Cardiac Skeleton (Fibrous Skeleton of the Heart)

The cardiac skeleton is a fibrous framework that provides structural support, electrical insulation, and attachment points for cardiac muscle and valves. Detailed anatomy of the fibrous rings, trigones, and membranous septum.

Heart Anatomy 6 min read
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The cardiac skeleton is a dense fibrous framework within the heart that provides structural support, serves as attachment points for cardiac muscle and valve leaflets, and electrically insulates the atria from the ventricles. Understanding its anatomy is essential for cardiac surgery and electrophysiology.

Overview and Composition

The cardiac skeleton consists of dense connective tissue (predominantly type I collagen with some elastic fibers). It is the remnant of the embryologic cardiac mesenchyme and gradually matures through childhood, reaching its adult density by adolescence.

Functions:

  • Provides rigid attachment for the four cardiac valves
  • Anchors the atrial and ventricular myocardium
  • Maintains the shape and integrity of the cardiac orifices
  • Electrically insulates the atria from the ventricles (the only normal myocardial connection through the atrioventricular bundle passes through the skeleton)
  • Distributes mechanical forces during the cardiac cycle
  • Serves as a barrier to the spread of infection (ring abscesses)

Components of the Cardiac Skeleton

Fibrous Rings (Annuli Fibrosi)

Four fibrous rings surround the cardiac orifices:

Mitral Annulus:

  • D-shaped fibrous ring around the left atrioventricular orifice
  • Anterior portion is interrupted by the intervalvular fibrosa
  • Weaker posterior portion is prone to dilation
  • Thin in cross-section compared to the aortic annulus

Tricuspid Annulus:

  • Oval or triangular fibrous ring around the right atrioventricular orifice
  • Less well-defined than the mitral annulus
  • Dynamic, with significant shape changes during the cardiac cycle
  • More prone to dilation (functional tricuspid regurgitation)

Aortic Annulus:

  • Crown-shaped, not a simple ring
  • Three scalloped attachments for the aortic valve cusps
  • Strongest of the four annuli (must withstand systemic pressure)
  • Fibrous continuity with the mitral annulus

Pulmonary Annulus:

  • Circular fibrous ring at the right ventricular outflow
  • Thinner than the aortic annulus
  • Less well-developed fibrous skeleton contribution
  • Must withstand lower pulmonary pressures

Fibrous Trigones

Two dense fibrous masses connect the annuli:

Right Fibrous Trigone:

  • Located between the aortic valve, mitral valve, and tricuspid valve
  • The largest mass of fibrous tissue in the cardiac skeleton
  • Contains the atrioventricular bundle (Bundle of His) passing through it
  • Dense collagen with some cartilage-like cells

Left Fibrous Trigone:

  • Located between the aortic valve and mitral valve
  • Smaller than the right trigone
  • Forms part of the aortomitral continuity
  • Less commonly involved in calcification

Intervalvular Fibrosa (Aortomitral Curtain)

The intervalvular fibrosa is the fibrous continuity between the aortic valve and the anterior mitral leaflet.

Anatomy:

  • Triangular sheet of fibrous tissue
  • Extends from the left and non-coronary cusps of the aortic valve to the anterior mitral leaflet
  • Base at the aortic annulus, apex at the anterior mitral leaflet
  • Forms the posterior wall of the left ventricular outflow tract
  • Lies anterior to the left atrium

Clinical Significance:

  • Site of aortomitral continuity (important for double valve replacement)
  • Can be involved in infective endocarditis (intervalvular fibrosa abscess)
  • Calcification can cause conduction abnormalities
  • Important landmark for transcatheter aortic valve replacement (TAVR)

Membranous Septum

The membranous septum is a thin, fibrous portion of the interventricular septum that is part of the cardiac skeleton.

Components:

  • Atrioventricular portion: Separates the right atrium from the left ventricle (superior)
  • Interventricular portion: Separates the right ventricle from the left ventricle (inferior)

Relations:

  • The atrioventricular bundle (Bundle of His) passes through the posterior margin
  • The septal tricuspid leaflet attaches to its right side
  • The aortic valve (right and non-coronary cusps) is related superiorly

Clinical Significance:

  • Common site of ventricular septal defects (membranous VSD)
  • Can be involved in aortic valve endocarditis with fistula formation
  • The Bundle of His is vulnerable during surgery for membranous VSD

Tendon of Todaro

A fibrous band within the right atrium that is part of the cardiac skeleton.

  • Extends from the central fibrous body to the Eustachian valve (valve of the inferior vena cava)
  • Forms one border of the triangle of Koch
  • Landmark for the atrioventricular node location
  • Composed of collagen and elastic fibers

Sinus Septum

The fibrous tissue separating the left atrial sinus from the pulmonary veins.

  • Incomplete in some individuals
  • Can be a substrate for left atrial arrhythmias
  • Contains the myocardial sleeves of the pulmonary veins

Anatomy of the Triangle of Koch

The triangle of Koch is a key anatomical landmark in the right atrium that marks the location of the atrioventricular node.

Borders:

  • Anterior: Attachment of the septal tricuspid leaflet (tendon of Todaro)
  • Posterior: Coronary sinus orifice
  • Apex: Central fibrous body (right fibrous trigone)

Contents:

  • Atrioventricular node (proximal)
  • Atrioventricular bundle (distal, at the apex)

The triangle of Koch is the target for AV nodal ablation during AV nodal reentrant tachycardia (AVNRT).

Development of the Cardiac Skeleton

The cardiac skeleton develops from the cardiac mesenchyme during embryonic development:

  • Weeks 4-8: Formation of the endocardial cushions
  • Week 8-12: Cushion fusion and condensation of mesenchymal tissue
  • Fetal period: Progressive collagen deposition and maturation
  • Childhood: Continued thickening and strengthening
  • Adulthood: Stable structure with slow age-related changes

Function

Mechanical Support

The cardiac skeleton provides a rigid scaffold that:

  • Maintains patency of the valvular orifices
  • Prevents overdilation of the annuli
  • Anchors the myocardium for coordinated contraction
  • Distributes contractile forces across the heart

Electrical Insulation

The fibrous skeleton is non-conductive and electrically isolates the atria from the ventricles:

  • The only normal electrical connection through the skeleton is the atrioventricular bundle
  • This ensures coordinated sequential contraction (atria first, then ventricles)
  • Disruption of this insulation can cause pre-excitation syndromes

Valve Anchoring

The fibrous skeleton provides the rigid attachment points necessary for valve function:

  • The annuli maintain the shape of the valvular orifices
  • Leaflets attach to the annuli
  • The intervalvular fibrosa maintains the relationship between the aortic and mitral valves
  • Neonatal: Fibrous skeleton is thin and pliable
  • Childhood: Progressive collagen deposition
  • Adulthood: Mature collagen structure with cross-linking
  • Elderly: Increased stiffness, calcification
  • Pathologic calcification: Involves the mitral annulus (most common), aortic annulus, or fibrous trigones

Clinical Significance

Calcification of the Cardiac Skeleton

Mitral Annular Calcification:

  • Common in elderly, especially women
  • Associated with chronic kidney disease, diabetes, hypertension
  • Can cause mitral regurgitation, stenosis, or both
  • May predispose to endocarditis
  • Can involve the conduction system causing heart block

Calcification of the Intervalvular Fibrosa:

  • Associated with aortic stenosis (post-inflammatory)
  • Can extend into the anterior mitral leaflet
  • May cause mitral stenosis independent of the mitral annulus

Conduction Abnormalities

Lev Disease:

  • Idiopathic calcification of the cardiac skeleton involving the conduction system
  • Causes progressive heart block in the elderly
  • Involves the central fibrous body and membranous septum

Lenegre Disease:

  • Primary sclerodegenerative disease of the conduction system
  • May involve the fibrous skeleton
  • Presents as progressive conduction disease in younger adults

Infective Endocarditis

Ring Abscess:

  • Infection extends from the valve into the annulus
  • Most common with aortic valve endocarditis
  • Can destroy the fibrous skeleton
  • May cause fistula formation (aorta to right atrium or right ventricle)
  • Requires surgical debridement and reconstruction

Surgical Considerations

  • Valve replacement: Sutures placed through the annuli must respect the fibrous skeleton
  • Maze procedure: Lesion sets are designed with knowledge of the fibrous skeleton as electrical barrier
  • Transcatheter valves: Understanding annular geometry is essential for sizing
  • Septal myectomy: Knowledge of the membranous septum protects the conduction system