The Structure and Mechanical Properties of the Mitral Valve Leaflet-Strut Chordae Transition Zone

[+] Author and Article Information
Ling Chen

Department of Mechanical Engineering, San Diego State University, San Diego, CA, 92182-1323e-mail: lchen@kahuna.sdsu.edu

Frank C-P. Yin, M.D., Ph.D.

Department of Biomechanical Engineering, Washington University, St. Louis, MO 63130 e-mail: yin@biomed.wustl.edu

Karen May-Newman, Ph.D.

Department of Mechanical Engineering, San Diego State University, San Diego, CA, 92182-1323 e-mail: kmn@kahuna.sdsu.edu

J Biomech Eng 126(2), 244-251 (May 04, 2004) (8 pages) doi:10.1115/1.1695569 History: Received February 12, 2003; Revised October 24, 2003; Online May 04, 2004
Copyright © 2004 by ASME
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Schematic showing the specimen preparation for biaxial testing: (a) A section of the anterior leaflet containing a single strut chordae was excised and markers attached spanning the distance from the annular muscle border to the strut chordae. The locations along the LCT zone were normalized by the total distance between the annular muscle border and the point at which the chordae separates from the leaflet and termed Normalized Annular-Chordae Distance (%NAD). Four regions of interest (ROI) are shown, each of which is defined by a group of 4 markers; (b) The specimen was mounted to the carriages of the biaxial test device such that the circumferential (Circ.) and radial directions were aligned with the axes of biaxial testing. Four to six continuous stitches of 5-0 silk sutures were placed on each of three sides of the specimen with a single tether attached to the chordae. lx and ly are the lengths along these directions over which the sutures exert force; (c) the biaxial test device stretched the tissue in two directions simultaneously using a video-feedback system.
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Histology preparation and analysis: (a) The orientation of the tissue specimens used for collagen fiber angle measurement is shown as a side view (left) and a ventricular side top view (right). The entire leaflet-chordae specimen was embedded, sectioned and stained to visualize collagen fibers, with care taken to include as much of the plane of interest as possible; (b) Digital microscopic images were taken at several locations along the centerline, and fiber angle measurements were made using the muscle-tissue border as a reference for 0 deg.
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Stress-stretch responses to a strip biaxial radial protocol for two different specimens are shown. Each curve corresponds to a different ROI, with the corresponding %NAD position as listed. The relations show that the behavior in the radial direction becomes less extensible with distance along the LCT zone.
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Stress-stretch responses to the strip biaxial circumferential protocol for two different specimens along the LCT zone. The data demonstrate that the response in the circumferential direction becomes more extensible near the chordae.
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Results from all of the strip biaxial radial testing: (a) The stretch at a stress value of 25 kPa was used as a measure of tissue extensibility in the radial direction along in the LCT zone. The results show a progressive decrease in extensibility from annulus to approximately 80% NAD; (b) Linear regression of pre-transitional stiffness demonstrates a progressive increase from the leaflet toward the chordae; (c) Post-transitional stiffness also increases along this direction.
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Collagen fiber angle distribution and its standard deviation in the LCT zone. Symbols are from four individual specimens, with each symbol and error bar corresponding to the measurement at each location (%NAD) along the LCT zone. Fiber angle measurements were made in 12 regions spanning the entire LCT zone for each specimen.
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Strip biaxial stress-stretch curves generated from the constitutive model at different %NAD using the collagen fiber data from Table 1 and the coefficients listed in Table 2. The circumferential direction becomes progressively more extensible with increasing %NAD, while the radial direction becomes less extensible.




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