Age-Related Changes in Whole-Bone Structure and Strength - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 13

The variation in bone size relative to body size is likely to be compensated,
otherwise slender structures will be under-designed (weak) and robust structures
will be over-designed (bulky). For diaphyseal structures, slender bones show a
proportionally greater amount of cortical bone to maximize mass, and greater
mineralization to increase tissue-modulus [ 29 , 30 , 34 , 35 ]. In contrast, robust
bones show proportionally less cortical area to minimize mass and lower miner-
alization. This functional adaptation process is not fully compensated, however.
Limitations in the degree to which bone cells are able to adjust tissue-modulus
results in slender tibiae being 2-3 times less stiff relative to body size compared to
robust tibiae [ 29 ]. The combination of naturally reduced stiffness plus the com-
pensatory increase in mineralization may help explain the increased risk of stress
fractures in military recruits and young-adult athletes having slender tibiae
[ 36 - 39 ]. Thus, the coordination between bone morphology and tissue-level
mechanical properties may be a critical factor responsible in establishing func-
tionality for habitual loading as well as fracture susceptibility.
Despite the fact that the periosteum is an important target for prophylactic
treatment, the amount of periosteal expansion required to maintain strength has
not been defined. Many factors contributing to variation in periosteal expansion
have been identified [ 14 , 16 , 40 - 46 ], including bone width [ 47 ]. In a theoretical
study, Lazenby showed that the amount of periosteal apposition depended on the
ratio of the inner to outer radii (i.e., r/R), which varies with external bone size
[ 29 , 35 ]. In general, slender bones do not need to expand as much as robust bones
in order to maintain stiffness over time. The sex-specific differences in periosteal
expansion support this phenomenon, however, whether this size-dependence is
observed within a single sex has yet to be determined. Importantly, the age-related
decrease in tissue-modulus [ 48 ] would have to be compensated by increases in
periosteal expansion beyond that expected for endocortical resorption. The
magnitude of the interactions among age-related decreases in tissue-modulus,
the natural variation in bone diameter, and periosteal expansion have yet to be
established.
3 Metaphyseal and Vertebral Changes with Age
Metaphyseal sites differ from diaphyseal sites by the presence of trabecular bone in
the medullary cavity and by thinner cortices. They also differ in the much higher
incidence of osteoporotic/age-related fractures. Metaphyseal sites that are sus-
ceptible to osteoporotic fractures include the femoral neck and distal radius. The
vertebral bodies of the spine are analogous to metaphyseal sites given their
proximity to joints (i.e., interverbral discs) and their high proportion of trabecular
bone. The age-related changes at each of these fracture-prone sites has been
evaluated using in vivo imaging, primarily QCT. The distal tibia has also been
well studied, probably because of the ease of access for peripheral QCT more so
than clinical significance.