Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 2.10
(a) The anterior view of a human femur
(www.doitpoms.ac.uk/tlplib/bones/images/intact.pdf ).
(b) Material directions aligned withthe principal strain
directions.
the epiphyseal region can be observed. High density values are also present in the
region of the femoral neck, following the line of compressive load.
It is important to highlight that the model shows a high sensitivity in relation
to the loading conditions. However, the results obtained confirm the use of the
optimum structure hypothesis as a possible approach for the determination of an
admissible distribution of densities.
The result in Figure 2.10 shows a microstructure orientation following a pattern
similar to that of a real femur. In this case, it is possible to note the vertical
alignment of the material, both in the medial cortex and the lateral cortex. In
the metaphyseal region of the femur it is not possible to identify preferential
orientations, consistent with a certain degree of isotropy of this region. In the
femoral neck, a certain alignment is recovered because of bending stresses.
This density distribution is now used as the initial condition for the simulation
of the adaptation of both the bone and the interface around the femoral shaft.
The test is performed starting from two extreme interface initial conditions: a total
encapsulation by fibrous tissue and a totally consolidated bone tissue. Figure 2.12
shows the final distribution of the product
α
b
ρ
n
(note that bone stiffness at the
interface is proportional to this product). It is possible to see small differences
in the final distribution for each case. The total percentage of interfacial bone
during time-steps starting from both initial conditions is shown in Figure 2.11.
The similarity of results for the two conditions points out that the low quality of the
bone tissue in the proximal part of the femur resulting from resorption does not
allow a biomechanically favorable environment for bone ingrowth in this region.
An important issue frequently related to bone ingrowth is the stiffness of the pros-
thesis. There are evidences that the more rigid the prosthesis, the more proximal
