Page 131 - Visions of the Future Chemistry and Life Science

120 M. C. H. VAN DER MEULEN AND P. J. PRENDERGAST

of trabeculae; bone length is seldom significantly affected, but curvature
may be altered.
Most experiments examine cortical bone responses, in contrast to the
historical interest in trabecular adaptation. Exercise often shows little or
no effect, presumably because the overall activity level is not substantially
elevated beyond the normal range. Demonstrating a definitive decrease in
physiological loads is more straightforward and has been accomplished by
casting, space flight, and hindlimb suspension. Hindlimb suspension was
developed as a ground-based model for space flight, demonstrating similar
skeletal effects. Compared to age-matched controls, suspended growing
animals continue to grow, but at a reduced rate, with lower age-related
increases in femur strength and cross-sectional area (Figure 7.4). Decreased
bone formation occurs on the outer cortical surface, exactly the location of
the greatest reduction in mechanical stimulus.
Although many experiments have been performed, quantitative rela-
tionships between mechanical loads and bone adaptation do not yet exist.
In vivo strain gauge studies have found a remarkable similarity of peak
surface strains: 2000 at the midshaft of different bones across different
animals at maximum activity. Measuring strains in adaptation studies
would allow us to relate in vivo load changes to altered surface strains to
adapted bone mass and strength.
Applying loads directly to a skeletal site has the advantage that the
load magnitudes, frequency, and duration are known or controllable. Loads

(a) (b)
Figure 7.4. Digitised cross sections of femora from 67-day-old rats: (a) normal
control and (b) four-week suspended animals. Cross-sectional area of (a) is 5.3 mm 2
2
and (b) is 3.8 mm , a 29 per cent reduction due to the unloading during growth.