Morphology and Embryology Obstruction at the valvar level is by far the most common lesion producing stenosis within the pulmonary outflow tract An understanding of the mechanisms responsible for stenosis requires a proper appreciation of normal valvar anatomy.2 At present such understanding is constrained by varying use of the term annulus in accounting for the structure of the normal valve Paradoxically, it is only when the valve is stenotic that the attachments of the leaflets approximate to an annular arrangement The essence of normal valvar anatomy is the suspension of the valvar leaflets in semilunar fashion within the sinuses of the pulmonary trunk Proper understanding then requires appreciation that the hinges of the leaflets cross the anatomic ventriculoarterial junction The arrangement is best seen when the normal outflow tract is spread open, having already removed the valvar leaflets (Fig 42.1) FIG 42.1 The normal pulmonary outflow tract has been opened and spread to show its full width (leaflets of the pulmonary valve have been removed) The dissection shows how each leaflet is attached distally at the sinutubular junction but proximally to the muscular infundibulum The semilunar line of attachment of each leaflet, marking the hemodynamic ventriculoarterial junction, crosses twice the anatomic junction between the wall of the pulmonary trunk and the muscular infundibulum, leaving triangles of fibrous wall (red stars) as part of the ventricle but sequestering crescents of musculature (white stars) as parts of the valvar sinuses In the normal heart, each valvar leaflet is attached by its two extremities at the sinutubular junction, with the basal attachment supported by the musculature of the subpulmonary infundibulum True anatomic rings can then be identified at the level of the sinutubular junction and also at the anatomic ventriculoarterial junction The latter structure is the locus over which the fibroelastic walls of the pulmonary trunk are supported by the muscular subpulmonary infundibulum A third ring can then be constructed by joining together the basal attachments of the leaflets This third ring, however, is a geometric construction rather than an anatomic reality The semilunar line of attachment of each of the leaflets marking the hemodynamic ventriculoarterial junction crosses twice the anatomic ventriculoarterial junction By virtue of this geometry, crescents of muscular infundibulum are incorporated at the base of each pulmonary valvar sinus, while three tapering triangles of fibrous pulmonary truncal wall extend beyond the anatomic ventriculoarterial junction as parts of the ventricular outflow tract, reaching to the level of the sinutubular junction (Fig 42.2) FIG 42.2 Effects of the hinge lines of the valvar leaflets assessed in three dimensions One hinge line (shown in purple) crosses the anatomic ventriculoarterial junction Triangles of arterial wall are incorporated into the ventricular outflow tract to the level of the sinutubular junction, and crescents of muscular infundibulum are sequestered at the base of each valvar sinus In the normal arrangement, the free edge of each valvar leaflet is appreciably longer than the cord of the sinus that supports it, thus permitting the three leaflets to fit snugly together when closed so as to produce a competent valvar orifice It is the semilunar nature of suspension of the leaflets, therefore, that permits competent closure and unobstructed opening of the valve When seen in closed position, the zones of apposition between the adjacent leaflets extend in triradiate fashion from the centroid of the valvar orifice to their peripheral attachments at the sinutubular junction (Fig 42.3)