Papillary muscle and annulus size effect on anterior and posterior annulus tension of the mitral valve: an insight into annulus dilatation.

Mitral valve (MV) annulus mechanics and its effect on annulus dilatation are not well understood. The objective of the current study was to understand annulus tension (AT) during valve closure. A porcine MV rested on top of annulus rings with papillary muscles (PMs) held at slack, normal and taut conditions. The annulus was held by strings in the periphery during valve closure under static trans-mitral pressures. String tensions were measured and further used to calculate the anterior and posterior ATs. Three rings of different sizes were used to simulate normal and dilatated annuli. Fourteen MVs were tested. The anterior ATs were 37.21+/-11.03, 53.86+/-14.98 and 58.87+/-15.72N/m, respectively, at the slack, normal and taut PM positions in the normal annulus at the trans-mitral pressure of 16.3kPa (122mmHg). The posterior ATs were 24.52+/-5.68, 36.29+/-8.89 and 42.32+/-11.82N/m, respectively, at the slack, normal and taut PM positions in the normal annulus at the trans-mitral pressure of 16.3kPa (122mmHg). AT increased as the PM changed from slack to normal, then to taut PM positions. The AT increases with the increase of annulus area and linearly with the increase of trans-mitral pressure. The AT increases with the increases of apical PM displacement and dilatated annulus area, and reduces the potential of annulus dilatation. Low trans-mitral pressure due to existent mitral regurgitation, and MV prolapse increase the potential of annulus dilatation.