Test Case Step¶
This step applies the experimentally measured loads to the node that is coincident with the fixed tibial coordinate system’s origin. The femur is fixed throughout this step, the flexion angle is maintained, 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 calibration test cases are applied to the knee model in this step.
Interactions¶
All interactions that are assigned for the simulated test are active during this step (Table 3).
Kinematic Boundary Conditions¶
Femur¶
The femur is fixed in all degrees of freedom throughout this step.
Tibia¶
The flexion angle of the joint is specified throughout this step, however, the tibia is free to move in the other five degrees of freedom. The flexion angle is defined as the experimentally measured flexion angles. Note that this is the absolute flexion angle, defined during experimental data processing.
Kinetic Boundary Conditions¶
Femur¶
No external loads were applied to the femur in this step.
Tibia¶
The experimentally measured tibial loads are applied to the node that is coincident that with the fixed tibial coordinate system. A transform was used to define the orientation of this node to be coincident with the fixed tibial coordinate system. The experimentally measured tibial forces are first processed, then used to define the profile of the tibial loads in 5 degrees of freedom. Each desired loading value will be applied as linear ramps between each loading case.
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.
** 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 is the magnitude of the corresponding load and the descriptions of the loads match the CSU convention (described in Kinetics Adjustment 1 - CSU convention). 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, and this corresponds with \(x_i\) in Table 1. The final integer is multiplied by the specified amplitude. This is used to adjust for a right or left knee (described in Kinetics Adjustment 2 - Right or Left Knee).
Note
The loads are applied relative to this node’s (JointCoordSys.1
) coordinate system. This coordinate system is likely not coincident with the global coordinate system because a transform was used to define the orientation of the node.
Tibia Loading Profile¶
There are a total of eight test cases that are used in model calibration. As described in the Simulation Steps section, there are separate simulations for each flexion angle included in the calibration test cases. Four of these test cases are from the \(0^\circ\) flexion laxity tests, and the other four are from the \(90^\circ\) flexion laxity tests.
A ramp-and-hold scheme is used to apply the desired loads from the calibration test cases (Fig. 5). The loading from each included laxity test is simulated over a step size 0.5 seconds. The load is linearly increased to the desired value at 0.3 seconds and subsequently held for 0.2 seconds (Fig. 5). These loading profiles apply the experimental feedback values in all 5 degrees of freedom. Note that the Fig. 5 highlights the dominate loading axes and every degree of freedom likely has a non-zero load throughout the simulation. Axis-specific load values will be determined during extraction of the relevant loading cases using the already developed in house tool.

Fig. 5 The succession of applied loads will include the varus-valgus torques and anterior-posterior drawer loads throughout a test case step. The vertical lines indicate the points in the step’s time where the simulation’s results are extracted, which will be used in the objective function. This example shows the applied loads for varus torque, valgus torque, anterior-drawer and posterior drawer tests at 0.5, 1.0, 1.5, and 2.0 seconds, respectively. These points are indicated with the black vertical lines. Note that the values shown in this figure are not the explicit feedback data from OpenKnee(s), but are meant to show the approximate loads along the dominate directions for each test case.