Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 4.8
(a) Initial finite element mesh and boundary
conditions in the fracture stabilized by a unilateral fixator;
(b) final finite element mesh: frontal view and transverse
plane.
(a)
(b)
Figure 4.9
Osteoblast evolution (number of cells per cubic
millimeter) in (a) the frontal plane; (b) lateral plane for the
fracture stabilized by a unilateral fixator.
The influence of bending loads was also analyzed. With this purpose, a
mid-diaphyseal fracture of a sheep tibia stabilized by a unilateral fixator was
simulated. The unilateral fixator was modeled with a 4 mm radius circular section
(Figure 4.8a). The fixator was attached only to one side of the bone by a screw of
finite stiffness. The bone was loaded by an axial load of 500 N [114]. In this case, a
clear bending effect could be observed. The evolution of bone cells and of the callus
geometry was not symmetrical in the screw plane as shown in Figure 4.9 [114, 115].
Bone callus bridged sooner, about six weeks after fracture, on the lateral- frontal
side (plane of the fixator, on the side close to the fixator) (Figure 4.9a). On the
medial side, the bridge occurred 10 weeks after fracture and a bigger callus resulted.
On the contrary, in the lateral plane (Figure 4.9b) the callus was symmetrical not
only in geometry, but also in the evolution of the different cells.
This unsymmetrical fixation resulted in high bending movements in addition to
the axial interfragmentary movement, causing a nonsymmetrical callus. A similar
effect was also observed in similar experimental tests [41].
Recently, bone healing models have been applied to simulate distraction osteo-
genesis [97, 112, 116]. Distraction osteogeneis is a surgical procedure aimed at
producing a large amount of bone; it is widely used in orthopedic and craniofacial
surgery. These models have successfully simulated the effect of the distraction
rate in the outcome of the process. They have predicted premature union for low
