The mitral annulus is a pliable 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.1-3 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. 34-1). The anterior mitral leaflet is in direct fibrous continuity with the aortic annulus under 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.2
Diagram from a pathologic perspective with division of the septum illustrating the fibrous continuity between the mitral and aortic valves. (Reproduced with permission from Wells FC, Shapiro LM: Mitral Valve Disease. England: Butterworth-Heinemann; 1996.)
The mitral valve has two major leaflets, the larger anterior (or aortic) leaflet and the smaller posterior (or mural) leaflet; the latter usually contains three or more scallops separated by fetal clefts or subcommissures.4 The three posterior leaflet scallops are termed anterolateral (P1), middle (P2), and posteromedial (P3) scallops3 (Fig. 34-2). 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.4 The two leaflets are separated by the posteromedial and anterolateral commissures.
The operative view of the mitral valve is shown (left), and the corresponding “surgical view” obtained with real-time three-dimensional transesophageal echocardiography volume rendering (right). Images are from patient with type IIIb dysfunction (see text). A1, A2, A3, anterior mitral valve scallops; AL, anterolateral; P1, P2, P3, posterior mitral valve scallops; PM, posteromedial. (Reproduced with permission from O’Gara P, Sugeng L, Lang R, et al: The role of imaging in chronic degenerative mitral regurgitation, JACC Cardiovasc Imaging. 2008 Mar;1(2):221-237.)
The histologic structure of the leaflets includes three layers: (1) the fibrosa, the solid collagenous core continuous with the chordae tendineae; (2) the spongiosa, which is on the atrial surface and forms the leaflet leading edge (it consists ...