Viscoelastic characterisation of chordae tendineae of the mitral valve: requirements for future replacement materials

Abstract

BackgroundChordal rupture can cause mitral valve regurgitation, which can be corrected by chordal replacement. Development of novel replacement chordae requires measurement of chordal properties under loads as they occur in vivo. However, frequency-dependent viscoelastic properties of chordae—ie, the storage E′ and loss E″ moduli which measure elastic recoil and energy dissipation, respectively—are unknown. This study aimed to characterise viscoelastic properties of chordae tendineae over physiological frequencies. MethodsDynamic mechanical analysis (DMA) was performed with a materials testing machine (Bose EnduraTEC 3200). 52 chordae were dissected from six porcine hearts and categorised as basal, marginal, strut, or commissural. After precycling, chordae were sinusoidally loaded at chordal-specific loads mimicking physiological loading (determined from literature). For example, different loading conditions were applied to marginal (0·6 N [± amplitude 0·5]) and basal (0·8 N [0·7]) chordae. Chordae were loaded at a range of frequencies between 0·5 and 5 Hz (hearts typically beat at around 1·2 Hz)—ie, physiological and pathophysiological heart rates. FindingsE′ (p<0·05), but not E″ (p>0·05), was frequency dependent for all chordae. E′ was greater than E″ for all chordae (p<0·05). E′ and E″ varied with chordal categories, with marginal chordae (E′ 816·9 MPa, E″ 80·9 MPa) being stiffest and strut chordae (E′ 137·1 MPa, E″ 12·3 MPa) the most compliant. This finding was consistent with E′ being inversely proportional to chordal thickness (p<0·05). InterpretationViscoelastic properties of chordae are dependent on both frequency and chordal type. The hierarchy of chordal stiffness (based on E′ values, highest to lowest) is marginal, commissural, basal, and strut. Differences in viscoelastic properties probably reflect chordal roles in the mitral valve: thinner marginal chordae are stiffer to ensure valve closure without regurgitation, and thicker basal and strut chordae transfer greater loads but are not responsible for valve competence and can, thus, be more compliant under dynamic loading. Future/novel replacement chordal materials must account for frequency dependent viscoelastic properties of chordae tendineae. FundingSociety of Biology, EU Seventh Framework Programme, and Arthritis Research UK.