Enhanced Aneurysmal Flow Diversion Using a Dynamic Push-Pull Technique: An Experimental and Modeling Study

Discussion

The current study found the following with DPPT: 1) The distal foreshortening of the FD was prevented by the pillowing technique; 2) higher FD metal coverage across the aneurysm orifice could be achieved by greater delivery wire advancement; 3) lower metal coverage at perforator regions could be achieved by smaller wire advancement and greater withdrawal of the microcatheter; and 4) high-compaction mesh resulted in greater inflow reduction (than low-compaction), which theoretically promotes aneurysm occlusion.

The FD foreshortening has been frequently encountered in practice.^4,5 Foreshortening occurs once the FD is released from the crimped state. The released FD radially expands and longitudinally foreshortens; this change makes the optimal placement of the FD relatively unpredictable, especially for a poorly sized FD. As a countermeasure, the pillowing technique was effective in preventing distal migration of the FD as it foreshortened.

To actively control the FD mesh, we demonstrated that the additional delivery wire/pusher advancement (approximately 10 mm) was sufficient to generate high-compaction mesh (metal coverage: 50% versus 36%). Similar results were found for the fusiform aneurysm in the previous study (metal coverage: 49% versus 40%, by a 6-mm difference in wire advancements).² In practice, high-compaction mesh can be achieved by pushing the delivery wire, either alone or together with the microcatheter, to radially expand and longitudinally compress the FD. However, when the FD is over important arterial branch ostia (eg, the anterior choroidal or posterior communicating arteries), we suggest additional pull of the microcatheter while holding the delivery wire to generate low-compaction mesh for the patency of small vessels.

Our experience shows that the above operations would work better around the orifice than inside parent vessels, because once a given FD fully expands to the vessel, its metal coverage is mainly determined by the vessel size and DPPT becomes less effective. This characteristic was confirmed by our result that mesh densities from the high- and low-compaction scenarios were similar in corresponding parent vessels (Fig 2).

One should also be mindful of potential risks of dense compaction: 1) Denser mesh increases the chance of occluding perforators and side branches, given that these vessels can only tolerate up to 50% ostial coverage as previously suggested⁸; 2) forceful pushing can lead to flattening, torsion, and even intussusceptions of the FD,⁹ compromising the vessel lumen with potentially devastating consequences; 3) controlled, selectively increased metal coverage over the aneurysm neck may take time to master and is more difficult to perform in tortuous vascular anatomies.

Flow simulations showed that high-compaction mesh was more effective (additional 12%–21% reduction) in diverting aneurysmal inflow than low-compaction mesh. Most interesting, the flow reduction by the high-compaction mesh of the sidewall aneurysm in the current study (62% inflow reduction) is comparable with the performance of overlapping 2 FDs (69% reduction of mean aneurysmal velocity) reported by Augsburger et al.¹⁰ However, further analysis is needed to verify this observation. With regard to the clinical implications of flow reduction, it is understood that more flow reduction leads to more immediate and complete thrombotic occlusion, therefore avoiding the complications related to aneurysm rupture.^11,12

The orifice coverage of the first clinical case matched the current high-compaction mesh of the sidewall aneurysm (46% versus 50%), indicating comparable flow reduction. The low-compaction mesh of the other case was favorable for branch vessel preservation, but with compromised flow reduction. These 2 cases showed high variability of the FD mesh and potentially distinct flow modifications under the DPPT operation.

Limitations of the current study include the following: 1) Only the hemodynamics of 2 aneurysm geometries were analyzed. However, these 2 were typical cases suitable for flow diversion (wide necks and complex geometries less optimal for coiling or clipping); 2) the 3.5 × 20 mm FD for high-compaction was oversized (3.5 versus 3 mm) due to limited FD sample availability. Oversized FDs were reported to achieve less flow reduction than nonoversized ones,¹³ indicating an underestimation to the flow reduction of the current high-compaction scenario; and 3) rigid wall was assumed through the deployment. Future direction would be to further examine the effects of DPPT on aneurysm occlusion and perforator preservation in clinical cases following flow diversion.