Differential collagen distribution in the mitral valve and its influence on biomechanical behaviour.

Surgical repair of the mitral valve primarily involves endogenous valve tissue, however, the intrinsic mechanical behaviour of the tissue is not well described. To address this issue, porcine mitral valve leaflets were examined histologically and engineering concepts were applied to understand the mechanical behaviour of the layered tissue. Rectangular portions were excised from the anterior and posterior leaflets, either parallel or perpendicular to the annulus, and sections were stained for collagen (Masson's trichrome). The cross sectional layers of the valve (atrialis/spongiosa, fibrosa, and ventricularis) were identified by differences in cellularity and collagen density. The fibrosa is composed of dense collagen, while the atrialis/spongiosa and ventricularis are composed of loose collagen. Layer thicknesses were recorded digitally across the section. These values were averaged within tissue groups to determine changes in layer thickness over the length of the sample and average thickness of each layer. In all tissue groups, the fibrosa was the thickest layer, and the atrialis/spongiosa layer was thicker than the ventricularis layer. The total and fibrosa layer thicknesses of the anterior leaflet were significantly thicker than in the posterior leaflet. Mechanical engineering analysis of the layered structures under tension indicated that the anterior leaflet would be able to support greater tensile loads than the posterior leaflet. The layered arrangement was then examined as a beam in bending, and was shown to decrease the resistance of leaflets to bending, and decrease the overall bending stresses on the leaflet. This type of analysis may be extrapolated to gain insight into changes in function in diseased valves as well.(ABSTRACT TRUNCATED AT 250 WORDS)