Chapter 6
Bones and Skeletal Tissues
191
6
but it takes much longer for large, weight-bearing bones and for
bones of elderly people (because of their poorer circulation).
Repair in a simple fracture involves four major stages
(Figure 6.15)
:
1
A hematoma forms.
When a bone breaks, blood vessels in
the bone and periosteum, and perhaps in surrounding tis-
sues, are torn and hemorrhage. As a result, a
hematoma
(he
0
mah-to
9
mah), a mass of clotted blood, forms at the frac-
ture site. Soon, bone cells deprived of nutrition die, and the
tissue at the site becomes swollen, painful, and inflamed.
2
Fibrocartilaginous callus forms.
Within a few days, several
events lead to the formation of soF
granulation tissue
, also
called the
sof callus
(kal
9
us; “hard skin”). Capillaries grow
into the hematoma and phagocytic cells invade the area and
begin cleaning up the debris. Meanwhile, fibroblasts and car-
tilage and osteogenic cells invade the fracture site from the
nearby periosteum and endosteum and begin reconstructing
the bone. Te fibroblasts produce collagen fibers that span
the break and connect the broken bone ends. Some precur-
sor cells differentiate into chondroblasts that secrete cartilage
matrix. Within this mass of repair tissue, osteoblasts begin
forming spongy bone. Te cartilage cells farthest from the
capillaries secrete an externally bulging cartilaginous ma-
trix that later calcifies. Tis entire mass of repair tissue, now
called the
fibrocartilaginous callus
, splints the broken bone.
3
Bony callus forms.
Within a week, new bone trabeculae
appear in the fibrocartilaginous callus and gradually con-
vert it to a
bony (hard) callus
of spongy bone. Bony callus
formation continues until a firm union forms about two
months later. Tis process generally repeats the events of
endochondral ossification.
4
Bone remodeling occurs.
Beginning during bony callus for-
mation and continuing for several months aFer, the bony
callus is remodeled. Te excess material on the diaphysis
exterior and within the medullary cavity is removed, and
However, mechanical forces determine which osteoclasts are
most sensitive to P±H stimulation, so that bone in the least
stressed areas (which is temporarily dispensable) is broken down.
Bone Repair
Despite their remarkable strength, bones are susceptible to
fractures
, or breaks. During youth, most fractures result from
exceptional trauma that twists or smashes the bones (sports in-
juries, automobile accidents, and falls, for example). In old age,
most fractures occur as bones thin and weaken.
Fracture Classification
²ractures may be classified by
Position of the bone ends aFer fracture: In
nondisplaced Frac-
tures
, the bone ends retain their normal position. In
displaced
Fractures
, the bone ends are out of normal alignment.
Completeness of the break: If the bone is broken through,
the fracture is a
complete Fracture
. If not, it is an
incomplete
Fracture
.
Whether the bone ends penetrate the skin: If so, the fracture
is an
open
(
compound
)
Fracture
. If not, it is a
closed
(
simple
)
Fracture
.
In addition to these three either-or classifications, all frac-
tures can be described in terms of the location of the fracture, its
external appearance, and/or the nature of the break
(Table 6.2)
.
Fracture Treatment and Repair
±reatment involves
reduction
, the realignment of the broken
bone ends. In
closed
(
external
)
reduction
, the physician’s hands
coax the bone ends into position. In
open
(
internal
)
reduction
,
the bone ends are secured together surgically with pins or wires.
AFer the broken bone is reduced, it is immobilized either by
a cast or traction to allow healing. A simple fracture of small or
medium-sized bones in young adults heals in six to eight weeks,
Hematoma
External
callus
Bony
callus of
spongy
bone
Healed
fracture
New
blood
vessels
Spongy
bone
trabecula
Internal
callus
(fibrous
tissue and
cartilage)
1
A hematoma forms.
2
Fibrocartilaginous
callus forms.
3
Bony callus forms.
4
Bone remodeling
occurs.
Figure 6.15
Stages in the healing of a bone fracture.
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