Relationships between anterior cruciate ligament elastic and viscoelastic properties and cartilage degeneration in osteoarthritic human knee joints

Knee joint osteoarthritis (OA) is characterized by alterations in articular cartilage and subchondral bone, but concurrent biomechanical changes in the bundles of human anterior cruciate ligament are poorly known. This study aimed at characterizing the anteromedial (AM) and posterolateral (PL) bundles' elastic and viscoelastic properties and relate them to knee joint OA. Small dogbone-shaped samples were cut from mid-substance of AM and PL bundles of human knees (n = 18 knees, N = 9 cadavers) and subjected to tensile sinusoidal and multi-step stress-relaxation testing. Phase difference and dynamic modulus were analyzed from the sinusoidal test, and equilibrium Young's modulus, peak-to-equilibrium stress ratio and fast and slow relaxation amplitudes and times were calculated to describe the elastic and viscoelastic properties. Cartilage degeneration was defined at eight sites in the knee joint by OARSI grading in our earlier study, and relationships between biomechanical properties and OARSI grades were investigated with Spearman's rank correlation. In the AM bundle, peak-to-equilibrium ratio increased (ρ = 0.525, p = 0.025), fast relaxation time decreased (ρ = -0.487, p = 0.040), and dynamic modulus decreased (ρ ≤ -0.501, p ≤ 0.034), with increasing OARSI grade of anterior medial femur. In both bundles, the phase difference increased (ρ ≥ 0.481, p ≤ 0.043) with OARSI grade of anterior medial femur. The AM and PL bundles become more viscous (i.e. resist better rapid loads) with anterior medial femoral cartilage degeneration, while also the material stiffness of the AM bundle decreased (i.e. restricts anterior tibial translation more compliantly). It could be that cartilage degeneration leads to chronic underloading with intermittent rapid straining of the ACL, causing the observed adaptive response.