Introduction: Knees are subjected to daily physical activities, injuries and diseases, such as osteoarthritis (OA). Such complications represent significant costs (billions and thousands of USD/year for countries and individuals, respectively). Moreover, there is no OA cure and its risk factors (obesity, malalignment and injury) affect joints’ mechanical loading. Thus, knees must be studied under realistic loading conditions. Unfortunately, due to joints’ complexity (geometry, mechanical properties and loading), current experimental methods seldom achieve this. Quantitative magnetic resonance imaging (qMRI) potentially offers a non-invasive evaluation of tissue structure, biochemistry and mechanics, thereby facilitating injury or disease tracking if links between these properties and imaging outcomes were well established. However, the connections between tissue health and mechanical properties remain unclear, as is the relation between tissue- and joint-level biomechanics.
Objective: Determine if tissue structure and joint function are related in whole cadaver knees under physiologically realistic loading conditions applied via a novel MRI-safe loading device.
Methods: A novel MRI-safe knee loading device was designed, built and its repeatability assessed. Physiologic loading conditions (simulating walking) suitable for mechanical tests were determined via musculoskeletal (MSK) modelling, verified and validated against published data, and applied to a cadaver knee. To measure tibio- and patello-femoral (T-F and P-F) contact responses, a pressure sensing system was used in conjunction with the instrumented loading device. Then, to search for T2 relaxation-deformation associations, tibial and patellar cartilage deformations and T2 relaxation responses of other six ex-vivo knees subjected to axial compression (simulating standing) were measured and correlation analyses performed.
Results & Discussion: The MRI-safe loading system developed was able to simulate healthy or pathologic gait with adequate repeatability (e.g., 1.23 to 2.91 CV% for compression, comparable to existing simulators), leading to generally consistent contact responses in agreement with published experimental and finite element studies. Cartilage thickness and T2 relaxation time magnitudes measured fell within expected values, while their loading-induced changes agreed with previous studies but exhibited larger variability. Moreover, a moderate negative correlation (r = -0.402, p = 0.019) was found between unloaded tibial cartilage thickness and T2 relaxation time, which may be linked to cartilage composition (relating collagen fibers and water content)
