Tensile Mechanics of the Knee Meniscus in the Context of Cracks: Failure and Fracture Mechanisms, Strain Concentrations, and the Effect of Specimen Shape

Knee meniscus tears (cracks) are a major cause of knee dysfunction and osteoarthritis, but little is known about how they grow or what effects they have on meniscus mechanics. The objective of this work was to investigate the mechanics and failure of crack-free and cracked meniscus in uniaxial tension, with specific attention to failure mechanisms (fracture and bulk rupture) and local strain concentrations. A finite element model was used to find a test configuration likely to cause fracture and crack propagation. Center cracks with a 45° crack–fiber angle were selected for producing large fiber stresses, and 90° edge cracks were selected for producing large inter-fiber shear stresses. The circumferential and radial tensile mechanics of the meniscus were quantified using ex vivo tensile testing. A fiber recruitment model was fitted to the test data, and a method was developed to quantify the inflection (yield) point and modulus based on the shape of the stress–strain curve. Comparison of tensile test specimen shapes showed that an expanded tab specimen shape produces more rapid and complete fiber recruitment, lesser yield strain, and greater peak stress (strength) than rectangle specimens, and, likely, dogbone specimens. Mechanical effects of meniscus cracks were quantified by comparing cracked and crack-free specimens in circumferential and radial tension. The cracks did not cause a decrease in peak stress, indicating fracture did not occur. However, significantly greater longitudinal strain and shear strain was found near the crack tip for circumferential tension specimens. In radial tension specimens, all strain field components were greater near the crack tip. Failure tended to proceed along fascicle boundaries. Circumferential specimens failed by widespread interdigitating fiber pull-out, which also caused crack deflection. Radial specimens failed by necking and fiber rotation. These data demonstrate the remarkable fracture toughness of the meniscus, but increased near-tip strain may cause sub-failure damage and dysfunction. These results provide functional targets for interventions to repair or regenerate the meniscus.