Cartilage Damage Is Related to ACL Stiffness in a Porcine Model of ACL Repair

Inferior anterior cruciate ligament (ACL) structural properties may inadequately restrain tibiofemoral joint motion following surgery, contributing to the increased risk of post-traumatic osteoarthritis. Using both a direct measure of ACL linear stiffness and an in vivo magnetic resonance imaging (MRI) T-2*-based prediction model, we hypothesized that cartilage damage and ACL stiffness would increase over time, and that an inverse relationship between cartilage damage and ACL stiffness would emerge at a later stage of healing. After either 6, 12, or 24 weeks (w) of healing after ACL repair, ACL linear stiffness was determined from the force-displacement relationship during tensile testing ex vivo and predicted in vivo from the MRI T-2*-based multiple linear regression model in 24 Yucatan minipigs. Tibiofemoral cartilage was graded postmortem. There was no relationship between cartilage damage and ACL stiffness at 6 w (R-2 = 0.04; p = 0.65), 12 w (R-2 = 0.02; p = 0.77), or when the data from all animals were pooled (R-2 = 0.02; p = 0.47). A significant inverse relationship between cartilage damage and ACL stiffness based on both ex vivo measurement (R-2 = 0.90; p < 0.001) and in vivo MRI prediction (R-2 = 0.78; p = 0.004) of ACL stiffness emerged at 24 w. This result suggests that 90% of the variability in gross cartilage changes is associated with the repaired ACL linear stiffness at 6 months of healing. Clinical Significance: Techniques that provide a higher stiffness to the repaired ACL may be required to mitigate the post-traumatic osteoarthritis commonly seen after ACL injury, and MRI T-2* can be used as a noninvasive estimation of ligament stiffness. (c) 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2249-2257, 2019