8.6 Mitral Valve Anatomy

Mitral Valve Anatomy refers to the structural organization and morphological characteristics of the mitral valve, a critical component of the heart’s left atrioventricular orifice. It functions to regulate unidirectional blood flow from the left atrium to the left ventricle during diastole and prevents backflow during systole. The mitral valve is a complex anatomical structure composed of multiple interconnected elements that work in concert to maintain cardiac efficiency and hemodynamic stability.

Mitral Valve Orifice

The mitral valve orifice is the opening between the left atrium and left ventricle, delineated by the valve leaflets and their attachments. It is oval or D-shaped in cross-section, with a dynamic area that varies throughout the cardiac cycle. The orifice size is crucial for efficient blood flow and is supported by the surrounding fibrous annulus, which provides structural integrity and attachment for the valve leaflets.

Mitral Attachment Line

The mitral attachment line, also known as the mitral annulus, is a fibrous ring that serves as the anchoring site for the valve leaflets. It is a saddle-shaped structure with heterogeneous composition, combining fibrous and muscular elements. This line forms the boundary between the left atrium and ventricle and maintains the geometric configuration necessary for leaflet coaptation and valve competence.

Anterior Mitral Leaflet

The anterior mitral leaflet is a large, triangular, and robust leaflet that occupies approximately one-third of the mitral valve circumference. It is contiguous with the aortic valve via the fibrous continuity known as the anterior leaflet aortic continuity. The anterior leaflet is thicker and less flexible than the posterior leaflet, playing a major role in valve closure and pressure distribution during systole.

Posterior Mitral Leaflet

The posterior mitral leaflet is longer but narrower than the anterior leaflet and is subdivided into three scallops designated as P1 (lateral), P2 (middle), and P3 (medial). These scallops increase the flexibility and adaptability of the valve to ventricular motion and loading conditions. The posterior leaflet is more compliant and allows for dynamic changes in valve orifice shape during the cardiac cycle.

Anterolateral Mitral Commissure

The anterolateral commissure is the junction where the anterior and posterior leaflets meet laterally. It is reinforced by chordae tendineae and papillary muscles, providing mechanical support for leaflet coaptation and valve closure. This commissure is located near the anterolateral papillary muscle and contributes to the valve’s three-dimensional geometry.

Posteromedial Mitral Commissure

The posteromedial commissure is the medial junction between the anterior and posterior leaflets. It is similarly supported by chordae tendineae and the posteromedial papillary muscle. This commissure is clinically significant because it is a common site of valve pathology and plays a vital role in maintaining valve competence.

Anterior Leaflet Aortic Continuity

The anterior leaflet aortic continuity is the fibrous connection between the anterior mitral leaflet and the aortic valve. This continuity forms part of the left ventricular outflow tract and is critical for synchronized function of these adjacent valves. It also contributes to the structural integrity of the left ventricular base and plays a role in the dynamic movement of the anterior leaflet.

Posterior Mitral Scallops P1, P2, P3

The posterior leaflet is divided into three scallops for enhanced flexibility and sealing capability:

• P1 (Lateral scallop): Located nearest the anterolateral commissure, participates in leaflet coaptation at the lateral aspect.
• P2 (Middle scallop): The largest and central scallop, it plays a pivotal role in sealing the valve orifice.
• P3 (Medial scallop): Adjacent to the posteromedial commissure, important for medial leaflet coaptation.

These scallops allow the posterior leaflet to adapt to ventricular shape changes and contribute to valve competence.

Mitral Leaflet Coaptation Zones

Coaptation zones are the regions where the anterior and posterior leaflets come into contact during systole to prevent regurgitation. These zones are critical for valve competence and are formed along the free edges of the leaflets, including the scallop borders and commissural junctions. Proper coaptation depends on leaflet size, flexibility, and the tension provided by chordae tendineae and papillary muscles.

Mitral Valve Three-Dimensional Form

The mitral valve exhibits a complex three-dimensional saddle-shaped geometry, which optimizes stress distribution and leaflet coaptation. The annulus is non-planar, with its highest points located at the anterolateral and posteromedial commissures and the lowest points near the anterior and posterior midpoints. This shape reduces leaflet stress during systole and enhances the durability of the valve. The interplay among leaflets, annulus, chordae tendineae, papillary muscles, and the ventricular myocardium creates a dynamic structure that adapts continuously to the cardiac cycle.

This comprehensive anatomical framework of the mitral valve supports its function as a highly specialized structure critical for maintaining effective cardiac output and preventing regurgitation in the left heart. Understanding these components is essential for clinical evaluation, surgical intervention, and the design of prosthetic or repair devices.