A comprehensive understanding of the normal and abnormal aortic root is paramount if we are to improve not only our assessment of the aortic root and its components but also the surgical approach to reconstructing this complex structure when congenitally malformed. Most anatomic and imaging-based classifications of the normal root recognize and describe the basic components, which include the shape and size of the three aortic sinuses and their three valvar leaflets, as well as the sinutubular junction and proximal ascending aorta. However, the three interposing fibrous interleaflet triangles, which share an intimate relationship with all elements of the root, are often ignored. In consequence, the important role the interleaflet triangles play in determining the function of the normal and congenitally malformed aortic root is underappreciated. Additionally, the subtle asymmetries found in the normal aortic root, such as differences between the sizes of the described components, underlie its hemodynamic efficiency. In this review the authors describe the complex structure of the normal aortic root, contrasting these normal characteristics with those found in the unicuspid and bicuspid variants of congenitally malformed aortic valves. Many of these features are readily recognizable using current imaging modalities and so should become a standard part of the description of aortic valvar disease. The authors believe that this thorough morphologic approach will provide a framework for the re-creation of a more normal aortic root at the time of repair or replacement, thereby improving current outcomes.
Highlights
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Comprehensive aortic root anatomy is reviewed.
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The interleaflet triangles determine valvar morphology in the aortic root.
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Imaging and surgical implications are discussed.
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Early Developmental Considerations
We still do not know the processes that are responsible for the formation of the arterial valvar sinuses as opposed to the valvar leaflets. We do know, however, that at the stage of formation and sculpting of the leaflets, the developing arterial roots are encased in their entirety within a turret of outflow tract myocardium, which extends distally to the level of the developing sinutubular junction. Given that the valvar leaflets are developed from the outflow cushions, the spaces between them on their ventricular aspects are initially confined by the myocardial wall of the outflow tract. It is also the case that as the subaortic root begins its development, the myocardium of the inner heart curvature interposes between the developing leaflets of the aortic and mitral valves. There is, therefore, continuous regression of the distal level of the myocardial walls of the arterial roots as the valvar sinuses develop to support the valvar leaflets. It is the regression of this myocardial border, with details as yet undetermined, that permits the apexes of the forming fibrous interleaflet triangles to separate the cavity of the left ventricle from the pericardial space. In similar fashion, it is the transformation of the initial myocardial inner heart curvature to fibrous tissue that produces the extensive aortomitral curtain in the roof of the left ventricle. The details of this process also remain fully to be determined.
Early Developmental Considerations
We still do not know the processes that are responsible for the formation of the arterial valvar sinuses as opposed to the valvar leaflets. We do know, however, that at the stage of formation and sculpting of the leaflets, the developing arterial roots are encased in their entirety within a turret of outflow tract myocardium, which extends distally to the level of the developing sinutubular junction. Given that the valvar leaflets are developed from the outflow cushions, the spaces between them on their ventricular aspects are initially confined by the myocardial wall of the outflow tract. It is also the case that as the subaortic root begins its development, the myocardium of the inner heart curvature interposes between the developing leaflets of the aortic and mitral valves. There is, therefore, continuous regression of the distal level of the myocardial walls of the arterial roots as the valvar sinuses develop to support the valvar leaflets. It is the regression of this myocardial border, with details as yet undetermined, that permits the apexes of the forming fibrous interleaflet triangles to separate the cavity of the left ventricle from the pericardial space. In similar fashion, it is the transformation of the initial myocardial inner heart curvature to fibrous tissue that produces the extensive aortomitral curtain in the roof of the left ventricle. The details of this process also remain fully to be determined.
Anatomy of the Normal Aortic Root
The anatomy of the congenitally malformed aortic valve cannot be understood in the absence of detailed knowledge of the structure of the normal aortic root. The normal root is composed of the three aortic sinuses, which support the valvar leaflets in semilunar fashion ( Figure 1 ). When these semilunar hinge lines are reconstructed, the valvar leaflets are seen to take the form of a crown or coronet, with a virtual plane, the virtual basal ring, drawn through their basal attachments to represent the enigmatic echocardiographic “annulus” ( Figures 2 and 3 ). The fibrous interleaflet triangles have as their base the true ventriculoarterial junction, or the plane of mitral-aortic continuity. They extend distally to the level of the sinutubular junction, are seen on the ventricular aspect of the root, and are bordered by the aortic components of the hinge lines of the aortic valvar leaflets. The free edges of the leaflets form three zones of apposition during diastole, with the level of coaptation formed at no more than half the overall height of the root ( Figure 4 ). The problems in defining the valvar “annulus” have previously been emphasized, with the different approaches used by surgeons, interventional cardiologists, and echocardiographers underscoring potential problems when quantitating the dimensions of the root. The only truly annular, or circular, anatomic structure found within the aortic root is the sinutubular junction. The anatomic ventriculoarterial junction is incomplete in the aortic root, because its posterior part is within the area of fibrous continuity between the leaflets of the aortic and mitral valves. The anterior portion of the junction is crossed by the semilunar hinges of the valvar leaflets and hence is not readily visible when the valve is intact. The junction, however, is readily revealed by both virtual dissection using, for example, high-resolution computed tomography ( Figure 2 ) or by anatomic dissection ( Figure 5 ).
Aortic Root Geometry: Asymmetry and Dynamic Changes with the Cardiac Cycle Are Key to Normal Function
Appreciation of the asymmetry found in the normal aortic root and its supporting structures can provide insight into what produces a competent aortic valve and what surgeons should be aiming to recreate when faced with the challenge of surgery involving the aortic valve and its supporting structures. The most obvious asymmetry is found with regard to the origin of the coronary arteries, with only two of the three sinuses giving rise to a coronary artery ( Figures 1, 2, 4 and 5 ). This feature permits the sinuses to be distinguished as being right and left coronary, as opposed to being noncoronary. The third sinus is usually named “noncoronary” because hardly ever does it give rise to a coronary artery. In the rare circumstances that this occurs, however, the sinus cannot sensibly be described as noncoronary. In these instances, the third sinus is better described as the nonadjacent sinus, because the sinuses that usually give rise to the coronary arteries are always adjacent to the pulmonary root. When considering the sinuses themselves, there is disproportion in their size. It is either the right or the noncoronary sinus that is largest, with the left coronary sinus usually being the smallest. The dimensions of the leaflets reflect the variations in size of the sinuses. These variations also correlate with studies assessing the biomechanics of the aortic root. Increased stress and strain have been identified in the noncoronary and right coronary leaflets and sinuses compared with the left coronary leaflet and sinus. The findings, in turn, lend support to the increased incidence of dilation or aneurysm involving the noncoronary and right coronary sinuses.
The fibrous interleaflet triangles also show normal variation in both their height and width, along with their underlying support. As we will show, it is significant asymmetric underdevelopment of one or more of these triangles that underscores the structure of the bicuspid or unicuspid aortic valves. As already emphasized, the triangles fill the distal spaces between the ventricular aspects of the semilunar hinges of the valvar leaflets, extending distally to the sinutubular junction ( Figures 1 and 2 ). In the normal root, it is only the two sinuses giving rise to the coronary arteries that have myocardial support. The entirety of the noncoronary leaflet, along with the posterior part of the left coronary leaflet, is in fibrous continuity with the aortic leaflet of the mitral valve. This area of continuity is commonly described as the mitral-aortic fibrous curtain. It is the interleaflet triangle abutting on this curtain, between the noncoronary and left coronary leaflets, that is the largest of the three. It separates the left ventricular cavity from the middle portion of the transverse pericardial sinus. The interleaflet triangle between the noncoronary and right coronary sinuses, which is the second largest, is continuous basally with the membranous septum ( Figure 2 ). The membranous septum itself is usually continuous with the right fibrous trigone, with these two latter structures forming the central fibrous body ( Figure 1 ). The atrioventricular conduction axis penetrates through the atrioventricular component of the membranous septum. This is an important landmark for interventions involving the aortic root. Complete heart block, necessitating permanent pacemaker implantation, was observed after isolated surgical replacement of the aortic valve in one in 20 patients in one adult study and can occur in up to one in five patients following transcatheter aortic valve implantation. It is the triangle interposing between the two sinuses supporting the coronary arteries that is the smallest. It separates the left ventricular outflow tract from the extracavitary space between the aortic root and the muscular infundibular sleeve of the right ventricle. The bases of the interleaflet triangles account for just over half of the circumference of the inlet of the aortic root, this being the virtual basal ring, which represents the “annulus” commonly measured by echocardiographers ( Figure 2 ). This virtual ring expands by up to one sixth during left ventricular ejection, with the aortic root changing its configuration throughout the entirety of the cardiac cycle from a cone to a cylinder and finally to an inverted cone. This feat is made possible only by the presence of the interleaflet triangles, with the very nature of these thin-walled structures making them vulnerable to dilation and aneurysmal formation.
Secondary to the subtle differences found in the arrangement of the valvar sinuses, the axis of the normal root between the sinutubular junction and the virtual basal ring tilts at an angle of 5° to 11°. Because of the tilt, the peripheral attachments of the zones of apposition between the leaflets, usually described as the valvar commissures, are found at different levels, with the commissure between the right and noncoronary leaflets positioned more inferior to the others ( Figure 2 ). The extent of tilting is dynamic, with the lowest angle occurring during ventricular ejection, effectively creating a straighter pathway from the left ventricular outflow tract to the ascending aorta during ventricular ejection. Parallel to the described aortic root tilt, there is a normal degree of aortic root torsion, or twisting of the sinutubular junction relative to the aortic root base, which is similarly maximal at end-diastole, decreasing during ventricular ejection. Finally, the diameter of the entrance to the aortic root, usually taken by echocardiographers as representing the “valvar annulus,” has its largest size, and most circular shape, in mid-systole, becoming smallest and most elliptical at end-diastole. Together, these normal cyclical changes result in efficient energy storage in diastole, followed by release of energy in systole.
Anatomy of the Congenitally Malformed Aortic Valve
To better understand the anatomy of the unicuspid and unicommissural aortic valve, and to distinguish it from the bicuspid valve, we need to explain further our own understanding of commissures, as opposed to zones of apposition. If defined in strictly anatomic fashion, a commissure is no more than the line of union between two adjacent structures, as in the lips or eyelids. In the setting of cardiac valves, however, as we have discussed above, cardiologists consider the commissure to be the peripheral end of such a zone of apposition ( Figure 4 ). The mitral valve, for example, which has but a solitary zone of apposition, is described as having two commissures. An appreciation of these differences in definition can illuminate the distinction between the unicuspid and bicuspid aortic valves. Both valves have but a solitary zone of apposition. In the setting of the bicuspid aortic valve, the zone of apposition extends at both ends to reach the sinutubular junction, with cardiologists then describing the valve as having two commissures ( Figure 6 ). It follows that there are also two well-formed interleaflet fibrous triangles, and if present, the third triangle is vestigial and is usually related to the raphe of the conjoined leaflet ( Figure 7 ). In the unicuspid valve, in contrast, the zone of apposition extends from the sinutubular junction only to the centroid of the valvar orifice. The extension to the sinutubular junction is typically produced by persistence of the interleaflet triangle between the noncoronary and left coronary leaflets. There are then two vestigial interleaflet triangles found on the ventricular aspect of the unicuspid and unicommissural valve. These are, on one hand, between the two coronary aortic leaflets and on the other hand between the right and noncoronary leaflets ( Figure 8 ). We speculate that the constant location of the zone of apposition as pointing posteriorly toward the aortic leaflet of the mitral valve in the material we have been able to examine reflects the fact that only the two coronary aortic sinuses of the aortic valve are supported by myocardium. The end result is an eccentric configuration of the valvar orifice relative to the short axis of the root ( Figures 9 and 10 , Video 1 ; available at www.onlinejase.com ). This is in contrast to the centrally located orifice usually seen in the critically stenotic pulmonary valve. It is our opinion that the “volcano” appearance reflects the uniform support provided by the completely muscular infundibular sleeve. In both valves, the lack of formation of the interleaflet triangles means that the persisting skirt of leaflet tissue is hinged in almost circular fashion within the arterial roots, producing a much more obvious “annular” arrangement. When considering the valve in its entirety, the free edge of the skirt of leaflet tissue is much shorter than the circumference of the sinuses. This is in stark contrast to the arrangement seen in the normal aortic valve, where the free edges of the leaflets are always longer than the circumference of their supporting sinus. Not surprisingly, the entrance to the bicuspid valvar root, or the echocardiographic “annulus,” has been demonstrated to be more elliptical in shape than the more circular normal root. Raphes, or ridges of tissue, as in the bicuspid aortic valve, are commonly present along the fused zones of apposition ( Figures 6 and 11 ). In the unicuspid valve, there may be two such raphes, in contrast to the solitary raphe typically found in the bicuspid valve. Unless attention is directed at the raphes, and to the vestigial nature of the interleaflet triangles, it is easy to mistake the unicuspid valve as being bicuspid. This is because the heights of the zones of apposition are difficult to appreciate by means of echocardiography, with raphes commonly mistaken for true zones of apposition when the valve is assessed during diastole. These subtle differences may even be overlooked at the time of surgical repair. Thus, distinguished surgical groups have considered the unicuspid aortic valve to be no more than one variant of the bicuspid valve. In our opinion, although sharing basic features, the two entities are discrete valvar phenotypes, distinguished by the differences seen in the underlying interleaflet triangles (compare Figures 8 and 11 ).
