The Food and Drug Administration on Tuesday approved an osteoporosis drug that represents the first new treatment approach in nearly two decades — a strategy based on a rare gene mutation in people with bones so dense that they never break.

About 10 million people in the United States have osteoporosis. Worldwide, about 200 million people have brittle bones; one in three women, and one in five men, will suffer a fracture because of osteoporosis, often of the hip or spine. For many, the break leads to a downward spiral of disability.

Standard treatments, drugs called bisphosphonates, stop the loss of bone but do not build it. The alternatives, parathyroid hormone and a derivative, build bone but also break it down, limiting the therapeutic effect.

The new drug, romosozumab (brand name Evenity), developed by Amgen in collaboration with the Belgian drug company UCB, restores bone without breaking it down, according to the findings of two large clinical trials.