Viscoelastic properties of the human medial collateral ligament under longitudinal, transverse and shear loading

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Abstract
Ligament viscoelasticity controls viscous dissipation of energy and thus the potential for injury or catastrophic failure. Viscoelasticity under different loading conditions is likely related to the organization and anisotropy of the tissue. The objective of this study was to quantify the strain- and frequency-dependent viscoelastic behavior of the human medial collateral ligament (MCL) in tension along its longitudinal and transverse directions, and under shear along the fiber direction. The overall hypothesis was that human MCL would exhibit direction-dependent viscoelastic behavior, reflecting the composite structural organization of the tissue. Incremental stress relaxation testing was performed, followed by the application of small sinusoidal strain oscillations at three different equilibrium strain levels. The peak and equilibrium stress-strain curves for the longitudinal, transverse and shear tests demonstrate that the instantaneous and long-time stress-strain response of the tissue differs significantly between loading conditions of along-fiber stretch, cross-fiber stretch and along-fiber shear. The reduced relaxation curves demonstrated at least two relaxation times for all three test modes. Relaxation resulted in stresses that were 60-80% of the initial stress after 1000 s. Incremental stress relaxation proceeded faster at the lowest strain level for all three test configurations. Dynamic stiffness varied greatly with test mode and equilibrium strain level, and showed a modest but significant increase with frequency of applied strain oscillations for longitudinal and shear tests. Phase angle was unaffected by strain level (with exception of lowest strain level for longitudinal samples) but showed a significant increase with increasing strain oscillation frequency. There was no effect of test type on the phase angle. The increase in phase and thus energy dissipation at higher frequencies may protect the tissue from injury at faster loading rates. Results suggest that the long-time relaxation behavior and the short-time dynamic energy dissipation of ligament may be governed by different viscoelastic mechanisms, yet these mechanisms may affect tissue viscoelasticity similarly under different loading configurations.