Biomechanical Behavior of Bioprosthetic Heart Valve Heterograft Tissues: Characterization, Simulation, and Performance

Abstract

The use of replacement heart valves continues to grow due to the increased prevalence of valvular heart disease resulting from an ageing population. Since bioprosthetic heart valves (BHVs) continue to be the preferred replacement valve, there continues to be a strong need to develop better and more reliable BHVs through and improved the general understanding of BHV failure mechanisms. The major technological hurdle for the lifespan of the BHV implant continues to be the durability of the constituent leaflet biomaterials, which if improved can lead to substantial clinical impact. In order to develop improved solutions for BHV biomaterials, it is critical to have a better understanding of the inherent biomechanical behaviors of the leaflet biomaterials, including chemical treatment technologies, the impact of repetitive mechanical loading, and the inherent failure modes. This review seeks to provide a comprehensive overview of these issues, with a focus on developing insight on the mechanisms of BHV function and failure. Additionally, this review provides a detailed summary of the computational biomechanical simulations that have been used to inform and develop a higher level of understanding of BHV tissues and their failure modes. Collectively, this information should serve as a tool not only to infer reliable and dependable prosthesis function, but also to instigate and facilitate the design of future bioprosthetic valves and clinically impact cardiology.

Keywords: Bioprosthetic heart valve; Constitutive modeling; Exogenous crosslinking; Fluid structure interaction; Heterograft; Mechanical testing; Modeling and simulation; Valve mechanics.

FIGURE 1

Increase in use of tissue valves for aortic valve replacement, reaching ~80% in recent years. Data from a total of 6648 patients treated at the Providence St. Vincent Hospital, Portland, Oregon, USA. Adapted with permission from Starr.

FIGURE 2

Carpentier-Edwards (a) BP and (b) PAV bioprostheses. With permission from http://www.edwards.com/.

FIGURE 3

History of technological developments in the processing of BHVs: note that current standards are relatively unchanged from the advent of commercial BHVs and that the future prospects point to tissue engineered heart valves.

FIGURE 4

Important considerations in BHV processing including design criteria and basic levels of tissue understanding.

FIGURE 5

Preferred fiber direction of the BP sac displayed as vectors superposed onto color representation orientation index. Adapted with permission from Hiester and Sacks.

FIGURE 6

Notable regions and orientation for the aortic valve.

FIGURE 7

AV cuspal stress-strain data for the (a) circumferential and (b) radial directions for a GL treated cusp demonstrating the effects of transverse loading (in-plane coupling). Number adjacent to curves indicate biaxial test protocol number.

FIGURE 8

Green strain presented as a mean ± standard deviation at a membrane tension of 60 N/m in the (a) circumferential and (b) radial directions with all statistically significant differences indicated by the corresponding p value, with n = 10 for each species and leaflet type. Adapted from Martin and Sun.

FIGURE 9

(a) Section of aortic valve used for flexural testing to capture unique interaction between transmural layers. (b) Schematic of the experimental determination of curvature with markers.

FIGURE 10

Representative moment-curvature data for both (a) natural and (b) heterograft bovine pericardium and porcine AV. Adapted from Mirnajafi.

FIGURE 11

Representative biaxial test data and Fung model fitting curves [with Eq. (4)] of glutaraldehyde treated (a, b) bovine pericardium and (c, d) porcine pericardium for 7 strain protocols (from Li and Sun).

FIGURE 12

Overview of two-tiered BP tissue sorting procedure with vector plots showing distribution of regional fiber preferred directions (from Sacks and Chuong). (a) A course small angle light scattering (SALS) scan of an anterior section of the BP sac showing where a 50 mm × 75 mm rectangular cutout regions were extracted, (b) a rescan of the cutout showing where the 25 mm × 25 mm biaxial testing specimen was selected, and (c) high spatial resolution scan of the biaxial test specimen overlaid on a gray scale OI values demonstrating high uniformity of both fiber preferred directions and OI, along with definition of the preferred and cross direction axes.

FIGURE 13

Biaxial mechanical behavior of glutaraldehyde treated bovine pericardium for five strain protocols (indicated beside each curve) for the (a) preferred direction (PD, 11 in the above nomenclature) and the (b) cross direction (CD, 22 in the current nomenclature). Also shown is the fit of the structural constitutive orthotropic model [cf. Eq. (4)], which demonstrated a very good fit to the data for all protocols (adapted from Sacks).

FIGURE 14

(a) The Elastic response of a single collagen fiber: comparing the standard model with no elastica effect, the analytical model based on Garikipati et al., and a FE simulation of 3D Neo Hookean fiber using the FEniCs project. (b) The simulated ensemble response of the Elastica model and the standard model shoes minimal difference.

FIGURE 15

Complete EXL structural model results or the S₁₁ and S₂₂ stress components for three protocols. The interactions produced the largest contribution to S₁₁ followed by matrix and collagen fibers; however, the contributions towards S₂₂ are dependent on loading path, with collagen dominating when λ₁ λ₂ and matrix dominating when λ₂ λ₁. The contribution of the matrix was much less loading path sensitive, owing to its near-linear, isotropic behavior (from Sacks et al.).

FIGURE 16

(a) A schematic of the nine sonocrystals placement on the mitral valve arterial leaflet surface, showing crystal positions in relation to valvular geometry. (b) Two three-dimensional reconstructed views of the nine sonocrystals in the unloaded reference state (t = 0 ms) and the fully coapted state (t = 500 ms). The evaluation of valve deformation is extremely difficult and, being crucial for simulation validation, still remains a significant challenge (from Sacks et al.).

FIGURE 17

Maximum in-plane principal strain magnitude plotted using the same color fringe scale for pressure levels of 40, 80 and 120 mm Hg. It is interesting to note that the free edge of one leaflet was slightly higher than that of the other two leaflets at 120 mm Hg and this feature was captured by the FE model (from Sun et al.).

FIGURE 18

Representative SALS data for three leaflets of an pericardial BHV. The vectors represent the local preferred fiber orientations, the color indicates the degree of collagen fiber orientation. Most leaflets have a ±45° preferred orientation and a fairly uniform degree of orientation throughout the leaflet (from Sun et al.).

FIGURE 19

Sequence of displacement of the BHV resultant shell model during the complete cardiac 6 cycle (from Kim et al.).

FIGURE 20

Volume rendering of the velocity field at several points during a cardiac cycle. The immersogeometric fluid structure interaction methodology applied to BHV modeling and simulation grants higher levels of automation, robustness, and realism than its standalone structural dynamics counterpart (from Hsu et al.).

FIGURE 21

(a) Quadrilateral mesh used, (b) fiber structure of three leaflets measured experimentally with SALS and then mapped onto valve geometry using spline technique, and (c) another view of the final valve mesh with fiber structure (from Aggarwal and Sacks).