Individual and combined effects of OA-related subchondral bone alterations on proximal tibial surface stiffness: a parametric finite element modeling study
Introduction
Subchondral changes during the osteoarthritic (OA) disease process have become a controversial and growing area of focus in OA research. OA-related changes to subchondral cortical and trabecular bone are theorized to initiate and accelerate cartilage degeneration by increasing subchondral bone surface stiffness (i.e., stiffness normal to the subchondral bone surface), which will alter load and stress distributions in cartilage, leading to cartilage degeneration and eventual OA [1], [2], [3]. Stiffness gradients arising from variations in local stiffness are also thought to increase cartilage shear stresses [2]. However, theories regarding the role of bone stiffness in OA have been based upon isolated analyses of individual tissues (e.g., only subchondral cortical bone), and it is not known what effect individual changes to different regions of bone (e.g., subchondral cortical, subchondral trabecular, epiphyseal trabecular bone) may have on surface stiffness.
In addition to the isolated effect of individual changes, it is not known what effect combined changes in different bony regions will have on stiffness. Interestingly, conflicting evidence exists regarding whether or not individual bony changes will increase local stiffness. With OA, bony regions near the cartilage surface have shown thicker subchondral cortical bone and higher trabecular bone volume fraction than normal bone [4], [5], [6], [7], [8], [9]. Although the OA subchondral cortical bone is thicker, it consists of younger hypomineralized tissue with an elastic modulus lower than that of normal bone [6], [8], [10], [11]. In addition, bony regions beneath the subchondral bone surface (epiphyseal trabecular bone) have thinner trabeculae and lower bone volume fraction and volumetric density with OA [12], [13], [14], [15], [16]. However, it is not possible to predict, intuitively, how these different bony changes in architecture and material properties will affect local stiffness.
To improve our understanding of the role of bone in OA we must evaluate how OA-related alterations in subchondral and epiphyseal bone morphology and mechanical properties (elastic modulus) are associated with local stiffness assessed directly at the subchondral bone surface. This site, as the immediate support of the overlaying cartilage, is most relevant for integrity and health of the cartilage. Parametric finite element (FE) modeling is a noninvasive, time and cost efficient technique which can be applied to investigate the effects of different mechanical and morphological alterations on stiffness while accounting for complex geometry like that of the proximal tibia. To date, a few studies have focused on OA stiffness using the FE method [17], [18], [19]. No studies, however, have evaluated the effects of individual and combined alterations in morphological and mechanical properties of subchondral and epiphyseal bone on local stiffness.
In this study we first developed a parametric FE model of the proximal tibia. Next, we assessed the FE model's ability to characterize surface stiffness via a validation study linking FE stiffness with experimental stiffness results obtained from in situ macro-indentation testing. We then used this validated model to characterize the individual (Objective 1) and combined (Objective 2) effects of regional OA-related morphological and mechanical alterations to subchondral and epiphyseal bone on local stiffness of the proximal tibia.
Section snippets
FE model geometry
We scanned 16 fresh frozen cadaveric, proximal tibial specimens from 10 donors (8 males, 2 females; ages ranging from 67 to 88 years (mean ± standard deviation (SD): 77.8 ± 7.4)) using quantitative computed tomography (QCT) imaging. Specimens included tissues 25 cm distal to the tibio-femoral joint line. Further details of the participants, sample preparation and imaging parameters can be found elsewhere [20]. Semi-automatic segmentation was performed to derive the overall geometry of each
Results
For the individual effects analysis, subchondral cortical and subchondral trabecular bone had small effects on stiffness (Fig. 5A–C). Among the 3 subchondral parameters (SCT, SCE, STE), K/Knorm values varied the least due to changes in SCT and varied the most due to changes in STE. For subchondral cortical bone thickness, K/Knorm ranged between 0.98 and 1.06 for minimum/maximum SCT values of 0.2 and 3.0 mm, respectively (Fig. 5A). For subchondral trabecular elastic modulus, K/Knorm ranged
Discussion
In this study we developed and validated a 3D parametric FE model with axisymmetric geometry of a human cadaveric proximal tibia to estimate the relative effects of different morphological and mechanical properties of different bony layers on local proximal tibial surface stiffness.
Results of the regression analysis for validation of our model suggest that it predicts stiffness comparable with other FE models used in the literature (R2 ranging from 0.59 to 0.92 [49], [50], [51], [52], [53], [54]
Funding sources
Natural Sciences and Engineering Research Council of Canada (NSERC).
Canadian Arthritis Network (CAN).
Conflict of interests
None.
Ethical approval
Not required.
Acknowledgments
This project was funded through support from the Natural Sciences and Engineering Research Council of Canada (NSERC Grant 371530) and the Canadian Arthritis Network (CAN).
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