In-vivo heterogeneous functional and residual strains in human aortic valve leaflets

Abstract

Residual and physiological functional strains in soft tissues are known to play an important role in modulating organ stress distributions. Yet, no known comprehensive information on residual strains exist, or non-invasive techniques to quantify in-vivo deformations for the aortic valve (AV) leaflets. Herein we present a completely non-invasive approach for determining heterogeneous strains – both functional and residual – in semilunar valves and apply it to normal human AV leaflets. Transesophageal 3D echocardiographic (3DE) images of the AV were acquired from open-heart transplant patients, with each AV leaflet excised after heart explant and then imaged in a flattened configuration ex-vivo. Using an established spline parameterization of both 3DE segmentations and digitized ex-vivo images (Aggarwal et al., 2014), surface strains were calculated for deformation between the ex-vivo and three in-vivo configurations: fully open, just-coapted, and fully-loaded. Results indicated that leaflet area increased by an average of 20% from the ex-vivo to in-vivo open states, with a highly heterogeneous strain field. The increase in area from open to just-coapted state was the highest at an average of 25%, while that from just-coapted to fully-loaded remained almost unaltered. Going from the ex-vivo to in-vivo mid-systole configurations, the leaflet area near the basal attachment shrank slightly, whereas the free edge expanded by ~10%. This was accompanied by a 10° −20° shear along the circumferential-radial direction. Moreover, the principal stretches aligned approximately with the circumferential and radial directions for all cases, with the highest stretch being along the radial direction. Collectively, these results indicated that even though the AV did not support any measurable pressure gradient in the just-coapted state, the leaflets were significantly pre-strained with respect to the excised state. Furthermore, the collagen fibers of the leaflet were almost fully recruited in the just-coapted state, making the leaflet very stiff with marginal deformation under full pressure. Lastly, the deformation was always higher in the radial direction and lower along the circumferential one, the latter direction made stiffer by the preferential alignment of collagen fibers. These results provide significant insight into the distribution of residual strains and the in-vivo strains encountered during valve opening and closing in AV leaflets, and will form an important component of the tool that can evaluate valve's functional properties in a non-invasive manner.

A functionally graded material model for the transmural stress distribution of the aortic valve leaflet

“…7) using our final transmurally varying parameters (Fig. 3a and 4) at a circumferential and radial stretch of 1.1 and 1.7 respectively, which corresponds to an approximate end-diastolic state in the leaflet’s belly region (Aggarwal et al, 2016). Our results show significant transmural variation in both the circumferential and radial components of the resulting Cauchy stress tensor (Fig.…”

“…Note that the fibrosa flattens from its initially wrinkled state radially following layer separation (Sacks et al, 1998; Stella and Sacks, 2007; Vesely and Lozon, 1993; Vesely and Noseworthy, 1992). As it is actually now known to be under substantial pre-stretch in vivo (Aggarwal et al, 2016), the fibrosa will be largely smooth in-vivo. Because this deformation is non-planar and thus cannot be precisely quantified through 2D fiducial marker tracking, we did not examine the response of individual fibrosa specimens.…”

Growth and remodeling play opposing roles during postnatal human heart valve development

“…Second, we did not implement prestretch, which is known to significantly influence tissue mechanics 47 . To the best of our knowledge, the only data on aortic and pulmonary leaflet prestretch has recently been obtained from valves from elderly patients (average age 63 yr) who underwent heart transplantation 48 . We used these novel findings in the generation of the leaflet geometry to ensure an appropriate circumferential-radial leaflet length ratio of 2 according to Fig.…”

A noninvasive method for the determination of in vivo mitral valve leaflet strains

“…To avoid the drawbacks of marker-based approaches, there have been some attempts to develop markerless methods to extract strains directly from images, using diffeomorphic tracking in conjunction with spline-based morphing. [49][50][51] These approaches have worked well when applied to geometrically tractable cardiovascular structures, such as the left ventricle and the aortic valve. The MV, however, has a significantly more complex shape, and correspondent landmark locations are insufficient and difficult to identify throughout the cardiac cycle.…”

“…While these breakthroughs have provided valuable insights into the effect of patient‐specific geometry and annuloplasty ring shape on repair success, there currently exists no reliable method to noninvasively extract the critical local strain information from clinically obtained in vivo images of the MV. To avoid the drawbacks of marker‐based approaches, there have been some attempts to develop markerless methods to extract strains directly from images, using diffeomorphic tracking in conjunction with spline‐based morphing . These approaches have worked well when applied to geometrically tractable cardiovascular structures, such as the left ventricle and the aortic valve.…”