6.5 Mitral Junction Anatomy

Mitral Junction Anatomy refers to the detailed structural and spatial configuration of the mitral valve complex at its junction with the left atrium and left ventricle. It encompasses the anatomical components that define the mitral orifice, its fibrous support, and the continuity and relationships between the atrial and ventricular myocardium around the valve. The mitral junction ensures the proper function of the mitral valve by maintaining its shape, flexibility, and anchorage during the cardiac cycle.

Left Atrioventricular (AV) Orifice

The mitral junction forms the left AV orifice, which is the opening between the left atrium and left ventricle allowing blood flow during diastole. This orifice is framed by the mitral annulus, a fibrous ring that provides structural support and maintains the orifice’s shape and size. The annulus is saddle-shaped in three dimensions, optimizing leaflet coaptation and minimizing leaflet stress.

The left AV orifice is characterized by:

• An oval shape with a major axis oriented anteroposteriorly.
• Surrounding myocardial tissue from the left atrium superiorly and left ventricle inferiorly.
• Attachment points for the anterior and posterior mitral leaflets.

Mitral Orifice Position and Orientation

The mitral orifice is positioned between the left atrium and left ventricle, located posteriorly and medially relative to the aortic valve. Its orientation is oblique, directed from the atrium to the ventricle with a tilt that aligns the inflow of blood towards the left ventricular outflow tract.

Key positional features include:

• The anterior mitral leaflet is adjacent to the aortic root, forming a fibrous continuity with the aortic valve.
• The posterior leaflet is anchored to the left ventricular free wall.
• The orifice lies at the junction of the atrial and ventricular myocardium with a fibrous annulus as the transition zone.

Atrium-Mitral Orifice Continuity

This continuity refers to the anatomical and functional interface between the left atrial myocardium and the mitral annulus. The atrial myocardium extends to the mitral annulus, blending into the fibrous tissue that forms the annular ring. This interface is critical for:

• Transmitting atrial contraction forces to the valve apparatus.
• Maintaining annular flexibility and allowing dynamic changes during the cardiac cycle.
• Supporting the anterior leaflet’s fibrous continuity with the aortic root, forming the aortomitral curtain.

Mitral Orifice-Ventricular Continuity

The ventricular side of the mitral junction comprises the interface between the mitral annulus and the left ventricular myocardium. This continuity includes:

• The insertion of the mitral leaflets via chordae tendineae into papillary muscles.
• The fibrous annulus transitioning into muscular ventricular tissue.
• The dynamic movement of the ventricular myocardium affecting annular shape and leaflet coaptation during systole.

The ventricular continuity ensures the mitral valve closes securely during ventricular contraction, preventing regurgitation.

Mitral Junction Circumference

The circumference of the mitral junction is defined by the annular ring that encircles the mitral orifice. It is typically 9 to 12 cm in length and exhibits a complex three-dimensional saddle shape with high and low points that correspond to leaflet insertion sites.

The circumference can be divided into:

• Anterior segment: fibrous, forming part of the aortomitral curtain.
• Posterior segment: muscular and partly fibrous, supporting the posterior leaflet.
• Septal margin: adjacent to the interventricular septum.
• Free-wall margin: adjacent to the lateral wall of the left ventricle.

Mitral Junction Fibrous Support

The fibrous support of the mitral junction consists mainly of:

• The mitral annulus: a dense collagenous structure that anchors the valve leaflets.
• The fibrous trigones: thickened regions of connective tissue at the junction of the mitral and aortic valves.
• The aortomitral curtain: a fibrous continuity between the anterior mitral leaflet and the aortic valve.

This fibrous framework maintains valve integrity, resists deformation, and provides a scaffold for the dynamic motion of the valve apparatus.

Mitral Junction Septal Margin

The septal margin of the mitral junction lies adjacent to the interventricular septum. It corresponds to the anterior part of the annulus where the anterior mitral leaflet attaches. This margin is closely related to the membranous septum and the left ventricular outflow tract.

Anatomically, this margin:

• Supports the anterior leaflet.
• Is part of the fibrous skeleton connecting the mitral and aortic valves.
• Is a critical region for conduction tissue proximity.

Mitral Junction Free-Wall Margin

The free-wall margin corresponds to the posterior and lateral segments of the mitral annulus adjacent to the left ventricular free wall. This margin supports the posterior mitral leaflet and is more muscular than fibrous compared to the septal margin.

Characteristics include:

• Greater flexibility compared to the anterior margin.
• Attachment of the posterior leaflet chordae and papillary muscles.
• Contribution to dynamic changes in annular shape during the cardiac cycle.

Mitral Junction Spatial Map

The spatial configuration of the mitral junction is a three-dimensional saddle-shaped ring that optimizes leaflet coaptation and valve competence. The annulus has high points at the anterior and posterior commissures and low points at the midpoint of the anterior and posterior leaflets.

This saddle shape reduces leaflet stress during closure and adapts dynamically during the cardiac cycle, influencing valve function and flow dynamics.

The mitral junction anatomy integrates complex structural and functional elements that maintain the mitral valve’s competence and coordinate its movement with atrial and ventricular contractions, essential for efficient cardiac hemodynamics.