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TECHNICAL PAPERS: Soft Tissue

In Vitro Dynamic Strain Behavior of the Mitral Valve Posterior Leaflet

[+] Author and Article Information
Zhaoming He

Wallace H. Coulter Department of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332-0535zhe@bme.gatech.edu

Jennifer Ritchie

Wallace H. Coulter Department of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332-0535gtg073k@prism.gatech.edu

Jonathan S. Grashow

Department of Bioengineering and the McGowan Institute for Regenerative Medicine,  University of Pittsburgh, Room 234, 100 Technology Drive, Pittsburgh, PA 15219Jsgst29@pitt.edu

Michael S. Sacks

Department of Bioengineering and the McGowan Institute for Regenerative Medicine,  University of Pittsburgh, Room 234, 100 Technology Drive, Pittsburgh, PA 15219msacks@pitt.edu

Ajit P. Yoganathan

Wallace H. Coulter Department of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332-0535 Phone : 404-894-2849ajit.yoganathan@bme.gatech.edu

J Biomech Eng 127(3), 504-511 (Jan 31, 2005) (8 pages) doi:10.1115/1.1894385 History: Received April 27, 2004; Revised November 23, 2004; Accepted January 31, 2005

Abstract

Knowledge of mitral valve (MV) mechanics is essential for the understanding of normal MV function, and the design and evaluation of new surgical repair procedures. In the present study, we extended our investigation of MV dynamic strain behavior to quantify the dynamic strain on the central region of the posterior leaflet. Native porcine MVs were mounted in an in-vitro physiologic flow loop. The papillary muscle (PM) positions were set to the normal, taut, and slack states to simulate physiological and pathological PM positions. Leaflet deformation was measured by tracking the displacements of 16 small markers placed in the central region of the posterior leaflet. Local leaflet tissue strain and strain rates were calculated from the measured displacements under dynamic loading conditions. A total of 18 mitral valves were studied. Our findings indicated the following: (1) There was a rapid rise in posterior leaflet strain during valve closure followed by a plateau where no additional strain (i.e., no creep) occurred. (2) The strain field was highly anisotropic with larger stretches and stretch rates in the radial direction. There were negligible stretches, or even compression $(stretch<1)$ in the circumferential direction at the beginning of valve closure. (3) The areal strain curves were similar to the stretches in the trends. The posterior leaflet showed no significant differences in either peak stretches or stretch rates during valve closure between the normal, taut, and slack PM positions. (4) As compared with the anterior leaflet, the posterior leaflet demonstrated overall lower stretch rates in the normal PM position. However, the slack and taut PM positions did not demonstrate the significant difference in the stretch rates and areal strain rates between the posterior leaflet and the anterior leaflet. The MV posterior leaflet exhibited pronounced mechanically anisotropic behavior. Loading rates of the MV posterior leaflet were very high. The PM positions influenced neither peak stretch nor stretch rates in the central area of the posterior leaflet. The stretch rates and areal strain rates were significantly lower in the posterior leaflet than those measured in the anterior leaflet in the normal PM position. However, the slack and taut PM positions did not demonstrate the significant differences between the posterior leaflet and the anterior leaflet. We conclude that PM positions may influence the posterior strain in a different way as compared to the anterior leaflet.

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Topics: Valves , Deformation

Figures

Figure 4

Three dimensional surface fit results for the marker array area of the posterior leaflet, with t=0 defined as the first frame where all markers are visible. Vectors represent the principal stretch direction and color fringe local major principal stretch magnitude (PS1). Here, the u and v axes are coincident with the circumferential and radial axes, respectively.

Figure 1

The Georgia Tech left heart simulator and flow loop were driven by a bladder pump. A data acquisition system recorded the transmitral pressure and flow rate. Two high speed cameras placed at an angle in the atrial side of the mitral valve recorded the marker array. Images and transmitral pressures were synchronized with a trigger signal from the pulse generator.

Figure 2

A marker array on the central region of the posterior leaflet between the annulus and the coaptation line. A sequence of images from camera A and B covering the period of valve closing and opening were recorded, digitized, and analyzed later to determine the principal stretches and areal strains of the central area of the posterior leaflet.

Figure 3

An example of flow rate and transmitral pressure of the tested valve. The valve closed and opened between 0 and 0.35 s. The valve closed during 0 s and 0.15 s, the valve was closed between 0.15 s and 0.25 s, and the valve opened during 0.25 s and 0.35 s. The transmitral pressure increased only a few mm Hg before 0.1 s. The mitral flow was negative during valve closure, which is predominantly the closing volume. It increased up to 15L∕min during valve opening.

Figure 5

An example of principal angle of the deformation of the central region of the posterior leaflet during valve closing and opening. It deviated slightly from 90°. This deviation rarely went above 20°, demonstrating that the principal angle usually aligned with the radial and circumferential directions.

Figure 6

An example of the shear angle α for the posterior leaflet, which represents the change in angle that two originally orthogonal lines undergo with deformation. The shear angle α was generally small (usually less than 5°), suggesting that the differences between the principal stretches and the stretch values resolved into the collagen fiber preferred directions were sufficiently small and did not yield any new information.

Figure 7

A representative example of the major and minor principal stretches and stretch rates during valve closing and opening in the normal PM position. The principal stretches demonstrate a rapid rise early in valve closure, followed by a plateau in systole, suggesting that the collagen fibers were fully straightened. Principal stretches decreased to the original state when the valve opened. There was a significant difference between the major and minor principal stretches. The peak major principal stretch rate was higher than the minor principal stretch rate during both valve closing and opening. The peak major principal stretch rate was higher during the valve unloading (i.e., opening) process than during valve loading (i.e., closing) process.

Figure 8

An example of the areal strain and areal strain rate during valve closing and opening in the normal papillary muscle position. The areal strain demonstrated a similar trend as principal stretches, and there was a plateau when the valve was closed.

Figure 9

An example of the transmitral pressure versus areal strain during the valve closure in the normal papillary muscle positions. These results show a dramatic stiffening of the central region of the posterior leaflet.

Figure 10

An example of SALS experiment results were superposed on the posterior leaflet, showing the mapping of preferred fiber orientation on the posterior leaflet. The posterior leaflet generally had a more irregular structure. The preferred fiber orientation is relatively uniform in the central region of the posterior leaflet.

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