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Disorders of the Skeletal System: Trauma, Infections, Neoplasms, and Childhood Disorders
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Bony Callus Formation.
Ossifi
cation represents the conversion of
the fibrocartilaginous cartilage to
bony callus. In areas close to well-
vascularized bone tissue, osteogenic
cells develop into osteoblasts, or
bone-building cells, which produce
spongy bone trabeculae. The newly
formed osteoblasts first deposit bone
on the outer surface of the bone
some distance from the fracture site.
The formation of bone progresses
toward the fracture site until a new
bony sheath covers the fibrocartilag-
inous callus. In time, the fibrocar-
tilage is converted to spongy bone,
and the callus is then referred to as
bony callus. Gradually, the bony
callus calcifies and is replaced by
mature bone. Bony callus formation
begins 3 to 4 weeks after injury and
continues until a firm bony union is
formed months later.
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Replacement of
callus with
mature bone
Proliferating
osteogenic cells
Trabeculae
of forming bone
Reorganization of
newly formed bone
along lines of stress
Osteoclasts
Remodeling.
During remodeling
of the bony callus, dead portions of
the bone are gradually removed by
osteoclasts. Compact bone replaces
spongy bone around the periphery
of the fracture, and there is reor-
ganization of mineralized bone
along the lines of mechanical stress.
During this period, the excess mate-
rial on the outside of the bone shaft
and within the medullary cavity is
removed and compact bone is laid
down to reconstruct the shaft. The
final structure of the remodeled
area resembles that of the original
unbroken bone; however, a thick-
ened area on the surface of the bone
may remain as evidence of a healed
fracture.
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