Cartilage-on-cartilage contact exhibits isotropic wear independent of fiber orientation: Experiments and biphasic finite element prediction

Articular cartilage wear contributes to structural degradation and functional impairment in synovial joints. Although collagen fiber anisotropy and fluid–solid interactions are known to influence cartilage tribology, their roles in cartilage-on-cartilage wear and wear prediction remain unclear. In this study, in vitro tribological experiments and finite element (FE) modeling were combined to characterize cartilage-on-cartilage wear and to compare monophasic and biphasic wear-prediction frameworks. Reciprocating cartilage-on-cartilage tests were performed on bovine specimens with counter-surface collagen fiber orientations of 0° and 90° relative to the sliding direction. Contrary to the pronounced anisotropy typically observed in cartilage-on-metal systems, the results revealed an isotropic wear behavior, with both coefficient of friction and wear rate exhibiting insensitivity to collagen fiber orientation. In wear prediction, the biphasic model proved essential for capturing the protective effect of fluid-supported load sharing during the early stage, whereas the monophasic model overestimated initial wear. Crucially, however, with prolonged loading and progressive fluid exudation, predicted wear depth and volume from the two models converged (relative difference < 10%). This study establishes an experimentally validated FE framework, providing a strategic guideline for modeling: while biphasic properties are critical for transient mechanical response, the computationally efficient monophasic formulation serves as an accurate surrogate for long-term wear prediction, significantly advancing the capability for efficient whole-joint simulations and clinical wear assessments.