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Ann Thorac Surg 1995;59:419-427
© 1995 The Society of Thoracic Surgeons
Department of Paediatrics, National Heart and Lung Institute, London, United Kingdom
Accepted for publication October 4, 1994.
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Surgeons and morphologists continue to battle with the most appropriate terminology to describe the proximal aorta and its origin from the left ventricle. Surgical texts suggest sewing prosthetics to the aortic ring or annulus [1, 2]. Morphologists, in contrast, describe the attachments of the valvar leaflets as semilunar, and stress the absence of a true ``ring'' of annular tissue supporting the hinge points of the leaflets in one straight circular plane [3–5]. What is the true situation, and how do we reconcile these differences? To clarify the morphology, we have reviewed the pertinent literature, assessed the gross anatomy of the aortic outflow tract, and observed both macroscopically and microscopically the consequences of placement of a prosthesis within the aortic root.
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Definition of Terms
The aortic root is that portion of the ventricular outflow tract that supports the leaflets of the aortic valve. It is a functioning unit with relations both to the aorta and to the left ventricle. The aortic root is made up of the sinuses of Valsalva, the valvar leaflets, and the interleaflet triangles.
The sinuses of Valsalva are the expanded portions of the aortic root confined proximally by the attachments of the valvar leaflets and distally by the sinutubular ridge. The sinuses are named according to the arteries arising from within them (right, left, and noncoronary).
Various names have been used for the portions of tissue separating the ventricular and aortic areas. These include valvule, cusp, scallop, and leaflet [3, 4, 6]. We prefer the term leaflet for use in description of the atrioventricular as well as the arterial valves, as the structures described subserve similar functions in both settings. The collagenous condensation at the point of attachment of each leaflet has been termed the annulus fibrosus [2, 7]Au: have renumbered references; Ref 7 was skipped. The body is the large weight-bearing surface of the leaflet. The free edge of the leaflet is constructed so that, when closed, the leaflets coapt over several millimeters. This margin of overlap defines the coapting surface of the valvar leaflet. It also has been termed the lunula.
When the aortic valve is closed, each leaflet of a trifoliate valve meets its neighbor along a junction extending from the periphery to the centroid of the valvar orifice (Fig 1
). There is no consensus as to the most appropriate description of this overall zone of apposition between adjacent leaflets. All of the area is important if the valve is to be a competent structure. If the word ``commissure'' was used in its vernacular meaning, then the entirety of these junctions of adjacent leaflets would justifiably be called commissures. Thus, the Shorter Oxford English Dictionary on Historic Principles [8] gives four definitions for commissure, at least two of which are pertinent for the aortic valve: (1) A joining together; the place where two bodies touch or unite; a joining, juncture, seam. (2) The line of junction or angles of the two lips, eyelids, etc. Both of these definitions would fit with the description of the zones of apposition between the leaflets of the aortic valve as shown in Figure 1. Neither is consonant with the traditional and current use of commissures in cardiac surgery. As stated by an anonymous referee of the first draft of this manuscript, conventional wisdom holds that the commissure applies only to the peripheral attachment of the valvar leaflets to the sinutubular junction of aortic root and ascending aorta. Therefore, to circumvent these semantic problems in this work, we will simply describe the zones of apposition of the leaflets (Fig 1). These zones have upper and lower lines of coaptation. Within this concept, the aortic root can contain either one zone of apposition, as seen in the bileaflet valve, or three zones of apposition as is the norm. This view of zones of apposition is equally applicable to the atrioventricular valves. If the word commissure was used in its vernacular meaning, then the zones should logically be described as commissures. Only time will tell if this can be achieved.
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). There was considerable variety in the architecture of musculature supporting the left and right sinuses. The noncoronary sinus had significant muscular support in only one instance. The transition from muscular to fibrous tissue at the membranous septum was also variable. Muscular bands could be seen crossing the interleaflet triangles in some specimens, whereas, in an equal number of specimens, the membranous septum passed under part of the right coronary sinus.
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). Just below the sinutubular ridge, the aortic root takes on a trifoliate and scalloped appearance when seen in cross section, this portion corresponding to the sinuses of Valsalva. The sinuses expand down to the base of the aortic root where the circumference narrows slightly. The coronary arteries generally arise within the sinuses or at the ridge, although they can originate above the sinutubular ridge as a normal variation. The origins of the arteries permit use of the descriptive terms of left and right coronary sinuses, although the arterial orifices can be paired or anomalous. It is exceedingly rare, however, for a coronary artery to arise from that sinus that is not adjacent to the pulmonary trunk (the nonfacing sinus). The sinuses are confined inferiorly by the leaflets.
The valvar leaflets are the portions of the aortic root that separate the hemodynamic components of aorta and ventricle, and are attached to the wall of the root in a semilunar fashion. The apices of these attachments are at the sinutubular ridge, whereas the nadirs are at or below the anatomic ventriculoarterial junction, this junction being the straight circle where the fibroelastic aortic wall joins with the supporting structures of the left ventricle. Each leaflet contains a hinge point, a body, and a coapting surface with a thickened central nodule. The hinge point is that area where the leaflet is attached to the aortic root. There is a condensation of collagenous tissue at the hinge point that follows the semilunar contour of the valvar attachment. Previously, this has been defined as the annulus fibrosus. This annulus, which takes the form of a three-pronged coronet rather than a ring, was thickest in all the hearts studied at the nadir of the semilunar attachments. A basal ring within the ventricle can be readily constructed by joining together these nadirs, but is not a complete straight circle (Fig 3).
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). At the level of the sinutubular ridge, three bulges can be seen internally in the elastic lamellas (see Fig 4A). Proceeding inferiorly, the usually circular lamellar architecture is interrupted by increasing amounts of interspersed collagenous tissue (see Fig 4B–D). The collagen then coalesces to take a primarily longitudinal orientation with interdigitating elastic tissue, with the bundles of longitudinally oriented collagen being surrounded on their luminal surface by additional elastic tissue (see Fig 5). Proceeding more inferiorly, the collagen becomes more defined, and the layers of elastic tissue on the luminal surface become thin and then disappear. At this point, the collagenous structure is seen almost to ``extrude'' into the lumen of the aortic root. The collagenous structure takes on a bicornuate appearance concomitant with separation of the limbs of longitudinally oriented collagen bundles (see Fig 4).
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), the elastic layers diminish in number, and the collagen bundles separate into two distinct structures. Between these limbs of thickened collagen (the so-called annulus), the leaflets of the valve take shape, and can be seen to have several layers. The more densely staining aortic side blends with the so-called annulus, forming the so-called fibrous layer (fibrosa). A less densely staining spongy portion is oriented toward the ventricle (spongiosa). The spongy layer blends with the tissue between the longitudinal limbs of the annulus to form the interleaflet triangle. Each layer is covered by segments of endothelial tissue, the aortic side coated with endothelium, and the ventricular side with endocardium.
Nearer the base of the sinuses, the orientation of the elastic tissue relative to the collagen is reversed (Fig 6). The elastic layers decrease in number and become oriented toward the lumen. A dense band of circularly oriented collagen fibers surrounds these tongues of elastic tissue. The area between the limbs of the annulus, corresponding to the interleaflet triangle, becomes large in relation to the area of the aortic root. This tissue of the interleaflet triangle is composed of a thinner, primarily circularly oriented, layer of fibers of collagen and light staining acellular material. A very thin layer of elastic tissue can be seen on the luminal surface that is continuous with the elastic layer in the ventricle beneath the endocardium. The fibrous portions at the base of the left coronary sinus and the noncoronary sinus can be seen to blend with the central fibrous portions of the heart.
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). This line corresponds to the anatomic ventriculoarterial junction. Histologically, large amounts of fibrous tissue could be seen surrounding the sewing ring and extending into the aortic wall.
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The aortic root is a simple structure viewed from the surface. It is bordered superiorly by the sinutubular ridge. It contains the semilunar leaflets. The attachments of the leaflets define the extent of the sinuses of Valsalva and the interleaflet triangles. The base of the aortic root is defined by the nadirs of the attachments of the leaflets. The base sits on partially fibrous and partially muscular ventricular support. This description is that of a three-dimensional structure (see Fig 3). Alterations in any part of the structure affect its function.
There are contrary views concerning the nature of the valve. For example, Silver and Roberts [11] define the aortic valve as those thin portions of tissue that allow the passage of blood into the aorta and prevent the reflux of blood back into the ventricle. In addition, they state that the structures contacting the valve are important to its function but are not, in themselves, part of the valve. Zimmerman [3], however, has recommended the study of cardiac anatomy with regard to structure and function. We agree with this point of view and prefer to discuss the aortic root as incorporating the entire aortic valvar complex. This complex can then be analyzed at three levels: sinutubular ridge, sinus, and base.
The sinutubular ridge is circular when seen from above in the autopsied heart (see Fig 1), and unequivocally constitutes a ring. This portion of the aortic root supports the peripheral attachments of the valvar leaflets. Lewis and Grant [12], using elastic stains, performed detailed histologic studies of the aortic root in the healthy heart and in the heart affected by endocarditis. Their findings are confirmed by our study. The apices of the attachment of the leaflet are surrounded by elastic aortic tissue. Gross and Kugel [7] also echoed their findings.
The parabolic shape of the aortic valve has been suggested to form a suitably even support similar to that of a suspension bridge [6]. This occurs both at the attachment of the leaflet to the wall of the aortic root and along the thickened coapting portions of the free edge (Fig 8). These two areas correspond to areas of collagenous thickening as seen using light and electron microscopy [13]. The peripheral attachments to the sinutubular ridge are several millimeters above the level of coaptation of the valvar leaflet [6]. Within the analogy of the bridge, these thickenings correspond to support cables. Brewer and colleagues [14] recognized that the collagenous nature of these free edges of the valvar leaflets would militate against repeated changes in their length. During systole, they showed the area of the aortic root at the level of the commissural attachments increased by about 16%. This increase in diameter corresponds to a loss of the parabolic shape of the free edge of the leaflet, and contributes in a large part to valvar opening. Again, carrying on with our analogy, as the support poles of the bridge tilt back, the cables are lifted, aiding in the opening of the valve.
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The sinuses of Valsalva occupy the greater part of the aortic root. The importance of these sinuses was recognized by Da Vinci [10]. The sinuses are composed primarily of elastic tissue, and usually support the openings of the coronary arteries [15]. In cross section, the aortic root at this level is trilobed [3]. The sinuses are the anatomic and functional units of the aortic root. They are the pivotal point of aortic radicular expansion [16]. They accommodate the open leaflets. The formation of vortexes within them is important to valvar function [3]. But, again, the sinuses are not the level of placement of prosthetic valves.
Toward the proximal end of the aortic root is the anatomic ventriculoarterial junction. This is also unequivocally of circular shape, and is below the origin of the coronary arteries. It is at this level that surgeons sew in prosthetic valves (see Fig 7). The circular ring, nonetheless, is crossed by the semilunar attachments of the valvar leaflets. These attachments dictate that portions of the aortic root are exposed to ventricular hemodynamics, whereas other portions are exposed to aortic hemodynamics (Figs 2, 9). These discordances are much more apparent in the pulmonary valve, but are also seen in the aortic valve. The unequivocal fibrous segments supporting the nadirs of attachment of each leaflet are interrupted by the interleaflet triangles. If joined together, these nadirs constitute a basal ring, but the ring does not form a continuous collagenous circle.
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). Indeed, Zimmerman [3] attempted to reconcile this problem by calling the fibrous tissue at the base of the aortic root a coronet. He argues that such a coronet is a stable fibrous structure. This suggestion received additional support from Reid [15]. Thubrikar and co-workers [16–18], however, have shown that the base of the valve is not a fixed structure, but that it expands as the ventricle fills and then shrinks as the ventricle contracts. This argues against the presence of a ring of fibrous tissue in the sense of Zimmerman's own structure–function continuum. Rather, the base of the aortic root can be said to behave in ventricular fashion. Thubrikar and colleagues [17] showed that the base of the aortic root contracts. They pointed out that the dog heart has 60% of the ventricular outlet composed of muscle. Anderson [5] has also emphasized the contribution of the ventricle to the aorta, and the contribution of the aorta to the ventricle. Sands and colleagues [19], in a comparison of species, found that the human aorta is supported in approximately 45% of its circumference by muscle. Our findings endorse this finding. That the base of the valve follows the ventricular pattern, therefore, is not surprising. The key to this behavior is in the interleaflet triangles.
The interleaflet triangles are of different histologic structure, and subserve different physiology when compared to the bases of the sinuses. They are thinner and less collagenous than the attachments of the leaflets, the latter forming their contiguous superior and lateral borders. They are generally bordered by muscle, although this is not true for the interleaflet triangle between the left and noncoronary sinuses. This portion is part of the subaortic curtain, and its more fibrous make-up is important for ventricular function [20]. Our histologic findings concerning the triangles are in keeping with those reported by Thubrikar and colleagues [17].
In the hearts we studied, the prosthetic valves were both seen sitting at the level of the anatomic ventriculoarterial junction in the base of the aortic sinuses. The peripheral attachments of the excised leaflets at the sinutubular ridge were well above the levels of surgical attachment. The sewing ring is attached just above the nadirs of the valvar leaflets and across the interleaflet triangles. The sewing ring of the stent-mounted valve followed the contour of the leaflet attachments and was not circular. McAlpine [4] stresses this relationship, and urges care when placing sutures in the region of the interleaflet triangles. Our review of the anatomic structure of the aortic root also emphasizes the proximity of the conduction system to the aortic root, as well as the lack of a true annular ring at its base.
In summary, therefore, the aortic root is a complex hemodynamic system. Its upper portion is exposed to aortic pressures and behaves as the rest of the aorta. It expands during systole, allowing the leaflets to retract and open. The base of the valve, on the other hand, is exposed to ventricular dynamics. It expands as the ventricle fills and, during the peak of systole, it contracts. This decreases the distance required for the valvar leaflets to travel to close the orifice. This action has been hypothesized as reducing the stress applied to the leaflets [17].
The aortic root is also a dynamic structure. Its overall configuration changes from a cone to a cylinder, and then to an inverted cone, as the ventricle fills and contracts. The sinus can be seen as the basic structural unit of the valve. The sinuses support the coronary arteries and the valvar leaflets. The sinuses also allow formation of vortexes, which likely aid in valvar closure. The sinus rocks out, then is dragged back toward the center, by the action of the ventricular myocardium [14, 15, 17]. Each sinus is separated from each other at its base by the interleaflet triangles.
It is the interleaflet triangles that are crucial for proper valvar function. Indeed, when one or more of the interleaflet triangles are vestigial, or very small, making the overall valve a more ringlike structure, the valve becomes stenotic. This situation can be seen in congenitally deformed valves with one or two leaflets [21]. Lewis and Grant [12] also remarked on this feature, but did not stress the importance of the triangles, preferring to focus on the attachments of the leaflets. From our review, it seems that the interleaflet triangles have been relatively ignored structures in the study of function of the aortic root. These triangles allow the sinuses to act independently. The mobility of this portion of the aortic root should be of interest considering the current construction of prostheses with rigid basilar sewing rings. Future designers of valvar prosthetics, and students of the aortic root, would do well to remember the forgotten interleaflet triangles.
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Doctors Siew Y. Ho and Robert H. Anderson are supported by the British Heart Foundation.
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R. H. Anderson and B. R. Wilcox Understanding Cardiac Anatomy: The Prerequisite for Optimal Cardiac Surgery Ann. Thorac. Surg., June 1, 1995; 59(6): 1366 - 1375. [Abstract] [Full Text]
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) is enlarged (B) to show the circular orientation of the fibrous tissue. (Vent. = ventricular.)
