A general method for estimating deformation and forces imposed in vivo on bioprosthetic heart valves with flexible annuli: In vitro and animal validation studies

Background and aim of the study: The use of flexible structures within card iovascular prostheses such as valves, stents and vascular grafts has been p roposed as a means of more closely modeling native mechanics, and thereby r educing the biomechanical problems associated with rigid materials. However , the design of such materials has been hampered by the paucity of quantita tive information on the in-vivo behavior of such structures. The aim of thi s study was to explore the use of 3D ultrasound imaging coupled with finite element analysis (FEA) as a tool to estimate deformation and forces impose d in vivo on a novel bioprosthetic valve design. Methods: The method was first tested using in-vitro static loading conditio ns, where good agreement between displacements seen on video and those obta ined from application of the identical force within the finite element prog ram was seen. The method was then tested in a porcine model with valves imp lanted in the mitral position. Images of the deforming annular ring were ob tained over the cardiac cycle using 3D intravascular ultrasound; these imag es were fed into the FEA program for calculation of reaction forces. Results: Results in vitro showed that a force of 2.7-8.0 Newtons (N) was re quired to produce a deformation of between 1.0 and 3.0 mm in the radial dir ection. A time history of deformation and force around the ring-of the valv e stent could be obtained for the in-vivo conditions. These results reveale d a maximum deformation of 0.5-1.7 mm along the short axis (anteroposterior ) of the mitral valve. Coupled to this, a peak reaction force of 4.4-13.9 N was found at the points corresponding to maximal deflection. Both deformat ion and reaction force reached maximum during atrial contraction. Conclusion This method provides an accurate means of estimating deformation and corresponding forces imposed in vivo on intracardiac prostheses. The r esults provide information on the dynamic behavior of the mitral valve annu lus. Such information should be useful in the design of flexible cardiovasc ular prostheses.