Multiaxial Viscoelastic Properties of Human Medial Collateral Ligament

Ligament viscoelasticity is an important determinant of tissue response to loading, and viscous dissipation modulates the potential for ligament damage or catastrophic failure. The nature of ligament viscoelastic response represents a combined measure of solid phase viscoelasticity and fluid movement. Both of these contributors are likely direction-dependent due to ligament anisotropy. Small sinusoidal perturbations about an equilibrium strain value allow application of linear viscoelasticity theory for determination of dynamic stiffness (a measure of modulus) and phase angle (a measure of energy dissipation). The objective of this study was to quantify the multiaxial strainand rate-dependent viscoelastic behavior of the human medial collateral ligament (MCL) in tension along its longitudinal and transverse directions, and under simple shear loading. It was hypothesized that the dynamic stiffness would increase modestly with strain rate and strain level and that the phase would remain constant with frequency but decrease with increasing strain level. Results demonstrated that dynamic stiffness of human MCL varies greatly with test mode and equilibrium strain, but the magnitude is relatively insensitive to strain rate. The phase lag had very similar magnitudes and variation with frequency across all three test configurations. The only exception was a higher phase lag at low frequencies for the longitudinal tests. This could be due to an energy dissipation contribution by uncrimped collagen at the lowest strain level. The increase in phase at higher frequencies indicates increased energy dissipation and may provide a protective