Pressure and flow fields in the hinge region of bileaflet mechanical heart valves

Recent clinical thrombotic experiences with the Medtronic Parallel (MP) bileaflet heart valve have highlighted the need for new methods to assess preclinical valve hinge flow. The aim of the current study was to investigate hinge pivot flow fields in bileaflet mechanical heart valves using flow visualization in scaled x5 magnification transparent polymer models and computational fluid dynamic (CFD) analysis using CFD 2000 STORM code.

Polymeric x5 flow models of the On-X, St. Jude Medical (SJM) and MP bileaflet heart valves were constructed using laser stereolithography to replicate the interior geometry while maintaining realistic manufacturing tolerances. Each hinge flow experiment was carried out by installing the transparent x5 model in a pulsatile flow loop, which was designed according to Womersley number similitude requirements. Motions of suspended microparticles in the valve hinge area, recorded by laser imaging techniques, were used to visualize hinge flow. Experimentally measured parameters were used as input for CFD analysis. CFD simulations were made by solving the Navier-Stokes equation using a finite volume method with the pressure-based algorithm for continuity, and a pressure-implicit with splitting of operators (PISO) algorithm for pressure-velocity coupling. Moving grid methodology was employed to simulate periodic motion of the valve leaflets. CFD hinge flow results were visualized on four parallel planes at different depths in the hinge socket. The hinge flow patterns of the three types of bileaflet heart valve design are discussed.

Prominent vortex formation and stagnant flow areas were noticed in the pivot region of the MP valve. Vortices persisted throughout both the forward- and reverse-flow phases. These flow structures were not observed in the hinge areas of the SJM and On-X valves.

Vortex formation observed in the MP valve may contribute to the high thrombogenic potential of this valve. The absence of such vortices and areas of stagnant flow in the On-X and SJM valves indicate that hinge flow conditions in these valves do not favor mechanically induced thrombogenesis or thromboembolic events.