= Mitral coaptation index
The \b[mitral coaptation index] is a dimensionless normalization of the <mitral valve>'s <coaptation area>. In the primary method of <Frederick C. Cobey> and colleagues, it is the three-dimensional end-systolic coaptation-zone area divided by the three-dimensional mitral-valve area used in that protocol:
$$
I_c=\frac{A_{\rm coaptation}}{A_{\rm MV}}.
$$
The denominator and surface-reconstruction convention are part of the definition.<Quantitative assessment of mitral valve coaptation using three-dimensional transesophageal echocardiography by Frederick C. Cobey and others>[Abstract, Methods, novel MV coaptation index formula]{ref}
The study used intraoperative transesophageal <echocardiography> in 25 patients with functional <mitral regurgitation>, followed by offline analysis of the end-systolic geometry.<Quantitative assessment of mitral valve coaptation using three-dimensional transesophageal echocardiography by Frederick C. Cobey and others>[Abstract, Methods]{ref} This supplies an experimental acquisition context for the ratio; it does not make every area called annular area interchangeable with the protocol's valve-area measurement.
An original normalization example uses $A_{\rm coaptation}=1.5$ cm$^2$ and $A_{\rm MV}=10$ cm$^2$, giving $I_c=0.15$. A second constructed valve with areas 3 and 20 cm$^2$ has the same index despite twice the absolute contact area. Conversely, keeping contact area 1.5 cm$^2$ while changing the denominator to 15 cm$^2$ gives $I_c=0.10$. Absolute area and normalized coverage answer different questions.
Uniformly scaling every length by a positive factor $s$ multiplies both areas by $s^2$, leaving $I_c$ unchanged. Inconsistent surface conventions do not cancel: counting two apposed faces in the numerator while counting their interface once in another reconstruction can change the ratio without a biological change.
Finally, the index does not describe the connectivity of any remaining gaps. Two contact patches with equal <coaptation area> and equal denominator can have different shapes and positions, as illustrated under <coaptation area>. The <mitral coaptation index> is a geometric descriptor to interpret alongside the reconstructed valve and <blood flow>, rather than a complete description of valve sealing.
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