Bone density and bone size at the two key factors in the development of osteoporosis. Bone density depends to some extent on the amount of calcium, phosphorous and other minerals that the body has absorbed.
There is a rapid decline in bone density in women after the menopause. This is due to the ovaries no longer producing oestrogen. Estradiol is the most potent form of oestrogen and appears to be a very strong factor in the deveolopment of osteoporosis. Estrone and estriol are other important oestregens. Oestregen's most important role appears to be in the prevention of bone breakdown. oestregen is also thought to be responsible for helping to maintain normal levels of vitamin D which has an important role in bone protection.
The risk of fractures rises with age with the incidence of hip fractures doubling for both genders between the ages of 55 and 75
Cigarette smoking inhibits the activity of the bone growing cells (osteoblasts). A sedentary lifestyle with little weight bearing exercise increases the risk of developing osteoporosis.
It is believed that oestrogen in some way controls the lifespan of the osteoclasts (the cells that break bones down)
In men, it is thought that declining levels of testostorone and oestrogen in elderly men contribute to bone loss.
Reduced physical activity as part of the ageing process may also be a significant factor.
Some studies have shown a link between vitamin D deficiency and osteoporosis, vitamin D is essential for the absorption of calcium into the bone and for normal bone growth. It is thought that hyperparathyroidism may also stimulate bone loss as high levels of parathyroid hormone stimulate bone loss. In one study women with low oestrogen levels were even more sensitive to this destructive aspect of parathyroid hormone.
Genetic factors are thought to have an influence on peak bone mass which is an important determinant of bone mass in later life. The BMJ makes the point that other factors such as "bone size and geometry and bone structure and material contribute to increased bone fragility".
Many people are treated with steroids for other conditions. Oral corticosteroids are known to reduce bone mass. Inhaled steroids have the same effect but to a lesser degree. The following medications are also known to increase the risk of bone loss.
Osteporosis can be secondary to the following conditions:
