SURGICAL ANATOMY OF MITRAL VALVE
DR. MOHAMED ELAWADI
PROF. OF CARDIOTHORACIC SURGERY
Fibrous Skeleton of the Heart
The overall structure and function of the heart depends on a widespread ‘honeycomb’ of connective tissue that courses throughout the heart cellular components.
This fine matrix is in turn supported by a more substantial network of dense connective tissue called the ‘fibrous skeleton of the heart.
This fibrous structure stabilizes the base of the ventricles, thus providing a relatively inflexible, but partially deformable, scaffold for the annulus of the mitral, tricuspid, and aortic valves.
The pulmonary valve is supported by the right ventricular
infundibulum, and is not directly related to the fibrous
skeleton of the heart.
In addition to providing mechanical support, the fibrous skeleton serves as an electrical insulator between the atrial and ventricular compartments of the heart.
This electrical insulation is interrupted only at the AV node, which is situated within the center of the fibrous skeleton.
A thorough comprehension of the fibrous skeleton is crucial to understanding the AV Functional Units, allowing recognition of the impact of both pathology and surgical intervention on valve function.
�Components of the Fibrous Skeleton�of the Heart
=The right fibrous trigone is situated at the center of the fibrous skeleton, Its superior boundary is positioned at the nadir of the non coronary sinus of the aortic valve, whilst inferiorly it relates to the posteromedial commissure of the mitral valve.
Four curved spines project from the right fibrous trigone called ‘fila coronaria,’ two partially surround the mitral annulus and two surround the tricuspid annulus.
The superior, posteriorly directed limb of the fila coronaria forms the anterior mitral valve annulus and unites with the left fibrous trigone
The Left FibrousTrigone
is positioned with its superior aspect at the nadir of the left-coronary sinus of the aortic valve whilst inferiorly it relates to the anterolateral commissure of the mitral valve.
From the left and right fibrous trigones the fibrous skeleton extends into ‘subaortic spans’ creating the aortic annulus.
These form a ‘coronet’ of fibroelastic tissue into which the aortic valve leaflets insert; the peak of each individual cusp within the coronet combines with the peak of the adjacent cusp to create the three commissures of the aortic valve situated between the left coronary cusp, the right coronary cusp, and the non coronary cusps, respectively.
The portion of the fibrous skeleton that is situated beneath the left coronary/non coronary commissure is referred to as the subaortic curtain which in turn is contiguous with the fibrous skeleton of the heart.
Its superior boundaries are the adjacent halves of the left coronary and non coronary aortic valve annulus, superiorly.
The curtain merges with the left and right fibrous bodies joined by inter-trigonal connective tissue inferiorly.
This structure stabilizes the interaction between the two valves, referred to as the aorto-mitral continuity
The anterior leaflet of the mitral valve hangs beneath the subaortic curtain, with the anterior mitral valve annulus formed by the inter-trigonal tissue.
The right fibrous trigone supports the posteromedial commissure of the mitral valve, whilst the left fibrous trigone supports the anterolateral commissure of the mitral valve.
The central fibrous body is the center-piece of the fibrous skeleton of the heart structurally and functionally.
It consists of the right fibrous trigone, the membranous septum, and the AV node and resides at the intersection of the mitral, tricuspid, and aortic valves.
The central fibrous body serves as a central hub, providing rigid support to the entire fibrous skeleton.
Age-associated calcification can occur in this area which can alter its functional properties.
Synchronized opening and closing of these three valves is vital for coordinated cardiac function and the fibrous skeleton is fundamental in stabilizing the dynamic processes involved.
Mitral Valve� The Functional Unit
The mitral or left atrioventricular valve separates the left atrium and left ventricle, optimizing the antegrade passage of blood to the left ventricle during ventricular diastole, whilst preventing retrograde flow during systole.
The mitral valve works as a Functional Unit, comprising numerous components, which provides the structure on which a dynamic series of physiological changes govern opening and closure throughout the cardiac cycle.
The Functional Unit consists of an annulus,two leaflets, atrial myocardium, chordae tendinae,papillary muscles, and ventricular myocardium.
Mitral Valve Annulus
When viewed in two-dimensions the mitral annulus is asymmetrical and elliptical in shape, bearing a resemblance to a kidney bean.
The anteroposterior dimension measures 0.75 of the lateral dimension.
However it has a non-planar saddle-shaped configuration, when viewed in three-dimensions with high points interiorly and posteriorly
The composition of the annular tissue from the left fibrous trigone, around the posterior aspect of the mitral valve annulus to the right fibrous trigone, has an even greater variability.
The fibrous tissue in some mitral valves extends almost completely around the annulus with gaps filled by less dense connective tissue whilst in others, very little fibrous extension is present beyond the inter-trigonal tissue, and trigones.
In these valves the annulus is composed of areolar tissue,
alongside ventricular and atrial myocardium.
Annular dilatation most commonly affects the area of the mitral valve annulus least supported by connective tissue and is therefore most frequently seen within the posterior mitral valve annulus.
Mitral Valve Leaflets
The mitral valve leaflets form a continuous curtain of tissue attached to the mitral annulus that guard the left atrioventricular orifice.
Although anatomical variations are reported, the mitral valve consists of two main leaflets,the anterior (aortic) and posterior (mural).
The anterior mitral valve leaflet separates the left ventricular inflow from the left ventricular outflow tract by hanging down from the fibrous skeleton between the left and right trigones.
The posterior leaflet is hinged on the posterior mitral valve annulus, extending between the left and right fibrous trigones.
The posterior mitral valve leaflet is attached to two thirds of the annular circumference, whilst the anterior leaflet is attached to only a third.
The junction of the anterior and posterior leaflets is formed by the mitral valve commissures.
The anterolateral commissure is located beneath the left fibrous trigone, whilst the posteromedial commissure is located beneath the right fibrous trigone.
Each leaflet has three scallops, divided by subcommissures.
The posterior subcommissures are more pronounced than those on the anterior leaflet.
For descriptive purposes, these scallops have been classified by Carpentier as A1, A2 and A3 on the anterior leaflet, and P1, P2 and P3 on the posterior leaflet.
This nomenclature starts from the anterolateral commissure,A1/P1 and progressing across the leaflet to the posteromedial commissure, A3/P3.
The A1 scallop, P1 scallop and the anterolateral commissure are supported by the anterolateral papillary muscle. The A3 scallop, P3 scallop, and the posteromedial commissure are supported by the posteromedial papillary muscle.
In contrast, the A2 and P2 scallops are supported by chordae tendinae from both papillary muscles.
This distinction is important when assessing and operating on the mitral valve.
This was recognized by Duran who proposed a differing classification of mitral leaflet anatomy, based on these functional considerations rather than the structural nomenclature described by Carpentier.
The anterior leaflet is semicircular in shape with a crescentic ridge along its length.
The area anterior to the ridge is called the clear zone, which is smooth and extends back to the annulus.
Distal to the ridge is the rough zone, so called because of its thickened nodular surface.
The ridge marks the “line of coaptation of the mitral valve leaflets,” the site beyond which the leaflet is in contact with the posterior mitral valve leaflet when the valve is closed.
The layer of coaptation between the two leaflets is known as the zone of coaptation .
The height of the anterior mitral valve leaflet ranges
between 20 and 25 mm, with a width of 30–35 mm.
Although the anterior leaflet attaches to the annulus around only one third of its circumference, its surface area is larger than the posterior leaflet, and it contributes to the majority cover of the orifice area during leaflet closure.
The posterior mitral valve leaflet is hinged from
two-thirds of the annulus and its three scallops P1, P2 and P3 are situated opposite the corresponding three anterior divisions.
Although there are three scallops in 90% of cases, there can be anatomical variation, with as many as five scallops reported.
The middle scallop is the largest in the majority of mitral valves.
The anterior mitral valve leaflet curves down to meet the posterior leaflet at the ‘line of coaptation.
During ventricular systole, the two leaflets are in contact with each other from the “line of coaptation” to the free edge of the leaflets, and this region is termed the “zone of coaptation.”
The posterior leaflet is slightly different in construction
when compared to the anterior leaflet.
It has a rough zone and a clear zone, similar to the anterior leaflet, but also an additional basal zone, which separates the annulus from the ‘clear zone’.
The ratio of rough zone: clear zone is also different when compared to the anterior leaflet with a considerably greater proportion of rough zone within the posterior leaflet.
This results from a much smaller clear zone area on the
posterior leaflet
Anterior leaflet: Rough zone and clear zone (rough:�clear = 0.6)
The collagenous fibrosa, together with the spongiosa, which is composed of proteoglycans, elastin, and connective tissue, makes up the core of the leaflets.
The outer surfaces are composed of elastin and are named as the atrialis and ventricularis layers, with both layers covered by a layer of endothelium.
Atrial and ventricular myocardium protrudes beneath the endothelial layers on the respective sides.
When the leaflet is exposed to stress during ventricular systole, the leaflets uncrimp, resulting in stretching out of the collagen fibers within the fibrosa and spongiosa layers, causing them to become linearly aligned. Subsequent pressure unloading results in recrimping.
These properties of the mitral valve leaflets reduce the stresses exerted upon the leaflets during opening and closure, thereby maintaining long-term function.
Also found within the leaflets are smooth muscle cells, myocardial cells, and contractile interstitial cells.
There is, in addition, leaflet neural innervation by both adrenergic and cholinergic nervous systems, both providing afferent and efferent nerves.
This innervation is most prominent on the anterior leaflet on the atrial side, proximally and medially, and potentially plays an important role in reducing valve leaflet stress during ventricular systole by increasing leaflet stiffness, thereby maintaining long-term valve function.
Mitral Valve Chordae Tendinae
The chordae tendinae connect the mitral valve leaflet to the papillary muscles.
The combination of the chordae and leaflet produces a parachute-like structure during ventricular systole, which prevents regurgitation of blood into the left atrium Blood passes around the chordae, after entering the left ventricle, thereby reaching the left ventricular outflow tract.
Three types can be described depending upon their attachment on the leaflets:
• Basal (or tertiary) chordae
• Intermediary (or secondary) also known as strut chordae chordae.
• Marginal (or primary) chordae are attached to the margin of the leaflets.
Primary Chordae
otherwise known as marginal or main chordae, insert into the free edge of the mitral valve.
Commissural chordae and cleft chordae are subcategories
of primary chordae.
Commissural chordae insert into the free edge of the anterolateral and posteromedial commissures, whilst cleft chordae insert into the free
edge of the leaflets spanning the subcommissures between the leaflet scallops.
Therefore, two arrangements of cleft chordae divide the leaflets of the mitral valve into three scallops.
Chordae inserting into the anterolateral commissural area and the adjoining half of the two leaflets arise from the anterolateral papillary muscle.
Chordae inserting into the posteromedial half of the leaflets and the posteromedial commissural area originate from the posteromedial papillary muscle.
�Secondary Chordae�
It insert into the body of the mitral valve leaflet at the junction of the rough zone and clear zone.
Tertiary chordae originate from the ventricular muscle and insert into the base of the leaflet.
Their point of insertion is into the basal zone of the leaflet and they are, therefore, not present in the anterior leaflet.
Secondary chordae tendinae branch into three, just after they originate from the papillary muscle.
They subsequently insert into the rough zone of the mitral valve leaflet.
The point of insertion is into the line of coaptation, at the border of the clear zone and rough zone.
These chordae are the thickest and strongest chordae.
Secondary chordae are exposed to the greatest chordal force as trans-mitral pressure gradients increase during ventricular systole.
They consist of connective tissue, fibroblasts, endothelial cells, and blood vessels, exhibiting a slightly elastic property under stress.
They are the narrowest, and thus the least strong just prior to inserting into the leaflet and this is often the site of rupture.
Primary chordae are 68% thinner than basal chordae, with posterior primary chordae 35% thinner than anterior primary chordae.
The chordal forces considerably greater on the anterior secondary chordae compared to the posterior secondary chordae.
These secondary chordae play a crucial role in maintaining left ventricular geometry, and thereby preserving left ventricular contractility.
Severing of the secondary chordae results in deterioration of left ventricular function, as measured by left ventricular elastance, a load-independent measure of systolic function, and preload recruitable stroke work, a load-independent measure of overall ventricular performance
This crucial role in maintaining left ventricular geometry and function underlines the importance of preserving the subvalvar apparatus when replacing the mitral valve with a prosthesis and also underlines the importance of mitral valve repair, which by definition preserves the secondary chordae in almost all the cases.
Secondary chordal division has been proposed in surgery for ischemic mitral valve regurgitation, but this approach has not been widely embraced, in part, because of the concern about its subsequent effect on left ventricular function.
Mitral Valve Papillary Muscles
There are two major papillary muscles that stabilize the chordae tendinae during ventricular systole, which together with the chordae are known as the subvalvar apparatus.
These papillary muscles are referred to as the anterolateral papillary muscle, and the posteromedial papillary muscle.
The anterolateral papillary muscle is situated superiorly on the anterior wall of the left ventricle and, in 70% of cases has only one head.
The anterior papillary muscle most often has a type I configuration with occasionally an adjacent type III papillary muscle attaching the commissural chordae.
The posterior papillary muscle usually has a type II configuration with one head attaching the chordae of the anterior leaflet, one head attaching the commissural chordae, and one head attaching chordae of the posterior leaflet.
Small arteries and veins penetrate either the base
of the papillary muscles or the body through trabecula
The anterolateral papillary muscle is vascularized by multiple branches from the anterior descending coronary artery and either the diagonal or a marginal branch of the circumflex vessels.
The posteromedial papillary muscle is vascularized by a small supply from the circumflex artery or the right coronary artery.
These differences between the vascularization of the papillary muscles explain why the posteromedial papillary muscle is more prone to necrosis and dysfunction than the anterolateral papillary muscle.
The posterior-medial papillary muscle is situated inferiorly on the posteroinferior wall of the left ventricle and has multiple heads, 60% of hearts having two to three heads.
The postero-medial papillary muscles are supplied by branches of the right coronary artery in 85% of cases or the left circumflex coronary artery in 10–15% of cases.
Acute ischemia of the papillary muscles can result in rupture leading to acute mitral valve regurgitation, whilst chronic ischemia and fibrosis can lead to papillary muscle dysfunction and chronic mitral valve regurgitation.
With the exception of type IV and V papillary muscles, which display a segmental distribution of vessels connected to intramyocardial vessels through trabecular attachments, the vascular branches have a central position within the papillary muscles.
One or two central arteries are long and terminal, dividing to form a network of rich anastomosis.
This arrangement has important implications in repair techniques involving the papillary muscles.
The papillary muscles are implanted on the muscular
wall of the left ventricle at a junction situated approximately 1/3 from the apex and 2/3 from the annulus .
The anterolateral papillary muscle is implanted at the junction between the septum and the posterior wall of the ventricle, either directly with a well-defined base or indirectly by several muscular bands.
The posteromedial papillary muscle is inserted on the lateral wall of the ventricle.