Pedestrian injuries : viscoelastic properties of human knee ligaments at high loading rates

ObjectiveAccidents involving pedestrians are very common, and often lead to severe injuries to the lower extremities. In a large portion of pedestrian-automobile collisions, knee ligament injuries aresustained. In this study, the viscoelastic properties of the four major human knee ligaments wereinvestigated at loading rates representative for pedestrian-automobile collisions.MethodsBone-ligament-bone specimens were tested in knee distraction loading. The collateral ligamentsand the separate functional bundles of the cruciate ligaments were tested in the anatomicalposition corresponding to a fully extended knee. A series of step-and-hold tests and ramp tests atdifferent rates were conducted in order to characterize the time-dependent behavior of the kneeligaments for deformation rates associated with the pedestrian impact loading environment. Thequasi linear viscoelastic (QLV) theory was used to describe the structural response of the kneeligaments and averaged parameters for this model were determined.ResultsThe QLV theory was found to be applicable for the time range that is relevant for pedestrianautomobilecollisions. The structural behavior of the knee ligaments was found to be particularlyrate-sensitive for high elongation rates, as occur during these collisions. The ligament stiffnesswas found to increase with age for both the collateral ligaments and with weight for the medialcollateral ligament.ConclusionsFor the loading conditions that are relevant for pedestrian-automobile collisions, the use of theQLV model for the description of the mechanical behavior of knee ligaments is appropriate. Therate-sensitivity is particularly important for these extreme loading conditions. The relaxationbehavior was found to be consistent between different ligament types and samples. Variationsdue to donor anthropometry were found predominantly for the instantaneous elastic behavior.