Cardiac Fibrous Skeleton #2: Aortic–Mitral Region

1. Anatomical Overview

The aortic–mitral region is a central component of the cardiac fibrous skeleton. It forms an integrated framework linking the aortic root, anterior mitral leaflet, fibrous trigones, membranous septum, central fibrous body, and atrioventricular conduction axis. This framework has two fundamental functions: it provides fibrous support for the valvar hinge lines and electrically separates atrial myocardium from ventricular myocardium at the atrioventricular junction. The normally insulated junction is crossed by the atrioventricular conduction axis, making the right-sided portion of the fibrous skeleton both a structural anchor and a critical electrophysiologic landmark [1].

The principal structures of the aortic–mitral region are:

1. The aorto-mitral curtain, or fibrous aortic–mitral continuity
2. The left fibrous trigone
3. The right fibrous trigone
4. The central fibrous body
5. The membranous septum
6. The penetrating atrioventricular bundle and adjacent branching conduction tissues

These components are not separate “rings.” They form a three-dimensional fibrous complex at the base of the ventricular mass. Their spatial relationships define the transition between the left ventricular inflow tract, the left ventricular outflow tract, the aortic root, and the septal atrioventricular junction [1,2].

2. Aorto-Mitral Curtain

The aorto-mitral curtain is the sheet of fibrous tissue extending between the anterior leaflet of the mitral valve and the posterior-leftward portion of the aortic root. It is also termed the aorto-mitral continuity, intervalvular fibrosa, or fibrous aortic–mitral curtain. Superiorly, it is related primarily to the noncoronary and left coronary aortic leaflets and the interleaflet triangle between them. Inferiorly, it is continuous with the hinge region of the anterior mitral leaflet [1].

This continuity is important because the posterior part of the aortic root is not supported directly by ventricular myocardium. Instead, fibrous tissue connects the aortic valvar hinge lines to the mitral valve. In contrast, the anterior and anterolateral portions of the left ventricular outflow tract are supported predominantly by ventricular muscle. The aorto-mitral curtain therefore forms the fibrous posterior wall of the left ventricular outflow tract, whereas the muscular ventricular septum and adjacent left ventricular myocardium provide much of its anterior and septal support [1].

The anterior mitral leaflet and curtain are related but distinct: the leaflet is mobile valvar tissue, whereas the curtain is the comparatively fixed fibrous tissue between the mitral and aortic hinge lines.

The curtain also explains why the mitral annulus is not a complete, uniform fibrous ring. The anterior portion of the mitral valvar attachment is incorporated into the aortic–mitral fibrous complex, whereas the mural annulus contains variable amounts of fibrous, fibroadipose, and muscular support. This distinction is important when interpreting imaging-defined annular planes or planning annular sutures [1].

3. Fibrous Trigones

The lateral ends of the aorto-mitral curtain are reinforced by the left and right fibrous trigones. These trigones are focal thickenings of the fibrous skeleton rather than isolated triangular plates.

Left Fibrous Trigone

The left fibrous trigone lies near the junction of the left coronary portion of the aortic root and the anterolateral attachment of the mitral valve. It reinforces the leftward end of the aorto-mitral curtain and anchors the posterior-leftward aortic root to the mitral apparatus and adjacent ventricular tissues [1].

The left trigone is less closely related to the atrioventricular conduction axis than the right trigone, but it remains surgically important. Deep sutures placed laterally around the anterior mitral annulus may extend toward the left atrioventricular groove, where the circumflex coronary artery can lie close to the mitral hinge line. The exact risk depends on coronary dominance, annular dimensions, and the individual three-dimensional relationship between the coronary artery and valve.

Right Fibrous Trigone

The right fibrous trigone forms the rightward end of the aorto-mitral curtain. It lies near the noncoronary portion of the aortic root and merges with the membranous septum and fibrous tissue associated with the septal tricuspid attachment. This convergence forms the central fibrous body [1,2].

The right trigone is therefore more than a lateral reinforcement of the curtain. It is the point at which the aortic, mitral, and tricuspid fibrous structures approach one another and where the atrioventricular conduction axis penetrates the insulating plane. This combination makes it a high-risk region during surgery involving the noncoronary aortic annulus, posteromedial mitral annulus, membranous septum, or perimembranous ventricular septum.

4. Central Fibrous Body

The central fibrous body is the dense fibrous confluence formed principally by the right fibrous trigone and membranous septum, with contributions from the aorto-mitral curtain, septal tricuspid attachment, and tendon of Todaro region [1,2]. It is best understood as a three-dimensional junctional complex rather than a single discrete mass.

Structurally, the central fibrous body stabilizes the neighboring valvar and septal attachments. Electrophysiologically, it contributes to insulation between atrial and ventricular myocardium. The penetrating atrioventricular bundle traverses this fibrous region before reaching the crest of the muscular ventricular septum [3].

Although mechanically suited to anchoring valvar structures, the central fibrous body contains or directly borders the conduction axis. Surgical injury may result from transection, deep sutures, compression, traction, ischemia, edema, hematoma, or thermal damage.

5. Membranous Septum

The membranous septum is a small fibrous component of the central skeleton situated beneath the interleaflet triangle between the right coronary and noncoronary aortic sinuses. Its inferior border is related to the crest of the muscular ventricular septum, while its superior aspect approaches the aortic root [1,2].

The attachment of the septal leaflet of the tricuspid valve divides the membranous septum into two functional components:

• Atrioventricular component: separates the left ventricular outflow tract from the right atrium.
• Interventricular component: separates the left ventricle from the right ventricle.

The atrioventricular component exists because the tricuspid hinge is normally displaced apically relative to the mitral hinge. Consequently, a portion of the left ventricular outflow tract is separated from the right atrium by fibrous tissue rather than ventricular myocardium. The size and relative proportions of the atrioventricular and interventricular components vary among individuals [2].

The membranous septum should not be viewed merely as a thin patch between the ventricles. It is a central landmark connecting the aortic root, ventricular septum, tricuspid valve, and conduction axis. Perimembranous ventricular septal defects develop at the margins of this region, and their relationship to the conduction tissues depends on the geometry of the defect and the surrounding septal structures [2,3].

6. Atrioventricular Conduction Axis

The atrioventricular node lies within the triangle of Koch in the right atrium. The conduction axis then continues as the penetrating atrioventricular bundle, passing through the central fibrous body and reaching the crest of the muscular ventricular septum. It proceeds as the nonbranching bundle before dividing into the right and left bundle branches [3].

The conduction axis does not have a fixed relationship to a simple two-dimensional annular line. Histologic studies show that, after entering the aortic root region, it is commonly closest to the hinge of the right coronary aortic leaflet. The axis usually lies on the crest of the ventricular septum, but it may deviate rightward or leftward. Its proximity to the aortic root is influenced by the depth of the inferoseptal recess and the angulation of the muscular ventricular septum [4].

The inferior margin of the membranous septum is frequently used as an imaging surrogate for the nearby conduction axis. This is useful but imperfect. The membranous septum identifies a region of increased conduction vulnerability; it does not display the exact course of the bundle or the left bundle branch in every patient. The superior fascicle of the left bundle branch may also approach the nadir of the right coronary leaflet more closely than conventional diagrams suggest [6].

For surgeons, the practical implication is that the conduction axis occupies a variable corridor extending from the right atrial septal junction, through the central fibrous body, and onto the crest of the muscular septum. Safe operative planning requires appreciation of this entire course rather than avoidance of a single visible point.

7. Three-Dimensional Geometry of the Left Ventricular Outflow Tract

The left ventricular outflow tract is bounded by both muscular and fibrous structures. Its posterior wall is formed by the anterior mitral leaflet and the aorto-mitral curtain. Its anterior and anterolateral walls are largely muscular. Its anteromedial and septal boundaries include the membranous septum and crest of the muscular ventricular septum [1].

This arrangement explains the close relationship between inflow, outflow, and conduction anatomy. The anterior mitral leaflet lies between the left ventricular inflow stream and the outflow tract. Superior to the leaflet, the aorto-mitral curtain links the mitral hinge to the aortic root. Rightward, the curtain thickens into the right trigone and central fibrous body, where the conduction axis penetrates. Thus, movement from left to right across the aorto-mitral continuity transitions from a predominantly structural region toward a region with increasing conduction-system relevance.

The aortic root itself can rotate within the base of the left ventricle. Variation in rotational position alters the balance between fibrous and muscular support beneath the root and changes the relationship between the right fibrous trigone, membranous septum, and conduction axis. Clockwise rotation has been associated with a wider central fibrous body and a greater extent of fibrous support, whereas counterclockwise rotation may place more of the conduction axis within the circumference of the outflow tract [5,6].

Consequently, patients with similar valves and annular dimensions may have different conduction risk. Root rotation, septal angulation, inferoseptal recess depth, and membranous septal dimensions should be assessed together.

8. Surgical Implications

Aortic Valve, Root, and LVOT Surgery

Operations involving the aortic valve, aortic root, or congenital left ventricular outflow tract may approach the conduction axis from above. Sutures or resection near the right–noncoronary commissural region, the nadir of the right coronary leaflet, or the membranous septum can injure the penetrating bundle or proximal left bundle branch [4,6].

Congenital LVOT surgery may be especially complex because abnormal root rotation, septal malalignment, prior operations, prosthetic material, and distorted ventricular geometry can alter conventional landmarks. In a recent cohort, preoperative CT-based estimation of the atrioventricular node, His bundle course, and left bundle branch origin was used to guide surgical avoidance. High-grade atrioventricular block occurred in 1 of 53 patients, or 1.9%, compared with a previously reported incidence of approximately 10% in a similar cohort without preoperative conduction-axis estimation [7]. This finding is promising but should be interpreted as single-center observational evidence rather than definitive proof of causality.

Mitral Valve and Aorto-Mitral Reconstruction

Anterior mitral annular sutures are placed within the aortic–mitral fibrous complex. Sutures directed excessively superiorly can enter the aortic root or distort an aortic leaflet. Lateral sutures near the left trigone may approach the circumflex coronary artery. Medial sutures near the right trigone and central fibrous body carry greater conduction risk.

Destruction of the intervalvular fibrosa may occur with invasive endocarditis, annular abscess, prosthetic valve infection, extensive calcification, or prior surgery. Complications include pseudoaneurysm, fistula, paravalvar leakage, and loss of structural continuity between the aortic root and mitral annulus [1]. Reconstruction must restore both aortic root support and mitral annular integrity while avoiding the conduction tissues on the rightward side.

Perimembranous VSD Closure and Septal Procedures

In the usual perimembranous ventricular septal defect, the conduction axis passes along the posteroinferior margin of the defect. Patch sutures in this region should avoid deep penetration into the fibrous–muscular junction [2,3]. However, this rule cannot be applied mechanically to every congenital heart defect. Malalignment, abnormal atrioventricular connections, ventricular topology, and septal deficiency can substantially alter the course of the conduction tissues.

9. Imaging and Preoperative Assessment

Cardiac CT and magnetic resonance imaging can delineate the aorto-mitral curtain, trigones, membranous septum, calcification, pseudoaneurysm, and relationships between the aortic root and ventricular septum [1,2]. CT is particularly useful for high-resolution assessment of calcification and three-dimensional root geometry, while magnetic resonance imaging provides complementary functional and tissue information.

For complex congenital or reoperative procedures, imaging should address:

• Rotational position of the aortic root
• Depth of the inferoseptal recess
• Angulation and crest of the muscular ventricular septum
• Dimensions and location of the membranous septum
• Relationship of prior patches or prostheses to the central fibrous body
• Extent of aorto-mitral discontinuity, calcification, abscess, or pseudoaneurysm

Routine imaging cannot directly display conduction tissue; its course is estimated from reproducible anatomic landmarks. Mapping should supplement, not replace, knowledge of congenital morphology and careful intraoperative technique [7].

10. Practical Surgical Framework

The aortic–mitral region can be conceptualized as a fibrous curtain suspended between two reinforced ends:

• The aorto-mitral curtain forms the central fibrous continuity.
• The left fibrous trigone reinforces its leftward end.
• The right fibrous trigone reinforces its rightward end and merges with the membranous septum.
• The right trigone and membranous septum form the core of the central fibrous body.
• The atrioventricular conduction axis penetrates this right-sided fibrous complex and continues along the crest of the muscular septum.

This model explains why the leftward portion is primarily a structural region, whereas the rightward portion combines structural and electrophysiologic significance. It also emphasizes the central operative principle: the aortic valve, mitral valve, tricuspid valve, membranous septum, ventricular septum, and conduction axis are anatomically integrated. A maneuver directed toward one component may affect several others.

Safe surgery therefore depends on three-dimensional understanding rather than reliance on a single annular line or surface landmark. The most vulnerable zone is not identical in every heart. Individual variation in root rotation, septal geometry, congenital morphology, and prior reconstruction must be incorporated into the operative plan [4–7].

References

[1] Saremi F, Sánchez-Quintana D, Mori S, Muresian H, Spicer DE, Hassani C, Anderson RH. Fibrous skeleton of the heart: anatomic overview and evaluation of pathologic conditions with CT and MR imaging. Radiographics. 2017;37(5):1330-1351.

[2] Saremi F, Hassani C, Sánchez-Quintana D. Septal atrioventricular junction region: comprehensive imaging in adults. Radiographics. 2016;36(7):1966-1986.

[3] Anderson RH, Ho SY, Becker AE. The surgical anatomy of the conduction tissues. Thorax. 1983;38(6):408-420.

[4] Macías Y, Tretter JT, Sánchez-Quintana D, Cabrera JA, Spicer DE, de Almeida MC, Anderson RH. The atrioventricular conduction axis and the aortic root—Inferences for transcatheter replacement of the aortic valve. Clin Anat. 2022;35(2):143-154.

[5] Tretter JT, Mori S, Saremi F, Chikkabyrappa S, Thomas K, Bu F, Loomba RS, Alsaied T, Spicer DE, Anderson RH. Variations in rotation of the aortic root and membranous septum with implications for transcatheter valve implantation. Heart. 2018;104(12):999-1005.

[6] Anderson RH, Spicer DE, Sánchez-Quintana D, Macías Y, Kapadia S, Tretter JT. Relationship between the aortic root and the atrioventricular conduction axis. Heart. 2023;109(24):1811-1818.

[7] Tretter JT, Dakik L, El-Assaad I, Ahmad M, Najm HK. Pre-operative cardiac computed tomographic conduction axis prediction to avoid damage in congenital left ventricular outflow tract and aortic valvar surgery. J Thorac Cardiovasc Surg. Published online March 2, 2026. doi:10.1016/j.jtcvs.2026.02.028.