Morphologic characteristics around the atrioventricular valve annulus relevant to the substrate for the annular disjunction in normal human hearts

The atrioventricular （AV） annulus is a part of the fibrous skeleton of the heart and is held between the AV valve leaflet and the AV junction. This fibrous structure deteriorates over time owing to exposure to long-term pressure stress. The morphological change associated with this deterioration is known as annular disjunction; however, the initial lesion and its related changes around the AV junction are partially described. We assessed 20 autopsied adult human hearts with no major structural abnormalities （three females）. The mean age was 70.8 （range: 31-88） years. After exposure to the endocardial surface of the right and left AV annuli, the annular circumferences were measured, and the ventricular summit of the AV junction was exposed by lifting the AV valve leaflets. Histological examination was performed on the perpendicular sections along the AV annulus. The mean heart weight was 330±54.2 （range: 235-400）g. No significant widening of the AV annulus circumference was observed. In the mitral annulus, small recesses assumed as initial lesions for annular disjunction were found at the anterior or posterior wall in 16 hearts （80％） and at the lateral wall in only six （30％） because of the structural support of the small trabeculation with the basal chorda. In contrast, the right ventricular summit of the AV junction was divided into three muscular patterns: 1） perpendicular to the annulus, 2） flat and fused trabeculation, and 3） parallel to the annulus. In the latter two patterns, a thin muscular layer was observed beneath the tricuspid valve attachment, regardless of age. The original structural differences between the right and left AV annuli may be the cause of morphological differences between the right and left AV annular disjunction.