The meniscus is a wedge-shaped fibrocartilaginous tissue located between the femur and tibia that helps stabilize the knee and protect the underlying cartilage. There are 2.5 million reported knee injuries each year, making it the most injured joint in the human body. Nearly twenty percent of these injuries are due to a torn meniscus, leading to over half a million meniscus surgeries performed in the United States annually. Therefore, it is critical to understand the failure modes of meniscus tissue to prevent these debilitating injuries. A failure mode that accounts for one-third of all meniscus injuries is repeated exposure to low-magnitude tensile loads, known as fatigue. One approach to gain physical insight into fatigue mechanisms is through cyclic tensile experiments performed in laboratories. An alternative approach is to use constitutive mathematical models that predict and describe the material's behavior. These models can avoid the expense and time required for experimental fatigue studies, but they also must be calibrated and validated using experimental data.
The aim of this study is to validate a constitutive model to predict human meniscus' observed fatigue behavior in force-controlled loading. Three variations of constitutive models were applied to test each model's ability to model fatigue induced creep. These models included a viscoelastic damage model, a continuum damage mechanics model, and a viscoelastic model. Using a custom program, each models' parameters were fit to stretch-time plots from previously performed fatigue experiments of cadaveric human meniscus. The quality of fit for each model was then measured.
The results of this study show that a viscoelastic damage formulation can effectively fit force-controlled fatigue behavior and, on average, performed the best of the three models presented. On average, the resulting NRMSE values for stretch at all creep stages were 0.22%, 2.03%, and 0.45% for the visco-damage, damage-only, and visco-only models, respectively. The requirement of including both viscoelasticity and damage to model all three creep stages indicates that viscoelasticity may be the driving factor for damage accumulation in fatigue loading. Further, the relatively low damage values, ranging from 0.05 to 0.2, right before exponential increases in stretch, indicate that failure may occur from fatigue loading without a considerable accumulation of damage. The validation results showed that the model could not completely represent pull to failure experiments when using material parameters that curve fit fatigue experiments. Still, they indicated that the combination of discontinuous CDM and viscoelasticity shows potential to model both fatigue and static loadings using a single formulation. To our knowledge, this is the first study to model force-controlled fatigue induced creep in the meniscus or any other soft tissue. This study's results can be utilized to further model force-controlled fatigue to predict and prevent meniscus tissue injuries.
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