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William Arthur Atkins

Osteoporosis

Definition

Osteoporosis is a chronic disorder in which the

mass of bones decreases and their internal structure

degenerates to the point where bones become fragile

and break easily.

Description

Bone is living material. It is constantly broken

down by cells called osteoclasts and built up again by

cells called osteoblasts. This process is called bone

remodeling, and it continues throughout an individu-

al’s life. Normally, more bone is built up than is bro-

ken down from birth through adolescence. In the late

teens or early twenties, people reach their peak bone

mass—the most bone that they will ever have. For

twenty or so years, bone gain and bone loss remain

approximately balanced in healthy people with good

nutrition. However, when women enter menopause,

usually in their mid to late forties, for the first 5to 7

years bone loss occurs at a rate of 1–5% a year. Men

tend to lose less bone, and the loss often begins later in

life. Osteoporosis occurs when bone loss continues

and bones become so thin and their internal structure

is so damaged that they break easily.

Bone remodeling occurs because bone is made

primarily ofcalciumand phosphorous. Calcium is

critically involved in muscle contraction, nerve impulse

transmission, and many metabolic activities within

cells. To remain healthy, the body must keep the level

of free calcium ions (Ca 2+) within a very narrow

concentration range. Besides providing a framework

for the body, bone acts as a calcium ‘‘bank.’’ When

excess calcium is present in the blood, osteoblasts

deposit it into bones where it is stored. When too little

calcium is present, osteoblasts dissolve calcium from

bones and move it into the blood. This process is con-

trolled mainly by parathyroid hormone (PTH) secreted

by the parathyroid glands in the neck. As people age,

various conditions cause them to take more calcium

out of the ‘‘bone bank’’ than they deposit, and osteo-

porosis (which literally means porous bones) eventually

develops. Osteoporosis is a silent disorder. It usually

Osteoporosis in the vertebrae

SOURCE: National Women’s Health Center, U.S. Department of

Health and Human Services

Osteoporosis is most common in the hips, wrist, and

vertebrae (spine). The vertebrae are most important because

these bones support the body to stand and sit upright. The

vertebrae on the left is normal and the vertebrae on the right

has been affected by osteoporosis.(Illustration by GGS Infor-

mation Services/Thomson Gale.)

Osteoporosis