The fibrous skeleton is a three-dimensional framework of dense connective tissue supporting the arterial and atrioventricular valves and electrically separating the atrial and ventricular myocardial masses. It is neither a single plate nor a set of four complete annuli. Its components vary in thickness and continuity and are closely related to the atrioventricular conduction axis, the aortic root, the membranous septum, and the inferior pyramidal space. [1-3]

1  General organisation

The principal components include the right and left fibrous trigones, the central fibrous body, fibrous continuity between the aortic and mitral valves, and variable fibro-fatty tissues supporting the remaining atrioventricular junctions. The arterial roots themselves have crown-shaped leaflet attachments rather than complete circular annuli. [1,2]

The skeleton provides mechanical support and electrical insulation, but its boundaries are irregular. In many regions, atrial and ventricular myocardium approach one another across thin fibrous or fatty tissue. This variability is essential for understanding accessory pathways and junctional interventions.

2  Central fibrous body and fibrous trigones

The central fibrous body is centred on the right fibrous trigone and the membranous septum, but this conventional description is incomplete unless it also acknowledges the fibrous continuity between the mitral and tricuspid valvar leaflets that forms the roof of the inferoseptal recess. It lies at the junction of the aortic root, mitral valve, and septal tricuspid region. The left fibrous trigone is situated at the opposite end of the aortic-mitral curtain and is less directly related to the conduction axis. [1,3,5]

The right trigone provides the fibrous environment through which the penetrating atrioventricular bundle passes. Its exact configuration varies with the dimensions of the membranous septum and the attachments of the adjacent valve leaflets.

3  Anatomy of the inferoseptal region

The inferoseptal recess is an intracavitary inferoseptal extension of the left ventricular outflow tract. It extends inferiorly from the rightward end of the aortic-mitral fibrous continuity and interposes between the skirt of the aortic leaflet of the mitral valve and the septal surface of the left ventricle. Its roof is formed by the area of fibrous continuity between the mitral and tricuspid valvar leaflets, which also supports the base of the anteroinferior buttress of the atrial septum. [4,5]

The three structures are adjacent but anatomically distinct. Although muscular rather than a component of the fibrous skeleton, the inferoseptal process is the basal inferoseptal prominence of left ventricular myocardium and forms the ventricular floor of the inferior pyramidal space. The inferior pyramidal space is an extracavitary wedge of epicardial fibro-adipose tissue extending superiorly from the inferior atrioventricular groove. It is bounded by the inferior atrial walls and the inferoseptal ventricular process and contains the atrioventricular nodal artery in close relationship to the coronary sinus and the atrioventricular conduction tissues. [4,6]

Superiorly, the apex of the inferior pyramidal space approaches and partly overlaps the inferior extent of the inferoseptal recess. The roof of the recess is formed by the mitral-to-tricuspid fibrous continuity, while the process provides the muscular floor beneath the pyramidal space. At this complex junction, the atrioventricular conduction axis transitions from its atrial components to the penetrating bundle. The depth of the recess, the prominence of the muscular process, and the amount of intervening fibrous and adipose tissue vary between individuals, altering the spatial relationship among the triangle of Koch, coronary sinus, aortic root, and ventricular conduction axis. [4-6]

4  Aortic mitral continuity

The anterior mitral leaflet is in fibrous continuity with the left and non-coronary leaflets of the aortic valve. This aortic-mitral curtain forms part of the roof of the left ventricular outflow tract. The interleaflet triangle between the left and non-coronary aortic sinuses merges with this fibrous continuity. [1,2]

Although often represented as a flat curtain, the structure is curved and changes during the cardiac cycle. Its left and right ends are anchored by the fibrous trigones. These relations are important for valve surgery, transcatheter intervention, and mapping near the superior mitral junction.

5  Membranous septum and conduction axis

The membranous septum lies beneath the region between the right and non-coronary aortic sinuses. Attachment of the septal tricuspid leaflet divides it into atrioventricular and interventricular components. The penetrating atrioventricular bundle enters the central fibrous body and emerges on the ventricular side near the crest of the muscular septum before dividing. [1-3]

The atrioventricular node itself lies in atrial myocardium at the apex of the triangle of Koch; it is not embedded wholly within the fibrous skeleton. Its penetrating continuation is the normal muscular connection crossing the atrioventricular insulating plane. This distinction is essential when relating anatomy to recordings of the His bundle.

6  Incomplete annuli and junctional insulation

The terms mitral annulus and tricuspid annulus are useful clinically but can imply complete fibrous rings that do not exist anatomically. Fibrous support is strongest near the trigones and aortic-mitral continuity and more variable along the mural portions of the atrioventricular junctions. [1,3]

Where the fibrous plane is thin or discontinuous, atrial and ventricular myocardium may be closely apposed. Accessory pathways are anomalous muscular bridges across this plane. The epicardial grooves add adipose tissue and coronary vessels, producing marked regional differences in the distance between the two myocardial masses.

7  Clinical relevance

The fibrous skeleton defines the central landmarks for valve implantation, septal procedures, mapping of the conduction axis, and ablation of paraseptal pathways. Injury risk depends not simply on a named valve segment but on the depth and direction of the conduction tissues relative to the device or catheter.

For transcatheter aortic valve implantation, the relation between device depth, membranous septal length, the extent of the inferoseptal recess, and the branching atrioventricular bundle is particularly important. A narrow or shallow recess, especially when associated with counterclockwise rotation of the aortic root and minimal outflow myocardium beneath the right coronary leaflet, can bring the conduction axis closer to the implanted prosthesis. At the atrioventricular junctions, recognition of incomplete fibrous insulation helps explain pathway distribution without implying that normal atrial and ventricular myocardium are broadly continuous. [4,5]