Response to: “Letter to the Editor, International Urology and Nephrology: in silico–in vitro–in vivo—can numerical simulations based on computational fluid dynamics (CFD) replace studies of the urinary tract?”

Abstract

We appreciate the Letter to the Editor by Kram and Buchbolz [1], which discusses the paper by Amitay-Rosen et al. [2]. The authors of the Letter summarize the nature of our computational fluid dynamics (CFD) study, and the results and insights gained from the simulations. The authors continue by reviewing the parameters and physical mechanisms from pathological responses that were, and were not, considered in our CFD analysis, as described within Amitay-Rosen et al. [2]. Kram and Buchbolz [1] then suggest that these pathological responses “...cannot be fully characterized with the simulation presented here.” In particular, Kram and Buchbolz [1] focus on the mechanisms of acute elevation of intrarenal pressure correlated to stent diameter, intrarenal reflux, peristalsis, and the temporal effects of extrinsic ureteral obstruction (EUO) as a function of its acute or chronic pathology. On this basis, Kram and Buchbolz [1] conclude that “CFD simulation is probably not suitable for describing the flow-oriented physiological conditions of the upper urinary tract. However, it may be suitable to simulate pressure-based closure mechanisms, i.e., in the urinary bladder”. We value these comments, as further discussion enables fuller consideration of the role of CFD simulations in the analysis and understanding of a wide variety of urologyrelated phenomena and pathologies. As noted by Kram and Buchbolz [1] in the title of their Letter, these phenomena can be studied with the aid of (i) in vivo, (ii) in vitro, and (iii) in silico (CFD) investigations. We agree completely. However, we respectfully but strongly disagree that the question is whether or not “simulations based on computational fluid dynamics (CFD) [can] replace studies of the urinary tract”. We similarly disagree with the conclusion that CFD simulation is “not suitable” for analysis of “flow-oriented physiological conditions of the upper urinary tract”. First, in Amitay-Rosen et al. [2], we make no claim of CFD superiority or exclusivity, and we relate our insights from CFD simulations to experimental and clinical information. Indeed, no researcher employing CFD would claim that CFD simulations are replacements for in vivo and in vitro studies. Rather, CFD simulations in the medical literature were introduced, and have always been employed, as a complementary tool to experiments that measure and analyze dynamics in real systems. While in vivo studies are the “ideal”, it is well-known that many pathologies and measurements cannot be analyzed with human subjects, while other pathologies measured in animal models lack full relevance to human physiology. Without analyzing the advantages and disadvantages of each of the specific studies cited by Kram and Buchbolz [1], suffice it to note that * Brian Berkowitz [email protected]