Osteoporosis is the most common generalized dis order of bone. It is estimated that in 2010 in the United States over 52 million people, 80% of whom are female, had some form of osteoporosis. It is most simply characterized as a state of insufficiently calcified bone and is the end result of a number of metabolic abnormalities that affect the skeleton. Some of the causes of osteoporosis that have been recognized or proposed include the following: (a) adrenal cortical hyperfunction, (b) prolonged dietary calcium deficiency, (c) reduced levels of estrogen, as in post menopausal women, (d) prolonged skeletal immobilization, and (e) chronic vitamin D deficiency. As a consequence of these “changes,” an imbalance results in the remodeling rate of bone, such that there is either an increase in the relative rate of bone resorption or a decrease in the rate of bone formation.
In osteoporosis, there can be an imbalance in the daily hormone mediated bone remodeling rates so that there is a small net daily loss of calcium from the skeleton. For example, if bone hormone mediated Ca2 + resorption exceeds hormone mediated bone accretion by only 10 mg/day, when extended for 20 years (e.g., from menopause at age ~50 to age ~70) there will be 10 mg/day × 365 days × 20 years = 73,000 mg of calcium lost from the skeleton or about 7.3% of the total skeletal calcium content.
The chief clinical sign of osteoporosis is thinning of the bones with a concomitant increase in the number of fractures (see Figure 1). Each year, in the US this disease leads to more than 1.5 million fractures, mostly of the hip, spine, and wrist. More than 350,000 osteoporosis-related life-threatening hip fractures occur annually and they all require hospitalization. The cost to the health care system associated with osteoporotic fractures is currently approximately $17 billion annually (2010). This converts to more than $45 million a day! Each hip fracture represents an estimated $40,000 in total medical costs.
Fig1. Effect of long-standing osteoporosis on the mineral content of lumbar vertebrae. Shown are the microradiographs of a lumbar vertebra from a young (A) and an elderly (B) female.
