Bma Illustrated Medical Dictionary

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.

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.

bone abscessA localized collection of

pus in a bone (see osteomyelitis).

bone ageA 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 cancerMalignantgrowth 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 osteosarcoma.

Other types include chondrosarcoma

and fibrosarcoma. Bone cancer can also

start in the bone marrow (see multiple

myeloma and leukaemia). 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-

therapyor chemotherapy, 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 hormone antagonists.

bone cystAn 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.

bone densityThe 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

BONE ABSCESS BONE DENSITY

B

Periosteum Compact bone

Blood

vessel

Nerve

Spongy

bone

BONE

Bone

marrow