2. Passive Flexion

This section describes the specific steps and boundary conditions that are being used during an Abaqus FE simulation of passive flexion.

There are four steps in the passive flexion simulation cycle:

  1. Initial settle
  2. Load Patella
  3. Initial orientation to 0 flexion
  4. Passive flexion

2.1. Initial settle

Before this simulation, the ligament and tendon meshes have been defined from the results of the reference simulation, so there is no overclosure between the ligaments, tendons and other bodies.

The total time for this step is 0.01 seconds.

2.1.1. Interactions

All of the desired interactions are active in this step.

Table 2.1 Active interactions between different bodies and structures during the Wrap soft tissue step.
Name Femur Tibia Patella Femoral Cartilage Tibial Cartilage Patellar Cartilage Medial Meniscus Lateral Meniscus
amACL x x   x x   x x
plACL x x   x x   x x
alPCL x x   x x   x x
pmPCL x x   x x   x x
sMCLProx x x   x x   x  
sMCLDist x x   x x   x  
dMCL x x   x x   x  
LCL x     x        
ALL x x   x x     x
PFL x x   x x     x
OPL x x   x x     x
MPFL x   x     x    
LPFL x   x     x    
Patellar Tendon x x   x        
Quadriceps Tendon x     x        
Femoral Cartilage         x x x x
Tibial Cartilage             x x

2.1.2. Kinematic Boundary Conditions

The rigid bodies are fixed in all degrees of freedom.

Femur

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

Tibia

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

Patella

The proximal nodes in the quadriceps tendon will be fixed in all degrees freedom in this step.

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

2.1.3. Kinetic Boundary Conditions

There are no external loads.

Femur

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

Tibia

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

Patella

No external loads are applied to the patella or the quadriceps tendon in this step.


2.2. Load Patella

The patella was fixed in the previous step. This step removes the kinematic constraint on the patella, and loads the quadriceps tendon with a nominal load.

The total time for this step is 0.25 seconds.

2.2.1. Interactions

The interactions from the previous step are still active. All of the desired interactions should still be active in this step.

2.2.2. Kinematic Boundary Conditions

Femur

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

Tibia

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

Patella

The proximal nodes in the quadriceps tendon are no longer fixed, they have no boundary conditions (note that their motion is still constrained with the patellar quadriceps connector).

No kinematic boundary conditions are specified for the patella.

2.2.3. Kinetic Boundary Conditions

Femur

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

Tibia

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

Patella

A 20 N load is applied to the patellar quadriceps connector. This load is linearly increased from 5 N to 20 N throughout this step.


2.3. Initial orientation to 0 flexion

This step moves the femur from it’s initial position to 0 degrees flexion. The femur’s flexion angle is controlled, and is free to move in other degrees of freedom. There is a nominal 20 N compressive load applied to the femur throughout this step.

2.3.1. Interactions

The orientation of the knee at the end of this step is orientation at the beginning of the passive flexion test. As such, all of the interactions that are desired for the passive flexion test are active during this step. These interactions are still active from the previous steps.

2.3.2. Kinematic Boundary Conditions

Femur

For the passive flexion test, the femur will be unconstrained in all directions except for rotation about the flexion axis. The angle about the femur’s flexion axis will be 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 (more details in Coordinate Systems). The initial flexion angle is used to define the kinematic boundary condition for the femur during the initial step. The rotation is applied to the connector element that corresponds to the flexion axis.

  • Change the flexion from the initial value to zero
    • The femur is free to move in other degrees of freedom

Tibia

The tibia will be fixed in all degrees of freedom.

Patella

No kinematic boundary conditions are applied to the patella or the quadriceps tendon.

2.3.3. Kinetic Boundary Conditions

Femur

To reduce the instability in the simulation, a nominal 20 Ncompressive force will be applied to the femur during this step. This force will be applied to the connector element that corresponds to the SI axis. No other external forces will be applied to the femur.

Tibia

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

Patella

To reduce the potential for instability, a 20 N load is applied to the patellar quadriceps connector element. This load is maintained at 20 N throughout this step.


2.4. Passive flexion

This step simulates passive flexion. The femur is kinematically controlled in flexion, and free to move in other degrees of freedom.

The total time for this step is 2 seconds.

2.4.1. Interactions

The interactions from the previous step are maintained in this step. All of the desired interactions for the simulation are active in this step.

Table 2.2 Active interactions between different bodies and structures during the Passive flexion step.
Name Femur Tibia Patella Femoral Cartilage Tibial Cartilage Patellar Cartilage Medial Meniscus Lateral Meniscus
amACL x x   x x   x x
plACL x x   x x   x x
alPCL x x   x x   x x
pmPCL x x   x x   x x
sMCLProx x x   x x   x  
sMCLDist x x   x x   x  
dMCL x x   x x   x  
LCL x     x        
ALL x x   x x     x
PFL x x   x x     x
OPL x x   x x     x
MPFL x   x     x    
LPFL x   x     x    
Patellar Tendon x x   x        
Quadriceps Tendon x     x        
Femoral Cartilage         x x x x
Tibial Cartilage             x x

2.4.2. Kinematic Boundary Conditions

Femur

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

  • Passive flexion step: Increase the flexion angle from 0 to 90 degrees
    • The femur is free to move in other degrees of freedom

Tibia

The tibia will be fixed in all degrees of freedom in this step.

Patella

No kinematic boundary conditions are applied to the patella or the quadriceps tendon.

2.4.3. Kinetic Boundary Conditions

Femur

To reduce potential instability in the simulation, a nominal compressive force of 20 N will be applied to the femur during this step. This force will be applied along the joint coordinate system’s SI axis, which is the tibia’s fixed SI axis. No other external forces will be applied to the femur.

Tibia

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

Patella

To reduce the potential for instability, a 20 N load is applied to the patellar quadriceps connector element. This load is maintained at 20 N throughout this step.