Functional In Situ Assessment of Human Cartilage Using Multiparametric MRI and Biomechanical Loading

Introduction: Detecting early osteoarthritis remains diagnostically challenging. A promising approach is the functional assessment of cartilage tissue by multiparametric magnetic resonance imaging (MRI) under simultaneous mechanical loading. In this respect, the comprehensive evaluation of loading-induced tissue changes by MRI and its reference to histologic and biomechanical parameters is still lacking. Therefore, an MRI-compatible total knee joint apparatus was developed for the in situ assessment of loading-induced changes in human cartilage tissue.

Materials and Methods: A formalin-fixed human knee joint was scanned in its native configuration via computed tomography, and the bony contours of the femur and tibia were segmented to create femoral and tibial bone models. The models were subsequently three-dimensional printed and coated with an artificial cartilage layer in the native configuration. Polyvinyl siloxane was used, which is biomechanically similar to native cartilage, devoid of MRI artifacts, and offers favorable processing properties. A circular 8-mm-diameter defect was punched out of the artificial cartilage layer of the medial femoral condyle, into which native cartilage specimens of corresponding dimensions were inserted. By control of displacement (i.e., 2.5 mm [δ_2.5] and 5.0 mm [δ_5.0]), the mobile tibia was axially displaced in relation to the fixed femur, and the loading-induced changes of macroscopically intact human knee-cartilage specimens (n = 10) were examined by MRI. Serial proton-density weighted/turbo spin-echo and T2-weighted multiple spin-echo sequences were acquired on a clinical 3-T MRI system (Achieva, Philips) and referenced to biomechanical (unconfined compression loading) and histopathologic standard evaluation methods (Mankin grading).

Results: The cartilage specimens could be visualized and evaluated in their entirety by MRI before and under loading. The forces acting on the entire joint were 141 ± 8 N (δ_2.5) and 906 ± 38 N (δ_5.0), and the corresponding pressures created at the site of the native cartilage specimen were 0.68 ± 0.09 MPa (δ_2.5) and 1.05 ± 0.10 MPa (δ_5.0). Loading induced significant decreases in sample height as well as a loss in T2 signal intensity, thereby indicating extensive and coherent sample pressurization. Reference evaluation revealed all cartilage samples to be histologically intact and biomechanically homogeneous.

Conclusion: This study demonstrates the feasibility of assessing loading-induced changes of human articular cartilage by multiparametric MRI in an experimental in situ setting and in reference to histology and biomechanics. Future studies will show whether the functional analysis of articular cartilage beyond mere static analysis allows the earlier detection of osteoarthritis and/or its more refined graduation.