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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.
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