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Skeletal structure 121
at sites or in directions that are not normally loaded have been demon-
strated to induce a greater response than increasing physiological loads.
Recent experimental models for noninvasive, controlled in vivo loading
have been developed to test weight-bearing bones in the rat. These new in
vivo approaches can be integrated with in vitro and ex vivo studies to
acquire a more complete understanding of load-induced adaptation. These
animal models can be used to examine loading parameters, to study gene
expression, and to validate computer simulations. The mouse has recently
become more relevant; our ability to manipulate the mouse genome has
led to the development of mutations and new biological markers and
assays. In vivo loading of mouse mutants will help identify critical genes
and regulatory factors in the mechanical response pathway.
Adaptation around bone implants has received considerable attention
clinically and experimentally. When a bone segment is replaced by a stiff
metal prosthesis, the implant becomes the primary load bearing structure,
reducing the mechanical stimulus to the surrounding bone. Severe bone
loss is one of the impediments to the long-term success of orthopaedic
joint replacements. Future developments will include active devices that
stimulate the surrounding bone and, ultimately, artificial organs engi-
neered in the laboratory.
7.3.2 Modelling
For a skeletal element to respond to its mechanical environment, the cells
in the tissue must regulate their environment in response to the mechan-
ical stimuli they receive. The regulatory process can be thought of as a
feedback loop (Figure 7.5) in which the osteocyte senses the stimulus and
Homeostatic
Stimulus, S o
Material Sensor Cell
Force Stimulus, S
Properties Cells Activity
Geometry
change in bone mass, M
Figure 7.5. Feedback diagram for skeletal mechanical regulation. When forces are
applied to a whole bone, the stimulus that results is sensed by the bone cells in
the tissue. The sensor cells then signal bone-forming and -removing cells to
change the geometry and material properties of the bone.