MATERIAL PROPERTIES OF HUMAN MEDIAL COLLATERAL LIGAMENT IN UNCONFINED COMPRESSION Jeffrey A. Weiss, Amy Lai, Stuart Loui, Jon Nisbet

The assumption of compressibility is often made when representing the material behavior of ligaments and tendons. The objectives of this study were to measure the changes in volume of ligament under a transverse load during unconfined compression loading, and to investigate time-dependent changes in the resulting load and stretch. Test specimens were harvested from human MCLs using a (lxlcm) steel punch, and measured using digital calipers. Four retro-reflective markers were attached to form a square to measure longitudinal and transverse stretch during transverse compression. A custom loading apparatus was built to apply transverse compression to the MCLs while allowing simultaneous measurement of the resulting transverse load and spreading of the MCL in the plane of the tissue using a motion analysis system. Thickness change was measured using an LVDT, and load was measured with a load cell. Each specimen was compressed, and stretch ratios were calculated from the measured marker coordinates. The volume ratio of the tissue was calculated as a function of time. The tissue underwent very small changes in length along the collogen fiber direction, while the tissue spread significantly transverse to the fiber direction. There was a significant increase in cross-fiber stretch with thickness changes but no change in fiber stretch with thickness change. The amount of volume change was large, with as much as a 35% reduction in volume under 40% transverse compressive strain. The observed behavior is similar to that predicted by the biphasic theory for tissues under unconfined compression. The magnitude of the stress relaxation strongly suggests fluid exudation, either along or transverse to the fiber direction, or in both directions. The asymmetry of the stretch ratios with applied compression is expected because of the tissue anisotropy. There is a strong preference for deformation in the cross-fiber direction rather than the fiber direction. The peak stresses were typically ten times as large as the equilibrium stresses. It is clear that significant changes in solid matrix volume can occur under the loading conditions used in this study. Data from this study will be used to formulate a constitutive representation for ligament that can accurately describe the time-dependent bulk material response.