Intrinsic Viscoelasticity of Type II Collagen Contributes to the Viscoelastic Response of Immature Bovine Articular Cartilage Under Unconfined Compression Stress Relaxation

Cartilage damage under loading is significantly affected by viscoelasticity. This study validates a finite deformation, nonlinear viscoelastic constitutive model for the collagen matrix of immature bovine articular cartilage, using reactive viscoelasticity. To examine the flow-independent viscoelasticity of cartilage collagen, tissue samples underwent proteoglycan (PG) digestion, losing more than 98% of their initial PG content to increase their hydraulic permeability. To verify that this PG-digestion eliminated flow-dependent viscoelasticity, PG-depleted samples were subjected to a gravitational permeation experiment, demonstrating that their hydraulic permeability, k=268 ± 152 mm4/N⋅s (n = 8), was five orders of magnitude greater than reported for untreated cartilage, confirming negligible flow-dependent viscoelasticity. Digested cartilage plugs were then subjected to unconfined compression stress relaxation (four consecutive ramp-hold profiles, each increasing the compressive strain by 10%) to fit the load response and extract material properties (RMSEfit=1.86 ± 0.61 kPa, n = 8). Successful curve-fitting served as a necessary condition for validating the model. Then, a separate unconfined compression stress-relaxation test was performed on the same samples, to 40% compressive strain at the same ramp rate. The model was able to faithfully predict this experimental response using fitted material properties (RMSEpred=3.95 ± 1.33 kPa, with 0≤ stresses ≤ 155 ± 37 kPa), providing a sufficient condition for validation in unconfined compression stress-relaxation. A computational model then showed that flow-independent viscoelasticity of cartilage collagen can enhance the stress response by ∼15% at fast strain rates, over flow-dependent effects. However, we estimate from prior studies that flow-independent viscoelasticity may enhance the stress response of cartilage by up to 200%, implying that PGs probably contribute significantly to the tissue's flow-independent viscoelasticity.