Published online by Cambridge University Press: 19 May 2010
Osteoporosis is a disorder of the skeletal system in which low bone mass and a deterioration of skeletal microarchitecture result in reduced bone strength and increased risk of fracture. Peak bone mass typically occurs in the third decade of life and is a result of genetic factors, nutrition, physical activity, and hormonal status. Age-related bone loss begins in the fourth or fifth decade of life and is accelerated in women during the early years of menopause. Factors that impair peak bone mass or accelerate bone loss can lead to osteoporosis. Medical conditions such as malabsorption and endocrine disorders that can impact peak bone mass and/or the rate of bone loss can increase the likelihood of osteoporosis.
The World Health Organization operationally defines osteoporosis as a bone mineral density (BMD) that falls 2.5 standard deviations or more below the mean for healthy young adults of the same race and sex. The T-score is the term given to express the number of standard deviations that an individual's BMD differs from the mean for healthy young adults of the same race and sex. A T-score lower than –2.5 is classified as osteoporosis and a T-score between –1.0 and –2.5 is considered to indicate osteopenia.
The primary clinical manifestations of this disorder are fractures of the spine and hip, although fractures may occur at any skeletal site. Osteoporosis exerts a significant toll on the quality of life and life expectancy of the elderly.