Patient specific finite element analysis results in more accurate prediction of stent fractures: application to percutaneous pulmonary valve implantation.

Stent fracture is a recognised complication following device implantation. Magnetic resonance data from a patient who underwent percutaneous pulmonary valve implantation (PPVI) and had subsequent stent fractures was used to create a finite element (FE) model of the patient's implantation site. Simulated expansion of the PPVI stent into this right ventricular outflow tract (RVOT) geometry was compared with free expansions of the PPVI stent up to a uniformly deployed configuration (conventional method employed in bench testing protocols), using FE analysis. PPVI biplane fluoroscopy images from the same patient were used to reconstruct the 3D shape and deformation of the stent in-situ and verify the FE geometrical results. Asymmetries were measured in all 3 orthogonal directions, in early systole and diastole. Although a simplified FE modelling of stent/implantation site interaction was adopted, this analysis gave useful information about the influence of the RVOT on the final geometry and mechanical performance of the stent. When deployed into the RVOT, the FE stent showed a non-uniform shape, similar to the geometry seen in the "real" fluoroscopy reconstructed stent, where the most expanded cells corresponded to the fracture locations. This asymmetrical geometry, when compared to the free-expanded stent, resulted in higher stresses in the portion of the stent where fractures occurred. Furthermore, fatigue fractures that were not predicted in the free-deployed stents, developed in the asymmetrically expanded device. In conclusion, the interaction between the PPVI device and the patient's RVOT is likely to be the crucial factor involved with this undesired event.