Even normal bone density tests in postmenopausal women may mask structural damage within, claims research.

Researchers found significant problems developing in just the space of a year.

Osteoporosis is a major health threat to particularly middle-aged or older women.

Weakening of bones, particularly at the hip, can lead to fractures in the event of a fall.

This can mean that a woman is left permanently disabled, or needs a radical operation such as a hip replacement.

The latest study, from Columbia University in the US, used 3D imaging techniques to look deep inside bone samples from the top of the hip bone.

Structure inside

They found that over a period of only a few months, not only did bone volume decrease, but so did important structures from the interior of the bone.

This "trabecular architecture" works like scaffolding, criss-crossing the cavity in the centre of large bones to increase their strength.

However, on average, the women had not suffered significant bone density reductions in the spine.

This means the problem may not have shown up on routine bone scans offered after the menopause.

On one group of patients, a drug called Actonel was tested to see if it could prevent the bone loss.

The researchers found that trabecular architecture was maintained while taking the drug.

Lifestyle and diet

Dr Jean Aaron, an osteoporosis specialist from the University of Leeds, said that a traditional bone density test might not be as reliable as women believed.

She told BBC News Online: "It's possible to get the situation where the bone mass does seem OK, but the architecture is deteriorating.

"There are some things that women should do, in terms of lifestyle and diet, to improve their bone health.

"However, the most important factor is the condition of the bones at their peak, at the age of 30, when they stop growing."

The research was presented at a meeting of the Endocrine Society in San Francisco.