Cartilage Stiffness and Knee Loads Distribution: A Discrete Model for Landing Impacts

Musculoskeletal simulations can be used to determine loads experienced by the ligaments and cartilage during athletic motions such as impact from a drop landing, hence investigating mechanisms for injury. An open-source discrete element knee model was used to perform a forward dynamic simulation of the impact phase of a drop landing. The analysis was performed for varying moduli: nominal stiffness based on the literature, stiffness increased by 10%, and decreased by 10%. As the cartilage stiffness increased, the medial compartment contact load decreased. Conversely, the lateral compartment load and medial collateral ligament (MCL) force increased, causing a shift in the load distribution. However, these changes were insignificant compared to the overall magnitude of the contact forces (<4% change). The anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and lateral collateral ligament (LCL) loads remain unchanged between varying cartilage stiffness values. The medial compartment bears a majority of the load (860 N in the medial compartment versus 540 N in the lateral) during the impact phase of a drop landing, which agrees with physiological data that the medial side of the knee is more affected by osteoarthritis (OA) than the lateral side. The model was then simplified using a linear Kelvin–Voight model for the cartilage and linear pretensioned springs representing the cumulative ligament bundles. This allowed for a validation of the system and the extrapolation of the results as the mass and cartilage stiffness varied. This is one of the few studies to quantify this load distribution and shows that the results are invariant to changes in cartilage stiffness. This effect is due to the precompression system created by the coordinated action of cartilage and ligaments.