The meniscus are crescent-shaped, fibrocartilaginous structures crucial to the normal function of the knee joint. They improve congruity and reduce contact stresses, serving to protect articular cartilage on the opposing joint surfaces. The circumferential orientation of collagen fibers in the meniscus gives rise to anisotropic behaviors of the solid matrix and enables it to withstand high tensile stresses generated by compression in vivo.

In this study, the mechanical properties of the normal and healing canine meniscus were experimentally determined. Tensile testing was conducted to determine the tensile moduli E₁ and E₂ and the Poisson’s ratios ν₁₂, ν₂₁, and ν₂₃ from optical strain analysis of three orientations of samples. There was a significant effect of anisotropy on the tensile moduli, with mean values for E₁ and E₂ of 67.8 MPa and 10.8 MPa, respectively, but not on the Poisson’s ratios, which ranged from 1.02 to 2.13. These properties were used to model the solid matrix as transversely isotropic. Biphasic finite element optimization was performed to determine the permeability of meniscal samples in a configuration for which the collagen fibers were engaged. The collagen fiber anisotropy did not have a significant effect on the permeability coefficients, which averaged 1 x 10^-17 m⁴/Ns. A surgically induced circumferential tear in a canine model was studied after 12 weeks to determine the stiffness of the healing tissue. Heterogeneous samples were tested in tension, and finite element optimization was used to determine the stiffness of the lesion tissue.

These mechanical properties were incorporated into a custom-written, biphasic finite element model of the meniscus with a partially healed tear. Results from the axisymmetric model were studied to determine alterations in the mechanical environment in the vicinity of the tear. Strains in the tear were observed to be lower than the surrounding tissue, and fluid pressures in the tear were greater than the solid stress components. The high pressurization of the meniscus for extended periods of time protected the solid matrix from high strains. These findings have implications for the healing of tears, as strain and fluid pressurization may be important for regulating cellular phenotype and biosynthesis.