Mapping Biomechanical Properties of Mouse Articular Surfaces Using Indentation: Preliminary Results

Introduction: The assessment of biomechanical properties of the cartilage is an important measure of its functional quality. We have previously developed and validated on human, sheep and rat knee joints a novel indentation technique ( 1, 2). As murine models are often used to study osteoarthritis (OA), the purpose of this study was to determine if this novel indentation technique could be used to map the biomechanical properties of the very small articular surfaces in mice knees. Materials and Methods: The biomechanical measurements were performed ex vivo, on the left and right femoral condyles and tibial plateaus of an 8-week-old C57BL/6 male mouse using a 3-axis mechanical tester (Mach-1 v500css, Biomomentum Inc., Montreal, Canada) equipped with a multiple-axis load cell. A spherical indenter of 0.35 mm diameter was used for the indentation measurement (40 µm of indentation depth in 1 second, 20 seconds for relaxation). For each position, it takes approximately 1 minute for the indentation measurements. Following biomechanical testing, the articular surfaces were fixed in 4% paraformaldehyde for histological assessment. Results and Discussion : The normal force versus time curve for the articular cartilage in a stress relaxation configuration showed typical behavior and spatial distribution similar to larger animals ( 3, 4 ). High-resolution mappings of the load at 10 μm of indentation depth were obtained for the tibial plateau and femoral condyles (Fig. 1) and the mean load was 5.7 ± 2.7 gf (n = 85 positions; ± SD) for the tibial plateau and 7.8 ± 4.1 gf (n = 49 positions; ± SD) for the femoral condyles. Histological results show that these measurements do not damage the cartilage which can then be analyzed histologically for various types of staining (ex. Safranin-O, immunohistochemistry), and histomorphometry. Conclusions: This study shows that mapping of biomechanical properties of cartilage using indentation is applicable to thin articular cartilages with non-uniform curvatures such as the tibial plateau and femoral condyle of mice. The mechanical properties mapping measured s how similar distribution patterns to those previously observed for larger species (human, sheep and rat) ( 3,4). The values obtained for the load at 10 mm are of the same order of magnitude (about 5 times higher) than the closer report of similar measurements (cartilage form mouse humeral head in flat indentation)( 5). These preliminary results suggest this method can assess the mechanical properties of entire cartilage surfaces. Studies are ongoing to further test and validate this indentation technique for its use with mouse OA models for testing the efficacy of drugs aimed to inhibit cartilage degradation or improving its healing in OA or following a joint injury.