Knee Implant Model - Computational Biomechanics of the Musculoskeletal System - page 99

with increasing trabecular stiffness. The rise was more apparent with greater flexion angles. The
stress increased from 0.22 MPa to 0.59 MPa in full extension and from 2.24 MPa to 4.20 MPa
at 60-degree flexion. The trend of the PR was similar to that of the AF. There was an increase of
0.56 MPa and 0.86 MPa in compressive stress when flexed at 0 degrees and 60 degrees, despite the
fact that the gain was apparently smaller than that of the AF. Around the SR site, the change was
not consistent in spite of the diminishing stress with decreasing trabecular stiffness. The range of
change was more persistent at lower flexion angles, while indeed the smaller flexion angles dis-
played a lower compressive stress than high flexion angles. The compressive stress increased from
0.68 MPa to 1.86 MPa at 0 degrees and from 1.36 MPa to 1.57 MPa at 60 degrees.
8.3.3 p arametric S tudy on t raBecular S tiffneSS and y ield
The yielding stress of the trabecular bone (Burgers et al. 2008) is incorporated in Figure 8.5 to eval-
uate possible material failure upon reducing trabecular stiffness. The range of yielding stress was
between 1.32 MPa and 5.26 MPa for the given range of trabecular stiffness, indicating that yielding
at the PR and SR was unlikely. The predicted compressive stress from full extension to 30-degree
flexion did not exceed the suggested yielding stress in general. Yet, the predicted compressive stress
overlapped the line of yielding stress for flexion angles greater than 45 degrees. The indication of
over-yielding became more apparent with lower trabecular stiffness and greater flexion angles.
The yielded volume in the cross-section and anterior views of the trabecular femur are shown in
Figure 8.6 under 45-degree and 60-degree flexion stiffness ranging from 25% to 125%. The mag-
nitude of yielded volume is also shown; it should be noted that a better discretization was used for
calculation and a slight inconsistency may be apparent between the values and preceding plot/graph.
The region of yield is located along the superior line of the AF. The yielding volume was 52 mm 3
under 45-degree flexion with 125% trabecular stiffness. It was gradually increased and reached
approximately 450 mm 3 under 60-degree flexion with 50% trabecular stiffness. A deeper flexion
angle and smaller trabecular stiffness generally preceded a larger yield volume, which was also sup-
ported by the wider gap between the predicted stress and yield in Figure 8.5.
In this study, the parametric effects of trabecular bone stiffness on compressive principal stress
at different susceptible regions were quantified with suggested yielding stresses. Pure load compres-
sion with different flexion angles was adopted in the simulations. The boundary condition mimicked
highly physical work with knee flexion to accommodate factors that may contribute to knee disor-
ders that may exaggerate the risk of periprosthetic fracture (Jensen and Eenberg 1996; McMillan
and Nichols 2005). In fact, this study applied an axial load in different flexion angles, which is also
a common practice as it complies with the material and standard testing for implant. Chu (1999)
applied 2200 N to evaluate the contact stress on the tibial component with different flexion angles.
Similar protocols were adopted by Villa et al. (2004) using different force-angle combinations. Pure
compression in the neutral position was also widely used. Liau et al. (2002) and Kim, Kwon, and
Kim (2008) applied 3000-N and 2000-N forces, respectively, despite the fact that most of the implant
studies focused on the response of the tibial component due to the importance of wear and tear.
8.3.4 V alidation of tHe m odel
The implant stress from the current study was compared to existing works. The contact stress of the
tibial component in our studies ranged from 23.28 MPa to 25.68 MPa, with less than 10% deviation.
Miyoshi et al. (2002) and Godest et al. (2002) presented a contact pressure of about 24 MPa in their
tibial components. An experiment-validated simulation conducted by Villa et al. (2004) reported
a contact pressure of 15 MPa at 15-degree flexion and 27.7 MPa at 45-degree flexion. The current
prediction was generally agreeable with the existing literature in terms of the magnitude of contact
pressure on the tibial component, with deviations arising from differences in implant designs, load-
ing conditions, and alignment protocols.