osteoporosis

osteoporosis is a condition in which the bones become porous and weak, and therefore fracture easily. The bone tissue is normal with a normal shape but it has lost mass and density and so lacks sufficient strength to withstand the forces which normally occur in daily life. There are no symptoms initially and the condition is often diagnosed only when a bone fracture occurs unexpectedly. These fractures occur in a third of all women and in one in twelve men. Post-menopausal and senile osteoporosis are recognized. The former is due to loss of oestrogen, the latter includes a time-dependent loss of bone common to men and women. Osteoporosis also occurs as a side-effect of some drug treatments, with oral corticosteroids for example, and occasionally, in a severe and little understood form at much younger ages.

Osteoporosis is often confused with osteoarthritis, another chronic problem of later life. However, osteoarthritis is a disease of the joints which is rare in those who have osteoporosis (unless they have been treated with corticosteroids).

Osteoporosis is not a disease like those caused by viral or bacterial infection, but a long term consequence of a small imbalance in the natural process of bone remodelling. Bone is a living tissue which constantly remodels itself through a process of resorption and formation known as bone turnover. Most processes in the body slow down as we grow older but bone turnover speeds up and the balance tips in favour of resorption, resulting in net bone loss. A greater imbalance develops in some people than in others and they are the ones who will suffer from osteoporosis, especially if they began with relatively low bone mass in middle age. This variation is to a large extent genetically determined, but lifestyle factors also contribute, including smoking, lack of exercise, low dietary calcium intake and, in the elderly, lack of vitamin D. Individuals who are small and thin are at greater risk because of their low bone mass compared with heavier individuals. Women who had an early menopause, or whose menstrual periods failed when they were young perhaps due to anorexia, are also at increased risk because the skeleton has had more prolonged exposure to a low oestrogen level. Although the process of resorption and formation is at the root of the osteoporotic condition, it is nevertheless a useful process which ensures that bone can repair minor damage and remodel itself in response to changing mechanical loads. It means that bone can respond positively to exercise and to drug treatments. Most of the effective drugs, such as bisphosphonates and hormone replacement therapy (HRT), act by slowing down resorption and therefore slowing the rate of loss of bone or tipping the balance in favour of formation.

The sites most commonly affected by osteoporosis are the wrist, the vertebrae in the spine, and the top of the femur (the hip). Vertebral fractures lead to collapse of the vertebrae which results in substantial loss of height or marked curvature of the spine (the Dowager's hump) and sometimes severe pain. Hip fractures occurring in the elderly in Britain cost the NHS nearly £1 billion in 1997 and the fracture rate has been rising faster than the increase in the number of elderly people in the population. The mortality rate following hip fracture is high and survivors usually suffer loss of independence and mobility. Both of these manifestations of osteoporosis were considered to be part of the normal ageing process until the middle of the twentieth century, and it was not until 1986 that the National Osteoporosis Society was established to provide support for sufferers, and advice and reliable information about the disease, which are still not widely available.

The osteoporotic condition develops slowly until so much bone has been lost that a threshold of vulnerability is reached and irreversible damage is likely. Preventative strategies are needed before this fragile state is reached. HRT is particularly useful for preventing post-menopausal loss in potentially vulnerable women. Adequate dietary calcium is essential. Dairy products such as cheese, yoghurt, and milk are rich in calcium. A pint of skimmed milk contains 700 mg which is the daily intake recommended in Britain. Smoking should be avoided, including passive smoking: it is known to interfere with the effect of oestrogen on bone. Excessive amounts of alcohol or caffeine (in tea, coffee, and coke) are also associated with a higher risk of osteoporosis.

The natural stimulus for bone to maintain its functional strength is the loading which results from gravitational forces and the tensions exerted by muscular activity. Astronauts lose bone while floating in space and so do patients who are confined to bed for long periods. Conversely, physically active people have higher bone mineral density compared with those who are sedentary. Exercise therefore has a role in reducing the long-term risk of osteoporotic fracture. The most effective exercise provides a regular series of varied short sharp loads to the sites which are most vulnerable for fracture. Brief exposure such as running up and down stairs a few times each day may be enough. Intermittent jogging (‘scouts' pace’) is useful, and so is weight-training, provided that over 70% of personal maximum effort is used in lifting slowly with a few repetitions. Research is still ongoing to find the best prescriptions. Improvements can probably occur at any age, but the increases appear to be largest before adolescence, and in later life vigorous exercise is obviously only safe for those who still have a robust skeleton. Bone changes slowly, improvements take months, and if the exercise is discontinued they are gradually lost again. In older people moderate exercise may prevent further loss of bone, and since fracture risk is only likely when bone density has fallen below a threshold value, maintenance is useful.

Joan Bassey

Bibliography

The National Osteoporosis Society publishes booklets. Helpline 01761 471771.

See also bone; hormone replacement therapy; menopause.