Osteoporosis is diagnosed by two criteria. The first is the occurrence of a fragility fracture in one of the major bones and the second is a DXA bone density test results with a T-score of –2.5 or less in the spine, hip or wrist.
Fragility Fractures Due to Osteoporosis
Until the advent of reliable bone densitometry measurements at the hip and spine the diagnosis of osteoporosis depended solely upon the patient experiencing fragility fracture typical of the disease. A fragility fracture is defined as a fracture occurring as a result of a fall from standing height or with minimal trauma or stress.
Osteoporotic fragility fractures commonly occur in the thoracic and lumbar vertebra, radius and ulna, proximal femur, bones of the pelvic girdle, humerus, ribs, tibia, and fibula. Rare sites for osteoporotic fragility fractures are the sternum and calcaneus.
A epidemiologic study showed that in U.S. whites, 80% of fractures occurring after age 45 irrespective of the circumstances related to the fracture were related to osteoporosis or poor bone quality.
As a general rule, fractures of the phalanges and metatarsals (fingers and toes), clavicles, and head (skull and jaw) are not considered due to osteoporosis.
Fractures occurring in growing children, especially the forearms of girls and boys are not due to osteoporosis.
Diagnosis of Osteoporosis by DXA Bone Densitometry
In 1993 the World Health Organization working group on bone and skeletal health issued a report on osteoporosis diagnosis using DXA bone densitometry. This report revolutionized the management of osteoporosis because it allowed clinicians to recognize people who were are high risk for fracture well before their bone broke in plenty of time for the judicious application of safe and effective treatment. The problem was in 1993, we had no safe and effective treatment.
The WHO diagnosis is based on the T-score, which is statistical device that compares the patient to young healthy people of the same gender, race and country. T-scores can be roughly translated into % change using the rule of thumb that a T-score of –1 is equal to a bone density that is 10% lower than the average person like the patient. A T-score of –1 was established by the WHO group as the threshold between normal and osteopenia.
Osteopenia is not a Disease
Osteopenia is a bone density range between normal and osteoporosis. I call it the “zero,” the “warning area,” or the “safe zone” depending on the context. When trying to explain what it is I will say that osteopenia is like the 0 on in a number series of +2 +1 0 –1 –2. You cannot go straight from +1 to –1. A zero was invented to occupy that place so we could work with imaginary numbers like –1. The other example was the clinical need to have a bone density that proceeded osteoporosis that people traveled through before becoming osteoporotic. That bone density area became osteopenia.
I call it the “warning area” when I want to get the attention of the patient who is loosing bone just how close they are to becoming osteoporotic. While it is semantics because risk for fracture is continuous, it is not linear. Fracture risk is an exponential variable whose inflexion point moves sharply upward around a T-score of –2.5. This means the fracture risk of someone at –2.75 is a whole lot higher than at –2.25 even though they are just 0.5 T-scores apart.
One the other hand, I call it the “safe zone” when discussing a patient’s successful response to treatment when their DXA bone density has risen progressively our of the “danger zone” into the “safe zone” and “technically” they no longer have osteoporosis, but they do because if they stop treatment they will fall back into the “danger zone”. We discuss that too but it is a time for rejoicing all the same.
Osteoporosis by Bone Density Begins When the T-score Falls Below –2.5
When the T-score is below –2.5, osteoporosis is said to be present even in the absence of a fragility fracture. This was a reasonable decision by the WHO working group because when you inspect fracture risk vs. bone density the rate of increase becomes exponential between –2 and –2.5. While only about 18% of white postmenopausal women ever become osteoporotic they suffer 50% of the fractures caused by this disease. Bone density testing is an excellent method of identifying them pre-fracture when safe and effective therapy can now be administered that prevent fractures before they occur.