Comments on: “Changes in wall shear stress magnitude after aneurysm rupture” by Kenichi Kono, Nagatsuki Tomura, Ryo Yoshimura and Tomoaki Terada, Acta Neurochir (2013) 155:1559–1563, DOI 10.1007/s00701-013-1773-2

Abstract

Dear Editor, We read with interest the article by Kenichi Kono et al ., "Changes in wall shear stress magnitude after aneurysm rupture" published in Acta Neurochir (2013) 155:1559–1563. We want to make some comments on the CFD analysis in this article. The number of unstructured tetrahedral elements that was used in this study is 650,000, without any evidence of conducting any grid sensitivity study to prove the solution independence of changing the grids, which may affect greatly the presented results in the paper. The effect of a grid sensitivity study on the CFD results can be found in many references such as [3, 7]. Although no special treatment is usually required for the boundary layer in the case of laminar flow problems, in this special case the use of only three prism layers (no details were given about these prism layers) was not enough to describe the complexity of the flow structure within the blood vessel boundary layer before and after aneurysm rupture. The use of more prism layers was necessary in this case to resolve all the flow characteristics in the boundary layer and calculate accurate values for WSS. The imposing of zero pressure (atmospheric pressure) at the blood vessel outlet by the authors especially in this flow problem was not precise. The use of inappropriate flow conditions at the outlet boundary will lead to inaccurate prediction for the flow pattern and the results by CFD [1]. The authors used a time step of 0.005 s for their CFD simulations, without mentioning why they used this time step and what the effects of varying the time step were on their CFD results before and after the aneurysm rupture. The use of an incorrect time step affects greatly the predicted CFD results [2, 5]. The velocity and WSS scales in Fig. 2 are not appropriate because they do not give any details about these two parameters inside the aneurysm (velocity distribution) or on the outer surface of the aneurysm (WSS distribution) before and after rupture; therefore, it is extremely difficult to perform any correct analysis or draw a conclusion. Additional figures are required for describing the flow pattern inside the aneurysm before rupture using velocity vectors, as well as pressure and WSS distributions on the aneurysm surface before [4] and after rupture. There is no time history for WSS on aneurysm proximal neck, dome and distal neck before rupture, where WSS can be maximum at the proximal and distal neck rather than the aneurysm dome [9]. The results in Table 1 give a very important and direct indication, where there was a reduction of the maximum CAWSS after the aneurysm rupture from 21.2 Pa to 17.4 Pa. This means that according to the principles of fluid mechanics [8], there is an increase of the pressure on the walls of the Y. M. Ahmed (*) Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia e-mail: yasser@mail.fkm.utm.my

DOI: 10.1007/s00701-013-1940-5

Cite this paper

@article{Ahmed2013CommentsO, title={Comments on: “Changes in wall shear stress magnitude after aneurysm rupture” by Kenichi Kono, Nagatsuki Tomura, Ryo Yoshimura and Tomoaki Terada, Acta Neurochir (2013) 155:1559–1563, DOI 10.1007/s00701-013-1773-2}, author={Yasser Mohamed Ahmed and Naema Y. Fatthallah}, journal={Acta Neurochirurgica}, year={2013}, volume={156}, pages={35-36} }