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For this study, we hypothesized that the depth-dependent compressive equilibrium properties of articular cartilage are the inherent consequence of its depth-dependent composition, and not the result of depth-dependent material properties. To test this hypothesis, our recently developed fibril-reinforced poroviscoelastic swelling model was expanded to include the influence of intra- and extra-fibrillar water content, and the influence of the solid fraction on the compressive properties of the tissue. With this model, the depth-dependent compressive equilibrium properties of articular cartilage were determined, and compared with experimental data from the literature. The typical depth-dependent behavior of articular cartilage was predicted by this model. The effective aggregate modulus was highly strain-dependent. It decreased with increasing strain for low strains, and increases with increasing strain for high strains. This effect was more pronounced with increasing distance from the articular surface. The main insight from this study is that the depth-dependent material behavior of articular cartilage can be obtained from its depth-dependent composition only. This eliminates the need for the assumption that the material properties of the different constituents themselves vary with depth. Such insights are important for understanding cartilage mechanical behavior, cartilage damage mechanisms and tissue engineering studies.

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Since both the static and the time-dependent mechanical properties have now become fully dependent on tissue composition, the model allows assessing the mechanical consequences of compositional changes seen during osteoarthritis without further assumptions. This is a major step forward in quantitative evaluations of osteoarthritis progression.

Osteoarthritis (OA) is a multifactorial disease, associated with articular cartilage degeneration and eventually joint destruction. The phases of the disease have been described in detail, and mechanical factors play an important role in the initiation of OA, but many questions remain about its etiology. Swelling of cartilage, one of the earliest signs of damage, is proportional to the amount of collagen damage. This strongly suggests that damage to the collagen network is an early event in cartilage degeneration. The goal of this study was to determine the mechanical cause of early collagen damage in articular cartilage after mechanical overloading. Both the shear strain along the fibrils and the maximum fibril strains were evaluated as possible candidates for causing collagen damage. This evaluation was done by comparing the locations of maximum shear and tensile strains with the locations of initial collagen damage after mechanical overloading in bovine explants as found using antibodies directed against denatured type II collagen (Col2-3/4M). Collagen damage could be initiated by excessive shear strains along the collagen fibrils, and by excessive fibrils strains. The locations of collagen damage after mechanical overloading were highly dependent on the cartilage thickness, with thinner cartilage being more susceptible to damage than thicker samples. ß

Reinforcing hydrogels with micro-fibre scaffolds obtained by a Melt-Electrospinning Writing (MEW) process has demonstrated great promise for developing tissue engineered (TE) constructs with mechanical properties compatible to native tissues. However, the mechanical performance and reinforcement mechanism of the micro-fibre reinforced hydrogels is not yet fully understood. In this study, FE models, implementing material properties measured experimentally, were used to explore the reinforcement mechanism of fibre-hydrogel composites. First, a continuum FE model based on idealized scaffold geometry was used to capture reinforcement effects related to the suppression of lateral gel expansion by the scaffold, while a second micro-FE model based on micro-CT images of the real construct geometry during compaction captured the effects of load transfer through the scaffold interconnections. Results demonstrate that the reinforcement mechanism at higher scaffold volume fractions was dominated by the load carrying-ability of the fibre scaffold interconnections, which was much higher than expected based on testing scaffolds alone because the hydrogel provides resistance against buckling of the scaffold. We propose that the theoretical understanding presented in this work will assist the design of more effective composite constructs with potential applications in a wide range of TE conditions.