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The mitral annulus is a pliable junctional zone of discontinuous fibrous and muscular tissue that joins the left atrium and left ventricle and anchors the hinge portions of the anterior and posterior mitral leaflets.1–8 The annulus has two major collagenous structures: (1) the right fibrous trigone, which is part of the central fibrous body and is located at the intersection of the membranous septum, the tricuspid annulus, and the aortic annulus; and (2) the left fibrous trigone, which is located near the aortic annulus under the left aortic cusp (Fig. 40-1). The anterior mitral leaflet spans the distance between the commissures (including the trigones) and is in direct fibrous continuity with the aortic annulus under the left and noncoronary aortic valve cusps, including the fibrous triangle between the left and noncoronary aortic valve cusps. The posterior half to two-thirds of the annulus, which subtends the posterior leaflet, is primarily muscular with little or no fibrous tissue.4
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The mitral valve has two major leaflets, the larger anterior (or aortic or septal) leaflet and the smaller posterior (or mural) leaflet; the latter usually contains three or more scallops separated by fetal clefts or subcommissures, which are developed to variable degrees in different individuals.9 The three posterior leaflet scallops are anatomically termed anterolateral (P1), middle (P2), and posteromedial (P3) scallops (Fig. 40-2). The portions of the leaflets near the free margin on the atrial surface are called the rough zone, with the remainder of the leaflet surface closer to the annulus being termed the smooth (or bare or membranous) zone. The ratio of the height of the rough zone to the height of the clear zone is 0.6 for the anterior leaflet and 1.4 for the posterior leaflet because the clear zone on the posterior scallops occupies only about 2 mm.9 The two leaflets are separated by the posteromedial and anterolateral commissures, which usually are distinctly developed but occasionally are incomplete.
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