Initial Orientation Step

This step moves the femur from it’s initial position to the flexion angle for the specific benchmark test case. The femur’s flexion angle was controlled, but the femur was free to move in other degrees of freedom. The initial flexion angle was defined by the joint’s position in the MR images, and the final flexion angle was defined by the flexion angle that is experimentally measured during the simulated test case. There was a nominal 20 N compressive load applied to the femur throughout this step.

The total time for this step is 0.5 seconds.

Interactions

The orientation of the knee at the end of this step is orientation at the beginning of the simulated test case. As such, all of the interactions that are desired for the simulated test are active during this step (Table 3).

Kinematic Boundary Conditions

Femur

For the simulated test, the femur was unconstrained in all directions except for rotation about the flexion axis. The angle about the femur’s flexion axis was assigned throughout this step.

The initial orientation of the knee is known after the femur and tibia coordinate systems are defined with respect to the MR image’s coordinate system. The initial flexion angle was used to define the kinematic boundary condition for the femur during this step. Rotation was applied to the connector element that corresponds to the flexion axis (see the Model Development documentation for details on the joint coordinate system connectors).

The final flexion angle of the knee was specified as experimentally measured flexion angle. Note that this is the absolute flexion angle, which is defined from the processed experimental data and linearly ramped from the imaged position. The knee is free to move in all other degrees of freedom.

Tibia

The tibia was fixed in all degrees of freedom. This ensured that the tibia’s origin was in the desired orientation at the beginning of the test case step.

Kinetic Boundary Conditions

Femur

To reduce the instability in the simulation, a nominal 20 N compressive force was applied to the femur during this step. This force was applied to the connector that defines internal-external tibial rotation axis (see the Model Development documentation for details on the joint coordinate system connectors). No other external forces are applied to the femur.

Tibia

No external loads were applied to the tibia in this step.