The anatomic substrate of mitral annular contraction

Background: Despite the absence of contractile elements, the mitral annulus undergoes sphincter-like “contraction” resulting in an area reduction of approximately 25%. Its anatomic basis has not, however, been delineated. Since annular contraction helps draw the mitral leaflets into apposition, an appreciation of its anatomic basis could enhance our understanding of the pathogenesis of mitral regurgitation. Methods: Gross dissection of >100 bovine, ovine and human hearts as well as histologic examination of 5 ovine hearts was performed to ascertain the origins, course and insertion points of the atrial and ventricular muscle bundles related to the annulus. Results: Significant circumferentially-oriented left atrial fibers derived from Bachman's bundle flank the annulus internally. These fibers encircle the base of the atrium and insert into the right fibrous trigone. Externally, the annulus is anatomically related to the superficial obliquely-oriented fibers of the left ventricular inlet which course from the subepicardium to the subendocardium. Conclusions: Intercalation of the annulus between the musculature of the left atrium and left ventricle subjects it to extrinsic contractile forces resulting in sphincter-like narrowing. The circumferential fibers of the left atrial base are favorably positioned such that their contraction imparts a centripetal force onto the inner aspect of the adjacent fibrous annulus which causes it to translate inward in late diastole. During systole, the superficial oblique fibers of the left ventricular inlet, impose a torsional force onto the outer aspect of the annulus causing it to translate inwards. These observations may have mechanistic implications in mitral regurgitation.