Residency · Residency · Cardiothoracic Surgery

Surgical Anatomy of the Cardiac Valves and Fibrous Skeleton

Overview

The four cardiac valves ensure that blood flows in one direction through the heart. Their surgical anatomy — including annular geometry, leaflet morphology, chordal attachments, and relationships to surrounding structures such as the conduction system and coronary arteries — is foundational knowledge for anyone performing valve repair or replacement.

The Fibrous Skeleton of the Heart

Components

The fibrous skeleton is a dense connective tissue framework that provides structural support for the valve annuli and serves as the electrical insulator between the atria and ventricles. Its key components include the central fibrous body, which sits at the confluence of the aortic, mitral, and tricuspid valve annuli, with the AV node and bundle of His passing through or adjacent to it. The right fibrous trigone is the thickest portion, located at the junction of all three of these valves. The left fibrous trigone marks the junction of the mitral and aortic annuli on the left side. The intervalvular fibrosa (also called the aortic-mitral curtain) is the region of fibrous continuity between the aortic and mitral valves. Finally, the membranous septum is a thin portion of the interventricular septum adjacent to the right fibrous trigone.

Surgical Significance of the Fibrous Skeleton

The fibrous skeleton provides secure tissue for suture placement during valve replacement. The aortic-mitral curtain is a key landmark during combined aortic and mitral valve procedures. Calcification of the fibrous skeleton (whether mitral annular calcification or aortic annular calcification) increases the technical complexity of surgery. Damage to the central fibrous body risks complete heart block, making precise anatomic knowledge critical for safe valve surgery.

Aortic Valve

Anatomy

The aortic valve has three semilunar cusps, named for their relationship to the coronary arteries: the right coronary, left coronary, and noncoronary cusps. Each cusp features a lunula (a thin crescentic margin) and a nodule of Arantius at the center of its free edge. Behind each cusp lies a sinus of Valsalva — outpouchings that house the coronary ostia. The right coronary artery originates from the right coronary sinus, and the left main coronary artery from the left coronary sinus. The noncoronary sinus is adjacent to the interatrial septum and right atrium.

The annular geometry of the aortic valve is crown-shaped rather than planar, with the commissures extending higher than the nadir of each cusp. The sinotubular junction (STJ) marks the transition from the aortic root to the tubular ascending aorta, while the ventriculoaortic junction is where the root meets the left ventricular outflow tract.

Surgical Relationships

Three anatomic relationships are critical during aortic valve surgery. The membranous septum and bundle of His are located beneath the commissure between the right and noncoronary cusps — sutures placed too deep here risk heart block. The left main coronary ostium typically sits 10 to 15 mm above the annulus in the left coronary sinus; a low-lying ostium increases risk during valve replacement. The mitral valve's anterior leaflet is in fibrous continuity with the noncoronary and left coronary cusps via the intervalvular fibrosa.

Bicuspid Aortic Valve

Bicuspid aortic valve is present in 1–2% of the population. The most common morphology (70–80% of cases) involves fusion of the right and left coronary cusps. It is associated with ascending aortopathy, including dilatation and dissection risk. The raphe — a ridge of tissue at the site of cusp fusion — is often calcified and may be mistaken for a true commissure.

Mitral Valve

Anatomy

The mitral valve has two leaflets: the anterior (aortic) leaflet and the posterior (mural) leaflet. The anterior leaflet occupies about one-third of the annular circumference but has a larger surface area. The posterior leaflet is divided into three scallops: P1 (lateral), P2 (middle), and P3 (medial), with corresponding segments of the anterior leaflet designated A1, A2, and A3.

The annular geometry is D-shaped (or saddle-shaped in three dimensions). The anterior portion is the fibrous (intertrigonal) segment, while the posterior portion is the muscular annulus. The two commissures — anterolateral and posteromedial — each have a corresponding papillary muscle. Chordae tendineae are classified as primary (marginal, inserting on the leaflet free edge), secondary (strut, inserting on the leaflet body), and tertiary (basal, running from the ventricular wall to the posterior leaflet base).

The two papillary muscles differ importantly in their blood supply. The anterolateral papillary muscle receives dual supply from the LAD and LCx, making it less prone to ischemic rupture. The posteromedial papillary muscle has a single blood supply (usually from the dominant artery, typically the RCA), making it more vulnerable to ischemic rupture — a distinction with major clinical significance.

Surgical Relationships

The circumflex artery courses in the left atrioventricular groove close to the posterior mitral annulus, particularly at the P1-P2 junction, where it is at risk of injury during annuloplasty suture placement. The AV node is located near the posteromedial commissure, in the triangle of Koch on the right atrial side. The coronary sinus runs in the left AV groove posterior to the mitral annulus and serves as an important landmark during mitral surgery approached from the left atrium.

Tricuspid Valve

Anatomy

The tricuspid valve has three leaflets: the anterior (largest), septal, and posterior (smallest). The septal leaflet is unique in having direct chordal attachments to the interventricular septum. The annulus is nonplanar and elliptical; the septal annulus is relatively fixed, while the anterior and posterior portions are more prone to dilatation. In functional tricuspid regurgitation, the annulus dilates primarily along the free wall (the anterior and posterior portions).

Surgical Relationships

The AV node and bundle of His are located in the triangle of Koch, which is bounded by the tendon of Todaro, the coronary sinus ostium, and the septal leaflet attachment. The bundle of His courses along the septal leaflet attachment near the anteroseptal commissure, and sutures in this area risk heart block. The posterior and anterior annulus are relatively safe zones for suture placement, while caution is required near the septal annulus, especially at the anteroseptal commissure. The right coronary artery courses in the right atrioventricular groove external to the tricuspid annulus, where deep sutures may injure it.

Pulmonary Valve

Anatomy

The pulmonary valve has three semilunar cusps (anterior, right, and left) with a structure similar to the aortic valve but with thinner cusps suited to the lower-pressure pulmonary environment. The pulmonary annulus is slightly larger than the aortic annulus, and the valve is positioned most anteriorly and superiorly of all four cardiac valves.

Surgical Relationships

The pulmonary valve is relatively isolated from critical structures, though the first septal perforator of the LAD passes nearby in some patients. It is most commonly addressed in congenital heart surgery (tetralogy of Fallot, pulmonary stenosis). In the Ross procedure, the pulmonary valve is used as an autograft to replace the aortic valve.

Summary of Cardiac Valve Anatomy

Spatial Relationships Between Valves

The Four Valves in Relation to Each Other

The aortic valve is centrally positioned and has been described as the "keystone" of the heart. Aortic-mitral fibrous continuity is a key surgical landmark. The tricuspid and mitral valves are separated by the membranous septum; the tricuspid annulus sits slightly more apical than the mitral annulus, an important echocardiographic landmark. The pulmonary valve is the most anterior and is separated from the other valves by the infundibular (conal) muscle.

<image>Surgeon's view of the aortic valve as seen from above during aortic valve replacement, showing the three cusps (right coronary, left coronary, noncoronary) with the coronary ostia visible in their respective sinuses. The membranous septum location is marked with a danger zone indicating proximity to the bundle of His beneath the commissure of the right and noncoronary cusps. The intervalvular fibrosa connecting to the mitral valve is shown.</image>

<image>Surgeon's view of the mitral valve as seen from the left atrial approach, showing the anterior and posterior leaflets with scallop nomenclature (A1-A3, P1-P3), anterolateral and posteromedial commissures, and the annuloplasty suture placement sites. The circumflex artery course is shown in relation to the posterior annulus with a highlighted danger zone at the P1-P2 junction. The fibrous trigones are labeled.</image>

<image>Cross-sectional view of the cardiac fibrous skeleton from above, showing all four valve annuli and their spatial relationships. The central fibrous body, right and left fibrous trigones, intervalvular fibrosa, and membranous septum are labeled. The conduction system pathway (AV node, bundle of His) is shown traversing through the central fibrous body. Color coding distinguishes fibrous from muscular portions of each annulus.</image>

<image>Anatomical illustration of the triangle of Koch as seen from the right atrial approach during tricuspid valve surgery, showing the tendon of Todaro, coronary sinus ostium, and septal leaflet of the tricuspid valve forming the triangle boundaries. The AV node and bundle of His are shown within the triangle with a safe-zone color gradient indicating where annuloplasty sutures can be safely placed versus areas of conduction injury risk.</image>

Clinical Pearls

The bundle of His is most vulnerable at two locations: the commissure between the right and noncoronary cusps of the aortic valve and the anteroseptal commissure of the tricuspid valve. During mitral valve annuloplasty, the circumflex artery is at greatest risk near the P1-P2 region of the posterior annulus. The posteromedial papillary muscle has a single blood supply and is more susceptible to ischemic rupture than the anterolateral papillary muscle — this is one of the highest-yield anatomy facts in cardiac surgery. Aortic-mitral fibrous continuity means that disease or surgery affecting one valve can impact the other. A bicuspid aortic valve diagnosis should prompt evaluation of the ascending aorta for associated aortopathy. In functional tricuspid regurgitation, the annulus dilates along the free wall while the septal annulus remains relatively fixed. In degenerative mitral valve disease, the P2 scallop is the most commonly prolapsing segment.

References

• Anderson RH, Becker AE. Cardiac anatomy: an integrated text and colour atlas. London: Gower Medical, 1980.
• Carpentier A, Adams DH, Filsoufi F. Carpentier's Reconstructive Valve Surgery. Saunders/Elsevier, 2010.
• Defined conduction system anatomy: Tawara S. Das Reizleitungssystem des Saugetierherzens, 1906. Modern review: Ho SY, Anderson RH. How constant is the orientation of the AV node relative to the triangle of Koch? J Cardiovasc Electrophysiol. 2012.
• Mitral annular anatomy and LCx relationship: Carpentier A. Cardiac valve surgery -- the "French correction." J Thorac Cardiovasc Surg. 1983;86(3):323-337.

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