On the in vivo deformation of the mitral valve anterior leaflet: effects of annular geometry and referential configuration

Abstract

Alteration of the native mitral valve (MV) shape has been hypothesized to have a profound effect on the local tissue stress distribution, and is potentially linked to limitations in repair durability. The present study was undertaken to elucidate the relation between MV annular shape and central mitral valve anterior leaflet (MVAL) strain history, using flat annuloplasty in an ovine model. In addition, we report for the first time the presence of residual in vivo leaflet strains. In vivo leaflet deformations were measured using sonocrystal transducers sutured to the MVAL (n = 10), with the 3D positions acquired over the full cardiac cycle. In six animals a flat ring was sutured to the annulus and the transducer positions recorded, while in the remaining four the MV was excised from the exsanguinated heart and the stress-free transducer positions obtained. In the central region of the MVAL the peak stretch values, referenced to the minimum left ventricular pressure (LVP), were 1.10 ± 0.01 and 1.31 ± 0.03 (mean ± standard error) in the circumferential and radial directions, respectively. Following flat ring annuloplasty, the central MVAL contracted 28% circumferentially and elongated 16% radially at minimum LVP, and the circumferential direction was under a negative strain state during the entire cardiac cycle. After valve excision from the exsanguinated heart, the MVAL contracted significantly (18 and 30% in the circumferential and radial directions, respectively), indicating the presence of substantial in vivo residual strains. While the physiological function of the residual strains (and their associated stresses) are at present unknown, accounting for their presence is clearly necessary for accurate computational simulations of MV function. Moreover, we demonstrated that changes in annular geometry dramatically alter valvular functional strains in vivo. As levels of homeostatic strains are related to tissue remodeling and homeostasis, our results suggest that surgically introduced alterations in MV shape could lead to the long term MV mechanobiological and microstructural alterations that could ultimately affect MV repair durability.

Figure 1

Arrangement of sonocrystal on the ovine anterior leaflet. Following the ring annuloplasty, crystals 1–5 were removed. The arrows show the circumferential (Circ.) and radial (Rad.) directions used in strain calculation at the midsection of the anterior leaflet. The midsection region surrounded by crystals 6–9 was used in bi-linear finite elements surface fitting and the region surrounded by crystals 1–7 and silhouetted was used in bi-quadratic finite element surface fitting.

Figure 2

(a) A single biquadratic finite element surface fitted to the nine crystals (darker surface) in comparison to the surface obtained from ultrasound data (lighter surface). (b) Fitting error defined as the shortest distance between the ultrasound surface and the fitted surface at each point on the surface.

Figure 3

A typical example showing that both the values and directions of maximum and minimum principal strains over the anterior leaf were fairly homogeneous (variation in the order of ~0.1). The principal strains were calculated using the nine crystal biquadratic element fit.

Figure 4

A typical example showing that both the values and directions of (a) maximum and (b) minimum principal strains over the anterior leaf midsection were fairly homogeneous (variation in the order of ~0.01). The principal strains were calculated using the four crystal bilinear element fit. (c) The variability in the areal changes calculated from four crystal data was also minimal.

Figure 5

A typical example showing the dynamic changes in the circumferential and radial stretches in the normal and annulus-restricted valves. The changes highlighted by roman numbers are further specified with null hypotheses for the collective data in Table 1.

Figure 6

Stretches in the circumferential and radial directions in the normal and annulus-restricted valves at the minimum and maximum functioning left ventricular pressures (LVP). Error bars are standard errors (n = 6).

Figure 7

Stretches in the circumferential and radial directions in the normal valve referenced to stress-free (excised valve) crystal configurations. Error bars are standard errors (n = 4).

Figure 8

Schematic of a single unit square element at the anterior leaflet midsection at minimum LVP undergoing average values of stretches shown in Fig. 6 and 7.