Muscle ridge between atrioventricular valves and malalignment of junction of these valves with ventricular septum.

A posteromedian muscle ridge is found in the left ventricle between the two atrioventricular valves in congenital malformations of the heart, where malalignment exists between the ventricular septum and the junction ofboth atrioventricular valve rings; this is seen in transposition with straddling tricuspid valve and double inlet ventricle with outflow chamber. In hearts with normal alignment of the atrioventricular valve and transposition of the great vessels, the same posteromedian muscle ridge was identified in all specimens with associated posterior ventricular septal defect without a muscular rim between the tricuspid valve and the edge of the ventricular septal defect. If this rim was present or if the ventricular septal defect was midseptal, only 50 per cent of the specimens studied showed a posteromedian muscle ridge. This muscle ridge was present in only 20 per cent ofthe hearts without a ventricular septal defect or with an anterior ventricular septal defect. The posteromedian muscle ridge has a narrow cranial part (head) which is just apical from the junction of the atrioventricular valve rings or apical from the septum membranaceum or its remnants. The middle part (body) is wider and occasionally double, extending down the posteromedian side of the anterior leaflet of the mitral valve, on the dorsal free wall of the left ventricle, or in the angle between those two structures. It then branches out and joins the base of the posterior papillary muscle and an accessory papillary muscle, which holds the left ventricular part of the tricuspid valve. The specimens studied showed a full range of malformations from uncomplicated transpositions, wherein the muscle ridge cannot or can scarcely be identified, to varying degrees of straddling tricuspid valve, wherein the valve moves further and further away from the septum, and finally to the double inlet ventricle of the left ventricular type, wherein the bulboventricular septum lies at the opposite side of the right atrioventricular valve. In double inlet ventricle of the right ventricular type with left posterior pouch, both atrioventricular valves and the posteromedian muscle ridge crossed the septum and were found in the right ventricle.