Variable porosity of the pipeline embolization device in straight and curved vessels: a guide for optimal deployment strategy

Abstract

Background and purpose: Low-porosity endoluminal devices for the treatment of intracranial aneurysms, also known as flow diverters, have been in experimental and clinical use for close to 10 years. Despite rigorous evidence of their safety and efficacy in well-controlled trials, a number of key factors concerning their use remain poorly defined. Among these, none has received more attention to date than the debate on how many devices are optimally required to achieve a safe, effective, and economical outcome. Additional, related questions concern device sizing relative to the parent artery and optimal method of deployment of the devices. While some or all of these issues may be ultimately answered on an empiric basis via subgroup analysis of growing treatment cohorts, we believe that careful in vitro examination of relevant device properties can also help guide its in vivo use.

Materials and methods: We conducted a number of benchtop experiments to investigate the varied porosity of Pipeline Embolization Devices deployed in a simulated range of parent vessel diameters and applied these results toward conceptualizing optimal treatment strategies of fusiform and wide-neck aneurysms.

Results: The results of our studies confirm a predictable parabolic variability in device porosity based on the respective comparative sizes of the device and recipient artery, as well as device curvature. Even modest oversizing leads to a significant increase in porosity.

Conclusions: The experiments demonstrate various deleterious effects of device oversizing relative to the parent artery and provide strategies for addressing size mismatches when they are unavoidable.

Figures

Fig 1.

A, Pipeline device delivery system components. B, Partially unsheathed device demonstrating the segment inserted into the capture coil.

Fig 2.

Representative sample of an experimental setup, with a 4.25 × 20 mm device inserted into plastic tubes of 0.5-mm incremental diameters. The corresponding parameters of cell length a and angle θ are shown, demonstrating that a remains relatively constant so that porosity is determined primarily by variance in θ.

Fig 3.

Morphologic effects of device oversizing and the corresponding solution. A, A model of a fusiform aneurysm with 3.0- and 5.0-mm landing zones, bridged by a single 5 × 20 mm device. A transition zone of minimum coverage is created as the device is constrained from its fully opened state into the 3-mm landing zone. Despite adequate length of the “landing zone” at the 3.0-mm end, the “shape memory” of the transition zone nevertheless produces a “lip” where the device remains unapposed to the inner wall of the tube. B and C, To address these issues, 2 devices are required, each of which is appropriately sized for its recipient artery. The first 3.0-mm device is deployed from the 3.0-mm-diameter vessel into the 5-mm recipient vessel (B), following which a second 5.0-mm-diameter device is telescoped into the first, with the 5.0-mm device anchored into its 5.0-mm vessel. Thus, the transition zone is shifted outside the aneurysm, while the aneurysmal segment receives the benefit of double coverage.

Fig 4.

The “fishmouth” configuration of oversized devices in scenarios of short landing zones. A 2.5-mm landing zone (A) leads to no appreciable fishmouth configuration, unless the forward load is applied to the freely expanded portion of the device, as might be done in an attempt to better seat the device into the recipient artery (B). This, in fact, has the effect of decreasing device apposition to the wall, because the foreshortened area of decreased coverage remains unchanged, while a degree of fishmouthing is now present (arrows), due to an increase in the centripetal force vector along the transition zone angle. These effects are magnified when the landing zone decreases to approximately 1.5 mm in length (C and D). Altogether, these images suggest that the deleterious effects of oversizing are only likely to be exacerbated by attempts to force the device into the undersized artery.

Fig 5.

In vivo illustration of the memory shape effects. A and B, A wide-neck ophthalmic segment aneurysm with associated ectasia of the parent vessel, which tapers down from the ophthalmic artery to the ostium of the posterior communicating artery (paired white lines). The carotid artery is, however, normal in caliber at the anterior genu (white arrow). C and D, Native and native + contrast lateral views following Pipeline deployment. Notice the reverse morphology of the devices with the maximal diameter at the distal end (C, paired white lines). Although the device was deployed with an appropriate load, the memory effect at the anterior genu resulted in incomplete opening of the device at the level of the ophthalmic artery (D, black arrows). It is necessary to make sure that the distal end is fully in contact with the vessel to prevent an endoleak in this scenario.

Fig 6.

Photograph of a 3.25 × 20 mm device along a 180° curvature taking up ∼10 mm of the device. The percentage of metal coverage at each of the stations along the curve is listed, with a corresponding illustration of cell shape, showing that both θ and cell side length a vary substantially along the curve.

Fig 7.

Illustration of double-coverage effects on the extent and morphology of metal coverage. High-magnification views of the cell structure of a single 3.0-mm device deployed in a 3.0-mm tube; a 5.0-mm device is then telescoped into the 3.0-mm device. Note the much larger cell size of the 5-mm device and different angles of overlapping pitch from each device.