A constitutive law for the pulmonary valve leaflet tissue

Includes bibliographical references (p. 80-83)Some people are born with a heart defect that requires replacement of the aortic valve (AV) during their childhood. A surgery called Ross Procedure has been developed in which a patient's AV is replaced with his/her pulmonary valve (PV) to avoid the degenerative changes that occur in mechanical and biological valves and provide a valve that will grow with the child into an adult. Despite the good success rate, the procedure is not always successful due to the difference in material properties between the AV and PV. The two valves function under different pressure loadings. The transvalvular pressure on the AV reaches a maximum of 80mmHg and on the PV it is only 10mmHg. Thus, we hypothesized that the tissue mechanical properties of the PV may not well match to the AV. The aim of this study was to derive a constitutive law for pulmonary valve leaflet tissue and compare the properties of the PV to the AV tissue using biaxial stretch studies and continuum mechanics analysis. A constitutive law is a mathematical description of the mechanical properties of a material over a range of loading. A three coefficient exponential strain-energy function has been previously developed in our laboratory to determine the mechanical behavior of mitral and aortic valve leaflet tissue. , W(I??[lower case alpha]) = c???(exp[upper case Q]--1)where Q=c???(I???--3)?? + c???([lower case alpha]--1)??? .c???, c???, c??? are the material coefficients that determine the material behavior. A set of constant invariant test are performed to identify a specific functional form. As the strain energy function is defined as the function of I??? and [lower case alpha], constant invariant stretch protocols are performed in which one of the invariants is held constant while the other is varied. Four leaflets from four porcine pulmonary valves (n=4) were mechanically stretched in a biaxial testing machine along the circumferential (C) and radial (R) directions. The material indices of the pulmonary valve are calculated from the stress-stretch data and compared to the aortic valve moduli.. Statistical comparison of the material indices was performed using ANOVA (Analysis of Variance) and paired t-test using Minitab Software (Minitab, Inc). The results show that the post-transitional stiffness (E[lower case "post"] C) of the pulmonary valve in circumferential direction is higher compared to the pulmonary valve. The "average" coefficients of the pulmonary valve leaflet tissue are calculated as c???= 0.075 g/cm??, c???= 0.311 and c???=1096. The three-coefficient exponential strain energy function which has been developed in our laboratory for mitral and aortic valve leaflets also describes the material behavior of the pulmonary valve leaflet tissue. Comparison of the material indices of the pulmonary valve leaflet tissue with aortic valve data suggests that the pulmonary valve is a suitable replacement for the aortic valve leaflet tissue, as in the Ross surgery