Avoid common mistakes on your manuscript.
Editor,
We read with interest the paper by Tal Amitay-Rosen et al. [1].
Numerical simulations based on computational fluid dynamics (CFD) were used to examine the degree of renal fluid pressure build-up and patterns of urine flow in a stented ureter in the presence of obstructions to evaluate conditions leading to stent failure. Obstructions were examined both, in the form of closure within the ureter lumen due to the physical effect of extrinsic ureteral obstruction (EUO), and to occlusion within the stent lumen due to mineral encrustation, either separately or simultaneously. The effects of up to 100% obstruction, in the stent and ureter lumina and/or the side holes, were compared.
The CFD simulations showed that stent failure under EUO tends to occur suddenly and only when both, ureter and stent lumina, are almost completely blocked.
Based on this CDF simulation, the authors attempt to describe complex physiological conditions in the renal pelvis and ureter with only a few parameters leading to stent failure.
The following parameters for Newtonian fluid are accepted:
-
Laminar flow,
-
Incompressible fluid.
Not considered were:
-
Dilatation of the ureter after obstruction and recent manipulation/ stent insertion;
-
Differences in elastic wall properties in different parts of the ureter;
-
Effects of peristalsis;
-
Vesicoureteral reflux;
-
Viscosity of urine and urinary sedimentation.
The authors noted that none of the reported CDF simulations has considered the effects of occlusions on renal fluid pressure. There are of course good reasons for this. With respect to urodynamic studies of the upper urinary tract, stented and obstructed ureters show a variety of pathological responses that cannot be fully characterized with the simulation presented here.
-
Acute ureteral intubation causes an elevation of intrarenal pressure which is directly correlatated to the stent diameter. The intrarenal pressure decreases back to baseline after 3 weeks [2]. Increased intrarenal reflux is noted in up to 90% of patients following stent placement for acute ureteral obstruction [3, 4] which also decreases back to baseline. Peristalsis gets diminished and usually recurs after about 8 weeks [5].
The specific pathological changes that occur during upper tract obstruction depend on the nature of the obstruction, and particularly whether it is acute or chronic in nature.
-
Acute obstruction results in an increase in hydrostatic intrarenal pressure and leads to stretching of the renal capsule and the ureteral wall. The frequency of the peristaltic waves increases, whilst their amplitude decreases. The degree of obstruction and the time of its development play an important role. Initially, there is an increase of intrarenal pressure and intrarenal blood perfusion, followed within up to 4 h by a decrease [6,7,8].
-
In extrinsic ureteral obstruction, i.e., due to a tumor or pregnancy (EUO), the obstruction usually develops gradually and is not comparable to an acute obstruction, i.e., by a ureteral stone. The increased pressure in the proximal tubule and at Bowman’s capsule leads to a decrease in glomerular filtration rate. Renal blood flow decreases, eventually leading to ischemia and chronic renal failure [9].
For all those reasons, 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.
References
Amitay-Rosen T et al (2021) Failure of ureteral stents subject to extrinsic ureteral obstruction and stent occlusions. Int Urol Nephrol https://doi.org/10.1007/s11255-021-02810-0
Payne SR, Ramsay JW (1988) The effects of double J stents on renal pelvic dynamics in the pig. J Urol 140(3):637–641. https://doi.org/10.1016/s0022-5347(17)41745-9
Ramsay JW, Payne SR, Gosling PT, Whitfield HN, Wickham JE, Levison DA (1985) The effects of double J stenting on unobstructed ureters. An experimental and clinical study. Br J Urol 57(6):630–634. https://doi.org/10.1111/j.1464-410x.1985.tb07021.x
Hübner WA, Plas EG, Stoller ML (1992) The double-J ureteral stent: in vivo and in vitro flow studies. J Urol 148(2 Pt 1):278–280. https://doi.org/10.1016/s0022-5347(17)36572-2
Patel U, Kellett MJ (1996) Ureteric drainage and peristalsis after stenting studied using colour Doppler ultrasound. Br J Urol 77(4):530–535. https://doi.org/10.1046/j.1464-410x.1996.09298.x
Shokeir AA (2001) Renal colic: pathophysiology, diagnosis and treatment. Eur Urol 39(3):241–249. https://doi.org/10.1159/000052446
Seitz C (2015) Therapie der akuten nierenkolik und konservative therapie. In: Michel MS, Thüroff JW, Janetschek G, Wirth M (eds) Die urologie. Springer Berlin Heidelberg, Berlin, Heidelberg, pp 1–10. https://doi.org/10.1007/978-3-642-41168-7_71-1
Alloussi S, Lang C, Hampel C (2012) Urodynamik der oberen Harnwege. In: Schultz-Lampel D, Goepel M, Haferkamp A (eds) Urodynamik. Springer Berlin Heidelberg, Berlin, Heidelberg, pp 53–67. https://doi.org/10.1007/978-3-642-13016-8_5
Kalu CO, Abudayyeh A (2020) Obstructive uropathy in critically Ill cancer patients. In: Nates JL, Price KJ (eds) Oncologic critical care. Springer International Publishing, Cham, pp 969–975. https://doi.org/10.1007/978-3-319-74588-6_78
Funding
Open Access funding enabled and organized by Projekt DEAL.
Author information
Authors and Affiliations
Corresponding author
Additional information
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Kram, W., Buchholz, N. 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?. Int Urol Nephrol 53, 1835–1836 (2021). https://doi.org/10.1007/s11255-021-02869-9
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11255-021-02869-9