Hip Model for Osteonecrosis - Computational Biomechanics of the Musculoskeletal System - page 118

10.2 model develoPment

10.2.1 G eometry a cquiSition
The model was developed from images of a 29-year-old male patient who was admitted in April
2012. He weighed 60 kg and was about 170 cm tall. The patient was diagnosed with bilateral femo-
ral head necrosis (steroid type) and systemic lupus erythematosus.
Both MRI and computer tomography (CT) were used to construct the geometry for the FE
model. MRI segmentation has the advantage of distinguishing bone structure. It was used to con-
struct the trabecular bone core and the cortical bone shell, whereas CT has a higher resolution for
differentiating the necrotic region from normal bone. The left side of the hip joint was constructed,
including the hemi-pelvis, proximal femur, articular cartilage, and capsule. The femur was divided
into trabecular, cortical, and necrotic regions. The necrotic volume accounted for about 31% of the
trabecular volume in the femoral head.

10.2.2 m aterial p ropertieS
The material properties of the trabecular and cortical bone were assigned orthotropically (Krone
and Schuster 2006). The necrotic region was parameterized with 10% to 100% Young's modulus
and shear modulus as the trabecular bone. The principal directions of the trabecular and necrotic
regions were modeled by the trajectory suggested by Skedros and Baucom (2007). The pelvis was
assigned a Young's modulus and Poisson's ratio of 15,100 MPa and 0.3, respectively (Brown and
Hild 1983). The material properties of the cartilage and capsule were 10.5 MPa and 12.4 MPa,
respectively (Grecu et al. 2010; Stewart et al. 2002).

10.2.3 B oundary c onditionS
Midstance of the gait cycle was simulated with ground reaction forces and muscle forces adopted
from the literature (Brand et al. 1982). The von Mises stress distribution of the bone is shown in
Figure 10.2.

10.3 aPPlICatIonS
The stress transfer within the femur was altered with the assignment of a necrotic region. Figure 10.2
shows the von Mises stress in a cross-sectional view. The stress at the superior surface of the femur
head was gradually transferred to the inferior side of the femoral head. However, the reduction in
stiffness in the necrotic region results in a sharp reduction in its ability to sustain stress. The stress
was transferred to the exterior of the femoral head. This could be reflected by the von Mises stress
of the cortical bone at the weight-bearing region. It increased from 15 MPa to 40 MPa when the
necrotic region was reduced to 10% stiffness.

10.3.1 t raBecular y ield
As the bone degrades, the Young's modulus of the trabecular bone in the necrotic region decreases.
Due to the effect of stress shielding, the load shared by the necrotic trabeculae also reduces.
However, the reduction of experienced load is coupled with a reduction in sustainability. When the
experienced load exceeds the sustainability, the bone will most likely fail. Figure 10.3 demonstrates
the increase in volume of the yielding region as the degradation progresses.

10.3.2 l oad t ranSfer a lteration
Figure 10.4 illustrates changes in load transfer as the bone progresses from a healthy to a necrotic
state. The compressive principal stress of the healthy trabeculae at the load-bearing area is of greater