Of the four heart valves, the mitral valve is the only valve that has a distinguishable annulus. However, the presence of a fibrous annular structure is variable and discontinuous. The firmest site of support for the mitral valve is the region of fibrous continuity between the aortic and mitral valves, the extent of which is delineated by the right and left fibrous trigones (Fig. 2). Nevertheless, for surgical purposes, the annulus is considered the area of attachment of the valve leaflets to the atrial muscle. The annulus is a functional component of the mitral valve. The annulus is quite flexible.
