Mitral valve (MV) diseases are the most common heart valve diseases (Writing Group Members et al., 2012). It can affect any components of the MV apparatus. Mitral valve regurgitation (MVR) affects 1.7 % of the global population (Yoon et al., 2017) and can result from mitral annulus (MA) alteration.

The MA is an irregular, fibrous, flexible and dynamical structure joining the leaflets of the valve and left ventricle (LV) (Angelini et al., 1988, Levine et al., 1989). Its shape changes during cardiac cycle (CC) from a saddle shape at the mid -diastole and a plane shape due to contraction in systole or relaxation of the LV in diastole (Levine et al., 1989). The orifice area at the basis of the leaflets also increases during CC and then regains its original size when MV closes to help leaflet coaptation (Ormiston et al., 1981). Thereby, healthy MA is capable of continuously deform without damaging itself.

Fibres organization in the MA has been described as a continuity between the aortic mitral leaflet (anterior part of MA) and the right and left fibrous trigone, a region less prone to dilatation (McCarthy et al., 2010). Collagen fibres of the posterior leaflets are parallel to the MA whereas those of commissural regions are orthogonal to the annulus (Rim et al., 2015).. X-ray patterns have also shown that the amount of collagen fibres circumferentially oriented from chordae tendineae towards the annulus is greater than the ones in the central area of the leaflets (Bigi et al., 1982).

Several studies on MA mechanics were conducted both in-vitro and in-vivo. He and Bhattacharya demonstrated that annular tension in play when MA is closed and contracted, is more important on anterior part (He and Bhattacharya, 2010). Such annular tension defined as the leaflet tension measured in-vitro by force transducer at the annulus per unit of annulus length, ranges from 53.86 N/m, and 36.89 N/m in anterior vs posterior part, in healthy individuals. The in-vivo forces increase from ventricular diastole and find their peak at mid-diastole (Siefert et al., 2012). Septal-lateral mean change of forces throughout CC reaches 4.4 N while transverse mean change reaches 1.9 N.

Previous researches using Tissue Doppler Imaging (TDI), an echocardiographic technique based on Doppler have proven that posterior part of MA is the most mobile part and is more affected by dilatation when MVR occurs (Victor and Nayak, 1994). Using feature tracking in MRI, Leng et al. measured the MA motion on six points on healthy subjects during all CC and obtained velocity and mitral annular plane systolic excursion (Leng et al., 2018). Additionally, MRI enable micro-structural investigation of the heart structure (McGill et al., 2015, Tunnicliffe et al., 2014). Mitral annulus morphology changes during CC differently depending on the MA part. The greater velocity and displacement (15 cm/s and 20 mm) were found posteriorly during systolic excursion. Velocity and displacement decrease alongside the MA when nearing anterior part.

In term of reported strain rate by echocardiography in 4-chamber view, reference values in healthy volunteers left ventricle were of − 0.68 ± 0.10 and 0.88 ± 0.23 1/s for systole and early diastole (Moreira et al., 2017). No regional reference values were found in the literature specifically for MA. While clinically measurement of the LV strain rate may be considered as relatively high, it has to be noticed that such strain rate in echography is define as the peak of the derivative of strain curve obtained within the myocardium. Moreover, patients undercoming mitral valve surgery are most likely to have altered LV contraction and more especially diastolic dysfunction resulting in lower strain rate value.

While MA dynamics are well documented (Rausch et al., 2011), its passive mechanical properties are not. Such material properties are however fundamental for understanding of the MA dynamics as well as developing prosthetic device for mitral valve repair or replacement (Vukicevic et al., 2022). Tensile test is the most common experimental procedure and aimed at characterizing the passive mechanical behaviour of MA. A study on the four parts of the MA reported 4 Young moduli at a strain rate of approximately 13 %/min between 1.007 MPa and 28.15 MPa (Gunning and Murphy, 2014). This study also shows that the septal segment of MA, in contact with the aortic valve, has a greater stiffness than the other parts of MA with a Young modulus value of 28.55 MPa at 2 % of deformation. The difference between the stiffness of other segments is less pronounced. However, the relation with the fibrous structure of the MA was not established and only one condition of strain rate was reported (2 mm/min) while speed differ according to the considered part of the MA (Leng et al., 2018).

Careful research in the literature highlighted the lack of description in both micro-structural and mechanical characteristics of the MA. Additionally, current advances in finite heart model have shown the importance to mechanically characterize passive tissue before modelling activation (Augustin et al., 2020). Specific constitutive model of the MA is still to be defined.

Therefore, this study aims at providing mechanical properties of MA in traction tests with several strain rates and at investing the possible correlation between these mechanical properties and the micro-structural fibre organization using MRI. Additionally, it questions the homogeneity as well as strain rate behavioural dependency of the MA through testing of four different segments at four different quasi-static strain rates. The results of this study will provide additional knowledge about the material properties of the MA enabling for both accurate experimental and numerical simulations. Applications could be transcatheter mitral valve development and testing as well as patient-specific surgical simulations.