Anisotropic wear behavior of meniscus: Influence of cross-shear and loading magnitude

The repetitive wear-and-tear of knee menisci contributes to chronic knee pain and disability, yet the mechanical factors driving this degenerative process are poorly understood. Here we characterize the effect of motion type and loading magnitude on the anisotropic wear behavior of bovine meniscus. Custom pin-on-plate systems applied 60,000 cycles of unidirectional motion or multidirectional (cross-shear) motion by translating a sectioned "plate" of meniscus under a fixed cartilage "pin" that was loaded to generate physiological stress conditions (0.5, 1.0, 1.5 MPa). Pin motion was applied either longitudinal or transverse to the circumferential fibers of the meniscal tissue. We measured the effect of wear testing on meniscal volume loss, compressive mechanical properties, fiber fraying, and superficial layer thickness. A three-fold increase in loading magnitude resulted in a 36% increase in volume loss and a significant increase in fiber fraying. Multidirectional motion resulted in 31% greater volume loss than unidirectional motion, however, this change was not significant. Transverse specimens exhibited 1.8x greater volume loss than longitudinal specimens. Multiple regression revealed that meniscal tissue was more resistant to wear when it had higher initial tissue stiffness and greater initial stress relaxation. For the first time, this study has demonstrated that the meniscus exhibits anisotropic wear behavior that is governed by the compressive loading magnitude. This study provides foundational data and mechanistic insights on the wear behavior of the knee meniscus.