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.
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