BONE ABSCESS
BONE DENSITY
B
for the muscles and protects certain
body organs. Bone consists of several
layers: a thin outer covering (the peri-
osteum), which contains blood vessels
and nerves; an inner shell of hard (com-
pact or cortical) bone composed of
columns of bone cells (osteoclasts and
osteoblasts), each with a central hollow
(haversian canal) that is important for
the nutrition, growth, and repair of the
bone; and a central, mesh-like structure
(known as spongy, cancellous, or trab-
ecular bone). The cavity in the centre of
some bones, and the spaces in spongy
bone, contain
bone marrow.
BO N E
Periosteum
Compact bone
Nerve
Spongy
bone
Blood
vessel
Bone
marrow
Bone is continuously reabsorbed by
osteoclasts and replaced by osteoblasts.
Osteoblasts encourage deposition of
calcium
phosphate
on
the
protein
framework of the bone, and osteoclasts
remove it. The actions of these cells are
controlled by hormones, which also
maintain the calcium level in the blood.
At birth, many bones consist mainly of
cartilage, which ossifies later (see
ossifi-
cation).
The
epiphyses
(growing ends of
the long bones) are separated from the
bone shaft
(diaphysis)
by the epiphyseal
plate. Some bones, such as certain skull
bones, do not develop from cartilage
and are known as membranous bones.
bo n e abscess A localized collection of
pus in a bone (see
osteomyelitis
).
bo n e age A measure of skeletal matur-
ity used to assess physical development
in children.
X-rays
, which show how
much bones have grown in a particular
body area, are used to determine bone
age. (See also
age
.)
bone can cer
Malignant
growth in bone,
which may originate in the bone itself
(primary bone cancer) or, more com-
monly, occur as a result of cancer
spreading from elsewhere in the body
(secondary, or metatastic, bone cancer).
Primary bone cancers are rare. The type
that occurs most often is
osteosarcom a
.
Other types include
chondrosarcoma
and
fibrosarcoma.
Bone cancer can also
start in the bone marrow (see
multiple
myeloma
and
leukaem ia
). The treatment
of primary bone cancer depends on the
extent to which the disease has spread.
If it remains confined to bone, amputa-
tion may be recommended; but it may
be possible to remove the cancer and
fill the defect with a
bone graft
.
Radio-
therapy
or
chem otherapy
, or both, may
also be needed
The cancers that spread readily to
form secondary bone cancer are those
of the breast, lung, prostate, thyroid,
and kidney. These bone metastases
occur commonly in the spine, pelvis,
ribs, and skull. Pain is usually the main
symptom. Affected bones are abnorm-
ally fragile and may easily fracture.
Bone cancer that affects the spine may
cause collapse or crushing of vertebrae,
damaging the spinal cord and causing
weakness or paralysis of one or more
limbs. Secondary bone cancers from the
breast and prostate often respond to
treatment with
horm one antagonists.
bo n e
cyst An abnormal cavity in a
bone. Bone cysts typically develop at
one end of a long bone and maybe dis-
covered only by chance after a bone
fracture at the site of the cyst. Minor
surgery to scrape out the cyst and fill
the the cavity with bone chips usually
cures the condition, although many
small cysts do not need treatment.
bo n e density The compactness of
bone
tissue in relation to its volume. A
decrease in bone density is a normal
part of aging. However, in some people,
excessive loss of density (see
osteo-
porosis
) can lead to fractures. Less
commonly, an increase in bone density
(see
osteosclerosis
) occurs in certain
86
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