Introduction: The primary drawback of endovascular coiling is aneurysm recurrence. The efficacy of coiling is dependent on both the packing density and permeability of the coil mass. While packing density is commonly used to characterize coiling, coil mass permeability has not been evaluated. We present an in vitro methodology to measure coil permeability.
Methods: A falling head permeameter setup  was constructed to measure the permeability of coiled samples (Figure). The time taken for fluid in the tube above the sample to fall under gravity is related to permeability. Using a first-order approximation, the generic equation for flow through porous media (Darcy’s Law) was modified for a spherical sample to define the permeability K (cm^2) (Figure). Four identical aneurysm models were manufactured from silicone. One model was used as control (no coil), two models were coiled to equivalent packing densities with Orbit (Codman) and Penumbra 400 (Penumbra) coils, and one model was coiled to a high packing density with Vortx (Boston Scientific) coils (Table). Sample permeabilities were measured repeatedly.
Results: The table lists the average permeabilities measured for the different coils. All permeabilities were statistically different from one another (ANOVA p values <0.0001) including the Penumbra and Orbit aneurysms coiled to equivalent packing densities.
Conclusions: Different coils with equal packing densities can have significantly different permeabilities. Thicker coils, intended to improve packing density, may result in higher coil mass permeability and potentially adversely affect treatment outcomes.
Patient Care: This research will improve patient care by alerting physicians and device manufacturers to the importance of permeability for effective treatment outcomes with endovascular coiling.
Learning Objectives: 1. By the conclusion of this session, participants should be able to: 1) Describe the relation between packing density and permeability, and explain the significance of permeability when using coils of various brands in order to treat cerebral aneurysms. 2) Discuss, in small groups, the value of using different packing densities; vis a vis the permeability. 3) Identify an effective treatment for cerebral aneurysms with platinum coils.
References:  Neithalath N, Sumanasooriya MS, Deo O. Materials Characterization. 2010;61(8):802-13.