Normal bone structure
Normal bones are composed of a shell of compact or
solid bone surrounding connecting plates and rods of bone (spongy bone)
within which lie the bone marrow. The thickness of the outer shell
of compact bone varies in different parts of the skeleton; for example,
it is much greater in the skull and bones of the legs and arms than
in the spine. Much of the strength of the skeleton is the result of
compact bone but the spongy bone also makes an important contribution.
Bone is actually made up mainly of a protein called collagen and bone
mineral, which contains calcium.
Bone is a living tissue which needs to be constantly
renewed to keep up its strength. All the time old bone is being broken
down and replaced by new stronger bone. If this process, which takes
place on the bone surface and is called bone remodelling, did not exist
our skeleton would begin to suffer from fatigue damage while we were
still young! There are two main types of cell in bone: the osteoclasts
which destroy bone and the osteoblasts which make new bone. Both of these
are formed in the bone marrow.
As we get older the osteoclasts become more active
and the osteoblasts less active, so more bone is removed and less formed.
Cross and long section of
bone (femur). |
|
Changes in bone in osteoporosis
In osteoporosis, the amount of both compact and spongy
bone is reduced. Thinning of the outer layer of compact bone greatly
reduces its strength and increases the likelihood of fracture. As bone
loss occurs in spongy bone, the thick plates and rods become very thin
and the continuity of structure is lost. These changes add to the weakening
of the bones caused by thinning of the compact shell around the bone.
Cross and long section of
typical vertebra. |
|
Bone mass changes throughout life
Peak bone mass
During childhood and adolescence, the bones not only
grow but also become more solid. By the age of about 25 years, the amount
of bone in the skeleton has reached its maximum; this is known as the
peak bone mass. Peak bone mass varies quite widely among individuals
and is generally higher in men than in women. As would be expected, it
is greater in those with a large body frame than in small, slim individuals.
The peak bone mass is very important in determining
whether an individual is at risk from osteoporosis later in life. If
it is low, then even small amounts of bone loss may result in fracture
whereas, if it is high, an individual will be protected from osteoporosis.
The factors that determine peak bone mass are not fully understood but
there is a strong genetic influence, and calcium intake and physical
exercise are also believed to be important. In addition, sex hormones
can influence peak bone mass, for example, amenorrhoea (absence of menstrual
periods) caused by anorexia nervosa or other illness will result in reduced
peak bone mass, whereas there is some evidence that oral contraceptive
use may result in a greater peak bone mass.
Front view of
skeleton. |
Side view of
skeleton. |
 |
 |
Age-related bone loss
In both men and women, age-related bone loss begins
around the age of 40 years and continues throughout life. In women about
35 per cent of compact bone and 50 per cent of spongy bone in the skeleton
is lost during a lifetime, whereas men lose about two-thirds of this
amount. The reason that women lose more bone than men is that, during
the menopause, the rate of bone loss increases for a few years. As women
have less bone to start with, lose increased amounts during the menopause
and live longer than men, they are more at risk from osteoporosis. In
fact, by the age of 80 years, nearly all women will have such a low bone
mass that they are likely to have a fracture if they fall. The causes
of age-related bone loss are not completely understood but oestrogen
deficiency is known to be mainly responsible for menopausal bone loss
in women.
In many individuals, age-related bone loss is sufficient
to result in osteoporosis in old age. In some cases, however, other factors
increase bone loss over and above that which would normally be expected
during ageing. These are considered in the next chapter.
Changes
in osteoporotic bone. |
 |
|
KEY POINTS
- Bone is composed mainly of protein and bone mineral,
which contains calcium
- During childhood and adolescence, the amount of
bone in the skeleton increases, reaching a maximum in the 20s
- From the age of around 40 years, the amount of
bone in the skeleton starts to decrease in both women and men; this
bone loss then continues throughout life
- The risk of developing osteoporosis depends on
how much bone a person has as a young adult and how quickly she or
he loses bone in later life
|
