The mitral annulus (MA) or mitral annular ring refers to a membrane-like structure of the atrioventricular junction, that forms a hinge for the mitral leaflets and separates the left atrial and ventricular myocardial walls.
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Gross anatomy
The mitral annulus is a ‘D-shaped’ structure resembling a kidney bean with a straight anteromedial and a curved posterolateral portion and its non-planar shape has been compared to a saddle 1-3. Each of the components reaches the right and left commissures and is characterized by different adjacent anatomical structures 1. It is often located in a 1-2 mm more apical position than the mitral hinge 1.
The mitral annulus appears largest during isovolumetric relaxation in diastole and smallest during isovolumetric contraction in systole 1,2.
Relations and boundaries
The anteromedial component is in continuity with the left atrial wall and the mitral-aortic curtain, a fibrous tissue structure containing the interleaflet triangle of the non-coronary and left coronary sinus that connects the aortic root with the anteromedial part of the mitral annulus. The anteromedial component with the mitral-aortic curtain is supported by the left and right fibrous trigones both composed of dense connective tissue 1.
The posterolateral component of the mitral annulus connects the left atrial wall to the left ventricular free wall at the hinge of the mitral leaflet and is immediately adjacent to the epicardial adipose tissue of the left atrioventricular groove which extends between the left atrial and the crest of the left ventricular myocardium up to the hinge line 1.
Histology
Histologically the mitral annulus is a fibrous structure composed of connective tissue lined around the mitral orifice. In its posterolateral part, it is often discontinuous and intermittently not discernible and varies in thickness 1.
Radiographic features
Echocardiography
Echocardiography can visualize the mitral annulus with its adjacent structures with an outstanding temporal resolution which can be used to assess annular changes over the cardiac cycle 3. 3D TEE allows visualization of the entire mitral valve with the mitral annulus from overhead perspective 1.
CT
Cardiac CT can identify the hinge line, the atrial and ventricular myocardium as well as the epicardial adipose tissue of the left atrioventricular groove, which is immediately located adjacent to the above structures and aids in their discernibility 1. Cardiac CT can also nicely depict the course of the circumflex artery in the left atrioventricular groove 1.
MRI
Cardiac MRI can depict the anatomy in particular the epicardial adipose tissue the hinge line and the left ventricular myocardium and less clearly the left atrial myocardium. The fibrous membrane cannot be directly visualized just as little as in other imaging modalities 1.
Clinical importance
Mitral annulus anatomy is of clinical importance in mitral valve repair or mitral valve replacement 1.
Related pathology
Clinical conditions and diseases related to the mitral annulus include 1-4:
- mitral annular calcification
- mitral annulus disjunction
- mitral annular dilation (concerns, in particular, the posterolateral portion)
- mitral insufficiency