The anatomy of heart valves is usually described with respect to transverse or longitudinal sections of the heart, which provide optimal viewpoints of the heart’s different structures ( Fig. 5-1 ). These anatomical views have limited practical value to surgeons because during an operation surgeons observe the mitral valve from the left atrium. The purpose of this chapter is therefore to describe the surgical anatomy of the mitral valve seen from an atrial view, with particular emphasis to anatomical details of significant importance in valve reconstruction.
With the left atrium widely exposed, three structures can be observed successively: the atrio-valvular junction, the leaflets, and the valve suspension system ( Fig. 5-2 ).
THE ATRIO-VALVULAR JUNCTION
The junction between the atrium and the valvular tissue is usually well delineated by the different colors of these structures: the atrium is slightly pink, and the leaflets are yellowish. This junction defines the hinge where the motion of the leaflets is initiated. The hinge allows demarcation of the annulus fibrosus , which is not visible from an atrial view. The part of the annulus fibrosus where the leaflets attach is located deeper and 2 mm external to the hinge. This anatomical relationship has important surgical implications in valve reconstruction since the sutures used to secure a prosthetic ring should be passed through the annulus and not through the hinge in order to preserve leaflet motion. Thus suture needles must be placed 2 mm away from the hinge and oriented towards the ventricle in order to pass through the resistant fibrous body of the annulus. Whenever endothelial thickening or jet lesions render the junction barely visible, an up-and-down mobilization of the leaflet can visualize the hinge.
The annulus fibrosus ( Fig. 5-3 ) is actually a discontinuous band of connective tissue that exists only in some parts of the attachment of the posterior leaflet. Mixed circular and oblique fibrous bundles ensure the continuity between the atrium, the posterior leaflet, and the crest of the ventricular wall while providing optimal mobility to the hinge of the leaflet (a) . The annulus in actuality does not exist at the attachment of the anterior leaflet because the leaflet tissue is continuous with the aorto-mitral curtain that extends from the aortic valve annulus to the base of the anterior leaflet (b) . At each extremity of the base of the anterior leaflet, the atrio-valvular junction is reinforced by two dense triangular fibrous structures: the anterolateral and posteromedial fibrous trigones.
The annulus of the mitral valve is not visible from the atrium. It is deeper and 2 mm external to the visible hinge of the leaflets.
The shape of the annulus varies throughout the cardiac cycle ( Fig. 5-4 ). During diastole, the shape is grossly circular (a) . During systole, the annulus has a kidney shape with the anteroposterior (septolateral) diameter significantly smaller than the transverse (commissure to commissure) diameter (b) ( Table 5-1 ). The 26% ± 3% reduction of the mitral valve orifice area during systole results from the contraction of the base of the heart and the displacement of the aorto-mitral curtain towards the center of the orifice.
| Echo Measurements of the Atrio-valvular Junction (Annulus) | Corrected Orifice Area for Body Surface Area | |
|---|---|---|
| Orifice area | ||
| Diastole: | 7.1 ± 1.3 cm 2 | 3.8 ± 0.7 cm/m 2 |
| Systole: | 5.0 ± 1.6 cm 2 | 2.9 ± 0.6 cm/m 2 |
| % reduction of orifice area | 26 ± 3% | |
| Circumference (max.): | 93 ± 9 mm | |
| % reduction of circumference: | 13 ± 3 | |
Instead of having a planar configuration, the annulus has a three-dimensional saddle-shape configuration ( Fig. 5-5 ). The two lowest points are located at the fibrous trigones and the two highest points are located at the midpoints of the anterior and posterior annuli. The midpoint of the anterior annulus is higher than that of the posterior annulus. The plane of the mitral valve annulus makes a 120° angle with the plane of the aortic valve annulus. This facilitates the filling of the inflow chamber of the left ventricle. Finally, as shown by magnetic resonance imaging (MRI), the annulus is displaced 5 to 10 mm apically during systole. The resulting increase in the dimension of the left atrium facilitates its filling. Pathological processes may alter this normal anatomy.
Four anatomical structures close to the annulus are at risk during surgery ( Fig. 5-6 ):
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The circumflex artery runs between the base of the left atrial appendage and the anterior commissure, 3 to 4 mm from the leaflet attachment, and then moves away from the rest of the posterior annulus.
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The coronary sinus skirts the attachment of the posterior leaflet. It is initially in a lateral position, and then crosses the artery and becomes medial, closer to the posterior leaflet attachment but 5 mm superior to the annulus.
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The bundle of His is located near the posteromedial trigone.
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The noncoronary and left coronary aortic cusps are in close relationship with the base of the anterior leaflet. Although slight variations exist (inset) , the nadir of these cusps is 6 to 10 mm away from the anterior mitral valve annulus, a distance that represents a safety zone for the placement of sutures in this area provided that the needle is properly oriented towards the ventricle.
THE LEAFLETS
The mitral valve comprises two leaflets—anterior and posterior—separated by two commissures. These leaflets are the opening and closing structures of the valve. The free motion of the hinge is necessary for optimal opening of the valve. Optimal closure implies a precise fitting between the surface area of the leaflets and the orifice area of the mitral valve.
The surface area of the leaflets and the surface area of the mitral orifice have a precise geometrical relationship, a condition required for optimal function.
Although the anterior and posterior leaflets have a different size and shape ( Table 5-2 ), with the anterior leaflet more extended vertically and the posterior leaflet more extended transversally, they have a similar surface area. The basal insertion of the anterior leaflet occupies approximately one third of the circumference of the mitral valve ( Fig. 5-7 ). The remaining two thirds of the circumference attaches to the posterior leaflet and the commissural tissue. The anterior leaflet is primarily related to the left ventricular outflow tract via the aorto-mitral curtain whereas the posterior leaflet is related to the muscular parietal base of the left ventricle. As a result of this configuration, the maximum stress during systole is concentrated at the midline of the posterior leaflet.
| Anterolateral Commissure | Anterior Leaflet | Posteromedial Commissure | Posterior Leaflet | |
|---|---|---|---|---|
| Insertion length (mm) | 12 ± 3.3 | 32 ± 1.3 | 17 ± 0.8 | 55 ± 2.2 |
| Height (mm) | 8 ± 1 | 23 ± 0.9 | 8 ± 1 | P1: 9 ± 1 |
| P2: 14 ± 0.9 | ||||
| P3: 10 ± 1.2 | ||||
| Coaptation zone height (mm) | 4 ± 0.5 | 8 ± 1.1 | 4 ± 0.6 | P2: 8 ± 0.9 |
The anterior leaflet , also called the aortic leaflet, has a trapezoidal shape ( Fig. 5-8 ). Its base, which measures 32 ± 1.3 mm, is inserted on the aorto-mitral curtain and the adjacent fibrous trigones. The free edge presents with a slightly convex curvature. At the midline, the height of the anterior leaflet averages 23 mm. From the base to the margin, two zones (proximal and distal) are clearly apparent. The proximal zone, called the atrial zone , is regular, thin, and translucent. The distal zone, called the zone of coaptation or rough zone , is irregular and thicker because of the numerous chordae attached to its ventricular side. The limit between these two zones is often highlighted by a thicker transverse line. The two zones have a similar surface area. The surface of coaptation, which has a height of 7 to 9 mm, ensures the competency of the valve during systole regardless of the physiological variations of ventricular volumes and pressures. During diastole, the anterior leaflet divides the left ventricle into two functional areas, the inflow chamber and the outflow tract (inset) .
The posterior leaflet is inserted approximately on two thirds of the annulus, to the crest of the ventricular wall. The free edge is deeply scalloped by two indentations, sometimes improperly called “clefts,” separating three segments: the anterior, middle, and posterior scallops are also called P1, P2, and P3, respectively, to facilitate valve analysis ( Fig. 5-9 ). By convention, the corresponding areas of the anterior leaflet are called A1, A2, and A3, and the commissures AC and PC. The size of the scallops of the posterior leaflet differs. The largest is the middle scallop (P2) and the smallest is the anterior scallop (P1). The indentations are supported by numerous chordae. They function like a commissure, facilitating a large opening of the posterior leaflet during diastole. The special configuration of the middle scallop evokes a spinnaker, with a large bulging surface area submitted to full systolic stress, explaining the higher frequency of P2 prolapse compared to P1 and P3, which have smaller surface areas.
The indentations of the posterior leaflet are normal anatomical structures that allow the leaflet to fully open during diastole.
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