Biomedical Engineering Reference
In-Depth Information
turnover to increase quickly at a similar rate (with the value for 90 × 4 slightly
greater than that for 360 × 1). After that, it is surprising that the bone turn-
over in the 90 × 4 scheme drops more quickly than that in the360 × 1 loading
scheme, implying less bone cell activity in samples from the 90 × 4 loading
scheme after the bone cells become desensitized from the 30th day.
Figure 7.4(d) presents the ratio OCA/OBA dynamics in the loading period
for the 360 × 1 and 90 × 4 loading schemes, starting from
OCA
/
OBA
≈ 1.25 in
healthy adults, which is consistent with Lemaire et al. [23]. Both ratios show
almost the same pattern and value: They drop quickly once the mechanical
loading applies, from 1.25 to about 0.1 on the 20th day, and then they remain
almost unchanged until the end of the experiment, predicting an osteogenic
outcome after loading.
Figure 7.5(a) shows the BMC dynamics during an extended loading period
(more than 16 weeks) for the 360 × 1 and 90 × 4 loading schemes. The BMC
remains almost at its initial value in the first week, which agrees with bone
remodeling cycle theory. Then, the BFR and BMC values for the loading
scheme 90 × 4 exceed those of the 360 × 1 loading scheme, which is consistent
with the experimental observation [12].
As we can see from the graph, BFR in both cases diminishes as time
elapses. We continue to draw the graph (which means the loading continues
in the experiment) and it is interesting to see that it takes less time for the
BMC to achieve peak value in the 90 × 4 loading scheme (150 days) than in
the 360 × 1 loading scheme (180 days). Unfortunately, there is no experiment
to verify this finding. By the end of the experiment (that is, 16 weeks' load-
ing), our simulation shows 5.5% and 8.8% increase in BMC for the 360 × 1 and
90 × 4 loading schemes, respectively (see the small circles and squares with
the words “Present model” on the solid and dotted curves). This compares
well with the measured changes in experiments, in which the increases were
by 6.9% and 11.7%, respectively [12] (see the small circles and squares with
words “Experiment [12]”).
This conclusion is significant to mechanical stimulus therapies, in that sep-
arating loading into short bouts such as 90 × 4 in this experiment not only
ultimately achieves greater BMC but also uses less time compared with one
long loading bout, such as 360 × 1 in this case.
Using our newly proposed standard of bone fracture energy, we calculate
the dynamics of BFE in the extended loading period (that is 365 days) in
Figure 7.5(b). The results show that the BFE increases almost linearly with
respect to time (days) until about the 150th day in the 90 × 4 loading scheme
and the 180th day in the 360 × 1 loading scheme, which matches the timing
when BMC peaks in Figure 7.5(a). Then, the BFE continues to grow linearly
at a slower rate, probably because the increase of BMC ceases in both cases.
The distinguishing difference between Figure 7.5(a) and 7.5(b) is that the BFE
continues to grow even after the BMC stops increasing, meaning that bone
strength continues to benefit from the mechanical stimulus although there
is hardly any gain in bone mass. Our simulation results indicate 72.8% and
