Before describing the specific anatomies of the atrioventricular valve, it is important to understand the anatomical cardiac framework that holds these valves in position at their respective atrioventricular junctions, and thus consequently the relationships of each valve to the others [6]. Figure 1.3 shows an anatomical plate of a human heart with the atria and great arteries removed highlighting the close proximity of all four cardiac valves to each other. Traditionally, the four valves of the heart have been described as being supported by a fibrous framework or cardiac skeleton made of dense connective tissue passing transversely through the base of the heart between the atria and the ventricles. However, current interpretations of the extent of the skeleton are often greatly exaggerated. As described by Wilcox, Cooke and Anderson [6] and by Bateman et al. [7] the strongest part of the skeleton is the area of fibrous continuity between the leaflets of the mitral and aortic valves. This fibrous strap, thickened at both ends by the fibrous trigones, anchors the aortic-mitral valvar unit within the base of the left ventricle (Fig. 1.4). The coronet-like support of the aortic valvar leaflets extends antero-cranially from the region of fibrous continuity, and is often considered to represent an aortic valvar annulus, but there are no anatomical structures supporting the semilunar hinges of the aortic valvar leaflets (Fig. 1.5) [7]. The right fibrous trigone is itself continuous with the membranous part of the ventricular septum, and is an integral part of the aortic coronet (Fig. 1.4). The trigone and membranous septum together are usually described as the central fibrous body. The so-called annular components of the atrioventricular valves then extend inferiorly and posteriorly from the central fibrous body and the left fibrous trigone, respectively (Fig. 1.5) [7].

It is the exception rather than the rule, however, for these fibrous cords to extend throughout the full circumferences of the left and right atrioventricular junctions. The annulus, as such, is better formed in the mitral as opposed to the tricuspid junction. Even in the mitral junction, nonetheless, it is common to find segments of the valvar leaflets hinged from the fibro-adipose tissue of the atrioventricular junction, rather than from a firm fibrous annulus [8]. In the tricuspid junction, the valvar leaflets are normally hinged from fibro-adipose tissue [9]. It is also the fibro-adipose tissues of the junctions that provide the greatest part of the insulation between the atrial and ventricular muscular masses, with the atrioventricular bundle of the ­conduction system being the only structure in the normal heart that crosses the ­insulating plane. The bundle penetrates through the atrioventricular component of the membranous septum. The annuluses, as part of the atrioventricular junctions and rarely being complete fibrous rings, are highly dynamic and change dramatically in shape and size throughout the cardiac cycle from systole to diastole [6, 7].

In the most basic anatomical sense, the atrioventricular valves are made up of three main components, as seen in Fig. 1.8:

The leaflets of the atrioventricular valves can be thought as forming a skirt that hangs from the annulus, and are divided into a series of sections that constitute the distinct leaflets of each valve. Due to the extent of variations between individuals with regard to leaflet morphologies, there has been much debate relative to nomenclature on the number of leaflets of both the mitral and tricuspid valves [10–12]. Traditionally, the division of the leaflets has been determined by the presence of commissures which can be described as the peripheral attachment of a break in the skirt [6].

The leaflets themselves are attached to the ventricles via the sub-valvar apparatus of each valve. In general, each apparatus consists of both the tendinous cords and the papillary muscle complexes of each valve. The tendinous cords are usually categorized by (1) those that support the free edges of the valves, (2) those that support the rough zones (the region between the free edge and each annulus), and/or (3) those that attach to the leaflets near to the annulus. Typically, the cords supporting the free edges of the leaflets are known as fan cords due to the presence of multiple fenestrations. Those that attach to the rough zone of the leaflets are distinguished by their larger size and are commonly defined as strut cords. Finally, those that attach near the annulus are known as basal cords. The strut cords are of specific importance as they bear the highest mechanical loads during systole [13]. Furthermore, the number and distribution of the tendinous cords across a given valve are critical to its function; it is well documented that dysfunction of these structures can lead to prolapse of the valves [2, 14, 15]. In general, the cords attach to the heads of the papillary muscles which themselves play an important role in the function of each valve by contracting during systole to cushion the valve closure.

The left atrioventricular valve, or mitral valve, named by Andreas Vesalius due to its structural resemblance to the cardinal’s mitre, is situated in the left atrioventricular junction and modulates the flow of blood between the left atrium and ventricle. Commonly, the valve consists of an annulus, two leaflets, two papillary muscle complexes, and two sets of tendinous cords, as seen in Fig. 1.8.

In 1976 Carpentier described the mitral valve as consisting of two apposing leaflets—a posterior leaflet with three scallops and an anterior leaflet with one scallop. Each region of the leaflets is designated an alphanumeric label to distinguish it from the rest of the valve (Fig. 1.9) [28]. However, when one considers these structures relative to the landmarks of the body (i.e., in an attitudinally correct nomenclature), the leaflets are located in postero-inferior and antero-superior positions. Confusion regarding positional nomenclature can be avoided when adopting the more traditional approach suggested by Vesalius for distinguishing between the leaflets, and recognizing that they are aortic and mural in their locations [29]. The junctions of the two leaflets are commonly referred to as the anterolateral and the posteromedial commissures, however these are more accurately described as superior and inferior. The line of apposition of the leaflets during valve closure is known as the fibrous ridge.

The simplicity and practicality of Carpentier’s anatomic description of the mitral leaflets led to its widespread use after being introduced in 1976 [28]; yet, while this description defines a majority of mitral valve anatomies, there can be wide variability in both the number of scallops within each leaflet and their relative positions [30].

In general, the aortic leaflet is found to be attached to approximately one-third of the annulus circumference and is supported by the aorto-mitral fibrous continuity, which terminates in the left and right fibrous trigones (Fig. 1.10). The mural leaflet is attached to the remaining two-thirds of the annulus and also to the fibrous extensions that continue from the trigones around the mitral valve. However, the lengths of these extensions can be highly variable. Furthermore, a fibrous-fatty tissue ­surrounds the valve in areas where the cardiac skeleton is not present. The mitral annulus is a highly dynamic feature of the heart, changing dramatically in shape and size throughout the cardiac cycle. It is often described as being saddle-shaped with the highest point of the saddle, the saddlehorn, being found at the midpoint of the area of aortic-to-mitral valvar continuity [31] (Figs. 1.6 and 1.11). Both Delgado and Veronisi and their colleagues reported a series of annular dimensions that were recorded using echocardiography in healthy patients; these data are summarized in Table 1.1 [32–34].