also SV as a predictive parameter and detected SV as the only independent predictor affecting stone passage based on multivariant analyses.
1
While explaining their rationale of advocating SV as a predictive factor, they have claimed that increased contact area between the ureteral stone and mucosa that increases friction force between the stone and the mucosa might be a causative factor. Contrary to their assertions, friction force between two objects is dependent on friction coefficient and the reaction force of the surface that directly affects the object, and it is independent of the areas of rubbing surfaces.
2
Perhaps increased SV and hence its weight also enhance reaction force of the surface together with friction force, which eventually may decrease the chance of SP.
Besides, the investigators did not include the patients who had experienced spontaneous stone passage from ipsilateral ureter, shockwave lithotripsy, ureteroscopy, and percutaneous nephrolithotomy because of the probability of formation of strictures and inflammation. However, various studies have demonstrated that stone passage is complicated by in situ ureteral edema and inflammation.
3
Based on the preference of the authors, these factors were not evaluated. In contrast, stone location (upper or lower ureter), which has been demonstrated to be an independent predictive factor, was not presented as a predictor based on the outcomes of this study.
3,4
We think that for a more accurate evaluation, these patients should also be included in the study.
