Tibiofemoral Model

The tibiofemoral model that is used in the optimization scheme is based on the model that was developed in the Model Development phase of this project. This section documents any changes that were made to the original model to facilitate the calibration procedure.

Given the model will likely be evaluated hundreds of times, specific anatomy were removed to increase computational efficiency. The patella is unloaded during the experimental laxity tests. As a result, the patella and the patellar ligaments were removed from the knee model. Additionally, the medial and lateral meniscus and mensical attachments were also removed. It is recognized that removing the meniscus may affect the estimate ligament slack lengths due to its potential to provide joint restraint. For the relatively low magnitudes of loading utilized in this work, it is assumed that minimal restraint is provided by the meniscus. This assumption will be tested during confirmation of the calibration procedure and updated if necessary.

Additionally, the additional experimental data that was made available for the Model Calibration phase was used to define the femoral and tibial fixed coordinate systems. These new coordinate systems are used to develop the joint coordinate system using the same methods described in the Model Development documentation. The difference in this model being the fixed femoral and tibial coordinate systems are defined using experimental data instead of manually digitized landmarks from the MR images.

The following sections offer more detail on the model that is used in the optimization scheme.

Ligament Properties

Each ligament bundle’s material properties include two slack length values, toe region percent, and stiffness. The calibration procedure will target bundle-specific slack lengths. The toe region percent and the stiffness values are not included in the calibration procedure and will be set to values defined during Model Development. In short, ligament stiffness values are defined based on literature values and all toe regions are be set at 6% strain.