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An aortic valve commissure is a region where adjacent aortic valve leaflets meet at their high peripheral attachment to the aortic root. In the usual three-leaflet valve, three commissures rise toward the sinotubular junction. They are high points of the crown-shaped attachment, distinct from the low landmarks defining the virtual basal ring.^[ref]

A commissural attachment should also be distinguished from coaptation. The commissure identifies a peripheral supporting location; coaptation describes apposition of mobile leaflet surfaces during closure. Knowing three attachment positions does not establish the entire contact area or exclude an intervening gap.

An original geometric model illustrates why the spatial arrangement matters. Put three attachment points on a circle of radius $R$ with equal angular spacing. The distance between neighbors isFor $R=15$ mm, $d\simeq 25.98$ mm. Keeping the same radius but moving two adjacent points to $90^{\circ }$ separation changes their distance to $2R\sin 45^{\circ }\simeq 21.21$ mm. A fixed circular diameter alone therefore does not determine every span between attachment sites.

Real aortic valve geometry includes leaflet shape, attachment height and deformation, not just points on a flat circle. The calculation explains which information is lost by replacing a spatial arrangement with one diameter; it does not provide a surgical target. Locating each aortic valve commissure helps make anatomical descriptions and imaging views comparable without identifying the whole valve with a single ring.

The sinotubular junction is the distal boundary of the aortic root, where the bulging aortic sinuses transition to the tubular ascending aorta. It lies above the virtual basal ring; high points of the crown-shaped leaflet attachment reach this region at the aortic valve commissures.^[ref]

Its diameter is consequently a level-specific geometric measurement, distinct from a maximum sinus diameter or a basal aortic annulus diameter. A constructed axial outline could have respective diameters of 24, 32 and 28 mm at the basal, sinus and sinotubular junction levels. Those numbers are stipulated geometry, not normal values or a diagnostic threshold. Even perfect measurement cannot make the three levels interchangeable.

A simple blood flow calculation illustrates the consequence of specifying a level. Consider two circular sections in an ideal unbranched conduit carrying the same incompressible volumetric flow rate $Q$. Their mean axial speeds satisfyFor stipulated diameters of 20 and 30 mm, ${\overline{u}}_2/{\overline{u}}_1=4/9$. Increasing a circular diameter by $50\%$ increases its area by $125\%$, so speed does not scale inversely with diameter alone. The calculation is a continuity example; actual aortic root transport has moving boundaries, branching coronary arteries and nonuniform velocity.

The sinotubular junction also describes part of the supporting geometry of the aortic valve. Distance between commissural attachment sites and the shapes of the valve leaflets are different descriptors from one diameter. Understanding the junction therefore helps relate an anatomical outline to leaflet coaptation, while a single diameter cannot establish the complete closing geometry.