Closer matching between flow diverter diameter and parent artery diameter is associated with improved aneurysm occlusion

Abstract

Background:

Selecting the flow diverter (FD) size is key to treatment outcomes in unruptured intracranial aneurysms. However, the impact of differences between the nominal and actual deployed device sizes on aneurysm occlusion is not fully understood. This study retrospectively evaluated the relationship between FD size and aneurysm occlusion.

Methods:

We retrospectively analyzed 54 patients with unruptured intracranial aneurysms who underwent FD placement at our institution from March 2023 to March 2025, where both length (Length_D) and diameter (Size_D) discrepancies were evaluated. Devices used were a Flow Redirection Endoluminal Device in 31 cases and a Pipeline Embolization Device in 23 cases. Length_D is the difference between the nominal device length and actual deployed length, while Size_D is the difference between the nominal device diameter and actual deployed diameter. The primary endpoint was aneurysm occlusion at 6 months, assessed using the O’Kelly–Marotta (OKM) scale. Adequate occlusion was defined as OKM grades C–D and incomplete occlusion as OKM grades A–B. Patients were divided into adequate and incomplete occlusion groups, and Length_D and Size_D were compared.

Results:

Adequate occlusion was achieved in 68.5% of patients. The mean Length_D was 2.9 ± 3.9 mm in the adequate occlusion group and 6.7 ± 5.1 mm in the incomplete occlusion group, with a significantly greater value in the incomplete occlusion group.

Conclusion:

Greater FD elongation post-deployment may be linked to lower aneurysm occlusion rates at 6 months. Choosing an FD size that matches the parent vessel diameter is crucial for optimal occlusion.

INTRODUCTION

Flow diverters (FDs) have been established as an effective treatment for intracranial aneurysms by promoting intra-aneurysmal thrombosis and occlusion through the redistribution of blood flow between the parent artery and aneurysm sac. Its efficacy has been demonstrated in wide-necked, complex, and large, unruptured intracranial aneurysms.[2,15] The Pipeline for Uncoilable or Failed Aneurysms trial reported high occlusion rates with the Pipeline Embolization Device (PED) (Medtronic, Irvine, CA, USA), and subsequent meta-analyses further supported the effectiveness of FD treatment.[1-3]

However, considerable variability exists in both the occlusion rates and time to occlusion following FD treatment. Several factors have been identified as determinants of treatment outcomes, including the aneurysm size, neck width, location, presence of branching vessels, antiplatelet responsiveness, device design, and deployment characteristics.[8,10] Recent reviews have also emphasized that treatment outcomes with FDs depend not only on device selection, but are also strongly influenced by appropriate sizing and deployment techniques.[16]

In clinical practice, FD size selection is typically based on the parent vessel diameter and the length of the landing zone. However, owing to factors such as vessel diameter variability, vascular tortuosity, device-specific braided structures, and deployment techniques, discrepancies between the nominal device size and actual deployed size frequently occur. In particular, longitudinal elongation of the FD has been suggested to reduce the mesh density, increase the pore size, and decrease the metal coverage, potentially diminishing the flow-diverting effect.[6,13]

Therefore, in this study, we focused on the discrepancies between the nominal and deployed FD sizes, particularly the longitudinal elongation, and investigated their impact on aneurysm occlusion in patients with unruptured intracranial aneurysms.

MATERIALS AND METHODS

Study population

This study was approved by the relevant institutional review board. The requirement for informed consent was waived due to the retrospective nature of the study. This was a single-center, retrospective, observational study. Patients with unruptured intracranial aneurysms who underwent FD placement at our institution between March 2023 and March 2025 were included in this study. Finally, 54 cases in which both length and diameter discrepancies of the FD could be evaluated were included in the analyses.

Patient data, including age, sex, clinical history, and comorbidities, were retrospectively extracted from the electronic medical records. Aneurysm size and location were obtained from radiological imaging data.

Imaging and measurements

Angiographic evaluations were performed using an ARTIS Q biplane system (Siemens Healthineers, Erlangen, Germany). Follow-up angiography was performed approximately 6 months after FD placement.

Measurements were performed using the Artis VD11C software (Siemens Healthineers, Erlangen, Germany), which enabled semi-automated analysis of the deployed FD. The software automatically calculated the representative mean diameter and deployed length of the FD over the analyzed stented segment based on three-dimensional angiographic reconstruction. These automatically generated values were used for the analysis [Figure 1].

Figure 1:

Representative case of a right internal carotid artery– posterior communicating artery aneurysm. (A) Pre-treatment angiography before flow-rediversion (Flow redirection endoluminal device [FRED]) placement. Red arrow showed the aneurysm. (B) Follow-up angiography 6 months after FRED placement. Red arrow showed. The red arrow showed the disappearance of the cerebral aneurysm. (C) Three-dimensional angiographic reconstruction 6 months after FRED placement. The yellow arrowheads indicated the proximal and distal ends of the FRED. (D) Semi-automated analysis of the deployed FRED using the dedicated ARTIS software. The yellow arrowheads indicated the proximal and distal ends of the FRED.

Based on these measurements, the following parameters were defined.

• Length_D (mm): (actual deployed length) – (nominal device length)

• Size_D(mm): (actual deployed diameter) – (nominal device diameter).

Outcome assessment

The primary endpoint was aneurysm occlusion at 6 months, as evaluated by angiography using the O’Kelly–Marotta (OKM) scale.[11] The OKM scale is a grading system for post-FD treatment evaluation, where Grade A indicates complete filling, Grade B indicates partial filling, Grade C indicates neck remnants, and Grade D indicates complete occlusion.

OKM grades C and D were defined as adequate occlusion, whereas grades A and B were defined as incomplete occlusion. The patients were divided into two groups, and the differences in LengthD and Size_D were compared.

Statistical analysis

Continuous variables are presented as mean ± standard deviation, and categorical variables are presented as counts and percentages. Comparisons between the two groups were performed using the Mann–Whitney U-test. The correlations between Length_D and Size_D were assessed using Pearson’s correlation coefficients.

Statistical analyses were performed using EZR version 2.13.0 (Saitama Medical Center, Jichi Medical University, Saitama, Japan).

RESULTS

Patient characteristics

Patient characteristics are summarized in Table 1. The mean age was 59.6 ± 12.0 years, and the study included 41 men and 13 women.

Table 1:

Patient characteristics in cases where a FD was placed for a cerebral aneurysm.

The mean aneurysm width, height, and neck diameter were 5.6 ± 3.6, 4.7 ± 3.9, and 4.9 ± 2.7 mm, respectively. Aneurysms were located in the anterior and posterior circulations in 44 and 10 patients, respectively.

The devices used were the Flow Redirection Endoluminal Device (FRED) (MicroVention, Tustin, CA, USA) in 31 cases and PED in 23 cases.

Occlusion outcomes

The occlusion outcomes are summarized in Table 2. Six months after treatment, adequate occlusion was achieved in 37 (68.5%) patients, whereas 17 (31.5%) showed incomplete occlusion.

Table 2:

Comparison of Length_D and Size_D between cases with adequate occlusion and cases with incomplete occlusion.

FD size discrepancy

The mean Length_D was 2.9 ± 3.9 mm in the adequate occlusion group and 6.7 ± 5.1 mm in the incomplete occlusion group, with a significantly greater value observed in the incomplete occlusion group (p = 0.0219).

In contrast, the mean Size_D was −0.9 ± 0.4 mm in both the adequate and incomplete occlusion groups, with no significant difference between the groups [Figure 2].

Figure 2:

Comparison of flow diverter (FD) size discrepancies between cases with adequate and incomplete occlusions. *indicates p < 0.05. OKM scale AB: Incomplete occlusion; OKM scale CD: Adequate occlusion.

No significant correlation was observed between Length_D and Size_D (r = −0.195, p = 0.157; [Figure 3]).

Figure 3:

Correlation between flow diverter size and length discrepancies. FD: Flow diverter, FRED: Flow redirection endoluminal device.

Additional exploratory analyses showed no significant association between nominal FD length and Length_D. Similarly, no significant association was observed between the difference in proximal and distal parent vessel diameters and Length_D.

Device-specific subgroup analysis

In the FRED subgroup, adequate and incomplete occlusion were observed in 19 and 12 patients, respectively. Length_D was significantly greater in the incomplete occlusion group than in the adequate occlusion group (6.6 ± 5.4 mm vs. 2.2 ± 4.4 mm, p = 0.035). In the Pipeline subgroup, adequate and incomplete occlusion were observed in 18 and 5 patients, respectively. Length_D was numerically greater in the incomplete occlusion group than in the adequate occlusion group (6.9 ± 4.9 mm vs. 3.7± 3.3 mm), although the difference was not statistically significant (p = 0.263) [Table 3].

Table 3:

Comparison of Length_D and Size_D between cases achieving adequate occlusion and those with incomplete occlusion, stratified by device type (Pipeline vs. FRED).

DISCUSSION

In this study, we demonstrated that in the treatment of unruptured intracranial aneurysms with FDs, a greater discrepancy between the nominal and deployed device sizes, particularly longitudinal elongation, was associated with a lower rate of aneurysm occlusion at 6 months postoperatively. Specifically, the mean length discrepancy (Length_D) was significantly larger in the incomplete occlusion group than in the adequate occlusion group (6.7 ± 5.1 mm vs. 2.9 ± 3.9 mm, p = 0.0219). These findings suggest that treatment efficacy is determined not only by device selection but also by the final deployed configuration within the parent vessel.

Our results are consistent with the emerging concept that the post-deployment configuration of FDs critically influences treatment outcomes. Using both experimental and computational fluid dynamics analyses, Ma et al. demonstrated that the dynamic push–pull technique can intentionally compact the FD at the aneurysm neck, thereby increasing the local metal coverage and significantly reducing the intra-aneurysmal inflow.[9] Their findings highlighted that FD performance is highly dependent on deployment characteristics rather than being solely an intrinsic property of the device.

A useful framework for understanding this mechanism is the deployed-to-nominal length ratio (DLR), as described by Gyürki et al.,[6] who showed that the hemodynamic effect of FDs is influenced not only by the device design but also by radial oversizing and longitudinal compression or elongation. When the device is elongated beyond its nominal length (high DLR), the braided structure becomes less dense, the pore size increases, and metal coverage decreases. Conversely, moderate compression increases the mesh density and enhances flow diversion at the aneurysm neck.

The present finding that a greater Length_D was associated with incomplete occlusion can be interpreted as a clinical manifestation of this effect. When an FD with a diameter larger than that of the parent vessel is selected, the device may fail to fully expand radially and instead elongates longitudinally. Consequently, the metal coverage at the aneurysm neck is reduced, leading to insufficient attenuation of the inflow jets and intra-aneurysmal vortex flow, thereby limiting the early aneurysm occlusion. Recent in vitro studies have also demonstrated that a greater mismatch between FD size and parent vessel diameter results in reduced flow-diverting efficacy.[4,7,12]

Sunohara et al.[13] reported that elongation relative to the nominal length was a predictor of incomplete occlusion in patients treated with a PED. While their study focused primarily on PED, the present study additionally included patients treated with the FRED. To address the potential influence of device-specific deployment characteristics, we performed exploratory subgroup analyses for each device separately. Greater Length_D remained significantly associated with incomplete aneurysm occlusion in the FRED subgroup, whereas a similar numerical trend was observed in the PED subgroup without reaching statistical significance. This difference may partly reflect device-specific deployment techniques. Both devices involve microcatheter withdrawal and delivery-wire manipulation during deployment. However, forward push during PED deployment can intentionally promote longitudinal shortening and mesh compaction. This procedural difference may therefore affect Length_D and potentially influence its relationship with aneurysm occlusion. Although these subgroup analyses were limited by the relatively small sample size, the findings suggest that the association between post-deployment elongation and aneurysm occlusion cannot be explained solely by differences in device type. Rather, the final deployed configuration of the FD may be a more important determinant of treatment efficacy than the choice of device itself.

More importantly, our results should not be interpreted as advocating the selection of smaller devices. Undersizing may result in poor wall apposition, device migration, excessive local compaction, and unintended branch vessel coverage. Rather, optimal outcomes likely require selecting an FD diameter that closely matches the true parent vessel diameter while ensuring appropriate neck coverage through careful deployment techniques. Recent studies have emphasized that FD sizing should incorporate not only the maximum vessel diameter, but also the characteristics of the entire landing zone, vascular tortuosity, and device-specific behavior.[5,10]

In practice, a two-step optimization strategy may be necessary: first, selecting a device size that closely approximates the parent vessel diameter preoperatively, and second, achieving adequate mesh compaction at the aneurysm neck intraoperatively through appropriate wire manipulation and unsheathing techniques.

Many operators tend to select slightly oversized devices to ensure safety and proper wall apposition. However, our findings suggest that excessive oversizing may be disadvantageous for aneurysm occlusion, particularly in unruptured aneurysms. Furthermore, recent studies have reported an association between oversizing and increased severity of in-stent stenosis,[14] indicating that size mismatch may influence occlusion rates, as well as vascular remodeling and re-endothelialization.

The significance of this study lies not only in demonstrating that elongated FDs are associated with poorer outcomes but also in highlighting the importance of incorporating the actual deployed configuration into outcome predictions. Future studies should integrate quantitative indices, such as DLR, into multivariate models along with conventional factors, including aneurysm size, neck width, location, branch vessels, and device type. Moreover, advances in preoperative 3D imaging, virtual stenting, and intraoperative imaging modalities, such as cone-beam CT and high-resolution angiography, may enable the prediction and optimization of the final mesh configuration, thereby improving treatment planning and outcomes.

Thus, when treating unruptured intracranial aneurysms with FDs, greater longitudinal elongation of the device after deployment may be associated with lower aneurysm occlusion rates at 6 months postoperatively. To achieve optimal treatment outcomes, it is important to select an FD size that closely matches the true parent vessel diameter and to ensure appropriate deployment with adequate mesh compaction at the neck of the aneurysm during the procedure.

Limitations

This study had several limitations that should be considered. First, it was a single-center retrospective study with a relatively small sample size, which limited the ability to fully adjust for potential confounding factors. Second, the measurement of the deployed FD length was based on angiographic imaging and may have been subject to measurement errors due to the projection angle and image resolution. Third, the primary endpoint was angiographic occlusion at six months, and long-term outcomes, including the durability of occlusion and retreatment rates, were not evaluated.

Finally, both Pipeline and FRED devices were included in the same analysis despite differences in their braided structures and deployment characteristics, which may influence device-specific behavior. Future studies with larger cohorts incorporating multivariate analyses that include DLR, oversizing indices, and device type, as well as device-specific subgroup analyses, are warranted to validate our findings.

Furthermore, neither nominal FD length nor the difference between the proximal and distal parent vessel diameters was significantly associated with FD elongation in the present cohort. These findings suggest that longitudinal elongation may be influenced by multiple factors, including deployment technique and device characteristics, rather than by these anatomical factors alone.

CONCLUSION

Our findings suggest that selecting an FD size that closely matches the parent vessel diameter and minimizes longitudinal elongation is critical for achieving favorable aneurysm occlusion.

Ethical approval:

The research/study was approved by the Institutional Review Board at Fukuoka Neurosurgical Hospital ethics committee, number 2020040, dated April 19, 2024.

Declaration of patient consent:

Patient’s consent not required as there are no patients in this study.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation:

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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