Osteoporosis is silent. People’s bones can gradually become weaker and weaker with no outward hints that there is a problem — until suddenly bones break. Spontaneous compression fractures of the spine can be quite painful and lead to deformity, chronic pain, and premature death. Broken hips are another risk factor for early death — approximately 20% of people with a broken hip die within one year of the fracture.
In an ideal world, osteoporosis would not occur. In theory, osteoporosis is entirely preventable. Since we don’t live in an ideal world, it is crucial that osteoporosis be identified early and treated aggressively.
DEXA is the gold-standard in osteoporosis testing. DEXA scanners (Dual-Energy X-ray Absorptiometry) (also called DXA) use two separate very low-radiation x-ray beams – about 1/10th the radiation of standard x-rays — to image the hips and spine to measure bone mineral density. As we would expect from the “dual energy” portion of the name, these two x-ray beams have different energy levels. Bone mineral density is calculated by measuring the difference between what is absorbed from the first beam and the second.
Test results will provide a variety of numbers.
- Bone Mineral Density
Bone Mineral Density (BMD) is a raw number indicating the average concentration of minerals in your bones. The higher the number, the higher the bone mineral density and the stronger the bones. Lower numbers indicate weaker bones.
T-score and Z-score are based on statistics. Compiling the results from many people has allowed scientists to determine what is normal bone mineral density, and what constitutes strong or weak bones. Graphing the data forms a picture shaped somewhat like a bell.
A brief aside about statistics: in statistics, the mean is the average — it tells us what is normal. The standard deviation tells us how far away something is from what is normal. 68% of all data will only deviate slightly from the average (will be within one standard deviation of the mean) — this makes sense because obviously most things should be close to what is normal. On a bell curve (pictured below), the mean does not deviate at all from what is normal, thus the center of the curve deviates zero (labeled 0), and most of the data clusters close to the middle — one standard deviation is labeled +1 (above zero) and -1 (below zero). 95% of the data will be within two standard deviations of the mean (labeled +2 and -2), and 99.7% of the data will be within three standard deviations of the mean. It is very rare for something to deviate significantly from what is normal.
T-score and Z-score numbers indicate standard deviations from the mean on a bell curve. A T-score compares your BMD with healthy young adults who have good bone mineral density. A Z-score compares your BMD with others of your age and ethnicity. Doctors are most concerned with the T-score.
A bone mineral density scan T-score that is more than one standard deviation below the mean is bad. Between one and 2.4 standard deviations below normal is osteopenia, while a T-score of 2.5 or more standard deviations below the mean is osteoporosis.
Who should get a bone density scan? The general rule is women at age 65 or men at age 70. Before age 65, the test is only considered if you have risk factors, and if treatment would occur based on test results. If you wouldn’t be treated, there’s no point in having the test done. Bone density scans are rarely done on premenopausal women; until menopause, high estrogen levels seem to provide protection against broken bones even in people with low bone mineral density. The question is, what are the risk factors?
Rheumatoid arthritis is just one of many risk factors for development of osteoporosis. Other risk factors include:
- history of taking 5mg or more of corticosteroids for more than three months
- taking methotrexate (other meds, too)
- family history of osteoporosis
- history of an immediate family member with a fragility fracture
- history of bone fracture as an adult
- loss of height
- weight of less than 127 pounds
- being a smoker
- eating a diet low in calcium
- avoiding sunlight (indicative of low vitamin D production)
Given these risk factors, it is no surprise than rheumatologists refer patients for bone density scans.
Everything published about DEXA says that it is painless. This information is obviously prepared by people who have never had the test. Although it is technically true that the x-rays themselves do not inflict pain, before the scan is taken you’re strapped to a table in an uncomfortable position and required to stay tied down for the duration of the 15-20 minutes of the test. Although the average person might not be physically injured by the scan, it is inaccurate to say that the test is painless. More accurately, the test is uncomfortable, but not unbearable.
After the test is complete, the referring doctor will receive a report showing your Bone Mineral Density, your T-score, and your Z-score. Your report might also include use of the World Health Organization’s Fracture Risk Assessment Tool (FRAX) . This attempts to calculate a person’s probability of fracture within the next ten years with the goal of frightening patients into taking osteoporosis seriously. A 28% risk of fracture within ten years is about 2.8% per year. 2.8 doesn’t sound nearly as scary as 28%, though, thus the use of ten-year risk estimates. If you click on the link, select “calculation tool” and then select your continent/country and complete the questionnaire. The calculation can be made either with or without results of a bone density scan.