Viscoelastic properties of anterolateral and posteromedial bundles of the posterior cruciate ligament change in osteoarthritic human knee joints

In the human posterior cruciate ligament (PCL), histological changes have been reported with knee joint osteoarthritis (OA). However, it is not known how the biomechanical properties of the anterolateral (AL) and posteromedial (PM) bundles of PCL change at different stages of knee joint OA. Dumbbell-shaped samples were cut from AL and PM bundles of 18 human knee joints and tensile-tested with a multi-step stress-relaxation protocol until failure, including a sinusoidal loading test at 9 % strain. The viscoelastic properties were characterized by the phase difference, dynamic modulus, peak-to-equilibrium stress ratio, and fast and long-term relaxation times and amplitudes, while the elastic properties were described by the equilibrium Young's modulus and strength. Cartilage degeneration was classified by OARSI grading at eight sites in the knee. Spearman's correlation was used to investigate the relationships between biomechanical properties of the bundles and OARSI grades. The elastic properties exhibited no correlation with cartilage OARSI grades in either of the bundles. The peak-to-equilibrium ratio in the AL bundle decreased (ρ = -0.541, p = 0.020) and the long-term relaxation became slower (ρ = 0.545, p = 0.019) with medial femoral OARSI grade. The peak-to-equilibrium ratio in the PM bundle increased (ρ = 0.707, p = 0.001) and the phase difference increased (ρ = 0.521, p = 0.027) with lateral femoral OARSI grade. The results imply that the elastic properties (i.e. principal mechanical function) of PCL are preserved in OA-affected knees. On the other hand, the viscoelastic properties of AL and PM bundles were associated either with medial or lateral femoral OA, further emphasizing the multi-tissue and site-specific nature of the disease.