The structures of the mitral valve encompass the annulus (i.e., annular attachment at the atrioventricular junction), leaflets, chordae tendinae, and posteromedial and anterolateral papillary muscles in the left ventricle.

The oval (D-shaped) flexible fibrous mitral valve annulus (hingeline of the valvular leaflets) enables the orifice to undergo complex shape changes during the cardiac cycle. It determines the opening area of the mitral valve. It is composed anteriorly of a fibrous component between the two fibrous trigones, the trigonum fibrosum dextrum and sinistrum, which separate the mitral valve from the aortic valve [481]. The right fibrous trigone and the membranous septum form the central fibrous body. The annulus edge opposite to the fibrous body can be altered by dilation and be the site of calcifications. In the anterior concave part of the annulus, fibers have a parallel and circular orientation. Its lateral and posterior parts are linked to the anterior part by the left and right fibrous trigone. The lateral part of the mitral annulus consists of collagen fibers. In the annulus transition zone, where the leaflets are anchored to the myocardium, the elastin and collagen fibers radiate into the myocardium [481]. In the hinge zone, where the atrial endocardium is thickened, the number of elastin fibers increases.

The anterior leaflet is commonly semicircular and the posterior leaflet has a crescentic shape. Both leaflets merge at the posteromedial and anterolateral commissures (Table 6.1). The tips of the papillary muscles point toward the commissures. A commissure corresponds to long invaginations of the free margin that limit adjacent cusps, the depth of which is larger than two thirds of the maximal width of the larger of the two adjacent leaflets. An indentation that separates two valvular cusp segments have a depth ranging from one third to two thirds of the maximal width of the larger of the two adjacent leaflets. Their number and position determine the number and size of valvular segments.

The anterior leaflet (a.k.a. aortic and septal leaflet) is the bigger cusp (Table 6.2). It is close to the aortic root. The aortic leaflet is divided arbitrarily into three regions (A1–A3) from the cardiac base to the apex. The anterior leaflet is close to the aortic valve (left and noncoronary cusps). It is much smoother than the mural cusp, usually having no indentations along its free edge. The atrial myocardium contacts the hinge of the aortic leaflet [486].

The narrow posterior leaflet (a.k.a. mural leaflet) is in 91 % of cases divided into three parts (posteromedial or right, intermediate, which is wider and higher than the others, and anterolateral or left) [481]. In adults, the mural leaflet has indentations that do not extend through the leaflet to the annulus. They generally form three segments¹ (scallops due to scalloped shape) along the elongated free edge according to Carpentier’s nomenclature (Fig. 6.1 and Table 6.3; [487]):
Each Pi segment faces Ai segment of the anterior leaflet. The posterior smaller leaflet is attached to nearly two thirds of the circumference around the left atrioventricular junction. The degree of myocardial extension toward the insertion of the hinge (or fulcrum) of the mural posterior leaflet varies between hearts and in a given heart, between regions [486].

The leaflets can be subdivided into a basal region, the zone of attachment to the annulus, and thin central translucent (clear) and thick rough zone from the attachment point of each leaflet at the annulus to the free edge. The chordae tendinae of the mitral valve are tethered to the ventricular face of the leaflet rough zone and free margin [481]. The rough zone is also the region of apposition (symmetrical overlap of the leaflet free edge, usually a minimum of 4–5 mm) and coaptation (region of between-leaflet contact when the mitral orifice is closed). Coaptation and correct apposition of leaflets prevent regurgitation.

Measurements of the leaflet geometry encompass:
The two leaflets of the mitral valve differ in structure. The mitral leaflets are composed of a fibrous skeleton. The mitral valve is made up of four layers: ventricularis, fibrosa, spongiosa, and atrialis [487]. Two main layers, a lamina spongiosa and a lamina fibrosa, face the atrial and ventricular side, respectively. They are covered by the endocardium. The latter is continuous with that of the atrium and ventricle. The thickness of each layer varies from the cusp insertion at the annulus to the free edge.

The atrialis is composed of mainly aligned elastic and collagen fibers. The spongiosa, a major component of the free edge, consists principally of proteoglycans and elastic fibers. The fibrosa, a major load-bearing layer, comprises a collagenous core. The collagen fibers are aligned and provide strength and stiffness to the leaflet. The fibrosa extends from the annulus into two thirds of the leaflet; it is absent at the free edge. The ventricularis covered by an endothelium consists of elastic and collagen fibers. Near the annulus, the fibrosa is the thickest layer; close to the free edge, it becomes thinner. On the other hand, the thickness of the spongiosa and atrialis rises distally to become the main component of the leaflet at the free edge [487].

The subendocardial connective tissue of the lamina spongiosa consists of fibroblasts, fibrocytes, histiocytes, and collagen fibers (caliber 15–35 nm) [481]. Fibroblasts, smooth myocytes, and cardiomyocytes in the proximal third reside in mitral valve leaflets. Cardiomyocytes extend a short distance into the base of the mitral leaflets without continuity with atrial and ventricular walls. At the atrioventricular junction, fibrofatty tissues indeed interpose.

The architecture of the fibrous meshwork, that is, the local fiber orientation, influences the valve rheology. Collagen in the leaflet comprises primarily type-1 (74 %), type-3 (24 %), and type-4 (2 %) [487].

Extra leaflets can be observed [490]. About two thirds of heart specimens exhibit the presence of an extra commissure or indentation, hence of additional leaflets or segments, respectively, at least in a small Indian population.

Mitral valve leaflets possess innervations from both sensory and autonomic components [481]. In the anterior leaflet, nerve density is twofold greater than that in the posterior leaflet. Nerves are situated in the atrial layer and extend over the proximal and medial portions of the leaflet.

Both leaflets are attached to the papillary muscles by the chordae tendinae. The posteromedial papillary muscle gives cords to the medial half of both leaflets (i.e., posteromedial commissure, A3, P3, and posterior half of A2 and P2 segments). The anterolateral papillary muscle sends cords to the lateral half of the leaflets (i.e., anterolateral commissure, A1, P1, and anterior half of A2 and P2 segments). Their position in the left ventricle varies among patients. In some subjects, one or both papillary muscles cannot be clearly defined; they are replaced by multiple small muscle bundles connected to the ventricular wall [487].

Thin chordae have a lower average fibril bore and a greater average fibril density than thick chordae. The surface of the chordae consists of a layer of endotheliocytes and a dense layer of elastic fibers [481]. These tendinous chordae are composed of elastic and collagen fibers parallel to the axis. They transmit the tension of the contracting papillary muscles to the valve leaflets. The majority of the chordae insert at the free margin or behind the free margin at the ventricular side (rough zone) of the leaflets. The base chordae insert at the leaflets near their attachment at the annulus originating from ventricular myocardium. The commissural chordae (interleaflet or commissural cords) insert at the free margin of two adjoining leaflets. Normally, only two commissural cords exist supporting each free margin of the commissural region. These cords arise from a single stem that branches, thereby allowing adjacent leaflets to coapt and to move apart.

In fact, numerous classifications of tendinous cords exist. Three types of chordae tendinae are also described [487]:
Among the secondary cords of the aortic (anterior) leaflet, two are the largest and thickest (strut cords). The latter originate from the tip of each papillary muscle. Hence, according to function, tendinous cords are categorized into commissural, indentation, and strut cords.

Mitral valve repair is more popular than mitral valve replacement. However, changes in cusp geometry alter valvular mechanics and outcomes, especially valve repair durability.

Annular flattening can result from myocardial infarction. On the other hand, the three-dimensional saddle shape of the mitral annulus changes during the cardiac cycle with systolic high points along the anterioposterior plane and low points at commissures. This shape enables a better leaflet curvature during systolic closure. The saddle shape influences the mechanics of the mitral leaflet. It lowers stresses on the chordae tendineae and strains on the leaflets, in particular in the posterior leaflet during systolic valve closure [492]. Maximal strain decreases from planar shape to 20 % saddle annulus.²

Estimation of regional stresses and strains enables to adapt the therapeutic strategy for valvular repair and replacement. In vivo deformations of the mitral valve anterior leaflet during the cardiac cycle was evaluated using sonocrystal transducers sutured to the valve [493]. Changes in annular geometry alter valvular strains in vivo.

The deformation of the insertion zone of chordae to porcine mitral valves endowed with a structured array of markers was measured during the cardiac cycle by tracking marker motions using high-speed cameras [495]. The insertion zone mechanics depends on the collagen architecture. The insertion region is stretched during the entire cardiac cycle.

Kinematics of the mitral valve annulus was explored in sheep using implanted sonocrystals [494]. Kinematic data collected can be fitted to a 3-D spline based on 5-order hermite shape functions with C continuity. Axial strain varies spatially and temporally (minimum and maximum -10 and 4 %, respectively) with strain rate changing up to 100 %/s contraction and 120 %/s elongation. Most of the strain energy is related to annular axial strain. However, implantation of sonocrystals may alter the annulus deformation, in addition to differences in behavior among mammalian species.

Simulations were carried out to assess the stress field in the mitral valve anterior leaflet, using different pseudo-hyperelastic constitutive models, especially a collagen fiber-mapped transversely isotropic model, and a given valve microstructure [496]. However, the collagen fiber arrangement and material constant may not represent perfectly in vivo data.