The unsaddled annulus: biomechanical culprit in mitral valve prolapse?

M asterful mitral valve (MV) repair demands thorough understanding of integrated mitral apparatus structure and mechanics. One consistently reported feature, the saddle shape of the mitral annulus, was recognized in the process of resolving the contradiction posed by an apparently high prevalence of MV prolapse (MVP) in otherwise healthy individuals: A normal saddle-like shape will show opposite leaflet curvatures with leaflets above or below the annulus in perpendicular 2-dimensional echocardiographic views, leading to the false impression of prolapse based on an assumed planar annulus. 3 Criteria recognizing annular nonplanarity substantially improved diagnostic specificity. Whether this saddle shape, by analogy to architectural structures , best supports ventricular-imposed stresses on the valve compared with a flat annulus 6 was first shown computation-ally 7 and then confirmed in vitro and in vivo. 8–11 In this issue of Circulation, Lee et al 12 revisit the implications of annular shape. They have used 3-dimensional echocardiography as a powerful technique to explore MV geometry in 100 subjects, 68 with degenerative MVP, with or without important mitral regurgitation (MR ≥ moderate), and 32 normal controls. Their most salient finding is that the mitral annulus of patients with more severe MR is not only greater in area but also flatter, with a decreased ratio of annular height to commissural width. That ratio is strongly associated with chordal rupture, which progressively increases in prevalence from 7% in those with the most saddle-shaped annuli to 42% in those with the most planar. Annular flattening is also associated with increased leaflet billowing volume, and together these variables determine MR severity. The authors therefore propose a potential role of annular flattening in MR pathogenesis. An initial reaction, acknowledged by the authors, is that annular flattening may simply be secondary to annular dilata-tion, increasing the denominator in the annular height-to-com-missural width ratio. Even the decrease in absolute annular height itself in MR patients may be secondary to out-of-plane annular stretching, reducing out-of-plane height (Figure 1). On further reflection, though, there may nevertheless be a true physical effect. Annular flattening, whatever its cause, exerts increased out-of-plane tension on the annulus, leaflets, and chords, as shown by Jensen et al 10,11 (Figure 1). With bio-mechanically weakened tissue, this can elongate the leaflets, promoting prolapse, and increase the likelihood of chordal rupture, leading to flail and more severe MR (Figure 2). Chordal rupture, in turn, increases stresses on adjacent unrup-tured chordal attachments and leaflet tissue 13 …