Multi-Structural Fibril-Reinforced Poro-Hyperelastic (MSFPH) Finite Element Modeling Approach for Advanced Understanding of Articular Cartilage Pathomechanics

Cartilage degeneration, a critical factor in joint diseases such as osteoarthritis, poses significant challenges in understanding the intricate biomechanical behaviors of articular cartilage under various physiological conditions. Despite extensive research, a comprehensive characterization of the mechanical environment of articular cartilage under daily activity loads remains elusive. Addressing this gap, our study introduces a novel multi-structural fibril-reinforced poro-hyperelastic (MSFPH) model, employing a physics-based approach to resolve the intricacies of cartilage mechanics.

This innovative model goes beyond the limitations of traditional computational multiscale modeling by incorporating multiple cartilage components such as the Extracellular Matrix (ECM), Pericellular Matrix (PCM), chondrocytes, and Collagen fibril types II while considering their varied orientations across different cartilage zones. The axisymmetric finite element model developed in this study provides a more detailed and accurate representation

of cartilage mechanics at the tissue level, particularly under unconfined compression tests that simulate daily activities.

The model, validated with experimental data, demonstrates effective replication of actual cartilage behavior. It elucidates the depth-dependent mechanical properties across superficial, middle, and deep zones of cartilage, focusing on stress, strain, and stiffness distributions. These findings are vital in understanding how mechanical forces are transmitted through cartilage components to chondrocytes, influencing tissue health and response to degenerative conditions.

Key findings include the highest von Mises stress concentration in the ECM, particularly in the superficial zone, indicative of its primary load-bearing role. Additionally, the model uncovers significant cellular strain, especially in the superficial zone, highlighting the sensitivity of chondrocytes to mechanical stimuli. This strain is critical in understanding the regulatory mechanisms of cartilage metabolism and health. Furthermore, the model

demonstrates the crucial role of collagen fibrils in resisting compressive forces, underscoring their importance in maintaining tensile strength and structural integrity of cartilage. These fibrils exhibit depth-dependent variations in stress distribution, contributing significantly to the mechanical environment of chondrocytes and the surrounding microenvironment.

Despite its comprehensive approach and significant findings, the model acknowledges certain limitations. Future research is directed towards refining the accuracy of the model in geometrical representation and material properties. Such enhancements are anticipated to deepen our understanding of cartilage mechanics, paving the way for advanced diagnostic and therapeutic approaches in managing cartilage-related diseases.

Plugs of bovine articular cartilage along with the underlying subchondral bone were gathered from three distinct fresh stifle joints, sourced from a local slaughterhouse. These joints were obtained from mature cows, each roughly 2 years old. They were preserved in a frozen state at -50 ◦C until needed for testing. The preparation of the samples adhered to a well-established experimental procedure. Before conducting tests, the thickness of each

cartilage plug was meticulously measured using the needle probe of a micromechanical tester, Mach-1 v500c (Biomomentum Inc.). This examination determined that the average thickness of the articular cartilage was 1.3 ±0.1 mm. The cartilage explants underwent compression tests, experiencing stress relaxation, with the use of a flat indenter in the Mach-1 testing device. Each specimen was compressed at a rate of 0.1% per second until it reached a total strain of 20%, a level typically seen under normal physiological conditions, followed by a relaxation phase.