Determination of damage criteria in medial collateral ligaments

The effect of consecutive and gradually increasing strain on the mechanical behavior of rat medial collateral ligaments (MCLs) was studied to characterize and understand the damage evolution process. Displacement-controlled tensile tests were conducted on femur-MCL-tibia complexes (FMTCs) harvested from 9 Sprague Dawley rats. Each FMTC was loaded to a given displacement, unloaded, and re-loaded to another increasing displacement several times until complete failure occurred. From the recorded tensile stress-strain data, two different phenomena indicative of initiation and propagation of damage were observed: an elongation of the toe region and a decrease in stiffness of the linear region. The threshold strain at which the elongation of the toe region first appeared was found to be (2:59±1:37)% while the threshold strain at which the decrease in stiffness first occurred was determined to be (5:11±1:25)%. These results suggest that there are two different mechanisms that control the damage process in ligaments and further investigation is needed to elucidate their micro-structural origin.