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Skeletal structure 119

the adult, remodelling serves to repair, renew, and adapt bone tissue. A
primary function of remodelling is to replace damaged tissue such as
microcracks resulting from repetitive functional loading. Without this
continuous repair process, a much larger skeleton would be needed to
prevent the accumulation of damage.
Repair of bone fractures is another important mechanically mediated
process. A fracture initiates a multistage sequence of tissue regeneration
which recapitulates tissue differentiation and development. This process
also occurs in individual, fractured trabeculae. Initially a large granuloma
forms containing undifferentiated mesenchymal stem cells whose diffe-
rentiation is regulated by genetic and epigenetic factors. Following this
immediate trauma response, the cells differentiate into cartilage to stabil-
ise the fracture. The initial bridging and immobilisation are performed by
tissues that can tolerate the high strains that preclude bone formation.
Thereafter, endochondral ossification of the cartilage occurs and bone
forms. Finally, the new bone is remodelled and integrated into the original
structure. The mechanical environment is critical to the ability of the
tissue to regenerate. Immobilisation of the fracture may enhance early
healing at a time when stability is critical. Fractured bones that are dynam-
ically loaded during healing regain strength more quickly, but if the applied
strains are too large, cartilage or fibrous tissue forms and a psuedo joint
may develop at the fracture site.

7.3 Mechanical regulation of bone structure

7.3.1 Adaptation experiments
The growth and development of organisms living within Earth’s gravita-
tional field are intricately linked to mechanical demands. Manipulation of
forces in animal experiments has provided insights into the overall nature
of the adaptation process. The characteristics of adaptation to increased or
decreased in vivo loading include changes in bone quantity, not material
quality; greater response in immature than mature tissue; and response to
cyclic, not static, loading. These results were first demonstrated in a series
of well-designed studies with loads applied to rabbit limbs and have been
confirmed by a variety of studies since then. In the adult, in general, when
the loads are increased over normal levels, bone mass is increased, and
when the loads are decreased, bone mass is lost. Changes occur in the
cross-sectional size and shape of cortical bone and in the apparent density