Anatomy of the so-called “Aortic Annulus”
Anatomy of the Aortic Valvar Complex
The aortic annulus is a fibrous ring at the aortic orifice to the front and right of the atrioventricular aortic valve and is considered the transition point between the left ventricle and aortic root. The annulus is part of the fibrous skeleton of the heart.
Figure 11. Ventriculo arterial junction
The anatomic ventriculo-arterial junction is the transition point where left ventricular muscular tissue is replaced by the fibroelastic walls of the aortic valvar sinuses (Figure 11). Two of the aortic valvar leaflets cross this ventriculo-arterial junction and take their basal origins from the muscular walls of the left ventricle (Figure 12). The semilunar hinge line of the coronary aortic leaflets, therefore, incorporates a small crescent of ventricular myocardium within the aortic sinuses. It is the virtual plane created by joining together the basal attachments of the leaflets that demarcates the frequently described "aortic annulus”. It is this diameter of the entrance to the root that is important for the purposes of transcatheter aortic valve sizing.
This is a close-up, long-axis view of the left ventricle and the aortic root. The aortic root is supported by partially fibrous and partially muscular walls with the anterior or aortic leaflet of the mitral valve in fibrous continuity with the aortic valve.
This area of fibrous continuity (black line) involves the left and non-adjacent leaflets. The ends of this area of fibrous continuity are thickened and form the right (yellow diamond) and left (red diamond) fibrous trigones with the right fibrous trigone continuous with the membranous septum. These two components then form the central fibrous body.
The attachments of the fibrous trigones to the crest of the musculature of the ventricular septum are what anchors the aortic root and the mitral valve within in the roof of the left ventricle.
In the left ventricle, the aortic root interposes between the mitral valve and the septum. The anatomic ventriculo-arterial junction is in the area where the fibrocollagenous wall of the aorta is supported by the musculature of the parietal wall of the ventriculo-arterial junction. There is epicardial fat marking the outer aspect of this junction.
The yellow dots mark the sinotubular junction which is the most distal extent of the aortic root. This area is typically represented by a discrete ridge between the aortic valvar sinuses and the tubular component of the aorta. The red lines represent a fibrous triangle between the right and non-adjacent aortic leaflets. These fibrous triangles extend between the semilunar hinge lines of the three aortic valvar leaflets to become the most distal extension of the ventricular outflow tract.
Figure 11. Ventriculo arterial junction (annotations)
1 - Left coronary artery
2 - Right coronary artery
3 - Non-coronary leaflet
4 - Atrial septum
5 - Coronary sinus
6 - Mitral valve
7 - Left ventricle
8 - Membranous septum
9 - Ventriculo-arterial junction
Histology of the aortic valvar complex (Figure 12) shows the anatomic ventriculo-arterial junction. Also, note that the basal attachment of the aortic valvar leaflets to the ventricular myocardium is proximal relative to the anatomic junction.The aortic leaflet displays an endothelial lining and a fibrous core30.
Figure 12. Histology of the aortic root
Introduction to the aortic annulus
The aortic rings
There are three circular and one crown-like rings within the extent of the aortic root (Figure 13)8. Since the leaflets are attached throughout the length of the root, it is their semilunar hinges which, in three dimensions, form the crown-like structure.
The base of this crown is not a distinct anatomical structure but is the virtual ring formed by joining the basal attachments of the leaflets within the left ventricle.
As discussed above, it is the diameter of this plane that is usually designated by echocardiographers as representing the valvar annulus. Many surgeons, in contrast, consider the crown-like construction to be the “annulus”. The sinotubular junction forms the superior aspect of the crown and is a true anatomical ring. It is demarcated by the sinus ridge, the distal attachment points of the leaflets to the aortic root, and represents the most distal apposition zone between the aortic valve leaflets.
However, despite its true annular morphology, it is rarely, if ever, defined as the “annulus”. The anatomic ventriculo-arterial junction also forms a ring between the base and superior aspects of the crown, but this locus is also not defined as the annulus.
Figure 13. The aortic rings (annotation)
A - Three-dimensional arrangement of the aortic root, which contains three circular "rings", but with the leaflets suspended within the root in crown-like fashion
B - The leaflets have been removed from this specimen of the aortic root, showing the location of the three rings relative to the crown-like hinges of the leaflets
Abbreviations
A-M: aortic-mitral
VA: ventriculo-arterial
Courtesy of Professor Robert Anderson.