Computational fluid dynamics analysis on recurrence of simple coiling intracranial aneurysms with remnant neck

Recently, hemodynamic research mainly simulated the blood flow characteristics of the remnant neck without considering the coils [16]. However, even when an aneurysm was completely embolized, the packing density might be only 25–35 % [18]. Though the aneurysm displayed no contrast material, in the immediate post-operative period, blood flow percolation could still occur in the aneurysm pouch. Therefore their method did not accord with the actual situation. Our study used a porous medium model as the equivalent of the coils [19], and combined with the reconstruction of the parent artery to simulate the hemodynamic characteristics of the aneurysm obtaining a more accurate result. Meanwhile, the porous model required the uniformity of the simulation material. In this study, in the immediate post-operative period, the velocity of the blood flow surrounding the coils was high, so that no thrombosis was formed in the aneurysm. As the material in the aneurysm approximated homogeneous distribution, it was reasonable to model the aneurysm with the porous medium. In the two follow-ups, significant thrombosis formed in the aneurysm, resulting in a change of porosity, so that the porous medium may not very accurately model the blood flow, while there was also percolation in the aneurysm. So the porous medium simulation largely replicated the blood flow situation, and the hemodynamic results were relatively accurate.

Sluzewski M [14] et al. demonstrated that coil compaction, resulting from high blood flow impingement, was an important factor in aneurysm recurrence. Some studies had also found that hemodynamic factors, such as high WSS and velocity, were highly related to aneurysm recurrence [16, 20]. In this study, for recanalized case, in the immediate post-operative period, the form of the aneurysm had no obvious change. However, a sizeable blood stagnation region was formed in the sac, and the WSS on the remnant neck decreased. In the first follow-up, the remnant neck area had no obvious change, the WSS on the remnant neck was higher than the immediate post-operative period, the flow impingement was stronger, while the impingement region was wide, and the blood stagnation region decreased. In the second follow-up, the remnant neck of the aneurysm enlarged, the aneurysm recurred, and while the WSS was lower on the remnant neck and aneurysm sac than the previous follow-up, the blood stagnation region became larger. For the recanalized case the WSS and velocity underwent dynamic changes during the aneurysm recurrence process after embolization, which influenced the progress of the aneurysm outcome. In conclusion this aneurysm recurrence process can be divided into two phases: in the first phase (from the immediate post-operative period to the first follow-up) the aneurysm did not recur, the coils experienced only minor compaction, resulting in local high blood flow velocity and a decrease in the blood stagnation area, the WSS on the remnant neck increased. In the second phase (from the first follow-up to the second follow-up) high WSS on the remnant neck resulted in the regrowth of the remnant neck and aneurysm recurrence.

In our study, for recurrent case 1, it was stable in the 6-month follow-up, but had recanalized in the 18-month follow up. For recurrent case 2, it recurred in the first follow-up on seventh month after procedure. Therefore, the different interval in different patients between the first follow-up and the second follow-up may induce much influence on the results. And it is necessary to conduct long-term follow-up for remnant-neck aneurysms. In clinical practice, the hemodynamic features of the intracranial aneurysms can also provide some references for the treatment and follow-up of the intracranial aneurysms.

The porous medium model also has some limitations, in the immediate post-operative period, the velocity of the blood flow surrounding the coils was high, so that thrombosis was difficult to be formed in the aneurysm. It is relatively reasonable to model the aneurysm with the porous medium. However, in the two follow-ups, significant thrombosis formed in the aneurysm, resulting in a gradual decrease of porosity, we could not accurately describe the thrombosis and calculate the porosity, but we gave a rough estimate of the porosity of the follow-up. Simultaneously this model represented blood flow in the aneurysm sac after coil embolization, it might be more reasonable than the previous method which directly deleted the aneurysm.