In vivo dynamic strains of the ovine anterior mitral valve leaflet

Abstract

Understanding the mechanics of the mitral valve is crucial in terms of designing and evaluating medical devices and techniques for mitral valve repair. In the current study we characterize the in vivo strains of the anterior mitral valve leaflet. On cardiopulmonary bypass, we sew miniature markers onto the leaflets of 57 sheep. During the cardiac cycle, the coordinates of these markers are recorded via biplane fluoroscopy. From the resulting four-dimensional data sets, we calculate areal, maximum principal, circumferential, and radial leaflet strains and display their profiles on the averaged leaflet geometry. Average peak areal strains are 13.8±6.3%, maximum principal strains are 13.0±4.7%, circumferential strains are 5.0±2.7%, and radial strains are 7.8±4.3%. Maximum principal strains are largest in the belly region, where they are aligned with the circumferential direction during diastole switching into the radial direction during systole. Circumferential strains are concentrated at the distal portion of the belly region close to the free edge of the leaflet, while radial strains are highest in the center of the leaflet, stretching from the posterior to the anterior commissure. In summary, leaflet strains display significant temporal, regional, and directional variations with largest values inside the belly region and toward the free edge. Characterizing strain distribution profiles might be of particular clinical significance when optimizing mitral valve repair techniques in terms of forces on suture lines and on medical devices.

Figure 1

Animal in decubitus position during videofluoroscopic imaging, left. Cameras C1 and C2 record two-dimensional X-ray images of the beating heart. Merging these two data sets, we obtain the three-dimensional coordinates of 23 markers from which we reconstruct the leaflet surface, right. We implanted seven markers close to the mitral leaflet free edge, grey, nine on the central belly region of the leaflet, black, and seven on the anterior section of the mitral annulus, white. The markers have an average weight of 3.2mg with an outer diameter of 1.1mm and inner diameter of 0.6mm.

Figure 2

Average left ventricular pressure with representative time points. Squares indicate the three analyzed reference states for the strain calculations referenced in Figure 3. Reference states R1, R2, and R3 correspond to minimum left ventricular pressure, black square, end-diastole, grey square, and leaflet separation, white square, respectively. The black diamond indicates maximum left ventricular pressure referenced in Figure 6. Black circles indicate the time points referenced in Figure 7.

Figure 3

Average peak strains and average peak strain rates in the belly region. Means ± standard deviations of areal, maximum principal, circumferential, and radial strains and their corresponding strain rates displayed for three different reference states. Reference states R1, R2, and R3 correspond to minimum left ventricular pressure, black, end-diastole, grey, and leaflet separation, white, respectively. Strains and strain rates display similar means and standard deviations, irrespective of the reference state R1, R2, or R3.

Figure 4

Average strains in the belly region over the normalized cardiac cycle. Time aligned areal, maximum principal, circumferential, and radial strains in the belly region are averaged over 57 animals. Average areal and maximum principal strains with peak values of 12.3% and 12.7% are significantly larger than average circumferential and radial strains with peak values of 3.8% and of 7.0%.

Figure 5

Average maximum principal strains upon successive mesh refinement. The original mesh shown on the left consists of 23 nodes and 30 triangular facet elements such that each node corresponds to the original marker position. The mesh is iteratively refined to 1017 nodes and 1920 elements in three consecutive subdivision steps. Upon mesh refinement, the resulting leaflet surface shows increased smoothness and resembles mitral valve leaflet geometry more closely. The convergence study of the surface subdivision algorithm shows that further refinement beyond the third subdivision does not reveal noticeable changes.

Figure 6

Average areal strains, maximum principal, circumferential, and radial strains at maximum left ventricular pressure. Geometric data of 57 animals are temporally interpolated and then averaged. Strains are calculated from the averaged data set on the approximated leaflet surface after three levels of subdivision.

Figure 7

Average maximum principal strains over the entire cardiac cycle. Geometric data of 57 animals are temporally interpolated and then averaged. Strains are calculated from the averaged data set on the approximated leaflet surface after three levels of subdivision. The first plot at t₁ corresponds to the leaflet in the reference state and is therefore strain free. Time points t₁ to t₁₅ correspond to the time points indicated as black circles in Figure 2.

Figure 8

Average maximum principal strain directions at end diastole and end systole. Strains are calculated from the averaged data set on the approximated leaflet surface after three levels of subdivision. At end diastole, maximum principal strains at the annulus and close to the free edge are oriented circumferentially, while maximum principal strains in the belly region are oriented radially. At end systole, maximum principal strains at the annulus and close to the free edge are oriented more radially, while maximum principal strains in the belly region are oriented more circumferentially.