Anatomical Structure of Cardiac Valves and Commissural Fusion
Basic Valve Anatomy
Cardiac valves consist of two distinct structural complexes: the atrioventricular valvar complex (mitral and tricuspid) and the ventriculo-arterial complex (aortic and pulmonic), each with unique anatomical components that are critical for understanding commissural fusion. 1
Atrioventricular Valvar Complex (Mitral and Tricuspid Valves)
The atrioventricular valves are composed of four integrated components 1:
- Annulus: The fibrous ring that anchors the valve at the atrioventricular junction 1
- Leaflets: The mobile tissue flaps that open and close (cusps in atrioventricular valves are called leaflets) 1
- Tendinous cords (chordae tendineae): Fibrous strings connecting leaflets to papillary muscles 1
- Papillary muscles: Muscular projections from the ventricular wall that provide tension to prevent leaflet prolapse during systole 1
The tension apparatus (chordae and papillary muscles) is essential because it holds the leaflets together against the force of ventricular systole, preventing them from prolapsing into the atrium 1.
Ventriculo-Arterial Complex (Aortic and Pulmonic Valves)
The semilunar valves have a simpler but equally important structure 1:
- Leaflets (cusps): Three semilunar-shaped tissue flaps that achieve snug closure during ventricular diastole 1
- Valvar sinuses: The dilated pockets behind each cusp that support leaflet function 1
- Sinutubular junction: The distal boundary where the sinuses meet the arterial trunk 1
- Commissures: The points where adjacent cusps meet at their attachment to the arterial wall 1
The semilunar nature of these leaflets is what enables their tight closure without requiring a tension apparatus 1.
Commissural Anatomy Specific to Fusion
Mitral Valve Commissures
The mitral valve has two commissures where fusion can occur 2, 3:
- Anterolateral commissure: Located anteriorly in the ventricle, positioned under the aortic valve's left coronary cusp 2, 3
- Posteromedial commissure: Located posteriorly in the ventricle, positioned under the aortic valve's noncoronary cusp 2, 3
Aortic Valve Commissures
The aortic valve has three commissures corresponding to the three cusps 2, 3:
- Left-noncoronary commissure: The fusion point between the left coronary cusp and noncoronary cusp 2, 3
- Right-left coronary commissure: The fusion point between the right coronary cusp and left coronary cusp 2, 3
- Right-noncoronary commissure: The fusion point between the right coronary cusp and noncoronary cusp 2, 3
Pathological Fusion Patterns
Classification by Severity
Commissural fusion severity directly impacts valve function and is classified as 2, 3:
- Absent: Normal cusp separation with valve motion (no pathology) 2, 3
- Partial: Cusps or leaflets do not separate fully during valve opening, creating functional stenosis 2, 3
- Complete: Two or more cardiac valve cusps do not separate at all with valve motion, causing severe stenosis 2, 3
Clinical Example: Unicuspid Aortic Valve
In unicuspid aortic valves (Sievers type 2), there is characteristic major fusion of the right-left coronary commissure with clefting, and minor fusion of the right-noncoronary commissure, occurring in 85% of cases 4. This demonstrates how commissural fusion can be congenital and involve multiple commissures simultaneously 4.
Developmental Origin
All valvar leaflets originate from endocardial cushions through endothelial-to-mesenchymal transformation 1:
- Atrioventricular valve leaflets develop from cushions in the atrioventricular canal 1
- Semilunar valve leaflets develop from outflow tract cushions that are excavated to form the final leaflet structure 1
- The papillary muscles derive from the trabecular layer of developing ventricular walls, not from the cushions 1
This developmental knowledge explains why commissural fusion can occur as a congenital malformation when cushion tissue fails to separate properly, or as an acquired condition when previously separated commissures become adherent 1.