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Simulation studies, such as finite element (FE) modeling, provide insight into knee joint mechanics without patient involvement. Generic FE models mimic the biomechanical behavior of the tissue, but overlook variations in geometry, loading, and material properties of a population. Conversely, subject-specific models include these factors, resulting in enhanced predictive precision, but are laborious and time intensive. The present study aimed to enhance subject-specific knee joint FE modeling by incorporating a semi-automated segmentation algorithm using a 3D Swin UNETR for an initial segmentation of the femur and tibia, followed by a statistical shape model (SSM) adjustment to improve surface roughness and continuity. For comparison, a manual FE model was developed through manual segmentation (i.e., the de-facto standard approach). Both FE models were subjected to gait loading and the predicted mechanical response was compared. The semi-automated segmentation achieved a Dice similarity coefficient (DSC) of over 98% for both the femur and tibia. Hausdorff distance (mm) between the semi-automated and manual segmentation was 1.4 mm. The mechanical results (max principal stress and strain, fluid pressure, fibril strain, and contact area) showed no significant differences between the manual and semi-automated FE models, indicating the effectiveness of the proposed semi-automated segmentation in creating accurate knee joint FE models. We have made our semi-automated models publicly accessible to support and facilitate biomechanical modeling and medical image segmentation efforts ( https://data.mendeley.com/datasets/k5hdc9cz7w/1 )., (© 2024. The Author(s).)

Background: Collagen fibrils of articular cartilage have a distinct organization in mature human knee joints. It seems that a mechanobiological process drives the remodeling of newborn collagen fibrils with maturation. Therefore, the goal of the present study was to develop a collagen fibril remodeling algorithm that describes the unique collagen fibril organization in a 3D knee model., Method: A fibril-reinforced, biphasic cartilage model was used with a cuboid and a 3D human knee joint geometries. An isotropic collagen fibril distribution was assigned to the cartilage at the start of the analysis. Each fibril was rotated towards the direction that resulted in a maximum stretch at each time increment of the loading cycle., Results: The resulting pattern for the collagen fibrils was compared with split line patterns of porcine knee joint cartilage and also data published in the literature. Fibrils on the articular surface had a radial pattern towards the geometrical centroid of the tibial and femoral cartilage. In the tibiofemoral contact regions of superficial zone, fibrils were oriented circumferentially and randomly. In the porcine samples, the split-line patterns were similar to those obtained theoretically. Depth-wise organization of fibril network was characterized by fibrils perpendicular to the subchondral bone in the deeper layers, and fibrils parallel to the surface of cartilage in the superficial zone., Conclusions: The maximum stretch criterion, coupled with a biphasic constitutive model, successfully predicted the collagen fibril organization observed in the articular cartilage throughout the depth and on the articular surface., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Background and Objective: Cartilage cracks and fissures may occur due to certain daily life activities such as sports practice, blunt trauma, and matrix fibrillation during early osteoarthritis. These cracks could further grow at the macroscopic level, alter the load distribution pattern in the matrix, limit the joint range of motion, and disturb chondrocytes synthesis. Cracks' shape and deformations in the loaded cartilage may affect the subsequent mechanobiological processes in the long term, likely because of the altered fluid exchange and excessive local deformations in the vicinity of the damage site. The fibrillar structure of the cartilage matrix appeared to have a protective effect against excessive deformations and tissue failure. Hence, in the present study, a fibril reinforced biphasic cartilage model was used to assess the potential role of different fibril orientations on the profile of a vertical crack in cartilage after applying a compressive load., Methods: A 20 × 20 × 1.5 mm 3 cartilage model was developed with a 0.7 mm length V-shape cut at the center. Using an impermeable indenter, a 27% compression was applied to immature, mature, and isotropic cartilage models. Each of immature and mature groups had 4 different split line directions with respect to the cut edges, including 90°, 45°, 0°, and random orientation. The latter represented the disrupted collagen fibril orientations in early osteoarthritis. The model was verified with the experimental results in the literature., Results: In the superficial zones, the larger angle between the split lines and cut edges resulted in a wider cut opening. In the absence of collagen fibrils, the isotropic model resulted in a closed edge profile. Also, under a consistently applied compression, the OA model, with random collagen fibril distribution on its surface, had the smallest load-bearing capacity compared to the other models., Conclusions: Findings highlighted a primary role of collagen fibrils on the cut profile, which was more pronounced at dynamic rather than static conditions. Split lines perpendicular to the cut edges had some protective effects against the large dislocation of cut edges. These findings could be utilized to develop engineered tissues less susceptible to rupture. Moreover, the outcome of the present study can explain the potential causes of the crack propagation path reported in the literature., Competing Interests: Declaration of Competing Interest None., (Copyright © 2020. Published by Elsevier B.V.)

Despite significant evidence regarding the increased risk of cartilage degeneration due to traumatic injuries to joints, there is still a lack of understanding of the mechanisms underlying osteoarthritis development following a joint injury. Injuries in knee cartilage are often characterized by lesions or tears. In addition to acute traumatic joint injuries, microscale damages, which may form because of wear, are thought to be a contributing factor in the development of osteoarthritis. While the overall function of a joint may not be affected by the presence of microcracks, we hypothesized that strain magnification in the vicinity of microcracks might be significant. We tested this hypothesis by creating partial cuts in articular cartilages and measuring the strain within 20 µm from the edge of these cuts. Measurements were made in the superficial and middle zones of articular cartilage extract samples. We found that local strain in the vicinity of cuts is magnified by a factor of 1.2-1.6 compared to strains in intact regions for nominal compressions exceeding 5%. For nominal compressions of less than 5%, no strain magnification was detected in the vicinity of the cracks. We concluded that articular cartilage cracks magnify local strains by damaging the structural integrity and decreasing the fluid pressure in the matrix., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Macro-cracks on the surface of articular cartilage are one of the hallmarks of early osteoarthritis and joint damage initiation. Macro-cracks negatively affect cartilage mechanobiology and load bearing capacity. The aim of this study was to quantify the changes in transient and steady-state force response of healthy cartilage in the presence of macro-cracks when compressed. Ten macro-cracks were created on the surface of intact articular cartilage. The force-time responses of intact cartilage and cartilage with macro-cracks (n = 22) were compared for multiple nominal axial compressive strain levels and strain rates. Experiments were simulated using a fiber-reinforced biphasic finite element model to gain insight into the possible mechanisms contributing to changes in the mechanical response of articular cartilage when introducing macro-cracks. We found a significant reduction in the transient and steady-state load bearing capacity of cartilage samples following the introduction of macro-cracks. Two mechanisms were identified as potential causes of this reduction: (1) an increase in permeability and associated decrease in fluid pressure, and (2) damage of the structural integrity of the solid matrix. The first cause was predicted by the finite element model, while the second cause was not.

The non-homogeneous, anisotropic material properties, and triphasic nature of articular cartilage enables diarthrodial joints to withstand large and complex physiological loading conditions. To develop biomaterials that provide similar functional properties as those found in articular cartilage, it is vital to have knowledge of the strain distributions in cartilage for a large range of loading conditions. Applied stress vs. strain properties of articular cartilage have been measured primarily for static conditions, but the dynamic properties are thought to be more relevant for joint function and cartilage biosynthesis. Furthermore, the dynamic stress-strain properties are expected to vary significantly from those obtained for static, steady-state conditions. Here, we present a method for the determination of axial strain fields throughout the depth of articular cartilage for static loading conditions and dynamic conditions performed at different loading rates. For the conditions tested here, the strain distributions throughout the cartilage depth were more uniform for the dynamic than the static loading conditions, and more uniform for high compared to low strain rates., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The biological activities of chondrocytes are influenced by the mechanical characteristics of their environment. The overall real-time mechanical response of cartilage has been investigated earlier. However, the instantaneous local mechano-biology of cartilage has not been investigated in detail under dynamic loading conditions. In order to address this gap in the literature, we designed a compression testing device and implemented a dual photon microscopy technique with the goal of measuring local mechanical and biological responses of articular cartilage under dynamic loading conditions. The details of the compression system and results of a pilot study are presented here. A 15% ramp compression at a rate of 0.003/s with a subsequent stress relaxation phase was applied to the cartilage explant samples. The extra cellular matrix was imaged throughout the entire thickness of the cartilage sample, and local tissue strains were measured during the compression and relaxation phase. The axial compressive strains in the middle and superficial zones of cartilage were observed to increase during the relaxation phase: this was a new finding, suggesting the importance of further investigations on the real-time local behavior of cartilage. The compression system showed promising results for investigating the dynamic, real-time mechanical response of articular cartilage, and can now be used to reveal the instantaneous mechanical and biological responses of chondrocytes in response to dynamic loading conditions., (Copyright © 2018. Published by Elsevier Ltd.)