= Cardiac valve annulus
A \b[cardiac valve annulus] is the supporting attachment region around a <heart valve>'s opening, used as an anatomical and imaging reference for its <valve leaflets>. The term should be interpreted for the particular valve: the <mitral annulus> and the operational reference geometry of the <aortic annulus> are not interchangeable structures. An annulus is not necessarily a rigid, flat circle.
The three-dimensional shape matters experimentally. A primary study mounted eleven human specimens of the <mitral valve> in a left-heart simulator with adjustable flat or saddle-shaped annular configurations. In selected experiments, transducers recorded <chordae tendineae> forces.<Effects of a saddle shaped annulus on mitral valve function and chordal force distribution: an in vitro study by Jorge Hernan Jimenez and others>[Abstract, Methods]{ref} The shape comparison redistributed chordal forces, showing that a two-dimensional diameter alone does not describe the full supporting geometry.
An original area example separates two common geometric descriptors. An <ellipse> with semiaxes $a=20$ mm and $b=10$ mm has area $\pi ab=200\pi$ mm$^2$. A circle of radius $\sqrt{200}$ mm has the same area, but its diameter is about 28.3 mm rather than the ellipse's 40 mm major diameter. An equal area therefore does not determine the width a leaflet must span in every direction.
A further three-dimensional model can retain the same projected outline while varying attachment heights:
$$
x=a\cos t,\qquad y=b\sin t,\qquad z=h\cos(2t).
$$
Changing $h$ leaves the projected <ellipse> unchanged but changes the spatial boundary. This is a schematic saddle-like curve, not a complete anatomical model. It illustrates why measurements of projected area, diameter and height can carry different information. The relation between annular geometry, leaflet deformation and <coaptation> must be assessed with consistent valve-specific landmarks and phases.
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