Biomedical Engineering Reference
In-Depth Information
This extended model is applied to determine from a computational perspective
the optimum rate of distraction in bone distraction prior to experimental perfor-
mance [
56
] and to investigate its ability to predict the main tissue patterns during the
course of the three dimensional lengthening procedure of the mandible ramus [
60
].
4.2.1
Long Bone Distraction Osteogenesis: Design of the Protocol
of Distraction
Since distraction osteogenesis is a mechanical-based pathology, mechanical factors,
such as axial alignment, stability and distraction rate affect both the quality and
quantity of the regenerated bone [
34
,
35
]. For example, nonunions may occur when
the bone ends move too much [
21
,
34
,
64
], the distraction rate is higher than a limit
value [
15
,
35
,
64
,
66
], the frequency of distraction is not adequate [
35
], there is no
latency phase [
34
,
35
,
66
] or due to damage of the bone marrow and periosteal soft
tissues [
34
].
Therefore, in this section, the extended mechanobiological model of Gomez-
Benito et al. [
56
] is applied to simulate different mechanical environments in order
to determine the optimum rate of distraction from a computational perspective
before experimental testing. Values of 0.3, 1 and 2 mm/day were applied to a sheep
model [
9
]. These rates are commonly used in the experiments found in the literature
[
1
,
3
,
14
,
20
,
34
-
36
,
42
]. The distraction length was kept constant (20 mm) and
therefore the distraction periods were modified accordingly for each distraction rate.
In order to test the reliability of the implemented model, the tissue distributions
obtained were compared with experimental results. Results agree with the exper-
imental ones: a moderate distraction rate is needed for successful bony bridging
whist insufficient or excessive mechanical stimulation is adverse for distraction
(Fig.
5
).
With moderate distraction rates (1 mm/day), bone tissue appears in the peripher-
ical zones, close to the cortical bone and periosteum (Fig.
5
a). This is in agreement
with the experimental findings, where bone forms from the host bones to the center
of the gap (Fig.
5
d) [
45
,
52
,
61
]. Similar computational results were presented
by Isaksson et al. [
38
] simulating the same distraction protocol with a different
evolutive model [
54
].
With insufficient mechanical stimulation (0.3 mm/day), there is a quick increase
of the bone density during the first days of the distraction process (Fig.
5
b, e).
Computationally, this lower rate of distraction is accompanied by lower mechanical
stimulation which promotes osteogenesis.
In contrast, excessive mechanical stimulation (2 mm/day) produces nonunion
[
48
]. The mechanical stimulus (deviatoric strain) increases in the gap during the
entire process of distraction osteogenesis, favoring the differentiation of MSCs to
fibroblasts (Fig.
5
c, f). Under this mechanical environment, the number of MSCs
in the gap is limited and, consequently, they cannot contribute to the proliferation
and differentiation processes producing a delay in bone tissue production. This is
in agreement with most clinical results which consider that a rate of distraction of
