Mitral annulus

The mitral annulus is the fibrous junctional zone at the atrioventricular junction that separates the left atrium from the left ventricle and serves as the insertion site for the mitral valve leaflets. It is more of a conceptual structure than a discrete, well-defined anatomic ring.

Anatomy and Structure

The mitral annulus has a D-shaped configuration, with the straight border formed by the aorto-mitral intervalvular fibrosa (the fibrous continuity between the mitral and aortic valves). It is divided into two distinct segments:

• Anterior annulus: Reinforced by the aorto-mitral curtain, which provides fibrous continuity with the aortic valve. There is no true discrete annular ring anteriorly; instead, the anterior mitral leaflet transitions continuously into the intervalvular fibrosa. Two triangular fibrous regions the anterolateral (left) and posteromedial (right) trigones anchor each end of the anterior annulus.

• Posterior annulus: Composed of a thin, discontinuous band of fibrous tissue at the attachment of the posterior leaflet, periodically interrupted by adipose tissue. This segment is less structurally robust and is the portion most prone to dilation in mitral regurgitation and to calcification in degenerative disease.

Three-Dimensional Geometry

The mitral annulus has a nonplanar, three-dimensional saddle shape, with the highest points (peaks) located along the anterior annulus (toward the left atrium) and the posterior annulus, and the lowest points (nadirs) near the commissures and fibrous trigones.

Dimensions

The normal dimensions of the mitral annulus show wide interindividual variation and differ by imaging modality, measurement technique, sex, and phase of the cardiac cycle. The key dimensional parameters and their reference values are summarized below.

Annular Diameters

The mitral annulus is characterized by two principal diameters:

• Anteroposterior (septolateral) diameter: By 3D echocardiography, the mean is approximately 28.5 ± 3.7 mm. By CMR (UK Biobank, n = 721 healthy adults), end-diastolic anteroposterior diameters are 29 ± 4 mm in men and 26 ± 4 mm in women, increasing at end-systole to 33 ± 4 mm and 30 ± 4 mm, respectively.

• Intercommissural (anterolateralposteromedial) diameter: Mean approximately 33.0 ± 5.3 mm by 3D echocardiography. The intercommissural diameter is typically larger than the anteroposterior diameter, contributing to the D-shaped annular geometry.

Annular Area

• By 3D echocardiography, normal mean mitral annular area ranges from 8.4 to 11.8 cm² (indexed 4.75.1 cm²/m²).

• By cardiac CT, normal values range from 8.4 to 10.2 cm² (indexed 4.55.5 cm²/m²).

• By 2D echocardiography, earlier studies reported smaller values, with a maximal annular area index of 3.8 ± 0.7 cm²/m².

Annular Circumference (Perimeter)

• Mean maximal annular circumference is approximately 93 ± 9 mm by 2D echocardiography, and approximately 107 ± 14.6 mm by 3D echocardiography. [1][5]

• The upper limit of normal for systolic perimeter is 125 mm, and for systolic area is 675 mm² (6.75 cm²).

Men have larger absolute annular dimensions than women across all parameters. However, after indexing to body surface area, most differences diminish, except for the septolateral distance, which remains relatively larger in women.