BACKGROUND AND AIMS OF THE STUDY
Porcine bioprosthetic heart valves (PBHV) continue to suffer from limited long-term durability. Failure of PBHV occurs mainly in the cusps and is characterized by mechanical damage, usually in conjunction with calcification. Mechanisms underlying calcification have received considerable attention, yet mechanical damage phenomena remain poorly understood. The structural response of PBHV cusps to in-vivo cyclic loading involves three primary factors: (i) mechanical properties; (ii) fiber architecture; and (iii) 3D geometry. Previous finite element studies have shown cuspal stress distribution to be highly sensitive to subtle changes in geometry, yet to date, cusp geometry has been largely ignored in studies of PBHV durability.
METHODS
A non-destructive method was developed to quantify PBHV 3D geometry using high-resolution magnetic resonance (MR) imaging. Images were obtained in three orthogonal planes from virgin and accelerated tested (50 x 10(6) and 200 x 10(6) cycles) PBHVs to fully capture 3D cuspal geometry. Surface curvatures were computed using a local biquadric surface patch approach.
RESULTS
Results indicated a tendency for cusps to permanently deform with accelerated testing, manifesting primarily as sagging of the cusp. This sagging induced areas of high curvature from the central belly region upwards to the nodulus of Aranti, corresponding to known locations of tissue failure.
CONCLUSIONS
It is likely that the observed changes in cuspal geometry induce deleterious alterations in the stress distribution, independent of those related to mechanical properties and fiber structure, and contribute to valve failure. Our results suggest that PBHV designers should attempt to compensate for the deleterious geometric changes that occur post-implantation.