Initial Orientation Step

This step moves the femur from it’s initial position to the flexion angle for the specific test case. The femur’s flexion angle is controlled, but the femur is free to move in other degrees of freedom. The initial flexion angle is defined by the joint’s position in the MR images, and the final flexion angle is defined by the flexion angle that is experimentally measured during the first simulated test case. There is 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 first 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 is used to define the kinematic boundary condition for the femur during this step. Rotation is 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 is specified as experimentally measured flexion angle at the first simulated test case. Note that this is the absolute flexion angle, which is defined during experimental data processing and linearly ramped from the reference state. The knee is free to move in all other degrees of freedom.

Tibia

The tibia was fixed in all degrees of freedom.

Kinetic Boundary Conditions

Femur

To reduce the instability in the simulation, a nominal 20 N compressive force is 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.