Ethel Siris, MD: The word osteopenia really means low bone mass. Well, what does that mean?Back in the early 1990s, the World Health Organization got together for the first time, and said how do we categorize people?We know a lot of people are fracturing, but how do we categorize them? So they said okay, we're going to measure bone density.We're going to measure bone density in a group of 30 year old women. Thirty year old women? These are women at their maximum bone mass.Your bones grow through puberty in height. And then, after that, they kind of thicken up, and when you're 30 or so, you've largely made as much bone as you're going to make.So if you take a large number of healthy 30 year olds, and you measure everybody's bone density, you're going to get an average value.Some are going to be above average, some are going to be below average, but you're going to get an average value. And that is given a score, a T score is what we call it, of zero.You now can measure a woman who's over the age of menopause. Do the bone density on her at the hip and the spine.If her measured bone density is no worse than about 10% below that average value in 30 year olds, which is called a T-score of minus one or better, we say she's normal.If that woman has a measured bone density that is 25% lower than that of the young normals, the score of minus 2.5 or more negative, that's called osteoporosis.Everything in between minus one and minus 2.5 is called osteopenia. It's lower than normal, but it's not yet lower enough to be given the word osteoporosis.