Test Case Step

This step applies the experimentally measured loads to the connector elements that define the joint coordinate system ([GS83]). The tibia is fixed throughout this step, the flexion angle is assigned throughout this step, and the femur 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 of the interactions that are assigned for the simulated test are active during this step (Table 3).

Kinematic Boundary Conditions

Femur

The femur is unconstrained in all directions except for rotation about the flexion axis. The angle about the femur’s flexion axis is assigned using the experimentally measured values throughout this step. The flexion angle is applied using the connector element that defines the joint’s flexion axis ([GS83]). See the Model Development documentation for more information on how the connector elements are used to define the joint’s coordinate system.

Tibia

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

Kinetic Boundary Conditions

Femur

The processed experimentally measured joint loads are applied to the appropriate connector elements throughout this step. See the Model Development documentation for more information on how the connector elements are used to define the joint’s coordinate system.

Tibia

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

Loading Profile

There are a total of eight test cases that are used in model calibration. As described in the Simulation Steps section,separate simulations at each flexion angle are 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. 6). The loading from each included laxity test is simulated over a step size of 0.5 seconds. The load is linearly increased to the desired value at 0.3 seconds and subsequently held for 0.2 seconds (Fig. 6). These loading profiles are used for all 5 degrees of freedom included in the kinetics. Note that the Fig. 6 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 and applied using the connector elements that make up the Grood and Suntay ([GS83]) based connector elements.

control variable

Fig. 6 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.

[GS83](1, 2, 3) E. S. Grood and W. J. Suntay. A Joint Coordinate System for the Clinical Description of Three-Dimensional Motions: Application to the Knee. Journal of Biomechanical Engineering, 105(2):136–144, May 1983. URL: http://dx.doi.org/10.1115/1.3138397, doi:10.1115/1.3138397.