Mitral Valve Anatomy #1: Leaflets and Annulus

The mitral valve is best understood not as two simple flaps suspended within the left atrioventricular junction, but as a three-dimensional and dynamically integrated valvular complex. Its normal function depends on the coordinated geometry of the annulus, anterior and posterior leaflets, commissures, chordae, papillary muscles, left ventricular myocardium, and the fibrous continuity with the aortic root. This architectural integration explains why even subtle disturbances in annular shape, leaflet geometry, or ventricular-atrial coupling can alter coaptation and produce clinically relevant regurgitation [1,2]. (PubMed)

1. Structural organization of the normal mitral valve

The normal mitral valve consists of two principal leaflets, two commissural regions, and a nonplanar annulus.

• Anterior leaflet

The anterior leaflet, often called the aortic leaflet, is in fibrous continuity with the aortic root through the aorto-mitral curtain. Although it occupies a smaller proportion of the annular circumference than the posterior leaflet, it has greater radial height and a broad coapting surface. Its fibrous support and proximity to the left ventricular outflow tract make it anatomically and surgically distinct [1,2].

• Posterior leaflet

The posterior leaflet, also called the mural leaflet, occupies a larger portion of the annular circumference and is usually shorter in height. It is attached along the mural aspect of the ventricle and demonstrates greater anatomic variability than the anterior leaflet. This leaflet is particularly important in repair because it is frequently the site of degenerative prolapse and annular dilatation [1-3]. (PubMed)

The two leaflets meet at the anterolateral and posteromedial commissures. These are not merely lines of leaflet separation; each commissural region contains dedicated commissural tissue that contributes to leaflet continuity, coaptation, and the transition between the anterior and posterior leaflets. In operative and imaging terms, the commissures should therefore be regarded as functional anatomic zones rather than only border points [2]. (PubMed)

2. Segmental anatomy: A1-A3 and P1-P3

For surgical description and echocardiographic communication, the mitral valve is divided into corresponding leaflet segments:

Anterior leaflet

• A1: lateral segment
• A2: middle segment
• A3: medial segment

Posterior leaflet

• P1: lateral scallop
• P2: middle scallop
• P3: medial scallop

This nomenclature remains fundamental because it allows precise localization of prolapse, flail, restriction, cleft-like indentation, tethering, calcification, or commissural pathology. A statement such as “A2 prolapse” or “P3 restriction” is not merely descriptive; it immediately guides operative reasoning, anticipated mechanism, and reconstructive options [2,5]. (PubMed)

A key anatomic refinement, however, is that the posterior leaflet is truly scalloped, whereas the anterior leaflet is usually not divided into true anatomic scallops in the same way. The A1-A3 terminology is primarily a segmental convention describing portions of the anterior leaflet opposite P1-P3. Modern morphologic studies have shown that leaflet variation is common, predominantly in the posterior leaflet, and often related to accessory scallops or variable indentations. This is important because the classic segmental model is highly useful, but it is still a practical framework rather than a rigid rule that every normal valve follows identically [3]. (PubMed)

3. The annulus: a dynamic saddle-shaped structure

The mitral annulus is not a flat fibrous ring. It is a complex, heterogeneous, nonplanar structure composed of fibrous, muscular, and adipose components, with a geometry that changes throughout the cardiac cycle [1,6]. In normal physiology, the annulus assumes a saddle-shaped configuration, with high points located anteriorly and posteriorly and lower points located near the commissural or medial-lateral axis. This nonplanar form becomes more accentuated in systole and tends to flatten in pathologic states such as annular dilatation, degenerative valve disease, or functional regurgitation [4,6]. (PubMed)

This geometry is not simply descriptive. Biomechanical data support the concept that saddle-shaped annular configuration reduces leaflet stress compared with flatter annular forms, thereby improving mechanical efficiency and potentially contributing to repair durability [4]. Accordingly, contemporary valve analysis increasingly treats annular shape as a functional variable rather than a passive background feature [4-6]. (PubMed)

4. Aorto-mitral continuity and the fibrous skeleton

The anterior portion of the mitral annulus differs fundamentally from the posterior portion because it is in fibrous continuity with the aortic valve through the aorto-mitral curtain, also termed the intervalvular fibrosa [1,2]. This region is anchored by the left and right fibrous trigones, which form part of the central fibrous skeleton of the heart and serve as critical structural transition points between the mitral annulus, the aortic root, and adjacent septal structures [1,2]. (PubMed)

This distinction has direct surgical implications:

1. The anterior annulus is predominantly fibrous and structurally constrained.
2. The posterior annulus has greater muscular association and is more susceptible to dilatation and geometric remodeling.
3. Disease processes that enlarge the annulus usually affect the mural/posterior component more prominently than the aorto-mitral continuity.

That heterogeneity is central to repair strategy, annular sizing, and interpretation of regurgitant mechanisms [1,6]. (PubMed)

5. Functional asymmetry of the leaflets

The anterior and posterior leaflets are not redundant mirror structures; they are intentionally asymmetric in size, support, and function.

Anterior leaflet

• Greater radial height
• Broad coapting surface
• Strong fibrous continuity with the aortic root
• Important interaction with LV outflow geometry

Posterior leaflet

• Larger circumferential annular attachment
• Shorter radial height
• Greater morphologic variability
• Frequent site of degenerative disease, particularly at P2

This asymmetry helps explain why posterior leaflet prolapse is often particularly amenable to repair, whereas anterior leaflet pathology may be more technically demanding because of its geometry, chordal distribution, and relationship to systolic coaptation and the LV outflow tract [2,5]. Excess tissue, annular flattening, or abnormal coaptation shift may also influence the risk of postoperative systolic anterior motion and therefore affect reconstructive planning [2,5]. (PubMed)

6. Contemporary imaging and modern anatomic understanding

A major contemporary advance has been the transition from predominantly two-dimensional description to three-dimensional quantitative analysis of mitral valve anatomy. Modern 3D echocardiography has clarified that the mitral valve should be evaluated as a spatial structure, not merely as a series of planar slices. This approach improves visualization of leaflet curvature, annular nonplanarity, commissural relationships, coaptation geometry, and the spatial relationship between the mitral valve and surrounding structures [5,6]. (PubMed)

Three-dimensional imaging has also demonstrated that normal mitral anatomy varies with body size and age while preserving key geometric principles, including saddle-shape relationships. In pediatric populations, this is especially relevant because annular and leaflet dimensions scale with growth, whereas the underlying three-dimensional configuration remains conserved [7]. This reinforces the concept that normal mitral anatomy is not static, but developmentally and dynamically organized [7]. (PubMed)

7. Practical surgical interpretation

From a surgeon’s standpoint, mitral leaflet and annular anatomy should be interpreted through three questions:

1. Where is the lesion?

Localize it accurately by segment and commissural zone.

2. What is the mechanism?

Determine whether the dominant problem is excess tissue, prolapse, restriction, tethering, annular enlargement, or distortion of leaflet-annular-ventricular coupling.

3. How does the geometry affect repair?

Assess whether the annulus is flattened or dilated, whether leaflet asymmetry is preserved, and whether the planned reconstruction may alter coaptation height or LVOT geometry.

This approach aligns anatomic description with operative decision-making and reflects current understanding that durable mitral repair depends on restoring three-dimensional valvular geometry, not simply correcting an isolated leaflet defect [2,4-6]. (PubMed)

8. Practical summary

The mitral valve should be understood as a dynamic, asymmetric, and three-dimensional valve complex rather than a simple bicuspid structure.

• It has two principal leaflets, anterior and posterior.
• The posterior leaflet is divided into true scallops, P1-P3.
• The anterior leaflet is described by corresponding A1-A3 segments.
• The leaflets meet at the anterolateral and posteromedial commissures.
• The anterior annulus is in fibrous continuity with the aortic root through the aorto-mitral curtain.
• The annulus is heterogeneous, nonplanar, and saddle-shaped.
• Annular shape is functionally important because it influences leaflet stress, coaptation, and repair durability.
• Contemporary 3D imaging has refined understanding of normal variation and has made mitral anatomy more quantifiable, more surgical, and more mechanistically interpretable [1-7]. (PubMed)

References

[1] Oliveira D, Srinivasan J, Espino D. Geometric description for the anatomy of the mitral valve. J Anat. 2020;237(2):209-224.

[2] Dal-Bianco JP, Levine RA. Anatomy of the mitral valve apparatus: role of 2D and 3D echocardiography. Cardiol Clin. 2013;31(2):151-164.

[3] Krawczyk-Ożóg A, Hołda MK, Sorysz D, Koziej M, Sorysz P, Duplaga M, Dudek D, Klimek-Piotrowska W. Morphologic variability of the mitral valve leaflets. J Thorac Cardiovasc Surg. 2017;154(6):1927-1935.

[4] Jimenez JH, Liou SW, Padala M, He Z, Sacks M, Gorman RC, Gorman JH 3rd, Yoganathan AP. A saddle-shaped annulus reduces systolic strain on the central region of the mitral valve anterior leaflet. J Thorac Cardiovasc Surg. 2007;134(6):1562-1568.

[5] Muraru D, Badano LP, Vannan M, Iliceto S. Mitral valve anatomy and function: new insights from three-dimensional echocardiography. J Cardiovasc Med (Hagerstown). 2013;14(2):91-101.

[6] Faletra FF, Leo LA, Paiocchi VL, Schlossbauer SA, Franzen O, Pedrazzini G, Moccetti T, Auricchio A. Anatomy of mitral annulus insights from non-invasive imaging techniques. Eur Heart J Cardiovasc Imaging. 2019;20(8):843-857.

[7] Jolley MA, Li JS, Rome JJ, Ivy DD, Maskatia SA, Rychik J. Three-dimensional mitral valve morphology and age-dependent trends in children with normal mitral valves. J Am Soc Echocardiogr. 2017;30(8):763-771.