MMCTS
(February 19, 2007). doi:10.1510/mmcts.2006.002147
Copyright © 2007 European Association for Cardio-thoracic Surgery
Critical Overview
Mitral valve repair: critical analysis of the anatomy discussed
Robert H. Anderson* and
Mazyar Kanani
Cardiac Unit, Institute of Child Health, University College, 30 Guilford Street, London WC1N 1EH, UK
* Corresponding author: * Tel.: +44-20-7905-2295 E-mail: r.anderson{at}ich.ucl.ac.uk
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Summary
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In this brief review, we discuss the anatomy of the mitral valve pertinent to surgical repair. First, we emphasise the need for diagnosticians to describe the valve in the context of the position of the heart within the body, following the standard rules of anatomy, and using attitudinally appropriate descriptions. It has become customary to describe cardiac structures as if the heart is positioned on its apex. This cannot be good in the current era, when the tomographic techniques increasingly used for diagnosis demonstrate the heart as seen in the body. We then discuss the overall valvar structure in terms of a complex made up of the annulus, the leaflets, their tendinous cords, and the supporting papillary muscles. After providing accounts of the salient structure of each part of the complex, we discuss potentially divisive issues, such as the number of leaflets, and the categorisation of the tendinous cords. We explain how most of the disagreements stem not from differences in observation, but rather from differences in definitions. We suggest that these can largely be dissipated if the valve is analysed in its closed, rather than its open, position. When seen in the closed position, it becomes obvious that the key feature is the solitary zone of apposition between the major components of the skirt of leaflet tissue, this being the major functional part of the valvar complex. Finally, we discuss the relationships of the valvar complex to the other cardiac structures, concentrating on the other cardiac valves, the conduction tissues, and the coronary arteries and veins.
Key Words: Atrioventricular valves • Cardiac surgery • Morphology
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Introduction
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It is axiomatic that knowledge of morphology provides the basis for optimal surgical practise. Nowhere is this truer than with the mitral valve. It is surprising, therefore, that several systems are currently in use for description of the components of the mitral valve [1,2,3,4]. In this review, we show how the different approaches can be reconciled when the valve is analysed in closed rather than open position, the valve needing, of course, to maintain its competence whilst in this closed position.
Attitudinally appropriate description
When describing the morphological arrangement of any structure within the body, convention dictates that the structure should be considered as it is oriented in space within the body in the so-called anatomical position, the subject standing upright and facing the observer. This position is then used to establish the coordinates for description, irrespective of whether the body, or the patient, is viewed in the upright position, or is lying on the operating table, as typically seen by the cardiac surgeon [5]. Within this standard position, all structures are then described on the basis of the three orthogonal planes, which run from anterior to posterior, from superior to inferior, and from medial to lateral. It is unfortunate, therefore, that it has become conventional to describe the heart, which is positioned within the body with a marked discrepancy between its own long axis and the bodily long axis, as if the heart is positioned on its apex, taking the cardiac long axis as representing the vertical plane. With the increasing use of diagnostic techniques that illustrate the heart appropriately in the context of the body, this cannot be helpful. Indeed, the inconsistencies engendered by this approach may now positively interfere with clinical understanding. A case in point is the positions of the papillary muscles supporting the leaflets of the mitral valve. In most textbooks, these muscles are currently described as being anterior and posterior, or else antero-medial and postero-lateral. Analysis of the arrangement as seen with the heart in the anatomical position (Schematic 1) shows the deficiencies of this approach. When compared to the coordinates of the body, the so-called anterior muscle is more posterior than its counterpart. In reality, the muscles are positioned superiorly and leftward, and inferiorly and rightward (Schematic 1). The future use of such attitudinally appropriate descriptions, or else describing the muscles as being positioned right and inferior, or septally, and left and superior, or parietally, will surely go some way to resolving the sometimes diverging interpretations reached by morphologists, surgeons, and echocardiographers.
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Schematic 1 The so-called anterior papillary muscle (solid star) is positioned superiorly and posteriorly relative to its counterpart (open star), the latter located inferiorly and anteriorly, when considered relative to the orthogonal planes of the body. Future surgical practise will dictate how best to describe these muscles in meaningful fashion.
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Structure of the mitral valve
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The mitral valvar complex [6] is made up of several individual parts, which need to function in harmony if the overall complex is to maintain its competency. The parts are the annulus, the leaflets, the tendinous cords, and the papillary muscles (Photo 1).
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Photo 1 As shown in this section of the human heart taken to simulate the parasternal long axis, echocardiographic cut, the mitral valve is a complex structure made up of the annulus, the leaflets, the tendinous cords, and the papillary muscles. Note that the superior leaflet of the valve, in fibrous continuity with the leaflets of the aortic valve, has greater depth than the inferior leaflet, which is hinged from the atrioventricular junction.
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We will discuss in turn the salient anatomy of each part.
The annulus
Although more kidney-shaped, or D-shaped, than circular, the annulus is a complete fibrous entity, which serves as the anchor for the leaflets. Although not readily obvious to the morphologist, it is also far from being a planar structure, possessing a distinct saddle-shaped configuration. Its parietal component coincides with the atrioventricular junction. The flattened portion of the D, however, extends away from the junction, incorporating as it does the extensive region of continuity with two of the leaflets of the aortic valve (Photo 2).
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Photo 2 The mitral valve has been removed from the heart together with the aortic root, and the contiguous parts of the fibrous structures supporting the leaflets of the tricuspid valve, and is viewed from the apex of the left ventricle, having removed the greater parts of the leaflets. The dissection shows how part of the annulus is formed by the region of fibrous continuity with the leaflets of the aortic valve (dotted yellow line). The ends of this area of continuity (triangles) are the fibrous trigones that anchor the valvar complex to the roof of the left ventricle. The right trigone merges with the membranous septum to form the central fibrous body.
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The two ends of this area of fibrous continuity anchor the overall valvar compex to the walls of the left ventricle. These ends are called the fibrous trigones, with the right trigone being larger and firmer than the left. In bovines, this structure is ossified, forming the so-called os cordis [7]. The right fibrous trigone is itself continuous with the membranous septum, the conjoined structures forming the central fibrous body. This body is pierced by the bundle of His, as this part of the atrioventricular conduction axis extends from the apex of the triangle of Koch to reach the crest of the muscular ventricular septum. The left fibrous trigone is attached to the parietal wall of the left ventricle (Photo 3).
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Photo 3 The left ventricular outflow tract has been opened from the front, showing how the area of fibrous continuity between the leaflets of the aortic and mitral valves is anchored at its ends by the fibrous trigones. Note the layered attachment of the ventricular aspect of the aortic leaflet of the mitral valve.
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The remainder of the annulus then continues around the parietal part of the left atrioventricular junction, where it provides flexible rather than a rigid support for the valvar leaflet. In places, the annulus takes the form of a collagenous rod, or shelf, which supports the mural leaflet of the valve. In other areas, the annulus is no more than a fibro-fatty fold. These features vary markedly from individual to individual [8].
The leaflets
The most important components of the valve, without question, are the leaflets. These are also the parts that attract the greatest divergence in description, particularly their number, albeit that this largely reflects the criterions used to define them [1,2,3,4, 9]. Traditionally, the mitral valve has been presumed to have two leaflets, hence its alternative title of the bicuspid valve. As far as we know, it was Andreas Vesalius, the famous anatomist who worked in Padova, who first likened the valve to the Episcopal mitre (Photo 4).
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Photo 4 The mitral valve is viewed from the apex of the left ventricle. The likeness to the Episcopal mitre is obvious.
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Subsequent to his descriptions, anatomists usually described the presumed two leaflets as being anterior and posterior, albeit that when viewed in attitudinally appropriate orientation, the overall valvar complex is seen to occupy a more oblique position within the cardiac short axis than these traditional names suggest (Photo 5).
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Photo 5 The heart has been sectioned in its short axis, and is viewed from the apex of the left ventricle. The leaflets of the mitral valve (black and red dotted lines) are obliquely orientated relative to the short axis of the body (green line). For this reason, we prefer to describe the leaflets as being aortic (black dotted line) and mural (red dotted line), following the precedent of Andreas Vesalius.
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Our own preference remains to describe the valve as possessing two leaflets, but following the precedent of Vesalius, and describing the so-called anterior leaflet as the aortic leaflet, and its counterpart as the mural leaflet. When viewed from the ventricular aspect, the defining feature of the aortic leaflet, and the reason for this name, is seen to be its fibrous continuity with two of the leaflets of the aortic valve. Its rounded leading edge hangs free as a curtain that separates the inflow and outflow components of the left ventricle (Photo 3). On either side of this dependent free edge, and throughout its extent, tendinous cords tether it to the paired left ventricular papillary muscles. Assessment from the atrial side in open position shows the significant depth of the leaflet, largely made up of the smooth zone, which is devoid of insertions of tendinous cords (Photo 6). The layered attachments of the tendinous cords, seen from the ventricular aspect (Photo 3), are to the so-called rough zone.
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Photo 6 The aortic leaflet of the mitral valve is seen from the atrial aspect. The leaflet possesses a rough zone, to which are attached the tendinous cords on the ventricular aspect (see Photo 3), and a smooth zone. The red dotted line shows the line of closure with the mural leaflet.
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Assessment from the atrial aspect, with the leaflets in closed position, shows that the aortic leaflet, positioned superiorly within the left ventricle (Photo 1), guards about one-third of the circumference of the annulus (Photo 7).
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Photo 7 The pattern of closure of the mitral valve as seen by the surgeon viewing the valve from its atrial aspect. The aortic and mural leaflets coapt along a solitary primary zone of apposition (red dotted line). There are several slits in the mural leaflet that permit snug closure of the valve (blue dotted lines), albeit that there is but one line of primary closure. Note that the two ends of this primary line of closure do not extend to the annulus (red brackets).
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But, because of its increased depth when compared to its mural counterpart (Photo 1), the overall orificial area covered by the two leaflets is more-or-less the same [9]. The leaflet positioned postero-inferiorly, best described as being the mural leaflet, when examined in a large series of specimens or patients, is found to have a much more variable appearance than the aortic leaflet. When viewed in closed position from the atrial aspect, the mural leaflet typically has at least three slits along its free edge. The problems relating to the number of components considered to exist within this leaflet, and how best to describe them, devolve from the usual practise of the pathologist, and anatomist, of examining the valve in its open position. When seen in this opened form (Photo 8), the indentations along the mural leaflet separate the so-called ‘scallops’, with the free margins of these components often running to within a half of a millimetre of the annulus [10].
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Photo 8 The mitral valve is photographed in open position, showing the indentations (arrows) in the mural leaflet that extend towards the annulus, albeit not quite reaching the atrioventricular junction. This arrangement gives the mural leaflet its scalloped appearance.
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The major issue is whether these scallops are to be considered as separate leaflets in their own right [1], or whether they are mere sub-divisions of one long, solitary mural leaflet [2, 3]. As we have already intimated, and as is shown in Photo 6, the variability in the size and number of these scallops is best appreciated when the closed valve is viewed from its atrial aspect, as it would be seen by the surgeon contemplating repair. The essential feature afforded by such examination is that the components of the overall skirt of leaflet tissue close along a solitary cavo-convex line of apposition. When considered in the literal sense, namely that of a zone of junction, this line represents the true valvar commissure. Indeed, it is the case that the mitral valve possesses only one such commissure, albeit that tradition dictates that the ends of this zone should be considered as paired commissures, reflecting the penchant of morphologists and pathologists to view the valve in its open state. The zone of apposition between the valvar leaflets, nonetheless, is directly comparable to the solitary commissure seen between the lips of the mouth, or between the eyelids [11].
When taking note of the line of closure, it becomes clear that, as emphasised by Victor and Nayak [3], the slits seen within the mural leaflet are the zones of apposition between its different parts, and are analogous to creases formed when a handkerchief is placed in a narrow napkin-ring, or to the multiple pleats of a skirt or kilt. It is also the case, of course, that each of the scallops may need to be managed by the surgeon, during repair of the valve, as a separate entity. The central scallop, for example, is well recognised as having a greater propensity to prolapse than do the others [2]. But does this justify nominating the overall valvar complex as having four leaflets, as suggested by Yacoub [1], or six as proposed by Kumar and colleagues [4]? In our opinion, it does not. We believe it is better to retain the time-honoured concept of dividing the valve into but two leaflets, and considering them as being aortic and mural, but acknowledging the multiple slits within the mural component of the leaflet, and recognising that the solitary zone of apposition does not extend to the annulus (Photo 6). The labels shown in Schematic 2, nonetheless, indicate how easy it is for the different observers to have assessed the same valve from their differing viewpoints.
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Schematic 2 The valve shown in Photo 6 has been copied to illustrate the concepts for description suggested by Yacoub (Panel A), arguing that the valve has four leaflets [1], and Kumar and colleagues (Panel B), who suggested that there are six leaflets when note is taken of the ‘commissural areas’ [4]. More commonly, however, surgeons adopt the suggestion of Carpentier (Panel C) who labelled the functional components of the valve in alphanumeric fashion [2]. Irrespective of the concept adopted, it remains the case that the valve possesses a solitary zone of primary closure between its component parts.
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The tendinous cords and papillary muscles
Since the earliest days, beginning with Leonardo da Vinci (Schematic 3), investigators have understood that the competence afforded by the leaflets of the mitral valve is only as good as the tendinous cords that support them. Over the last several decades, some investigators have studied in detail the morphology of these structures [12], while others have demonstrated their function in the living heart [13, 14]. Only now are these two approaches being married to delineate how form produces function. The impetus for this union has arisen through experience with the repair of the mitral valve, and the occasional need to manipulate the cords in order to alter the behaviour of the leaflets [15, 16]. Once considered a homogeneous entity providing support only in terms of preventing prolapse of the leaflets during ventricular systole, the cords may now be classified into broad groups according to their point of attachment to the leaflets, and their attachment to the papillary muscles. Groups of cords sprout from both papillary muscles, and support the leading edges of both leaflets (Photo 6). Fan-shaped cords are found beneath the two ends of the solitary line of apposition between the aortic and mural leaflets, and also beneath the subsidiary zones of apposition of the components of the mural leaflet. It had previously been suggested that the location of such cords provided an identification of the beginning of one leaflet, and the end of its neighbour [12]. We find this approach much less than satisfactory, since if such a definition is used, it is difficult to deny the state of leaflets to the scallops of the mural component of the valve, as suggested by Yacoub [1]. As already discussed, and in light of the marked variability between individuals, we find it much more satisfactory to define the extent of the leaflets on the basis of the positions of the ends of the solitary zone of apposition between them.
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Schematic 3 Cartoon by Leonardo da Vinci, believed to have been drawn around 1513, and showing the varied nature of the tendinous cords supporting the aortic and mural leaflets of the mitral valve. Note in particular the laminated appearance that Leonardo da Vinci depicts for the ventricular surface of the aortic leaflet (see also Photo 3).
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The papillary muscles themselves lie directly beneath the two ends of the solitary zone of apposition between the leaflets. Contrary to the names usually given to them nowadays, they are located inferiorly and septally, and superiorly and parietally (Photo 9).
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Photo 9 The left ventricle has been dissected so as to remove its posterior wall, and is viewed from behind in anatomically appropriate orientation. As shown, the papillary muscles supporting the leaflets of the mitral valve, contrary to conventional wisdom, are positioned postero-superiorly and antero-inferiorly.
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Their positions ensure that the cords arising from the tips of the two muscles are able to provide support to the adjacent parts of both leaflets. Those seeking details of the individual variation to be found in the arrangement of the tendinous cords and papillary muscles should study the excellent investigation of Becker and de Wit [17]. It is the location of the insertions of the cords on the leaflets that provides the basis for classification. Primary cords, or cords of the first order, normally insert in uniform fashion along the free edge of both leaflets (Photos 6 and 8). Secondary cords, or cords of the second order, insert in layered fashion along the rough zone of the aortic leaflet, as illustrated so long ago by Leonardo da Vinci (compare Photo 3 and Schematic 3). They ensure that the coapting surfaces meet during systolic closure. Some of these secondary cords inserting into the ventricular aspect of the aortic leaflet are thickened, and are appropriately described as the strut cords. There are also cords supporting the ventricular aspect of the mural leaflet, the so-called basal cords. These cords, however, differ from all the others in that they do not take origin from the papillary muscles, but rather possess their own small muscular bellies, or else extend directly from the parietal ventricular wall to insert on to the undersurface of the mural leaflet. In the past, the basal cords have been considered to represent cords of a third order. This seems an unnecessary complication. It makes more sense, we would suggest, to reserve the title of first order, or primary, cords to all those supporting the free edges of the leaflets, and to view the remaining cords, including the basal cords, which insert into the ventricular aspects of the leaflets, as belonging to the second order. The studies performed on isolated hearts have shown that it is the primary cords that are involved in preventing prolapse of the leaflets, and hence, preventing valvar regurgitation [13]. This notion is further supported by the finding that rupture of these primary cords underscores the unnatural history of acute mitral valvar regurgitation. The secondary cords, being more important in maintaining ventricular geometry, and subsequent ventricular performance, may therefore, be translocated surgically without compromising the distribution of cordal stress or coaptation of the leaflets [15]. It would be inappropriate to close our discussion of the arrangement of the papillary muscles without considering the alignment of the myocytes within the muscles. The overall arrangement is that of a parallel array in line with the long axis of the muscles. Much has been written recently concerning the so-called ‘unique myocardial band’ [18]. Those wishing to use this concept as the basis for cardiac surgical procedures should be aware that it has never received independent anatomical validation, and that studies of the histology of the ventricular mass produce absolutely no supporting evidence for the existence of a tract of ventricular myocardium running from the aortic to the mitral valve [19]. In short, the ventricular myocardium is arranged as a mesh, with the myocytes supported by a matrix of fibrous tissue. There is no reliable anatomic evidence to support the mistaken concept of the ‘unique myocardial band’ [20].
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Relationships of the mitral valvar complex
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Hinged as it is from the atrioventricular junction, with the mural part of the annulus being an integral part of the left atrioventricular junction, and in continuity antero-superiorly with the aortic root, the mitral valve has crucial relationships to the left atrium and ventricle, the atrial septum, the coronary vasculature, and the atrioventricular axis of conduction tissue. As is seen from the short axis section of the left ventricle, the overall line of closure of the valvar leaflets is obliquely orientated within the left ventricle, with the aortic root interposed between the valvar components and the septum (Photo 5). Unlike the tricuspid valve, therefore, the leaflets of the mitral valve have no direct attachments to the ventricular septum, a feature well seen in the echocardiographic four-chamber view, this view also emphasising that the left atrioventricular junction is off-set relative to the right junction supporting the leaflets of the tricuspid valve (Photo 10).
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Photo 10 The heart has been sectioned to replicate the echocardiographic four-chamber plane. Note the off-setting of the left atrioventricular junction, supporting the leaflets of the mitral valve, as opposed to the right junction supporting the tricuspid valve. Note also that the mitral valvar leaflets, unlike those of the tricuspid valve, have no attachments to the ventricular septum (yellow and red arrow). The yellow lines show the area of overlapping of the atrial and ventricular musculatures in the floor of the triangle of Koch.
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The atrioventricular conduction axis penetrates from the atrial musculature to reach the crest of the muscular ventricular septum through the area where the inferior margins of the atrioventricular junctions diverge from one another (Photo 11).
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Photo 11 The inferior aspect of the atrioventricular junctions has been prepared by removing the atrial myocardium from the base of the ventricular mass. The dissection shows how the left atrial (yellow dotted line) and right atrial (green dotted line) walls diverge as the extend inferiorly away from the aortic root. The location of the atrioventricular conduction axis has been superimposed on the dissection (see Photo 12).
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Photo 12 The dissection is similar to the one shown in Photo 11, except that the atrial walls have not been removed. The non-coronary sinus of the aortic rot, however, has been cut away to show how the subaortic outflow tract is interposed between the mitral valve and the septum, permitting the atrioventricular conduction axis (red stippled area) to penetrate from the apex of the triangle of Koch to reach the crest of the muscular ventricular septum.
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We had previously considered this area to represent a muscular atrioventricular septum. We now know that it is no more than a sandwich, with a superior extension from the inferior atrioventricular groove interposing between the right atrial wall and the crest of the muscular ventricular septum. The atrial wall in this area is the floor of the triangle of Koch. The bundle of His penetrates the atrioventricular fibrous septum at the apex of this septum, passing into the subaortic outflow tract, where it branches on the crest of the muscular ventricular septum. From the stance of the mitral valve, this crucial area is related to the rightward end of the zone of apposition between the two leaflets (Photo 12).
If we examine the relationships as shown in Photos 11 and 12 as they would be seen by the surgeon approaching through the roof of the left atrium (Schematic 4), we are then able to emphasise the remaining crucial relationships of the mitral valve.
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Schematic 4 The cartoon shows the relationships of the mitral valve to the atrial and ventricular structures as would be seen by the surgeon approaching through the roof of the left atrium. Note also the relationships of the atrioventricular node (green), the coronary arteries (pink) and the coronary sinus (purple). The precise relationships of the circumflex and right coronary arteries depend on which artery is dominant (see Photos 13 and 14). The blue structure is the central fibrous body.
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Photo 13 In this individual, the right coronary artery (RCA) was dominant, giving rise to the artery feeding the atrioventricular node, and continuing to supply the diaphragmatic surface of the left ventricle. The circumflex artery supplies the obtuse margin of the left ventricle, but has a limited relationship to the hinges of the leaflets of the mitral valve. Compare with Photo 14.
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Photo 14 The left atrioventricular junction has been dissected in an individual in whom the left coronary artery was dominant. Note how much closer the artery is related to the hinge of the mural leaflet of the mitral valve (arrows) than when the right coronary artery is dominant (Photo 13).
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The precise relationships of the coronary arteries depends on which artery gives rise to the inferior interventricular artery, the latter still described in most instances incorrectly as being posterior and descending. When the right coronary artery gives rise to the inferior interventricular branch, then the circumflex artery typically feeds only the obtuse margins of the left ventricular musculature. This is the arrangement known as right coronary arterial dominance, and the branches of the arteries are relatively distant from the hinges of the mitral valve (Photo 13).
In contrast, when it is the circumflex artery that gives rise to the inferior interventricular artery, so-called left coronary arterial dominance, then the circumflex artery itself is much more closely related to the hinges of the mitral valve as it encircles the left atrioventricular junction (Photo 14).
The final structure occupying the left atrioventricular junction, and bearing an important relationship to the hinge of the mural leaflet of the mitral valve, is the coronary sinus. In most instances, this structure is considered to begin at the site where the oblique vein of the left atrium joins the great cardiac vein (Photo 15).
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Photo 15 The cavities of the left side of the heart have been cast in red to produce this specimen, with the veins draining to the right atrium cast in blue. The specimen shows the relationship of the venous structures within the left atrioventricular junction, photographed from the left side. The great cardiac vein becomes the coronary sinus at the point where it receives the oblique vein of the left atrium.
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The sinus then courses through the left atrioventricular groove to open into the right atrium. Although we thought for some time that there was a common wall between the sinus and the left atrium, we now know that venous and atrial structures have their own walls [21]. Although the sinus courses through the atrioventricular groove, it is rarely located within one centimetre of the attachment of the mural leaflet of the mitral valve, and hence should not be at risk from sutures placed within the left atrioventricular junction.
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Acknowledgements
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Many of the illustrations used for this review are modified from those appearing in 'Surgical Anatomy of the Heart', published by Cambridge University Press. We thank the co-authors of this book, Drs Benson Wilcox and Andrew Cook, for permission to modify the images for the purposes of our current work.
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Footnotes
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 The authors have chosen to retain the copyright on this work. 
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References
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Summary
Introduction
