Modeling Fatigue and Damage Development in the Annulus Fibrosus Using a Reactive Viscoelastic Framework.

purposeTo develop a constitutive model of the annulus fibrosus (AF) that predicts both acute and chronic damage development during repetitive loading.

methodsImplemented a reactive viscoelasticity framework combined with a reactive fatigue constitutive model for the AF, which was calibrated under both damaging and non-damaging cyclic loading, then probabilistically validated using experimental data collected from porcine AF specimens. The annulus was modeled using a three-component mixture to represent the ground matrix, collagen fibers, and elastin fibers.

resultsThe developed material model could replicate experimental data. Correlations were observed between key parameters for collagen and elastin fibers. Calibration results demonstrated low mean absolute error and successfully replicated experimental force variation under both damaging and non-damaging cyclic loading. Probabilistic validation confirmed alignment with experimental data, capturing key behaviors such as relaxation ratios and minimal elastin fiber damage at low strains, while identifying increasing elastin damage under damaging conditions

conclusionThe model's generalizability and probabilistic validation provide a robust foundation for understanding soft tissue damage under cyclic loading, while highlighting the need for further investigations into healing mechanisms and full-disc validation studies. These advancements hold potential for applications in pilot and clinical spine health management and activity-based recovery standards.