Osteoporosis is defined as a proportional decrease of both bone mineral and bone matrix, leading to fracture after minimal trauma. It differs from osteomalacia in which there is a normal amount of bone matrix (osteoid) but decreased mineralization. There are two clinical syndromes of osteoporosis. Type I, or postmenopausal osteoporosis, occurs in women aged 51 to 75; it involves primarily trabecular bone loss, and presents as vertebral crush fractures or fracture of the distal radius. Type II, or senile osteoporosis, occurs in both men and women, particularly after the age of 60; it involves trabecular and cortical bone loss, and more commonly presents with hip and vertebral wedge fractures. Postmenopausal osteoporosis is associated with decreased serum levels of parathyroid hormone and a secondary decrease in activation of vitamin D, whereas senile osteoporosis is associated with a primary decrease in activation of vitamin D and increased parathyroid hormone.

Osteoporosis is an enormous public health problem, responsible for at least 1.2 million fractures in the United States each year. Fractures of the vertebral bodies and hip comprise the majority, and the complications of hip fracture are fatal in 12 to 20 percent of cases. Nearly 30 percent require long-term nursing home care. The direct and indirect costs of osteoporosis in the United States are estimated at over 8 billion in 1989 dollars annually.