The World Health Organization "WHO" has established criteria for making the diagnosis of osteoporosis, as well as determining levels which predict higher chances of fractures. These criteria are based on comparing bone mineral density (BMD) in a particular patient with those of a 25 year old female. BMD values which fall well below the average for the 25 year old female (stated statistically as 2.5 standard deviations below the average) are diagnosed as "osteoporotic". If a patient has a BMD value less than the normal 25 year old female, but not 2.5 standard deviations below the average, the bone is said to be "osteopaenic" (osteopaenic means decreased bone mineral density, but not as severe as osteoporosis).
Osteoporosis is different from most other diseases or common illnesses in that there is no one single cause. The overall health of a person's bones is a function of many things ranging from how well the bones were formed as a youth, to the level of exercise the bones have seen over the years. During the first 20 years of life, the formation of bone is the most important factor, but after that point it is the prevention of bone loss which becomes most important. Anything which leads to decreased formation of bone early in life, or loss of bone structure later in life will lead to osteoporosis and fragile bones which are subject to fracture.