Biomedical Engineering Reference
In-Depth Information
3.2.3 Remodelling
Eventually, fracture healing is completed during the remodelling phase, in
which cooperation between osteoblasts and osteoclasts occurs, progressively
converting the fracture callus to a bony structure capable of supporting
physiological mechanical loads. Unlike long-bone
expansion, in which there
is a balance between periosteal appositional
growth and bone resorption at
the endosteal surface, remodelling
of fracture callus occurs from the outer
surface inward, balancing external removal with the addition
of bone to
internal surfaces.
Trabecular bone is first resorbed by osteoclasts, creating a shallow
resorption pit known as Howship's lacunae. Each individual cell acidifies
the local extracellular space and secretes active lysosomal enzymes which
break down the bone matrix enzymatically. these enzymes include serine
proteases, collagenases and tartrate-resistant acid phosphatase. the enzymatic
destruction of bone matrix releases various proteins, including growth factors
previously stored during bone formation. these, in addition to cytokines
manufactured by osteoclasts and other cells, recruit adjacent osteoprogenitor
cells to become osteoblasts. osteoblasts enter the resorption pits created
by the osteoclast and manufacture new bone matrix of either the woven or
lamellar type. as the cells become entrapped within the bone matrix, they
evolve to osteocytes.
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Some osteoblasts eventually become flat surface cells
lining the quiescent bone surfaces, that is, bone lining cells.
With load bearing, most forces across the diaphysis are axial, therefore
the healing area is compressed while outside tension is created. Since
compressive 'physiological' forces result in chondroblast development,
whereas moderate tensile stress may stimulate bone formation, the fracture
callus of a diaphyseal fracture healed by secondary bone union shows bone
primarily on the outside of the callus, where tensile stress is applied, whereas
in the callus centre mesenchymal precursors experience compressive forces
with cartilage formation.
angiogenesis is closely associated with bone resorption and bone
formation. angiogenic factors, such as vascular endothelial growth factor
and endothelin, are regulators of osteoclast and osteoblast activity, but the
formation of blood vessels also serves as a way of transporting circulating
osteoblast and osteoclast precursors to sites undergoing active remodelling.
the close association between bone and vessels plays a pivotal role in the
regulation of bone remodelling and fracture repair.
the interface between
bone forming surfaces and bone marrow is lined by vascular structures, the
paratrabecular sinusoidal capillaries. an osteoblastic layer makes up the
osseous wall of the capillary, the opposite wall being formed by endothelial
cells.
Bone remodelling is also related to the existence of an increased flow
