Mechanics of Porcine Heart Valves’ Strut Chordae Tendineae Investigated as a Leaflet–Chordae–Papillary Muscle Entity

Abstract

Proper blood flow through the atrioventricular heart valves (AHVs) relies on the holistic function of the valve and subvalvular structures, and a failure of any component can lead to life-threatening heart disease. A comprehension of the mechanical characteristics of healthy valvular components is necessary for the refinement of heart valve computational models. In previous studies, the chordae tendineae have been mechanically characterized as individual structures, usually in a clamping-based approach, which may not accurately reflect the in vivo chordal interactions with the leaflet insertion and papillary muscles. In this study, we performed uniaxial mechanical testing of strut chordae tendineae of the AHVs under a unique tine-based leaflet–chordae–papillary muscle testing to observe the chordae mechanics while preserving the subvalvular component interactions. Results of this study provided insight to the disparity of chordae tissue stress-stretch responses between the mitral valve (MV) and the tricuspid valve (TV) under their respective emulated physiological loading. Specifically, strut chordae tendineae of the MV anterior leaflet had peak stretches of 1.09–1.16, while peak stretches of 1.08–1.11 were found for the TV anterior leaflet strut chordae. Constitutive parameters were also derived for the chordae tissue specimens using an Ogden model, which is useful for AHV computational model refinement. Results of this study are beneficial to the eventual improvement of treatment methods for valvular disease.

Appendix: Polarized Spatial Frequency Domain Imaging (pSFDI) for Quantification of the Collagen Fiber Architecture of the Chordae–Leaflet Insertion Region

In this section, we briefly discuss about the imaging method used to obtain the collagen fiber orientations of the chordae-leaflet insertion region, as described in “Potential Advantages of the Proposed CT-Entity Testing Method” section. The polarized spatial frequency domain imaging (pSFDI) technique takes advantage of the polarized light-dependent birefringent properties of the collagen fibers. This birefringent property of the collagen fibers can be used in conjunction with a polarizer, a camera, and a light projector to quantify the collagen fiber orientations based on the intensity of reflected light from the fiber. The imaging is performed in the sequential order (see Figs. 1 and 2 of Jett et al.11):

• (i) Projected incident light through a polarizer at an angle θ_polarizer onto the tissue specimen

• (ii) The incident light is reflected from the collagen fibers

• (iii) The reflected light passes through a polarizer at the same θ_polarizer as in step (i)

• (iv) The light intensity at the specific θ_polarizer is captured by a camera

This imaging procedure is conducted for θ_polarizer ranging from 0° to 180° (at an increment of 5°) to capture the intensity of the collagen fibers. Owing to the birefringent response of the collagen fiber networks, the peak intensity of reflected light represents the quantified collagen fiber orientation angle θ_polarizer. In addition, the captured light intensity, say I_out, can be described by the following three-term Fourier series:

$$\frac{{I_{\text{out}} }}{{\tau_{\text{sys}} }} = a_{0} + a_{2} [2(\theta_{\text{fiber}} - \theta_{\text{polarizer}} )] + a_{4} [4(\theta_{\text{fiber}} - \theta_{\text{polarizer}} )],$$

(5)

where τ_sys represents the non-birefringent intensities, and a₀, a₂, and a₄ are the three Fourier coefficients. Specifically, a₀ describes the mean reflected light intensity from the sample, whereas a₂ and a₄ represent the optical anisotropies arising from the sample’s birefringence.

After obtaining the orientation angle of the collagen fibers, the degree of fiber alignment can be quantified through an optics-based metric – the degree of optical anisotropy (DOA):

$${\text{DOA}} = \frac{{a_{2} + a_{4} }}{{a_{0} + a_{2} + a_{4} }} .$$

(6)

The technology also allows for investigating the collagen fiber orientation and dispersion at varied depths of the tissue based on the spatial frequency domain imaging (SFDI) theory. However, the preliminary result presented in this study (Fig. 5c) only contains the tissues’ collagen fiber architecture through the full tissue depth. For more details about the SFDI theory, the pSFDI imaging procedures, and the corresponding data analyses, the reader is referred to Jett et al.11 and Goth et al.7