Bone density tests quickly became a rite of passage for many postmenopausal women in their 50s. When doctors started scanning these women, however, a problem emerged, says Cummings, who has co-authored some of the largest studies of osteoporosis in the past three decades. Instead of regarding “low bone density” as one sign of risk, doctors equated it with full-blown osteoporosis. Even worse, under the banner of early detection and prevention, bone density that was slightly lower than average got its own medical label—osteopenia—and some doctors started treating that condition with drugs, too.
The conflation of disease with disease risk might not be so bad, Cummings says, if bone density tracked tightly with the incidence of serious fractures at every age, under every condition. But it does not. Among 16,000 postmenopausal women in Manitoba who received baseline bone scans at age 50 or older, for example, most of those who eventually suffered fractures had normal bone density, according to a 2007 study in the Canadian Medical Association Journal. As the studies piled up, Cummings notes, “it quickly became evident that in a group of people with the same bone mineral density, some got fractures and others didn't. Clearly, some other feature of bone plays an important role here.”
That should not come as a surprise, says Markus Seibel, who studies bone metabolism at the University of Sydney. Much of modern medicine is about treating risk instead of symptoms, he notes. Doctors attempt to lower bad cholesterol in hopes of preventing a heart attack, for example. But relying strictly on numbers to predict health outcomes is tricky. “The more we move away from actual disease, the harder it is to predict what will happen in a particular patient,” Seibel says.
So far, Seibel observes, scientists have not identified the underlying physiological features that make a bone resistant or prone to cracks. Large epidemiological studies, however, have revealed more characteristics of people that, when taken together with measurements of bone density, can help improve predictions about who will suffer a major fracture. In 2008 the World Health Organization integrated 12 of the most influential of these risk factors into an algorithm that is the basis for an easy-to-use online risk calculator known as FRAX.