Biomedical Engineering Reference
In-Depth Information
Figure 1. Two murine femora, one showing development from the cartilage anlage after transplantation to
the spleen, devoid of mechanical loading and showing the form associated with the genetic template alone.
The second shows the functionally related refinements to this template in a bone from the normal functional
location.
Thus, persistent abnormal loading or change in patterns of loading will induce changes in
the normal mass and architecture of the associated site of the skeleton. Changes in normal
loading may be a consequence of abnormalities in muscle function or neural control and gravi-
tational loading. The consequences of injury and subsequent orthopaedic procedures will also
influence the loading of the skeleton both in terms of the presence of orthopaedic devices in
relation to specific sites on specific bones, and also changes in load-bearing of the affected limb
or region of the skeleton.
Gross overload resulting in fracture is followed by a repair process in which, under favourable
conditions, bone undergoes a remarkable process of regeneration to restore the anatomical and
functional capability of the damaged skeletal element. In contrast to many other tissues and
organs this repair process does not involve scarring.
The process of repair is also modulated by the mechanical environment, initially to induce
one of two distinct types of repair, either direct or primary repair under conditions of
inter-fragmentary stability with rigid fixation, resulting in intra-cortical osteonal remodelling
and no periosteal callus or, indirect or secondary repair, sometimes referred to as biological
repair, in which the fixation methods allows greater inter-fragmentary motion that induces a
periosteal callus and inter-fragmentary endochondral repair.
Perhaps it is now evident that this may be a continuum and in addition the process of
indirect repair itself can be modulated by specific mechanical conditions at the fracture site,
which may enhance or inhibit osseous union of the fracture fragments.
The process of indirect fracture repair replicates many of the processes of embryonic endo-
chondral bone development. A fracture haematoma forms following disruption of the
intra-medullary and periosteal blood supplies at the time of injury; this then organises into
granulation tissue and progresses. Although a cascade of tissue differentiation through the en-
tire spectrum of connective tissues, indicating the potential to regenerate these tissues even in
the adult. The presence of mesenchymal stem cells within the medullary cavity together with
the release of growth factors in the early “inflammatory” phase of the repair process needs to be
understood not only to enhance the healing of fractures but also to explore the potential for
engineering or regenerating other connective tissues.
Search WWH ::
Custom Search