A condition of reduced bone mass, with decreased outer thickness and a decrease in the number and size of the spongy structures in the bone (but normal chemical composition), resulting in increased fracture incidence. Osteoporosis is primarily classified as an age-related decline in bone mineral density. Secondary osteoporosis results from an identifiable disease (for example, diseases of the parathyroid glands, or, for instance, certain medications). See the Osteoporosis page for additional information.