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Caused by physical trauma,
bone fracture is one of the most common abnormalities of bone.
The degree of
fracture can vary widely from a simple crack in the cortical bone to a complex
multiple fracture with fragmentation and displacement of the bone pieces,
associated with severe damage to the surrounding soft tissues and, sometimes,
exposure of the bone fragments to the exterior through a large gaping wound
(open 'compound' fracture).
For a fracture to heal efficiently it is important that all conditions are
optimal; there are many factors that interfere with the satisfactory healing of
(a) Due to tearing of blood
vessels in the medullary cavity, cortex and periosteum, haematoma forms at the
site of fracture. The periosteum is stripped from the bone surface.
(b) Organization of the haematoma is associated with the migration of
neutrophils and macrophages into the fracture haematoma; these cells phagocytose
the haematoma and necrotic debris. This is followed by ingrowth of capillaries
and fibroblasts from surrounding tissue, producing fibrovascular granulation
tissue. New osteoprogenitor cells develop from mesenchymal precursor cells.
(c) Osteoblasts derived from osteoprogenitor cells migrate into the granulation
tissue and differentiate into osteoid-synthesizing units, which proceed to
deposit large quantities of osteoid collagen in a haphazard way, producing a
woven bone pattern. External callus bridges the fracture site outside the bone
and, if there is a significant gap between the bone ends, it may include
cartilage. Internal callus bridges the fracture in the medullary cavity and
rarely contains cartilage. When bone ends are closely apposed, direct
ossification between fractured ends occurs.
(d) Callus is usually well-established by the third week after fracture, but
initial bony union is by woven bone, which is mechanically weak. Re-modelling of
callus occurs once the defect between the two bone ends is bridged by bony
callus, taking many months. A combination of osteoclastic erosion and organized
osteoblastic osteoid synthesis removes surplus calcified callus, replacing the
inefficient bulky woven bone with compact organized lamellar bone.
(e) With time, re-modelling creates new lamellar trabecular bone, which is
orientated in a direction determined by the stresses to which the bone is
exposed with mobilization. Even with re-modelling, cortical irregularity and
minor marrow space fibrosis persist at the site of fracture.
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