ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

joint computer model was driven with force and moment control in five DOF, while the
flexion was displacement driven (Figure 1C). The tibia bone was constrained in all
degrees of freedom via its rigid body reference node.
The structural connection between the femur and tibia bone as existing in the human
knee joint via ligaments and the capsule was incorporated using a phenomenological
ligament model [14] that was calibrated using the experimental data from the resection
study as determined during the in-vitro robot experiments. The ligament model
interpolated ligament forces and moments for an actual femorotibial relative position
using a neuro-fuzzy model [15] based on the experimentally determined kinematics and
kinetics and was incorporated into the computer model using the user subroutine
functionality of Abaqus/Explicit. Internally the subroutine processed the femorotibial
relative motion deploying the sensor functionality of the history output within
Abaqus/Explicit.
Finite element (FE) analyses of all six implanted specimen specific models were
performed using Abaqus/Explicit. Consistent with the robot experiments VV, IE and AP
laxity tests were analyzed for compression load levels at 44 N and 500 N [16] at
different flexion angles (extension, 30°, 45°, 60°, 90°, 120°). The initial extension angle
was found to be dependent on the specimen and the implantation of the components,
and therefore the flexion angles applied numerically were based on model specific
initial orientations.
Table 1: Applied loads and moments for the different laxity tests
Boundary Condition
Laxity test
Varus / valgus Internal / external Anterior / posterior
Compression load [N] 44 44; 500 44; 500
Varus (+) valgus (-) moment [Nm] 10 0 0
Internal (+) external (-) moment [Nm] 0 6 0
Anterior (+) Posterior (-) load [N] 0 0 100
Laxity test loads and moments (Table 1) were applied to the rigid body reference nodes
of the femur component rigid body. Resulting kinematics in terms of translations and
rotations of the femur relative to the tibia were post processed. Kinematics was
quantified by tracking femur flexion facet centers (FFC) [17] of the femoral condyles as
well as the midpoint between the FFCs called the “Origin”. Resulting femorotibial
relative translations and rotations of these tracking points were post processed in clinical
directions of the knee joint based on the definition of an anatomical bone coordinate
system.
3.3 Model Validation
Validation of the specimen specific implanted computer models against the implanted
in-vitro robot experiments was quantified by comparing the resulting kinematics
(translations and rotations) of the analyzed laxity tests. In particular, the root mean
square of the differences (RMSD) between model predictions and experimental data
was evaluated across all specimens.
4 RESULTS
AP translations, VV and IE rotations predicted by the computer models agree well with
the experimental data in trend and magnitude for both compression load levels (Figure
2, Figure 3).