Mechanical characterization of articular cartilage by combining magnetic resonance imaging and finite-element analysis - a potential functional imaging technique

Magnetic resonance imaging (MRI) provides a method for non-invasive characterization of cartilage composition and structure. We aimed to see whether T-1 and T-2 relaxation times are related to proteoglycan (PG) and collagen-specific mechanical properties of articular cartilage. Specifically, we analyzed whether variations in the depthwise collagen orientation, as assessed by the laminae obtained from T-2 profiles, affect the mechanical characteristics of cartilage. After MRI and unconfined compression tests of human and bovine patellar cartilage samples, fibril-reinforced poroviscoelastic finite-element models (FEM), with depthwise collagen orientations implemented from quantitative T-2 maps (3 laminae for human, 3-7 laminae for bovine), were constructed to analyze the non-fibrillar matrix modulus (PG specific), fibril modulus (collagen specific) and permeability of the samples. In bovine cartilage, the non-fibrillar matrix modulus (R = -0.64, p < 0.05) as well as the initial permeability (R = 0.70, p < 0.05) correlated with T-1. In bovine cartilage, T-2 correlated positively with the initial fibril modulus (R = 0.62, p = 0.05). In human cartilage, the initial fibril modulus correlated negatively (R = -0.61, p < 0.05) with T-2. Based on the simulations, cartilage with a complex collagen architecture (5 or 7 laminae), leading to high bulk T-2 due to magic angle effects, provided higher compressive stiffness than tissue with a simple collagen architecture (3 laminae). Our results suggest that T-1 reflects PG-specific mechanical properties of cartilage. High T-2 is characteristic to soft cartilage with a classical collagen architecture. Contradictorily, high bulk T-2 can also be found in stiff cartilage with a multilaminar collagen fibril network. By emerging MRI and FEM, the present study establishes a step toward functional imaging of articular cartilage.