Karl, R.; Recco, D. P.; Kizilski, S.; Tharwat Abdelmoneim, I.; Kneier, N.; Hon, K.; Bergt, C. S.; del Nido, P. J.; Hoganson, D.; Engelhardt, S.; Hammer, P.

Purpose: Secondary chordae tendineae (CT) are frequently disregarded during mitral valve (MV) repair. However, their contribution to MV biomechanics remains poorly understood. We hypothesized that secondary CT play an important role in maintaining leaflet geometry during valve closure. Methods: Eleven explanted porcine MVs were micro-CT scanned in a systolic configuration under three conditions: (1) physiological baseline with all secondary CT intact, (2) transection of the A2 strut CT, and (3) transection of all secondary CT. Billowing height (BH), coaptation height (CH) along the parasternal long-axis view, and coaptation area (CA) were quantified and compared between conditions. An increase in BH [≥]2 mm from baseline was defined as pathological. Results: Transection of the A2 strut CT increased BH by 3.1 mm, exceeding the pathological billowing threshold by 1.1 mm (padj = 0.003). CH decreased by 0.6 mm (padj = 0.003), whereas CA showed no significant change compared with baseline. Subsequent transection of all secondary CT further increased BH to 4.8 mm relative to baseline, corresponding to 2.8 mm above the billowing threshold (padj = 0.003). CH did not differ significantly, while CA decreased by 0.35 cm2 compared with baseline (padj = 0.029). All values represent medians. Conclusion: Secondary CT play a critical role in stabilizing MV leaflet geometry. Their transection resulted in increased leaflet billowing and reduced coaptation metrics associated with unfavorable MV repair outcomes. Preservation or reconstruction of secondary CT may therefore be an important consideration in MV repair strategies.