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What forces act on a flat rigid mitral annuloplasty ring?

BACKGROUND AND AIM OF THE STUDY
Increasing mitral valve repair durability requires successful restoration and support with annuloplasty rings. The stress distribution in these devices indirectly determines the success of the repair. It is hypothesized that changes in annular geometry throughout the cardiac cycle result in adverse strain distribution in stiff, flat annuloplasty rings, and hence non-physiological loading of the myocardium. The study aim was to identify the three-dimensional (3-D) force distribution in mitral annuloplasty rings.


METHODS
Eight animals were included in an acute porcine study. The mitral annulus 3-D dynamic geometry was assessed with sonomicrometry prior to ring insertion. Strain gauges mounted on dedicated D-shaped rigid flat annuloplasty rings enabled dynamic force measurements to be made perpendicular to the annulus plane.


RESULTS
The average systolic annular height to commissural width ratio before ring implantation was 13.7 +/- 1.4%. Following ring implantation, the annulus was fixed in the diastolic flat configuration (p <0.01). Force accumulation was seen from the anterior (0.7 +/- 0.4 N) and commissural (1.4 +/- 1.0 N) annular segments; both forces were acting in opposite directions and were statistically significantly larger than zero (p <0.01; n = 8).


CONCLUSION
These data demonstrate highest strains at the anterior and commissural areas of flat mitral annuloplasty rings, and support the hypothesis that the mitral valve annulus and its attached valvular and subvalvular structures apply systolic torque onto the flat annuloplasty ring in an attempt to conform it into the saddle-shaped configuration.