Biomedical Engineering Reference
In-Depth Information
Figure 2. The distribution of connective tissues in the mid phase of indirect bone healing.
Indirect Bone Repair
When less rigid methods of stabilisation are used ranging from casting and functional brac-
ing to various configurations of external fixators and modalities of intra-medullary nailing,
there is more interfragmentary motion, particularly in the early stages of the repair process.
This initial mobility at the fracture site, induces a different pattern of tissue repair, called indi-
rect, secondary or biological fracture repair. This was described elegantly by McKibbin, 1978. 55
In this complex biological response to damage there are several processes occurring in parallel.
Periosteal proliferation occurs by intra-membranous ossification and initiates a bridging pro-
cess between the fragments (Fig. 2). Within the fracture gap, the haematoma that forms as a
consequence of the rupture of intra-medullary and periosteal vessels organises into granulation
tissue, rich in blood vessels. With time the granulation tissue differentiated into fibro-cartilage
and areas of hyaline cartilage which is avascular. The hyaline cartilage is then invaded by blood
vessels and undergoes endochondral ossification, a process very similar to that seen in the
embryonic development of the skeleton.
Thus, the fracture fragments undergo osseomechanical integration through both
intra-membranous ossification and a cascade of connective tissue differentiation from haematoma
through endochondral ossification to lamellar bone. Some reports also suggest a process within
this cascade termed transdifferentiation, 76 in which cells of a defined phenotype such as
chondrocytes express message for matrix proteins of a different tissue, for example, osteocalcin
a typical marker of bone forming cells.
Indirect bone repair is also a rapid process in establishing osseous union, the large mass of
reparative periosteal callus responsible for restoration of functional mechanical integrity then
undergoing a prolonged period of remodelling toward restoration of anatomical integrity.
The process of indirect bone healing is sensitive to the mechanical environment at the
fracture site. Both the inherent mechanical environment imposed by the combination of fixa-
tion device and patient activity and any imposed mechanical stimulation can modulate the rate
and extent of the indirect bone healing process.
 
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