Test Case Step

This step applied the experimentally measured loads to the node that is coincident with the fixed tibial coordinate system’s origin. The femur was fixed throughout this step, the flexion angle was specified, and the tibia is free to move in the other five degrees of freedom.

See the loading profile section for more information on how the benchmarking test case loads were applied to the knee model in this step.

Interactions

All interactions that are assigned for the simulated test are active during this step (Table 1).

Kinematic Boundary Conditions

Femur

The femur is fixed in all degrees of freedom throughout this step.

Tibia

The flexion angle of the joint was specified throughout this step, however, the tibia was free to move in the other five degrees of freedom. The flexion angle was defined as the experimentally measured flexion angles. Note that this is the absolute flexion angle, defined using processed experimental data.

Kinetic Boundary Conditions

Femur

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

Tibia

The experimentally measured tibial loads were applied to the node that was coincident with the fixed tibial coordinate system. As described in the Tibial Origin Orientation section of the Model Calibration documentation, a transform was used to define the orientation of this node as coincident with the fixed tibial coordinate system. The processed experimental tibial forces used to define the profile of the tibial loads in 6 degrees of freedom. Each loading value was applied as linear ramps between each load point. Note that the tibia is fixed in flexion about the JCS coordinate system, however the JCS flexion axis is not necessarily coincident with any of the axes of the tibia’s fixed coordinate system, therefore, loads in all 6 degrees of freedom were applied.

Below is an example of the loads used for a left knee specimen (such as oks003):

** Medial tibial drawer force, -1. for right knee and 1. for left knee
*CLOAD, amplitude=medialTibialDrawerForce, follower, op=new
JointCoordSys.1, 1, 1.
** Anterior tibial drawer force, 1. for right and left knee
*CLOAD, amplitude=anteriorTibialDrawerForce, follower, op=new
JointCoordSys.1, 2, 1.
** Distraction force, -1. for right and left knee
*CLOAD, amplitude=distractionForce, follower, op=new
JointCoordSys.1, 3, -1.
** Flexion torque, -1 for a right and left knee
*CLOAD, amplitude=flexionTorque_Posteriordrawer_oks003, follower, op=new
JointCoordSys.1, 4, -1.
** Varus torque, 1. for right knee and -1. for left knee
*CLOAD, amplitude=varusTorque, follower, op=new
JointCoordSys.1, 5, -1.
** Internal tibial rotation torque, 1. for right knee and -1. for left knee
*CLOAD, amplitude=internalTibialRotationTorque, follower, op=new
JointCoordSys.1, 6, -1.

Where each amplitude was the magnitude of the corresponding load and the descriptions of the loads match the CSU/WSU convention (described in Experimental Data). These values can be found in the corresponding test’s .inp file. The follower option ensures the applied loads follow the orientation of the node JointCoordSys.1 as the tibia moves throughout the simulation.

The node JointCoordSys.1 is the node that is coincident with the origin of the tibia’s fixed coordinate system. The integer following JointCoordSys.1 indicates the degree of freedom that the load is applied to. The final integer is multiplied by the specified amplitude. This is used to adjust for a right or left knee (see the Kinetics Adjustment 2 - Right or Left Knee section in the Model Calibration documentation for more information).

Note

The loads were applied relative to this node’s (JointCoordSys.1) coordinate system. This coordinate system is likely not coincident with the global coordinate system.

Tibia Loading Profile

A ramp-and-hold scheme was used to apply the desired loads from the benchmarking test case (Fig. 1). The given test point was simulated over 4.0 seconds, where the load was linearly increased to the desired value at 3.5 seconds and subsequently held for 0.5 seconds (Fig. 1). These loading profiles apply the experimentally measured loads in all 6 degrees of freedom. Note that Fig. 1 highlights the dominate loading axes, however every degree of freedom likely has a non-zero load throughout the simulation.

control variable

Fig. 1 The succession of applied varus and internal rotation loads throughout a combined loading test. The vertical lines indicate the points in the step’s time where the simulation’s results are extracted. Note that loads were applied in the other four degrees of freedom.