Halting osteoporosis, the inevitable weakening of bone, is the best way to avoid the hip and spine fractures that are the leading cause of health problems in the elderly. Current drugs for osteoporosis work by blocking the effect of bone-destroying cells, which increase in number as people age. But a new compound under review by the FDA tackles the problem in a different way by curbing the formation of the bone-gnawing cells. That tilts the balance in favor of bone-building. In two studies published in August, the experimental compound denosumab was shown to reduce the risk of fractures in postmenopausal women as well as men being treated for prostate cancer, the two largest patient populations at risk for bone loss. What's not clear, however, is how the new drug, if approved, would compare with existing osteoporosis drugs like Fosamax, Boniva and Reclast.