Impact of Design and Deployment Technique on the Hydrodynamic Resistance of Flow Diverters

Abstract

Purpose Despite the high efficacy of flow diverters (FD) in treating sidewall intracranial aneurysms, failures are reported. One of the physical factors determining efficacy is the flow reducing capacity of the FD that is currently unknown to the operator. Our aim was to measure the flow reducing capacity expressed as the hydrodynamic resistance (HR), the metallic surface area (MSA) and pore density (PD) of two different FD designs and quantitatively investigate the impact of sizing and the deployment technique on these parameters. Methods Altogether 38 Pipeline (Medtronic) and P64 (Phenox) FD‑s were implanted in holder tubes by a neurointerventionist in nominally sized, oversized and longitudinally compressed or elongated manners. The tubes were placed in a flow model with the flow directed across the FD through a side hole on the tube. HR was expressed by the measured pressure drop as the function of the flow rate. Deployed length, MSA and PD were also measured and correlated with the HR. Results Both PD and MSA changed with varying deployment length, which correlates well with the change in HR. Oversizing the device by 1 mm in diameter has reduced the HR on average to one fifth of the original value for both manufacturers. Conclusion This study demonstrates experimentally that different FD designs have different flow diverting capacities (HR). Parameters are greatly influenced by radial sizing and longitudinal compression or elongation during implantation. Our results might be useful in procedure planning, predicting clinical outcome, and in patient-specific numerical flow simulations.

Effects of different stent size selection on pipeline embolization device treatment of intracranial aneurysms

“…Indeed, in an in vitro study, Shapiro et al 4 found that even moderate oversizing could lead to a significant increase in porosity (reduction in metal coverage), thereby lowering the blood flow diversion effect and further treatment efficiency of PED. Gyürki et al 6 found that oversizing the device by 1 mm in diameter has reduced the hydrodynamic resistance, which was a more decisive factor because it more accurately reflects the capacity of flow diversions, on average to one-fifth of its original value. Oversizing for treating an aneurysm over a nominally sized stent may produce an insufficient resistance through the aneurysm neck; therefore, the thrombosis in the sac may not be complete.…”

“…The braided structure of PED promotes high plasticity; its advantage is its high adaptability to various intracranial vessels, which also leads to the selection of a stent model for the same aneurysm that fluctuates within a specific range, thereby introducing the problem of under- or over-sizing. Several studies have reported that the size of flow diversion is strongly associated with metal coverage (amount of metal surface area covered by the device), pore density (number of pores per unit surface area), and hydrodynamic resistance, 3 – 6 both of which play an important role in the occlusion of aneurysms. 7 Previous research has shown that an inappropriate flow diversion model may cause accidental coverage of lateral branches, eccentric stenosis, poor wall apposition, or delayed migration, all of which could further lead to branches occlusion, endo-leak formation, disordered growth of the neointima, in-stent stenosis, or even death.…”

Pipeline embolization device dynamics: prediction of incomplete occlusion by elongation from nominal length

“…19 This TZ was reported to potentially compromise aneurysm occlusion as well. [19][20][21] Based on these reports, a strategy for optimal PED diameter and selection based on computer software was reported. 34,35 However, these conclusions on incomplete occlusion of the PED were derived from benchtop studies or computed flow simulation studies; clinical data are lacking.…”

“…This TZ was also reported to inhibit complete occlusion. [19][20][21] However, these reported sizing effects were based on benchtop studies, and no clinical data support the effects of these factors on clinical outcomes.…”

Review: Flow Diversion for the Treatment of Middle Cerebral Artery Aneurysms

“…Growing clinical experience suggested that such stents not only protect the parent vessel from coil dislocation ex aneurysm, but also influence the hemodynamic situation in the aneurysm-harboring vessel in a beneficial way. Increased metal surface coverage due to dual layer or triple layer stent constructs showed enhanced flow modifying properties [3,4], and braided stents, with even greater surface coverage than combined lasercut stents, already demonstrated a moderate flow redirecting effect [5,6]. Further developing this concept, densely braided flow diverting stents were introduced and became one of the cornerstones of modern endovascular therapy for cerebral aneurysms.…”