Non-planarity of the Mitral Valve and Annulus

Based on the studies conducted in an effort to refine the diagnosis of mitral valve prolapse, the geometric shape of the mitral valve and its annulus was defined. It has been documented that the mitral annulus and leaflets are non-planar saddle-shaped structures which are equivalent with the so-called hyperbolic paraboloid – a geometric surface of which all sections parallel to one co-ordinate plane are hyperbolas and all sections parallel to another co-ordinate plane are parabolas (see Figure 2). There are two high points (peaks) lying anteriorly and posteriorly at the aortic insertion and posterior left ventricular wall and two low points (troughs) closest to the apex located medially and laterally (see Figure 3). According to Levine et al. the maximum deviation from planarity, i.e. the distance between the highest and lowest points of the mitral annulus, is on average 1.4cm ± 0.3cm.4 Regarding the leaflet-annular relations, in mediolateral view (fourchamber view in 2-D echocardiography) the leaflets can appear above the mitral annulus but in anterolateral view (parasternal long-axis view in 2-D echocardiography) they do not ascend the annulus. That is the reason for the misdiagnosis of superior leaflet displacement in otherwise normal individuals, in the 2-D echocardiography era. The leaflet–annular non-planarity is rational in two ways. Firstly, as the base of the LV decreases in circumference during systole but the leaflets do not contract, the mitral annular area can decrease in some way by folding, which is achieved by lowering of the distance between high and lower points of the annulus. (5,6) Secondly, the saddle-shape provides a configuration capable of withstanding the stresses imposed by left ventricular pressure in systole. Salgo et al. studied the effect of non-planarity on stress reduction.(7) Two shape factors that have synergistic effect on stress reduction have been identified – leaflet billowing and annular non-planarity. The saddle-shape of the mitral annulus was preserved across three mammalian species (human, sheep and baboons) with an annular height commissural width ratio of approximately 15%. Their data suggest that nature conserves the saddle-shaped configuration of the annulus for a mechanical benefit.

Figure 2: Hyperbolic Paraboloid

Dynamics of the Mitral Annulus

There are two studies by Flachskampf et al. and Kaplan et al. which provide insights into the dynamics of the mitral annulus.5,6 According to these studies the mitral annular area was on average 5–6cm2/m2 corrected for body surface area (because of non-planarity an area projected into the least square plane was measured). This area decreased in systole by approximately 24%. The mechanis