1. Introduction
The aortic valve is not an isolated three-leaflet structure mounted in a rigid circular annulus. It is part of a three-dimensional, dynamic aortic root complex extending from the left ventricular outflow tract (LVOT) to the sinotubular junction (STJ). Its components include the semilunar leaflets, their crown-shaped attachments, the sinuses of Valsalva, the interleaflet triangles, the commissures, and the supporting ventricular and fibrous structures. Their spatial relationships determine effective valve opening, central leaflet coaptation, coronary perfusion, and the safety of surgical or transcatheter intervention [1–4].
This series examines three complementary perspectives: #1 leaflet and root structures, #2 root configuration and surrounding structures, and #3 basal attachments and ring geometry. The central principle is that an anatomical leaflet hinge line, a virtual basal ring, and the anatomical ventriculoarterial junction represent different structures and should not be used interchangeably.
2. Aortic Leaflets and Root Structures
2.1. Three leaflets and their coronary relationships
The normal trileaflet aortic valve contains the right coronary cusp (RCC), left coronary cusp (LCC), and noncoronary cusp (NCC). Each leaflet has a curved basal attachment, a flexible body, and a free margin that meets the other leaflets during diastole. The right and left coronary sinuses ordinarily contain the ostia of the right coronary artery (RCA) and left coronary artery (LCA), respectively. The noncoronary sinus normally has no coronary ostium. Coronary origins, sinus dimensions, commissural orientation, and leaflet morphology vary among individuals, particularly in congenital valve disease [1,2].
The term cusp is sometimes used for both the leaflet and its corresponding sinus; distinguishing the leaflet from the sinus avoids ambiguity during imaging interpretation and operative descriptions. The three leaflets are unequal in some otherwise normal valves, and perfect symmetry is not a requirement for competence.
2.2. Free margin, lunulae, and nodules of Arantius
The free edge of each leaflet is bordered by a delicate coaptation region. The lunulae are crescent-shaped areas adjacent to the free margin that participate in leaflet apposition. Near the midpoint of each free edge, the nodule of Arantius forms a localized fibrous thickening. Together, the central nodules and adjacent coaptation surfaces help complete the central seal when the valve closes [2,3].
Valve competence depends on sufficient leaflet tissue, mobile leaflet bodies, adequate coaptation height, and appropriate relationships between the effective leaflet margins and the supporting root. The nodules alone do not create competence, and the lunulae are not independent rigid sealing devices. Redundant, retracted, fenestrated, or prolapsing free margins can change the position and extent of coaptation. Small fenestrations near commissures may be incidental, whereas extensive tissue deficiency can contribute to regurgitation.
2.3. Commissures and sinuses of Valsalva
A commissure is the junction between adjacent leaflets at the superior end of their insertion lines. Each commissure is supported near the STJ. During systole, leaflet excursion creates a central outflow orifice; during diastole, the leaflet bodies coapt as the sinuses provide space for leaflet motion.
The sinuses of Valsalva are three outward bulges of the proximal ascending aorta behind the leaflets. Their geometry accommodates cusp movement and coronary origins. The sinuses must be assessed together with commissural height and orientation. Restriction at the STJ, sinus distortion, or asymmetric commissural displacement can impair opening or reduce effective coaptation even when leaflet tissue appears intact [2–4].
3. Root Configuration and the Cardiac Fibrous Skeleton
3.1. The aortic root as an integrated structure
The aortic root occupies a central position between the LVOT and ascending aorta. The pulmonary root and right ventricular outflow tract (RVOT) lie generally anterior and leftward, although the exact spatial relationships depend on cardiac anatomy. Posteriorly, the aortic root is connected to the anterior mitral leaflet by the aortic–mitral curtain. Rightward and posteriorly, it lies close to the atrial septum, membranous septum, and septal aspect of the tricuspid valve [1–3].
The fibrous skeleton connects the aortic, mitral, and tricuspid valvar regions. This arrangement provides mechanical support while separating portions of atrial and ventricular myocardium. The aortic and mitral valves share a fibrous continuity rather than being separated everywhere by a muscular outflow tract. The aortic and pulmonary roots, conversely, are close neighbors but do not form a continuous fibrous aortic–pulmonary annulus.
3.2. Aortic–mitral curtain and fibrous trigones
The aortic–mitral curtain, or intervalvular fibrosa, extends between the aortic root and the anterior mitral leaflet, predominantly beneath the left and noncoronary portions of the root. Its lateral boundaries are associated with the left and right fibrous trigones. The right trigone contributes to the central fibrous body, adjacent to the membranous septum and the penetrating atrioventricular conduction axis [1,2,5].
This continuity matters clinically because operative manipulation of the aortic root can influence adjacent mitral structures. Root enlargement, annular debridement, reconstruction after endocarditis, and valve replacement must respect the different tissues supporting the root. Surgical descriptions should identify whether an incision enters ventricular muscle, fibrous continuity, or the membranous septal region rather than simply describing all three as the “annulus.”
3.3. Membranous septum and conduction tissue
The membranous septum is a small fibrous structure beneath the region of the right–noncoronary commissure. Its relation to the septal tricuspid leaflet creates atrioventricular and interventricular components. The penetrating bundle of His runs in close relation to the central fibrous body and inferior margin of this septal region. Its exact course and the dimensions of the membranous septum vary [5,6].
Consequently, deep sutures, extensive debridement, valve prosthesis expansion, or septal reconstruction near the RCC–NCC region may injure conduction tissue. In transcatheter aortic valve implantation, the relationship between the virtual basal ring, membranous septum, and device implantation depth is particularly relevant to conduction disturbances. These observations are primarily drawn from adult acquired aortic valve interventions; risk profiles differ in pediatric congenital operations [6].
4. Basal Attachments and Ring Geometry
4.1. A crown-shaped leaflet insertion
One of the most important features of aortic root anatomy is that leaflet insertions are semilunar rather than circular. Each leaflet is attached along a scalloped, crown-like hinge line, reaching its lowest point at the nadir of that leaflet and ascending on both sides toward the commissures. Together, the insertion lines create a three-peaked crown in three-dimensional space [1–4].
The basal nadirs lie below the commissural tips. The attachments therefore traverse different anatomical levels and cross the plane of the anatomical ventriculoarterial junction. A single horizontal line drawn around the root cannot represent the entire leaflet insertion. This explains why descriptions of a single solid anatomical “aortic annulus” are potentially misleading.
4.2. Virtual basal ring versus anatomical ventriculoarterial junction
The virtual basal ring (VBR) is an imaginary plane connecting the three lowest points of the leaflet attachments. Although often called the aortic annulus in clinical imaging and transcatheter practice, it is a geometric construct, not a discrete continuous fibrous band. In a dynamic root, its cross-sectional profile need not be perfectly circular [2–4,7].
The anatomical ventriculoarterial junction identifies the transition between ventricular structures and the arterial wall. It is anatomically distinct from the VBR. Because the crown-shaped leaflet insertion rises above its basal nadirs, parts of the leaflet attachments are on the arterial side of the anatomical junction while other portions are supported by ventricular or fibrous tissues. The root therefore contains different structural tissues at the same axial level.
The STJ is the more distal transition between the sinuses and the tubular ascending aorta. It constrains the upper extent of commissural support and is an essential determinant of root configuration. In simplified reconstruction, the VBR can be represented as the inferior reference plane and the STJ as a superior reference ring, connected by three crown-shaped leaflet insertion lines.
4.3. Interleaflet triangles
Between neighboring leaflet insertion lines lie three interleaflet triangles, whose apices approach the commissures and whose bases lie closer to ventricular structures. Despite their apparent location within the root, these regions are not equivalent to ordinary sinus wall. Their morphology reflects the unusual interface between the ventricular outflow, fibrous skeleton, and arterial structures [1,3,8].
The triangle between the right and noncoronary sinuses relates particularly closely to the membranous septum and conduction axis. The triangle between the left and noncoronary sinuses is associated with aortic–mitral fibrous continuity. The region between the right and left coronary sinuses is related to the muscular outflow region. These differences determine the consequences of root incisions and suturing [1,8].
5. Functional Geometry and Imaging
5.1. Valve competence depends on root–leaflet matching
Normal aortic closure requires compatibility between leaflet surface area and the geometry of their supporting structures. Leaflet prolapse may lower the effective free margin, producing an eccentric regurgitant jet. Leaflet restriction or insufficient tissue can prevent adequate coaptation. Basal ring enlargement, sinus dilatation, or STJ enlargement can also cause aortic regurgitation despite relatively preserved leaflet tissue [4,9].
This framework separates primary leaflet pathology from functional regurgitation due to root geometry. Both may coexist. Repair planning should therefore assess leaflet quality, geometric and effective heights when relevant, commissural symmetry, the basal ring, and STJ dimensions rather than treating all aortic regurgitation as a leaflet lesion.
5.2. Echocardiography and cross-sectional imaging
Transthoracic or transesophageal echocardiography evaluates leaflet number, mobility, restriction, prolapse, coaptation, Doppler gradients, and regurgitation mechanism. Short-axis imaging helps define commissural orientation and congenital leaflet fusion; long-axis imaging displays LVOT–root–ascending aortic continuity. Three-dimensional echocardiography can improve characterization of complex valve morphology.
ECG-gated computed tomography (CT) provides detailed multiplanar assessment of the basal ring, sinuses, coronary ostia, STJ, ascending aorta, and the relationship to the membranous septum. It is particularly useful when procedural planning requires measurements of coronary height, root dimensions, or the geometry of an implant landing zone. Cardiac magnetic resonance imaging offers complementary information regarding flow, ventricular remodeling, and aortic dimensions without ionizing radiation [7,10].
Measurements must identify the level and modality used. A basal-ring diameter on CT should not be equated uncritically with an echocardiographic measurement at another root level. In children, growth-related changes and body-size normalization introduce additional considerations.
6. Operative and Interventional Implications
6.1. Valve repair and root reconstruction
Aortic valve repair begins with an assessment of leaflet tissue, free-margin position, commissural geometry, and root support. In valve-sparing reconstruction, preserving native leaflets is insufficient if basal or STJ dimensions remain incompatible with durable coaptation. Conversely, altering the basal ring or commissural support without evaluating free margins may expose previously unrecognized leaflet prolapse [4,9].
A useful operative assessment includes (1) identifying leaflet and coronary anatomy, (2) inspecting the nadirs, commissures, and interleaflet triangles, (3) defining the relationship of the proposed suture line to the membranous septum and aortic–mitral curtain, (4) restoring appropriate root and commissural geometry, and (5) testing the completed valve for coaptation, obstruction, and coronary compromise.
6.2. Root enlargement and structures at risk
Posterior root enlargement approaches operate near the noncoronary and left coronary aspects of the root and the aortic–mitral continuity; anterior or septal enlargement approaches encounter different ventricular and septal structures. The anatomy of the interleaflet triangles determines safe incision boundaries and patch configuration. The conduction axis is particularly relevant near the right–noncoronary commissural region, while coronary ostia and proximal branches must be preserved whenever root tissue is resected or reconstructed [1,5,8].
In congenital hearts, associated LVOT obstruction, bicuspid or unicuspid morphology, abnormal coronary patterns, and prior repairs may substantially alter these relationships. The normal trileaflet model is therefore a reference anatomy, not a universal operative map.
7. Key Anatomical and Clinical Principles
- Three-dimensional valve complex: The leaflets, sinuses, commissures, interleaflet triangles, VBR, and STJ form one functional root.
- Central coaptation: Leaflet body mobility, lunulae, free margins, and nodules of Arantius contribute to valve closure.
- Coronary relationship: The RCA and LCA ordinarily originate from the right and left coronary sinuses, respectively.
- Crown-shaped attachments: Leaflet hinge lines ascend from basal nadirs to commissural peaks.
- Distinct boundaries: The VBR, anatomical ventriculoarterial junction, and STJ are not synonymous.
- Fibrous continuity: The aortic root shares a supporting fibrous complex with the anterior mitral leaflet and fibrous trigones.
- Conduction safety: The membranous septum and conduction axis are particularly important near the right–noncoronary region.
- Procedural planning: Valve competence and intervention safety require assessment of both leaflet morphology and root geometry
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