1. Introduction
The aortic valve is frequently described as a three-leaflet structure seated within a fibrous annulus. This terminology is convenient clinically but does not accurately describe the underlying anatomy. Unlike the mitral valve, the normal aortic valve has no discrete, continuous anatomical ring supporting the entire circumference of its leaflets. Instead, three semilunar lines of leaflet attachment extend from their basal nadirs within the ventricular outflow tract toward their commissures near the sinotubular junction (STJ). Together, these attachments form a three-dimensional crown-shaped structure. [1–3]
Three distinct geometric references must therefore be differentiated: the virtual basal ring, the anatomical ventriculoarterial junction, and the sinotubular junction. They describe different parts of the root, do not lie in a single plane, and have different applications in imaging and surgery. Their relationship to the interleaflet triangles, membranous septum, fibrous trigones, and aortic–mitral curtain explains why interventions at different points around the apparent “annulus” carry different anatomical risks. [1–4]
2. The Virtual Basal Ring
2.1 Definition and anatomical basis
The virtual basal ring (VBR) is an imaginary plane or contour constructed by joining the lowest insertion points, or nadirs, of the right coronary, left coronary, and noncoronary leaflets. It is commonly termed the aortic annulus in echocardiography, computed tomography (CT), transcatheter valve planning, and operative discussions. However, the VBR is a geometric reference rather than a separately identifiable fibrous structure. [1,2]
The three nadirs define the basal level of the valve complex, but the tissue surrounding this level is heterogeneous. Portions of the root are attached to ventricular myocardium, while posterior portions are supported by fibrous continuity with the mitral valve and central fibrous skeleton. Accordingly, the basal ring should not be conceptualized as a rigid, circular sewing cuff with uniform mechanical properties. [2,3]
The VBR is useful because it defines a reproducible reference for assessing root dimensions and comparing them before and after intervention. Nevertheless, measurements require attention to imaging modality and geometric method. A single two-dimensional diameter cannot completely describe an elliptical or asymmetrical basal orifice. CT can provide area, perimeter, minimum and maximum diameters, and their corresponding derived diameters, whereas echocardiographic measurements depend on the selected imaging plane. [5,6]
2.2 Implications for congenital morphology
In congenital aortic valve disease, the lowest leaflet attachments may not have the usual symmetrical arrangement. Bicuspid valves with two functional commissures, a raphe, or asymmetric sinuses may have substantially different basal and commissural geometry from a typical trileaflet valve. The number and distribution of true interleaflet triangles help distinguish morphological subtypes. The convention of joining three nadirs must therefore be interpreted according to the actual valve morphology; it should not be imposed on every congenitally abnormal valve as though three equivalent leaflets were present. [7]
3. Crown-Shaped Leaflet Attachments
3.1 Semilunar hinge lines
Each aortic leaflet is attached along a curved hinge line that descends toward its basal nadir and ascends on either side toward the neighboring commissures. In an opened root, the three hinge lines form a scalloped or crown-shaped pattern rather than a flat annular circle. The free leaflet margins lie above these basal insertions, where adjacent leaflets meet in diastolic coaptation. [1–3]
The hinge lines are not merely a passive means of anchoring the tissue. Their height, symmetry, and spatial relationships influence the geometry available for leaflet opening and closure. Adequate competence requires coordinated geometry among leaflet tissue, commissural orientation, the VBR, and the STJ. Consequently, a structurally intact leaflet may fail to coapt when the supporting root becomes dilated or distorted. [8]
The commissures lie near the distal portion of the root, in relation to the STJ, while the basal nadirs are much closer to the left ventricular outflow tract (LVOT). Thus, the actual attachment of an individual leaflet occupies a considerable vertical distance. Labeling all portions of this attachment as the “annulus” obscures this clinically important longitudinal arrangement.
3.2 The root is not a simple cylinder
The normal aortic root contains three outwardly expanded sinuses of Valsalva, a proximal basal reference, and a distal STJ. Between these levels, leaflet attachments and interleaflet triangles produce a complex interface between the ventricular and arterial compartments. The three sinuses are not interchangeable in their adjoining structures: the right and left coronary sinuses are associated with the coronary origins, whereas the noncoronary sinus is adjacent to the fibrous skeleton and interatrial region. [1,3]
Anatomical measurements also demonstrate that interleaflet triangles and root dimensions are not necessarily equal or perfectly symmetrical. In a morphometric investigation of 16 normal human hearts, Contino and colleagues documented variability among the triangles and a systematic relationship between the STJ and basal geometry. Such findings support careful individualized assessment rather than reliance on a perfectly cylindrical or uniformly symmetrical model. [4]
4. Anatomical Ventriculoarterial Junction
4.1 Definition and distinction from the VBR
The anatomical ventriculoarterial junction (VA junction) is the boundary where ventricular structures—myocardial or fibrous—meet the arterial wall of the aortic root. It is not identical to the VBR. The semilunar leaflet insertions cross this tissue boundary as they ascend from the nadirs to the commissures. [1–3]
Below portions of the hinge lines, small crescents of ventricular myocardium are incorporated into the bases of the aortic sinuses. Conversely, the regions between ascending leaflet attachments contain extensions of arterial wall that descend toward the ventricular side of the valvar complex. This arrangement is responsible for the distinctive interleaflet triangles. The boundary between ventricular and arterial tissue is therefore different from both the valvar attachment line and the plane used for annular measurement. [1–3]
The distinction is especially important in root operations. A suture placed at the VBR, a stitch following the leaflet hinge, and a dissection at the anatomical VA junction may pass through different tissues and depths even when all are informally described as annular work.
4.2 Hemodynamic versus anatomical boundaries
A related concept is the hemodynamic ventriculoarterial junction, reflecting the level of the competent valve that separates ventricular and arterial pressure compartments in diastole. This boundary follows the functional closing apparatus and should not be assumed to match the histological junction of myocardial and arterial tissue. The interleaflet triangles, although composed of arterial wall, form part of the structure exposed to ventricular pressure below the leaflet coaptation. [1,4]
This apparent mismatch between tissue identity and pressure environment is a defining feature of the aortic root and helps explain why a purely planar description cannot capture its functional anatomy.
5. Interleaflet Triangles and the Surrounding Fibrous Skeleton
5.1 Interleaflet triangles
The three interleaflet triangles lie between neighboring semilunar leaflet attachments, with their apices extending toward the commissures. They are important morphological landmarks rather than empty spaces between valve leaflets. Their precise configuration reflects the arrangement of the arterial sinuses and underlying ventricular structures. [3,9]
The right–noncoronary interleaflet triangle is especially important because of its proximity to the membranous septum, central fibrous body, and penetrating atrioventricular conduction axis. The left–noncoronary triangle relates to the aortic–mitral fibrous continuity, while the right–left triangle is associated with the muscular outflow septal region. These relationships provide a framework for planning root incisions and understanding the consequences of deep suturing. [3,9]
In congenital valves, abnormal commissural spacing, fused leaflets, and absent or rudimentary triangles may reveal developmental differences that are not fully characterized by counting leaflet free edges. Assessment should therefore include the number of commissures and the underlying arrangement of interleaflet triangles. [7]
5.2 Membranous septum and conduction axis
The membranous septum is located beneath the right–noncoronary commissural region. The atrioventricular conduction bundle penetrates the central fibrous body and courses near the inferior margin of the membranous septum before entering the ventricular septal conduction system. Consequently, deep sutures, prosthetic fixation, root enlargement, or extensive debridement in this region may injure the conduction axis. [3,9]
The proximity is not represented by the VBR alone: the risk depends on the actual tissue encountered, the depth and direction of the intervention, and the relationship between the septal structures and leaflet attachment. This is particularly relevant in congenital heart disease, in which malalignment or associated septal defects may modify the expected surgical anatomy.
5.3 Fibrous trigones and aortic–mitral curtain
The posterior aortic root is supported by the aortic–mitral curtain, the fibrous continuity between the aortic valvar complex and the anterior mitral leaflet. The right and left fibrous trigones define the boundaries of this continuity and link it to the cardiac fibrous skeleton. The right trigone is associated with the central fibrous body, whereas the left trigone forms the opposite limit of the fibrous intervalvular tissue. [2,3,10]
This continuity is fundamental to posterior root enlargement procedures. Expansion through selected portions of the fibrous curtain can increase the effective outflow dimensions, but the reconstruction must preserve or restore the relationship between the anterior mitral leaflet and the aortic root. An incision that extends toward the right trigone additionally approaches the conduction-sensitive region. Accordingly, the feasibility of enlargement depends on the specific operative anatomy, not simply the measured basal diameter. [10]
6. Sinotubular Junction and Root Geometry
The sinotubular junction (STJ) marks the distal transition from the sinuses of Valsalva to the tubular ascending aorta. It forms an important geometric boundary for the commissural level and thus contributes to the spatial configuration of leaflet coaptation. In the normal root, STJ dimensions are related to, but not necessarily identical to, those of the basal region. [4,8]
Dilatation of the STJ may displace commissures outward and reduce leaflet coaptation, producing aortic regurgitation despite relatively preserved leaflet tissue. Conversely, excessive narrowing or an incorrectly reconstructed commissural configuration may restrict leaflet opening, cause distortion, or produce residual stenosis. Root and valve repair therefore require assessment of both proximal and distal geometry. [8]
Aortic valve competence depends on the combined geometry of the VBR, commissures, STJ, and leaflet tissue. Correction of a dilated basal ring without addressing significant STJ enlargement—or correction of the STJ without identifying leaflet restriction or prolapse—may leave an important mechanism of regurgitation untreated.
7. Implications for Imaging
7.1 Echocardiography
Transthoracic and transesophageal echocardiography allow dynamic assessment of leaflet motion, coaptation, stenosis, regurgitation, and the adjacent LVOT. The measured “annulus” is a selected imaging representation of the basal attachment region. Two-dimensional imaging can miss asymmetry and may produce a dimension different from the maximal or area-derived diameter measured with three-dimensional imaging. [5,6]
In congenital aortic valve disease, imaging should describe commissural orientation, leaflet fusion or raphes, leaflet mobility, and associated LVOT or supravalvar obstruction. When the mechanism of regurgitation is uncertain, geometric assessment must be integrated with leaflet tissue quality and coaptation rather than relying on annular diameter alone.
7.2 CT and three-dimensional reconstruction
ECG-gated CT can characterize the aortic root as a three-dimensional structure, including the basal plane, sinuses, STJ, coronary ostial positions, and their spatial relationships. Such measurements are essential for transcatheter valve planning and may also support selected surgical reconstruction strategies. [5,6]
CT-based annular sizing is performed in a plane defined relative to the lowest leaflet attachments, rather than at the entire crown-shaped insertion line. For interventions involving prosthetic devices, planning should also consider the adjacent LVOT, coronary ostia, and septal structures. An adequate basal dimension does not by itself guarantee safe implantation or appropriate valve performance.
8. Surgical and Procedural Relevance
The distinction among the crown-shaped attachment, VBR, anatomical VA junction, and STJ has practical implications across several procedures.
Aortic valve replacement. Prosthetic fixation occurs within a nonuniform annular region comprising fibrous and muscular components. Suture depth should reflect the anatomical sector, particularly around the right–noncoronary region and conduction system. Implant geometry and the risk of patient–prosthesis mismatch cannot be inferred from a single annular measurement alone.
Aortic valve repair and valve-sparing root replacement. Repair must balance basal stabilization, appropriate commissural height and orientation, STJ geometry, and effective leaflet coaptation. Contemporary anatomical and surgical analyses emphasize that repair of the root framework is distinct from correction of intrinsic leaflet prolapse or restriction. [8,11]
Aortic root enlargement. Anterior or posterior enlargement crosses different tissue interfaces. The muscular outflow septum, membranous septum, right fibrous trigone, and aortic–mitral continuity determine the safe extent of each incision and the potential for conduction injury or mitral distortion. Surgical strategy should be individualized according to the intended enlargement and associated ventricular outflow morphology. [10]
Transcatheter interventions. The VBR is a useful device-sizing reference, but the implantation zone also includes variable LVOT morphology, septal relationships, and coronary anatomy. Device dimensions, positioning, and interaction with adjoining tissues influence paravalvular regurgitation and conduction disturbances. Anatomical measurements must therefore be interpreted as part of a three-dimensional landing-zone assessment. [5,6]
9. Key Anatomical and Surgical Principles
- The aortic annulus is virtual. The commonly measured basal ring joins leaflet nadirs and is not a continuous fibrous anatomical ring.
- Leaflet attachments are crown-shaped. Each semilunar hinge line rises from the basal nadir toward adjacent commissures near the STJ.
- The anatomical VA junction is a separate structure. It marks the ventricular-to-arterial tissue transition and is crossed by the leaflet hinge lines.
- Interleaflet triangles are essential landmarks. They define root morphology and establish relations to the septum, fibrous skeleton, and surrounding cardiac structures.
- The right–noncoronary region is conduction-sensitive. Intervention must account for the membranous septum and penetrating atrioventricular bundle.
- The aortic–mitral curtain is a reconstructive interface. Posterior enlargement can exploit fibrous continuity but must preserve mitral geometry.
- Root function depends on coordinated geometry. VBR size, STJ diameter, commissural position, and leaflet tissue collectively determine valve competence.
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