Excellent news: The rate of hip fractures among older Americans has declined.

Two epidemiologists, analyzing a large government survey of patients discharged from hospitals between 1990 and 2006, found significant decreases among men over 85 and even more significant drops among women over 75.

This is major. Breaking a hip can be devastating; the prospect justifiably inspires dread among the elderly and their families.

“It has such long-term consequences,” said Judy A. Stevens, co-author of the study and a specialist in falls (which cause 90 percent of hip fractures) at the Centers for Disease Control and Prevention. She pointed out a few aftereffects: a one in four chance of spending a year or more in a nursing home, a doubled or tripled risk of dying, serious threats to mobility.

“Many people have seen friends and family members suffer hip fractures and never fully recover,” she said.

The only reason she’s not dancing a happy jig: she doesn’t know why the rate is dropping.

“I wish I could come up with something brilliant, but there isn’t a particularly good explanation,” Dr. Stevens said.

Without being able to pinpoint a cause, it’s hard to know what this decline tells us about prevention or whether this trend will continue.

Maybe better treatment for osteoporosis, a major risk factor, should get the credit? Dr. Stevens doesn’t think so. Medicare records show that women underuse screening tests and often abandon their drug regimens.

Another hypothesis is that old people have become healthier and more functional, and therefore fall less often — or that they fall less because they’re taking fewer psychoactive drugs, like tranquilizers and sedatives. It’s even possible that our country’s much-remarked-on obesity epidemic means older people have developed stronger bones (since just walking around becomes weight-bearing exercise) and benefit from more padding when they do fall.

Then there’s the cohort explanation. The older women who broke hips in the early 1990s — 1997 was the year that fracture rates peaked — were young during the Depression. Perhaps, Dr. Stevens speculates, because of food shortages and malnutrition, they never developed sufficient bone density.

So let’s celebrate good tidings — fewer hip fractures is certainly cause — while we can. And as the epidemiologists try to figure out what’s going on, let’s keep taking our calcium and vitamin D, removing scatter rugs from our homes, and doing tai chi.