On the basis of aetiology, osteoporosis can be classified as idiopathic or secondary.
This is essentially osteoporosis in the absence of another condition that can predispose to osteoporosis. It is therefore mainly due to natural changes in bone density that occur over time.
The normal pattern of bone density changes over a lifetime for men and women are shown below.
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In women, after the menopause there is greater rate of decline in bone density than in men for the first 5-10 years. Similarly due to women having an earlier growth phase in puberty than men, their peak bone density is less than men. These factors make women more likely to suffer osteoporosis than men, but its occurrence in men as well should be noted.
This post-menopausal change in women seems to be due to a lack of oestrogen. Low oestrogen levels have been shown to inhibit the osteoblast cells that form bone and stimulate the osteoclast cells that resorb bone (see 'What is bone remodelling?') This probably accounts for the rapid loss of bone density seen in women. Indeed, the fact it may be managed by Hormone replacement Therapy (HRT) using oestrogen supports this idea.
Other factors that predispose to idiopathic osteoporosis include:
This is where other medical conditions or medications can cause osteoporosis. These include:
1. Endocrine disorders
2. Metabolic problems
3. Specific conditions affecting bones