Torsional Characterization of Braided Flow Diverter Stents : A New Method to Evaluate Twisting Phenomenon

In the interventional treatment of cerebral aneurysms, flow diverter (FD) stents have played a significant role for more than a decade. Many studies have shown good aneurysm occlusion rates and low complication profiles. However, feared complications include acute thrombotic vessel occlusion due to stenotic deformation of the FD during release, the so-called twisting. This work investigates the behavior of different stent types to causative torsion forces in a mechanical model.

Torsion characterization equipment was custom built, and two different FD stents (Derivo, Acandis and P64, Phenox) with n = 3 were tested. One end of the FD was fixed while the other end was twisted while measuring the torsion force.

In torsional force vs. the twisting angle graph, a very sharp decrease and increase in force was recorded when the stent collapsed or reopened, respectively, making it possible to characterize for twisting. All six devices showed partial/complete collapse on torsion and showed significant delayed reopening on untwisting. Interestingly on repeated testing, the stent collapsed at earlier angles, probably due to microscopic material defects. Slight variations between stents of the same type suggest that more extensive data sets are needed.

We report a new method to characterize torsion for braided FD stents, which is reliable and reproducible. Additionally, the delayed reopening and the tendency to collapse at earlier angles on consequent testing maneuvers can be significant for clinical usage.