Biomedical Engineering Reference
In-Depth Information
fibril elongation, with part of the fibrils relaxed back to their unstressed length
and the remainder elongated by more than 4- 5%. The peak splitting occurs
shortly after the mechanical transition from approximately linear elastic to inelastic
behavior in the stress-strain curve.
1.4
Remarks and Conclusions
The structure of a bone is discussed in this chapter; this discussion applies to
a wide range of species, families, and orders, since, in a rough treatment, the
bones of all mammals could be described in a similar manner. Figures and
photographs of human bones and those of different animals are presented here
without differentiation, although one should be aware of the fact that every species
has its own characteristic features.
The bone components described above are essential for development and main-
tenance of a whole bone. For example, clinical data indicate that the nervous system
influences skeletal development. Sciatic nerve injury in infants is sometimes fol-
lowed by subnormal foot growth [267]. Moreover, Janet McCredie reported that
the limbs of children with thalidomide malformations show changes analogous
to those that can occur in the adult as a consequence of pathological alterations
to peripheral nerves. The original animal tests did not show indications of this
unexpected and serious side effect [268, 269].
Scurvy is a disease resulting from a deficiency of vitamin C, which is required for
the synthesis of collagen in humans. Collagen is an important part of the bone, so
bone formation is also affected. Most plant and animal species synthesize vitamin
C. Notable exceptions in the mammalian group include most or all of the order
chiroptera (bats), and one of the two major primate suborders, including human
beings.
Vitamin D deficiency results in impaired bone mineralization and leads to
bone-softening diseases including, rickets, a childhood disease characterized by
impeded growth and deformity of the long bones [270].
Astronauts lose significant bone mass during spaceflight, in conditions of
microgravity, despite rigorous musculoskeletal conditioning exercises. Femoral
neck-bending strength index decreased 2.55%/month for spaceflights lasting 4-6
months.
Similarly, in a study on the influence of long-term immobilization in dogs, 32
weeks of disuse resulted in significant bone loss in both young and old dogs. After
28 weeks of remobilization, young dogs (respectively, old dogs) recovered only 70%
(respectively, 40%) of the cortical bone lost during disuse.
Bears and humans have similar lower limb skeletal morphology and bears are
plantigrade like humans.
In bears, during hibernation disuse, the bone mass and strength losses are small,
if any. It is hypothesized that bears maintain bone cross-sectional properties and
strength during annual periods of disuse because they maintain bone formation.
